U.S. patent application number 15/786907 was filed with the patent office on 2018-05-31 for roofing membrane for mitigating passive intermodulation.
The applicant listed for this patent is JOHNS MANVILLE. Invention is credited to Thomas Bell, Christopher Griffin, Jacob Heaps, Jordan Kortmeyer.
Application Number | 20180147812 15/786907 |
Document ID | / |
Family ID | 62192848 |
Filed Date | 2018-05-31 |
United States Patent
Application |
20180147812 |
Kind Code |
A1 |
Bell; Thomas ; et
al. |
May 31, 2018 |
ROOFING MEMBRANE FOR MITIGATING PASSIVE INTERMODULATION
Abstract
Materials and methods for mitigating passive intermodulation. A
membrane for reducing passive intermodulation includes a first
polymeric layer, a second polymeric layer, and a continuous metal
layer encapsulated between the first and second polymeric layers. A
self-adhesive radio frequency barrier tape includes a waterproof
polymeric top layer, a metal-containing layer adhered by an
adhesive layer to the polymeric top layer, a pressure sensitive
adhesive layer adhered to the metal-containing layer, and a release
liner on a bottom surface of the pressure sensitive adhesive layer.
A method of mitigating passive intermodulation includes passing a
probe over an area of interest, the probe being sensitive to an
intermodulation frequency of interest, and identifying a suspected
source of passive intermodulation when the amplitude of the probe
output exceeds a threshold at the frequency of interest. The method
further includes covering the suspected passive intermodulation
source with a radio frequency barrier material.
Inventors: |
Bell; Thomas; (Colorado
Springs, CO) ; Griffin; Christopher; (Aurora, CO)
; Heaps; Jacob; (Edgewater, CO) ; Kortmeyer;
Jordan; (Parker, CO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JOHNS MANVILLE |
Denver |
CO |
US |
|
|
Family ID: |
62192848 |
Appl. No.: |
15/786907 |
Filed: |
October 18, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62426673 |
Nov 28, 2016 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04B 17/104 20150115;
B32B 2307/212 20130101; B32B 2395/00 20130101; B32B 11/10 20130101;
B32B 27/304 20130101; B32B 2323/16 20130101; C09J 7/22 20180101;
H01Q 3/267 20130101; B32B 9/045 20130101; B32B 2250/40 20130101;
B32B 2262/0215 20130101; C09J 7/29 20180101; H01Q 15/14 20130101;
B29C 48/0021 20190201; B32B 7/08 20130101; B32B 3/266 20130101;
B32B 7/06 20130101; B32B 21/08 20130101; B32B 2255/12 20130101;
B32B 2250/05 20130101; B32B 2307/732 20130101; B32B 2307/20
20130101; C09J 195/00 20130101; H04B 17/345 20150115; B32B 7/12
20130101; B32B 2255/205 20130101; B32B 2307/748 20130101; B32B
15/085 20130101; C09J 7/20 20180101; B32B 5/028 20130101; H04B
15/02 20130101; B29C 48/154 20190201; B32B 11/06 20130101; B32B
15/20 20130101; B32B 27/08 20130101; B32B 2307/206 20130101; C09J
7/38 20180101; C09J 2301/40 20200801; C09J 2423/166 20130101; B32B
27/36 20130101; B29C 48/022 20190201; B32B 11/00 20130101; C09J
2301/302 20200801; G21F 1/125 20130101; C09J 2495/006 20130101;
B32B 5/18 20130101; B32B 2262/0276 20130101; B32B 2311/24 20130101;
B32B 11/042 20130101; B32B 15/082 20130101; H01Q 17/00 20130101;
B32B 27/12 20130101; B32B 2255/10 20130101; C09J 2427/006 20130101;
H04B 15/00 20130101; B32B 27/32 20130101; B32B 2327/06 20130101;
C09J 2495/00 20130101; B29C 48/0011 20190201; B32B 15/08 20130101;
B32B 15/14 20130101; B32B 2307/402 20130101; B32B 7/04 20130101;
B32B 11/08 20130101; B32B 27/065 20130101; C09J 2400/163 20130101;
H01Q 1/246 20130101; B32B 11/04 20130101; B32B 11/046 20130101;
B32B 27/18 20130101; B32B 2419/06 20130101; B32B 2262/101 20130101;
B32B 27/10 20130101; B32B 7/02 20130101 |
International
Class: |
B32B 7/02 20060101
B32B007/02; B32B 27/32 20060101 B32B027/32; B32B 27/30 20060101
B32B027/30; B32B 15/085 20060101 B32B015/085; B32B 15/20 20060101
B32B015/20; B29C 47/00 20060101 B29C047/00; G21F 1/12 20060101
G21F001/12 |
Claims
1. A membrane for reducing passive intermodulation, comprising: a
first polymeric layer; a second polymeric layer; and a continuous
metal layer encapsulated between the first and second polymeric
layers.
2. The membrane of claim 1, wherein at least one of the first and
second polymeric layers comprises thermoplastic polyolefin
(TPO).
3. The membrane of claim 1, wherein at least one of the first and
second polymeric layers comprises ethylene propylene diene monomer
(EPDM).
4. The membrane of claim 1, wherein at least one of the first and
second polymeric layers comprises polyvinyl chloride (PVC).
5. The membrane of claim 1, wherein at least one of the first and
second polymeric layers comprises modified bitumen.
6. The membrane of claim 1, wherein the metal layer is a layer of
aluminum.
7. The membrane of claim 6, wherein the layer of aluminum has a
thickness between 10 and 100 nanometers.
8. The membrane of claim 1, wherein the metal layer has a thickness
of at least 1 micron.
9. The membrane of claim 1, wherein the metal layer comprises
aluminum vapor deposited on a film, and wherein the aluminum and
the film are encapsulated between the first and second polymeric
layers.
10. The membrane of claim 9, wherein the film comprises one or more
materials selected from the group of materials consisting of
polyethylene, polyester, and polypropylene.
11. The membrane of claim 1, wherein the metal layer comprises a
metal foil bonded to a reinforcement mat.
12. The membrane of claim 1, comprising: two metal layers
encapsulated between the first and second polymeric layers; and a
plastic film separating the two metal layers.
13. The membrane of claim 1, wherein the metal layer comprises
copper.
14. The membrane of claim 1, wherein the membrane has an overall
thickness of between 0.020 and 0.250 inches.
15. The membrane of claim 14, wherein the membrane has an overall
thickness of between 0.040 and 0.150 inches.
16. The membrane of claim 1, wherein the upper surface of the
membrane is white.
17. A membrane for reducing passive intermodulation, comprising: a
first polymeric layer; a second polymeric layer; and a metal layer
encapsulated between the first and second polymeric layers; wherein
the first and second polymeric layers comprise one or more
materials selected from the group of materials consisting of
thermoplastic polyolefin (TPO), ethylene propylene diene monomer
(EPDM), and polyvinyl chloride (PVC).
18. The membrane of claim 19, wherein the metal layer is
perforated.
19. A method of making a membrane for mitigating passive
intermodulation, the method comprising: obtaining a metallized
sheet material having a continuous layer of metal supported on a
plastic film, the metallized sheet material having first and second
sides; extruding a first polymeric layer onto the first side of the
metallized sheet material; and extruding a second polymeric layer
onto the second side of the metallized sheet material.
20. The method of claim 19, wherein extruding the first or second
polymeric layer comprises extruding thermoplastic polyolefin
(TPO).
21. The method of claim 19, wherein extruding the first or second
polymeric layer comprises extruding ethylene propylene diene
monomer (EPDM).
22. The method of claim 19, wherein extruding the first or second
polymeric layer comprises extruding polyvinyl chloride (PVC).
23. The method of claim 19, wherein extruding the first or second
polymeric layer comprises extruding modified bitumen.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of provisional U.S.
patent application No. 62/426,673, filed Nov. 28, 2016 and titled
"PIM Reducing RF Barrier", the entire disclosure of which is hereby
incorporated by reference herein for all purposes.
BACKGROUND
[0002] An essential element of modern mobile communications systems
is the "cell site." The cell site includes one or more directional
base station antennas aimed at a desired geographical area of
coverage with coaxial cable connecting the antennas to base station
radio equipment. The base station radio equipment acts as a
transceiver, transmitting high power signals (37 dBm to 49 dBm
channel power) in the direction of mobile users and receives low
power signals (as low as -106 dBm channel power) from mobile user
equipment. Often the range of a cell site is uplink limited,
meaning that the distance that a particular cell site is able to
cover is limited by the transmit power of the mobile equipment and
the path loss between the mobile equipment and the cell site. The
signal received at the cell site from mobile equipment reduces by 6
dB each time the distance between the mobile equipment and the cell
site is doubled. Eventually, the signal from the mobile equipment
drops to a level where the base station receiver is not able to
distinguish the signal from background noise, and the call is
dropped.
[0003] Mobile operators typically design their network of cell
sites to achieve a signal-to-interference plus noise ratio (SINR)
of >6 dB across the region of coverage. This means that the
signal level arriving from mobile equipment at each base station
receiver should be at least 6 dB higher than all sources of noise
generated within or received by the base station receiver. For data
networks, the network of cell sites may be designed to achieve even
higher SINR levels to enable faster data transmission.
[0004] In order to achieve the strongest possible signal from
mobile equipment, it is desirable to locate the cell site antennas
at an elevation that is higher than the average clutter (trees,
buildings, etc.) in the area to be covered. This can be achieved by
erecting a tower at the cell site to support the antennas at the
desired elevation or can be achieved by mounting the antennas on
existing infrastructure, such as building rooftops. When mounting
antennas on buildings, the building owner may require for aesthetic
reasons that the operator only install the antennas at locations
that are not visible from the ground outside the building. This
often forces the antennas to be installed near the center of the
building rooftop rather than near the edges of the building.
[0005] For example, FIG. 1 shows a ground-level perspective view of
a building 100, showing that no cell site is visible. However, FIG.
2 shows building 100 from an upper oblique view, revealing cell
site 200 on roof 201, set back from the edges of roof 201.
[0006] The performance of cell sites installed on building rooftops
is often limited due to passive intermodulation (PIM). Passive
intermodulation occurs when the high-power downlink signals
broadcast by the base station antennas mix at passive, non-linear
junctions in the RF path, creating new signals. If these new
signals (intermodulation products) fall in an operator's uplink
band, they act as interference and reduce SINR. As the SINR is
reduced, the geographic coverage of the cell site is reduced and
the data capacity of that cell site is reduced.
BRIEF SUMMARY
[0007] According to one aspect, a membrane for reducing passive
intermodulation comprises a first polymeric layer, a second
polymeric layer, and a continuous metal layer encapsulated between
the first and second polymeric layers. In some embodiments, at
least one of the first and second polymeric layers comprises
thermoplastic polyolefin (TPO). In some embodiments, at least one
of the first and second polymeric layers comprises ethylene
propylene diene monomer (EPDM). In some embodiments, at least one
of the first and second polymeric layers comprises polyvinyl
chloride (PVC). In some embodiments, at least one of the first and
second polymeric layers comprises modified bitumen. The metal layer
may be a layer of aluminum. The layer of aluminum may have a
thickness between 10 and 100 nanometers. The metal layer may have a
thickness of at least 1 micron. In some embodiments, the metal
layer comprises aluminum vapor deposited on a film, and the
aluminum and the film are encapsulated between the first and second
polymeric layers. In some embodiments, the film comprises one or
more materials selected from the group of materials consisting of
polyethylene, polyester, and polypropylene. In some embodiments,
the metal layer comprises a metal foil bonded to a reinforcement
mat. In some embodiments, the membrane comprises two metal layers
encapsulated between the first and second polymeric layers, and a
plastic film separating the two metal layers. The metal layer may
comprise copper. In some embodiments, the membrane has an overall
thickness of between 0.020 and 0.250 inches. In some embodiments,
the membrane has an overall thickness of between 0.040 and 0.150
inches. The upper surface of the membrane may be white.
[0008] According to another aspect, membrane for reducing passive
intermodulation comprises a first polymeric layer, a second
polymeric layer, and a metal layer encapsulated between the first
and second polymeric layers. The first and second polymeric layers
comprise one or more materials selected from the group of materials
consisting of thermoplastic polyolefin (TPO), ethylene propylene
diene monomer (EPDM), and polyvinyl chloride (PVC). The metal layer
may be perforated.
[0009] According to another aspect, a method of making a membrane
for mitigating passive intermodulation comprises obtaining a
metallized sheet material having a continuous layer of metal
supported on a plastic film, the metallized sheet material having
first and second sides, extruding a first polymeric layer onto the
first side of the metallized sheet material, and extruding a second
polymeric layer onto the second side of the metallized sheet
material. In some embodiments, extruding the first or second
polymeric layer comprises extruding thermoplastic polyolefin (TPO).
In some embodiments, extruding the first or second polymeric layer
comprises extruding ethylene propylene diene monomer (EPDM). In
some embodiments, extruding the first or second polymeric layer
comprises extruding polyvinyl chloride (PVC). In some embodiments,
extruding the first or second polymeric layer comprises extruding
modified bitumen.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 shows a ground-level perspective view of a
building.
[0011] FIG. 2 shows the building of FIG. 1 from an upper oblique
view, revealing a cell site on the roof of the building.
[0012] FIG. 3 illustrates the generation of passive
intermodulation.
[0013] FIG. 4 illustrates the building of FIG. 1 with an RF barrier
installed on the roof of the building, in accordance with
embodiments of the invention.
[0014] FIGS. 5 and 6 show measurements of a typical cell site
installation, before and after placement of an RF barrier in
accordance with embodiments of the invention.
[0015] FIG. 7 illustrates a PIM barrier membrane, in accordance
with embodiments of the invention.
[0016] FIG. 8 illustrates a process of making a PIM barrier
membrane, in accordance with embodiments of the invention.
[0017] FIG. 9 illustrates a PIM barrier membrane in accordance with
other embodiments of the invention.
[0018] FIG. 10 illustrates an exploded view of a self-adhesive RF
barrier tape in accordance with embodiments of the invention.
[0019] FIG. 11 shows an edge view of the self-adhesive RF barrier
tape of FIG. 10, in its assembled state.
[0020] FIG. 12 illustrates a technique for locating PIM sources, in
accordance with embodiments of the invention.
[0021] FIG. 13 shows suspected PIM sources covered with respective
pieces of barrier material, in accordance with embodiments of the
invention.
[0022] FIG. 14 illustrates large pieces of barrier material applied
to the entire area of interest on a roof, in accordance with
embodiments of the invention.
[0023] FIG. 15 illustrates a mat RF barrier, in accordance with
embodiments of the invention.
[0024] FIG. 16 illustrates the installation of the mat RF barrier
of FIG. 15, in accordance with embodiments of the invention.
[0025] FIG. 17 illustrates a mat RF barrier in accordance with
other embodiments of the invention.
[0026] FIG. 18 illustrates an RF barrier membrane in accordance
with other embodiments of the invention.
[0027] FIG. 19 illustrates the installation of the RF barrier
membrane of FIG. 18, in accordance with embodiments of the
invention.
DETAILED DESCRIPTION
[0028] Loosely touching metal-to-metal surfaces as well as rusty
metal surfaces behave in a non-linear fashion and can generate PIM
when illuminated with RF energy. On rooftop cell site installations
where the antennas are placed near the center of the roof, many
non-linear objects may be present in front of the antenna. PIM
sources may be located within the cellular network equipment or
externally. Common external sources include loosely contacting
sheet metal flashing used for weatherproofing, loose connections at
overlapping metal decking members, rusty bolts, vent pipes, and the
like. Common internal sources may include loose mechanical
junctions, oxidized or contaminated surfaces, connections or
contact between dissimilar metals, and the like. Loose
metal-to-metal contacts can also occur at the fastener locations,
where due to time and thermal expansion, the heads of screws or
nails are no longer applying high contact pressure. Since steel is
a common material used in roof construction, rust is likely to form
at locations where moisture has been able to penetrate the roof
membrane.
[0029] FIG. 3 shows an enlarged view of a portion of FIG. 2, and
illustrates PIM in more detail. Cell site 200 includes one or more
antennas 301, connected to control electronics 302. Cell site 200
may be surrounded by an RF-transparent screen 303 for aesthetic
purposes. Antenna 301 transmits relatively high-power downlink
signals 304, and receives relatively low-power uplink signals 305
from a number of cellular phones in the vicinity.
[0030] In FIG. 3, various PIM sources 306 are shown emitting RF
noise 307 in response to exposure to downlink signals 304. Some of
the RF noise is directed toward antenna 301, and some may fall
within the uplink band, thus reducing the uplink SINR. Many PIM
sources 306 may be encompassed in the relatively wide area of roof
201 exposed between cell site 200 and the edges of roof 201 and
subject to downlink signals 304.
[0031] Rooftop PIM sources at cellular installations have been
shown to produce 3rd order intermodulation products (IM3) as high
as -50 dBm when the site is tested by injecting 43 dBm test tones
into the cell site antenna. This level of PIM is almost 50 dB
(100,000 times) higher than the typical "passing" PIM level
required for acceptable site performance.
[0032] In accordance with embodiments of the invention, covering
the non-linear objects on the rooftop with an RF barrier
significantly reduces the PIM generated by these non-linear
objects. For every 1 dB that the RF barrier is able to attenuate
the signal arriving at a non-linear junction, the IM3 level seen at
the base station receiver is typically reduced by 3.5 dB.
[0033] FIG. 4 illustrates building 100 and cell site 200 with an RF
barrier 401 installed on roof 201, over PIM sources 306. RF barrier
401 reduces the RF energy reaching PIM sources 306, and
consequently reduces the amount of PIM generated at sources
306.
[0034] RF barrier 401 preferably reduces the RF energy reaching PIM
sources 306 by at least 10 dB. This will reduce the magnitude of
PIM generated by PIM sources under the barrier by approximately 35
dB. In other embodiments more or less attenuation may be provided.
For example, FIGS. 5 and 6 show measurements of a typical cell site
installation, before and after placement of an RF barrier such as
RF barrier 401. In the example of FIGS. 5 and 6, a reduction in PIM
magnitude of approximately 36 dB was achieved, from -61.1 dBm to
-97.8 dBm.
[0035] The attenuation may be achieved through absorption,
reflection or a combination of the two. For example, a reflective
material such as RF barrier 401 reflects at least some of the
original RF signals 304 from antenna 301, reducing the induced
current in the underlying PIM sources, and ultimately reducing the
magnitude of the PIM generated.
[0036] Preferably, the materials used to construct RF barrier 401
do not themselves become a PIM source. For example, RF barrier 401
should not contain loose metal-to-metal contacts, and should be
installable in a way that prevents loose metal-to-metal contacts
from being formed. Preferably, RF barrier 401 can be cut in the
field to custom shapes without generating burrs that could touch
other metal surfaces and generate PIM.
[0037] In cases where the RF barrier is applied to existing rooftop
surfaces, it is desirable that RF barrier 401 and any mounting
system for RF barrier 401 not penetrate the existing roof membrane,
so that any warranty of the existing roof is not voided. In
addition, penetrations are preferably avoided because they are
typically made with metal fasteners, which could become PIM sources
themselves.
[0038] Preferably, the materials of RF barrier 401 provide good
environmental stability and durability, similar to typical roofing
materials. For example, RF barrier 401 may provide up to 10 years
or more of operating life when installed in a rooftop environment.
This includes exposure to ultraviolet radiation, wind, rain, snow,
extreme heat and extreme cold. The material of RF barrier 401
preferably will not rot, support fungal growth or delaminate after
long term environmental exposure. In addition, an RF barrier 401
applied in an area subject to foot traffic should be able to
withstand people walking on the barrier without physical damage or
deterioration in performance.
[0039] FIG. 7 illustrates a PIM barrier membrane 700, in accordance
with embodiments of the invention. Example membrane 700 includes a
metal layer 701 sandwiched between two polymeric layers 702 and
703. Metal layer 701 may include, for example, a layer of aluminum
having a thickness of 10 to 100 nanometers. Other metals may be
used, for example copper, and other thicknesses of metal may be
used if desired. For example, in other embodiments, the thickness
of the metal layer may be up to 40 microns or more. The metal of
metal layer 701 may be vapor deposited or otherwise coated on a
film 704 such as a 0.0005 to 0.025 inch thick polyester film or
other plastic film for ease of handling. In some embodiments, the
metal layer 701 may be enclosed between two similar plastic films
704 and 705. In some embodiments, metal layer 701 may be a metal
foil, which in turn may be bonded to a reinforcement mat made of
polyester, glass, or another suitable material.
[0040] Metal layer 701 may be continuous, and formed without folds
or other features that may cause metal-to-metal contact. In other
embodiments, the metal layer may be perforated, having very small
holes to improve adhesion of the polymeric layers. Any holes should
preferably have a size of no more than 1/50.sup.th of the
wavelength of the lowest operating frequency of the cell site.
[0041] Polymeric layers 702 and 703 may be made of thermoplastic
polyolefin (TPO), ethylene propylene diene monomer (EPDM),
polyvinyl chloride (PVC), modified bitumen, or another suitable
polymer with sufficient durability for use on a roof. Polymeric
layers 702 and 703 may each have a thickness of between 0.010 and
0.100 inches, or another suitable thickness. Membrane 700 may have
an overall thickness of between 0.020 and 0.250 inches, or another
suitable thickness. In some embodiments, membrane 700 may have an
overall thickness of between 0.040 and 0.150 inches, and may
preferably have a thickness between 0.050 and 0.100 inches.
Polymeric layers 702 and 703 may be made of the same material or of
different materials. For example, polymeric layer 702 could be made
of TPO and polymeric layer 703 of EPDM. Many combinations are
possible.
[0042] Polymeric layers 702 and 703 may include a fire retardant
such as magnesium hydroxide, calcium carbonate, talc, zinc borate,
or another fire suitable retardant. Preferably, the fire retardant
is non-halogenated. In some embodiments, the fire retardant may
comprise 20-40 weight percent of the polymeric layers.
[0043] Polymeric layers 702 and 703 may also include various
stabilizers such as high and low molecular weight hindered amine
light stabilizers, phosphate antioxidants, phenolic antioxidants,
or other suitable stabilizers. In some embodiments, one of
polymeric layers 702 and 703 may be designated as a top layer of
membrane 700, and may include more stabilizer than the bottom
layer. For example, top layer 702 may include 1-5 weight percent of
a stabilizer or stabilizers, while bottom layer 703 may include 1-2
percent of a stabilizer or stabilizers. The formulations of top and
bottom layers 702 and 703 need not be identical. For example, top
and bottom layers 702 and 703 may use different fire retardants or
different stabilizers, or may differ in other respects. In other
embodiments, the formulations of top and bottom layers 702 and 703
may be the same.
[0044] For the purposes of this disclosure, the words "top" and
"bottom" refer to the orientations of the various materials in the
figures. While these orientations correspond to orientation of the
materials as they would be installed on a rooftop, it is to be
recognized that the materials may be installed in other
orientations, for example on a vertical wall, and that the claims
are intended to encompass these other orientations.
[0045] FIG. 8 illustrates a process of making a PIM barrier
membrane such as membrane 700, in accordance with embodiments of
the invention. Metal layer 701 and its support film or films 704
and 705 are paid off of a supply roll 801, and passed between
calendering rollers 804 and 805. Extruders 802 and 803 inject the
material of polymeric layers 702 and 703. The completed membrane
700 is wound into a roll 806 for storage and shipment to a job
site.
[0046] The process shown in FIG. 8 is known as co-extrusion,
because both layers 702 and 703 are formed at the same or nearly
the same time. In other embodiments, only one of layers 702 or 703
may be formed in a first pass. The half-completed membrane may then
be sent through a second extrusion pass to form the other polymeric
layer.
[0047] The extruder of FIG. 8 may have a length to diameter ration
of 40:1, 50:1, 60:1, or another usable ratio. The components of
polymeric layers 702 and 703 may be fed into extruders 802 and 803
in neat or pre-pelletized form. Each extruder may include an
extruder screw and transfer zones that are optimized to mix the
formulation components. From the extruder, the formulation can go
through a melt pump which prevents pressure fluctuation of the melt
going into the die. The die lip is used to control the amount of
formulation being applied to the metallic film.
[0048] Calendering rollers 804 and 805 form a calendering stack,
and may be stainless or treated galvanized rollers. More than two
rollers may be used, for example four rollers. The thickness of
membrane 700 is controlled by the die lip and the gap or opening of
the calendering rollers. The gap or pressure generated from the
calendering rollers helps control adhesion of the polymeric layers
to the metallic film.
[0049] Completed membrane 700 may be considered a "single ply"
membrane, even though it comprises various layers. In the field of
roofing, the term "single ply" distinguishes a roofing membrane
such as membrane 700 from a "built up roof", in which multiple
components such as asphalt and ballast are separately applied to
the roof. Membrane 700 may be supplied in any workable size, for
example in rolls up to 50 feet wide or more and 100 feet long or
more. In some embodiments, membrane 700 may be provided in rolls
about 6 feet wide.
[0050] Metal layer 701 provides the desired RF attenuation in the
500 MHz to 6000 MHz frequency range. Plastic film layers 704 and
705 on both sides of the metal layer 701 provide electrical
insulation to prevent unwanted metal-to-metal contact when the
membrane contacts other metal surfaces or is folded back on itself.
Because the metal layer is very thin, burrs do not form when the
membrane is cut with a razor blade.
[0051] In some embodiments, for example when higher RF attenuation
is required, a second layer of aluminum or other metal can be added
to the stack. For example, FIG. 9 illustrates a PIM barrier
membrane 900 in accordance with other embodiments of the invention.
In membrane 900, two metal layers 901 and 902 are interleaved with
three plastic film support layers 903, 904, and 905. Layers 901-905
are then encapsulated between polymeric layers 906 and 907, for
example in an extrusion process similar to that described above.
Metal layers 901 and 902 may be similar to metal layer 701
described above. For example, metal layers 901 and 902 may be made
of aluminum, copper, or another suitable metal, and may be 10 to
100 nanometers thick, or another suitable thickness. Plastic film
layers 903-905 may be similar to film layers 704 and 705 described
above, and polymeric layers 906 and 907 may be similar to layers
702 and 703 described above. Intermediate plastic film layer 904
prevents the two metal layers 901 and 902 from touching each other,
so that the two metal layers do not become a PIM source.
[0052] In other embodiments, additional shielding may be provided
using a single, thicker layer of metal.
[0053] A PIM barrier membrane such as membrane 700 or membrane 900
may be attached to a roof in any suitable way. In some embodiments,
the edges of the PIM barrier membrane are heat welded to the
existing roof membrane. In other embodiments, the PIM barrier
membrane may be adhered to the existing roof using an adhesive or
using a tape at the edges of the membrane. A PIM barrier membrane
in accordance with embodiments of the invention may also be used in
re-roofing applications and in new construction.
[0054] The polymeric layers of a membrane embodying the invention
may be of any available color. For example, top layer 702 or 906
may be colored to match an existing roof, either by coloring the
material of which the layer is made to match the existing roof, or
by adding a coloring layer to the top of the membrane. In other
embodiments, the color may be selected for different reasons. For
example, top layer 702 or 906 may be made white or otherwise highly
reflective to reduce the absorption of heat into the building.
[0055] While a membrane such as membrane 700 or membrane 900 is
effective in mitigating PIM, other forms of RF barrier may be
useful as well, alone or in conjunction with a barrier such as
membrane 700 or membrane 900. For example, a self-adhesive barrier
may be helpful for vertical or irregularly shaped surfaces.
[0056] FIG. 10 illustrates an exploded view of a self-adhesive RF
barrier tape 1000 in accordance with embodiments of the invention.
Self-adhesive RF barrier tape 1000 includes a polymeric top layer
1001, made of a waterproof, durable material suitable for exposure
to the elements on a rooftop. For example, polymeric top layer 1001
may be made of TPO thermoplastic polyolefin (TPO), ethylene
propylene diene monomer (EPDM), polyvinyl chloride (PVC), or
another suitable material. Polymeric top layer 1001 may have a
thickness of 0.030-0.060 inches, or another suitable thickness. The
top surface of polymeric top layer 1001 may be of any suitable
color, for example black, white, gray, or another color. The top
surface may be the natural color of the material of polymeric top
layer 1001, or may be painted or otherwise coated to impart a
different color. A highly reflective color such as white may be
preferable in cool roof applications.
[0057] The underside 1002 of polymeric top layer 1001 is coated
before assembly with an adhesive, for example an acrylic
adhesive.
[0058] Self-adhesive RF barrier tape 1000 further includes a metal
layer 1003. Metal layer 1003 may include, for example, a layer of
aluminum 10 to 100 nanometers thick, vapor deposited on a plastic
film (not separately shown). Other metals and thicknesses may be
used. For example, in other embodiments the thickness of metal
layer 1003 maybe up to 40 microns or more. In some embodiments,
metal layer 1003 may be about 18 microns thick. The plastic film
may be made of polyethylene, polyester, polypropylene, or another
suitable material, and may have a thickness of 0.0005 to 0.025
inches, or another suitable thickness. In some embodiments, metal
layer 701 may be a metal foil, which in turn may be bonded to a
reinforcement mat made of polyester, glass, or another suitable
material.
[0059] Self-adhesive RF barrier tape 1000 further includes an
adhesive layer 1004. Adhesive layer 1004 may be made of an adhesive
composition that can adhere to at least some common roofing
materials. Examples of common roofing materials may include TPO,
EPDM, PVC, modified bitumen, metals, brick, fiberglass, and wood.
Adhesive layer 1004 may include a pressure sensitive adhesive. In
some embodiments, adhesive layer 1004 is made of bitumen (asphalt)
or modified bitumen and has a thickness of between 0.030 and 0.060
inches. Other adhesives and thicknesses may be used as well.
[0060] A release liner 1005 is preferably temporarily attached to
the bottom surface of adhesive layer 1004. Release liner 1005 may
be a thin plastic, a silicone-coated paper, or another material
that lightly adheres to the adhesive of adhesive layer 1004, but
can be easily removed without damaging adhesive layer 1004.
Self-adhesive RF barrier tape 1000 may be shipped to a job site
with release liner 1005 in place, and release liner 1005 is then
removed just prior to installation of tape 1000.
[0061] FIG. 11 shows an edge view of self-adhesive RF barrier tape
1000, in its assembled state. In order from the top, self-adhesive
RF barrier tape 1000 includes polymeric top layer 1001, and a layer
of adhesive 1101 on the underside of polymeric top layer 1001.
Metal layer 1003 is adhered to adhesive 1101, and is supported by
plastic film layer 1102. Adhesive layer 1004 is adhered to plastic
film layer 1102, and is covered on its bottom side by release liner
1005. In other embodiments, the positioning of metal layer 1003 and
plastic film layer 1102 may be reversed.
[0062] Self-adhesive RF barrier tape 1000 may be manufactured in
any workable size, for example in rolls 60 inches wide or more.
Narrower tapes may be made, or may be cut from wider tapes. For
example, widths of 6, 12, 18, or 24 inches may be produced, or
other widths.
[0063] Prior to attaching self-adhesive RF barrier tape 1000 to a
rooftop surface, the roof surface should be clean, dry and free of
oils or grease. Dusty surfaces may be primed with a water or
solvent based primer prior to applying the RF barrier to improve
adhesion. Self-adhesive RF barrier tape 1000 should be un-rolled
and the pressure sensitive adhesive side pressed against the
surface to be covered. A roller can be used to help press the
adhesive into the roof surface to help maximize adhesion.
[0064] Referring again to FIG. 3, PIM sources such as sources 306
may not be apparent from a visual inspection of roof 201, because
they may be buried within roof 201. The presence of PIM at a
particular cell site may be suggested by a statistical analysis of
the performance of the cell site. For example, a cell site with
significant PIM may have a shorter average call duration or a
higher percentage of dropped calls than another similarly-situated
site. If the differences in performance are not explained by other
factors, PIM may be suspected.
[0065] It is desirable to have a method of finding and mitigating
PIM sources, in accordance with embodiments of the invention. Such
a method may be practiced when PIM is specifically suspected, or as
a matter of general practice for network optimization.
[0066] FIG. 12 illustrates a technique for locating PIM sources, in
accordance with embodiments of the invention, in the context of
building 100 and cell site 200. Two signals of known frequency are
injected into antenna 301, which produces radio frequency (RF)
radiation 1201 containing the two known frequencies toward part of
roof 201. The RF radiation excites passive intermodulation (PIM)
sources 306, which re-radiate broad spectrum RF radiation.
[0067] An intermodulation frequency is calculated from the known
frequencies of the two injected signals. Preferably, this is the
third-order intermodulation frequency IM3, but other frequencies
could be used.
[0068] A worker 1202 "walks" the roof area of interest, carrying a
probe 1203 sensitive to the RF radiation produced by PIM sources
306. Probe 1203 is passed over the roof and receives
intermodulation frequencies generated by PIM sources generated at
locations on the roof. Probe 1203 may be, for example, a
PIMHunter.TM. probe manufactured by Anritsu Corporation of
Kanagawa, Japan, or another suitable kind of probe.
[0069] The signal received by probe 1203 is fed to a spectrum
analyzer 1204. A spectrum analyzer is an instrument that can
separate an electronic signal into its frequency components, and
measure the amplitude of the signal as a function of frequency.
Spectrum analyzer 1204 may be, for example, a model MS2720T
Spectrum Master.TM. spectrum analyzer manufactured by Anritsu
Corporation, or another suitable kind of spectrum analyzer.
[0070] Worker 1202 sweeps the area of interest while monitoring the
output of spectrum analyzer 1204. Preferably, at least the area of
roof 201 within the half power beamwidth .theta. of antenna 301 is
swept. Worker 1202 monitors the output of spectrum analyzer 1204 to
see if the amplitude of any sensed signal at the intermodulation
frequency exceeds a predetermined threshold. For example, worker
1202 may monitor a graphical display of spectrum analyzer 1204.
Depending on the model of spectrum analyzer used, spectrum analyzer
1204 may be able to generate an audible signal when the threshold
is exceeded at the frequency of interest.
[0071] When the threshold amplitude is exceeded, it may be
suspected that a PIM source has been detected. The roof location
may be marked for mitigation.
[0072] Once potential PIM sources such as sources 306 have been
located, each potential source can be covered with a radio
frequency barrier material configured to mitigate passive
intermodulation. FIG. 13 shows suspected PIM sources 306 covered
with respective pieces of barrier material 1301, in accordance with
embodiments of the invention. Barrier material 1301 may be
temporary and put in place for additional testing and verification,
or may be a permanent application.
[0073] Suitable barrier materials may include absorptive materials
such as RF absorbing foams, or reflective materials such as metal
foil encapsulated between nonconductive insulation, for example
polymeric layers as described above.
[0074] Roof 201 may be re-measured with barrier materials 1301 in
place to test whether the PIM has been satisfactorily mitigated.
Re-measurement may verify a reduction in PIM caused by PIM sources
306, or may reveal that additional mitigation measures are needed.
For example, the re-measurement may detect additional PIM sources
that were not previously detected, as PIM is a dynamic phenomenon
and may be affected by foot traffic on roof 201, nearby metal
objects, wind, or other factors. Any additional PIM sources may be
mitigated similarly to PIM sources 306. Any mitigated PIM sources
that still radiate significantly at the intermodulation frequency
may have additional layers of barrier material 1301 applied. Any
number of test-and-mitigate iterations may be performed.
[0075] If barrier materials 1301 are temporary, they are preferably
removed and replaced with more permanent materials, for example
barrier membranes having a metal layer encapsulated between
polymeric layers, as described above. The barrier material may be
self-adhesive, may be adhered to the existing roof with an added
adhesive or by heat welding, or may be affixed to the roof in
another suitable way. Preferably, the existing roof is not damaged.
For example, the barrier material is preferably not attached using
penetrating fasteners.
[0076] The relatively small pieces of barrier material 1301 shown
in FIG. 13 may be called "walkpads." In other embodiments, larger
areas of roof 201, possibly including multiple PIM sources, may be
covered with PIM barrier material, rather than covering the PIM
sources individually. FIG. 14 illustrates larger pieces of barrier
material 1401 applied to the entire area of interest on roof 201.
Barrier material 1401 is preferably supplied in a roll, so that
installation of larger areas may be simplified.
[0077] In applications where PIM mitigation would be desirable but
the building owner does not want the RF barrier permanently
attached to the roof surface, a "mat" RF barrier 1500 can be
deployed as shown in FIG. 15. The mat may be constructed using the
same rugged outer layer 1501 and insulated metal layer 1502 as
self-adhesive RF barrier tape 1000 described above, or similar
materials suitable for exposure to the elements on a roof. However,
the bottom layer 1503 in this form is a thick rubber mat such as is
commonly used for lining gym floors or horse stalls. Additional
layers such as adhesive layers, film backing for metal layer 1502,
or other layers may be present. The mat material 1503 may have a
thickness of 0.5 to 0.75 inches, or another suitable thickness. The
weight of the rubber mat provides ballast to hold the RF barrier in
place. Additional ballast in the form of concrete blocks can be
applied to the top of the mat if desired or necessary, for example
in areas with higher wind requirements.
[0078] Such mat type RF barriers may be supplied in square sections
measuring 36 inches by 36 inches, or in another suitable shape and
size. Larger sections are possible to manufacture but are not
practical for transporting onto rooftops due to their size and
weight. To cover large surface areas, multiple mats can be deployed
and mechanically joined together as shown in FIG. 16. To prevent RF
energy from passing through the gap between matts and exciting PIM
sources below, self-adhesive RF barrier material 1000 can be
applied over the seams. While this tape does not create a
continuous electrical connection between the individual barriers,
it does provide sufficient capacitive coupling between barriers
provide a continuous RF barrier. Additional mat connector
mechanisms 1601 may be provided if desired.
[0079] An alternate method for producing the mat style RF barrier
material 1700 is shown in FIG. 17. In this configuration, the outer
rugged layer 1701 is replaced with a 0.25 inch to 0.38 inch rubber
mat layer and the lower mat layer 1703 is reduced in thickness to
0.25 inch to 0.38 inch, yielding the same over-all mat thickness.
Metal layer 1702 is encapsulated between mat layers 1701 and 1703.
Other layers may be present as well.
[0080] In some cases, it may be desirable to replace an existing
roof membrane with an RF barrier membrane rather than apply
separate RF barriers on top of the existing membrane. This might be
the case when an RF barrier needs to be applied to reduce PIM, but
the existing roofing system is close to the end of its useful life.
For this application, an RF barrier 1800 as shown in FIG. 18 may be
used. RF barrier 1800 comprises an outer rugged layer 1801 with
inner insulated metal layer 1802 as previously described with a
second outer rugged layer 1803. The thicknesses of the outer rugged
layers may be reduced to 0.015 inch to 0.030 inch per layer to
maintain the overall membrane thickness. Outer layers 1801 and 1803
may be made of, for example, EPDM, TPO, or PVC. Installation of
this membrane may utilize the existing installation processes and
bonding materials. The self-adhesive tape RF barrier material 1000
as shown in FIG. 10 can be used at the seams to achieve a
continuous RF barrier, as shown in FIG. 19.
[0081] The invention has now been described in detail for the
purposes of clarity and understanding. However, those skilled in
the art will appreciate that certain changes and modifications may
be practiced within the scope of the appended claims.
* * * * *