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.
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.
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.
For example,
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.
According to one aspect, a self-adhesive radio frequency barrier tape comprises a waterproof polymeric top layer having an upper surface and a lower surface, an adhesive coating on the lower surface of the polymeric top layer, a metal-containing layer adhered to the adhesive coating on the lower surface of the polymeric top layer, a pressure sensitive adhesive layer adhered to a lower side of the metal-containing layer, and a release liner on a bottom surface of the pressure sensitive adhesive layer. In some embodiments, the metal-containing layer comprises a continuous layer of metal. In some embodiments, the metal-containing layer comprises a perforated layer of metal. In some embodiments, the metal-containing layer comprises a continuous aluminum layer that has been vapor deposited on a film, and the aluminum and the film are encapsulated between the polymeric top layer and the pressure sensitive adhesive layer. In some embodiments, the metal of the metal-containing layer is adhered to the adhesive coating on the lower surface of the polymeric top layer and the film is adhered to the pressure sensitive adhesive layer. In some embodiments, the film is adhered to the adhesive coating on the lower surface of the polymeric top layer and the metal of the metal-containing layer is adhered to the pressure sensitive adhesive layer. In some embodiments, the pressure sensitive adhesive layer comprises modified bitumen. In some embodiments, the metal-containing layer comprises copper. In some embodiments, the metal-containing layer comprises a metal foil bonded to a reinforcement mat. In some embodiments, the metal of the metal-containing layer has a thickness between 10 and 100 nanometers. In some embodiments, the metal of the metal-containing layer has a thickness of at least 1 micron. In some embodiments, the metal of the metal-containing layer has a thickness of about 18 microns.
According to another aspect, a method of making a self-adhesive radio frequency barrier tape comprises applying an adhesive coating to a lower side of a polymeric layer, and adhering a metal-containing layer to the adhesive coating. The metal-containing layer includes a layer of metal. The method further comprises adhering a pressure sensitive adhesive layer to the metal-containing layer, and placing a release liner on a lower surface of the pressure sensitive adhesive layer. In some embodiments, the metal-containing layer comprises aluminum that has been vapor deposited on a plastic film. In some embodiments, the metal-containing layer comprises a metal foil bonded to a reinforcement mat. In some embodiments, the layer of metal is continuous. In some embodiments, the layer of metal is perforated. In some embodiments, the polymeric layer comprises thermoplastic polyolefin (TPO). In some embodiments, the polymeric layer comprises ethylene propylene diene monomer (EPDM). In some embodiments, the polymeric layer comprises polyvinyl chloride (PVC). In some embodiments, the polymeric layer comprises modified bitumen.
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.
In
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.
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.
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,
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.
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.
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.
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.
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 1/50th of the wavelength of the lowest operating frequency of the cell site.
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.
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.
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.
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.
The process shown in
The extruder of
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.
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.
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.
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,
In other embodiments, additional shielding may be provided using a single, thicker layer of metal.
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.
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.
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.
The underside 1002 of polymeric top layer 1001 is coated before assembly with an adhesive, for example an acrylic adhesive.
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.
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.
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.
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.
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.
Referring again to
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.
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.
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™ probe manufactured by Anritsu Corporation of Kanagawa, Japan, or another suitable kind of probe.
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™ spectrum analyzer manufactured by Anritsu Corporation, or another suitable kind of spectrum analyzer.
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 Θ 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.
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.
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.
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.
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.
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.
The relatively small pieces of barrier material 1301 shown in
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
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
An alternate method for producing the mat style RF barrier material 1700 is shown in
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
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.
This application is a division of pending U.S. application Ser. No. 15/786,982 filed Oct. 18, 2017 and titled SELF-ADHESIVE MEMBRANE FOR MITIGATING PASSIVE INTERMODULATION, which claims the benefit of provisional U.S. 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.
Number | Name | Date | Kind |
---|---|---|---|
3349396 | Reed | Oct 1967 | A |
3900644 | Sackoff et al. | Apr 1975 | A |
4386354 | Watson | May 1983 | A |
4585682 | Colarusso | Apr 1986 | A |
4640851 | Pusch | Feb 1987 | A |
4647714 | Goto | Mar 1987 | A |
4749625 | Obayashi | Jun 1988 | A |
4755409 | Harkness | Jul 1988 | A |
5456779 | Sinha | Oct 1995 | A |
5458162 | Sinha | Oct 1995 | A |
5493771 | Sinha et al. | Feb 1996 | A |
5494755 | McCloskey | Feb 1996 | A |
5517206 | Boone et al. | May 1996 | A |
5598989 | Ross et al. | Feb 1997 | A |
5757246 | Johnson | May 1998 | A |
5885906 | Reynolds et al. | Mar 1999 | A |
5886294 | Scrimpshire et al. | Mar 1999 | A |
6067053 | Runyon et al. | May 2000 | A |
6222503 | Gietema et al. | Apr 2001 | B1 |
6479119 | Simpson | Nov 2002 | B1 |
7227508 | Wang et al. | Jun 2007 | B2 |
7391381 | Wang et al. | Jun 2008 | B2 |
7429961 | Sievenpiper et al. | Sep 2008 | B2 |
7639207 | Sievenpiper et al. | Dec 2009 | B2 |
9112283 | Shoemaker | Aug 2015 | B2 |
9314994 | Strait | Apr 2016 | B2 |
10615885 | Bell | Apr 2020 | B2 |
20020121061 | Rippey | Sep 2002 | A1 |
20070196610 | O'Rourke | Aug 2007 | A1 |
20070196671 | Kobayashi | Aug 2007 | A1 |
20090220720 | Mohseen | Sep 2009 | A1 |
20110050530 | Hnatiw | Mar 2011 | A1 |
20110120764 | Kelley | May 2011 | A1 |
20130063393 | Kurishima | Mar 2013 | A1 |
20140292609 | Schadler | Oct 2014 | A1 |
Number | Date | Country |
---|---|---|
1571623 | Jan 2005 | CN |
102468529 | May 2012 | CN |
203406418 | Jan 2014 | CN |
20 2004 012 762 | Jan 2005 | DE |
2 573 865 | Mar 2013 | EP |
3 032 642 | Jun 2016 | EP |
2065156 | Jun 1981 | GB |
2002076740 | Mar 2002 | JP |
2008028549 | Feb 2008 | JP |
2012129772 | Jul 2012 | JP |
Entry |
---|
Anritsu Company, Identifying Sources of External PIM, pp. 1-10. |
Anritsu Company, Test Probe for More Efficient and Effective Location of External PIM Sources, Nov. 22, 2016, pp. 1-2. |
Johns Manville Sales GmbH, Evalith® Glasfaservlies / glass fiber nonwoven, May 17, 2017, p. 1. |
Isola a.s., Laying instructions—High quality single layer Roofing System, pp. 1-12. |
Isola a.s., Isola Mestertekk Robust roof covering suitable for our tough Nordic climate, Sep. 29, 2017, pp. 1-2. |
Isola a.s., Isola Mestertekk Single layer roofing felt for flat and pitched roofs, Sep. 3, 2013, pp. 1-3. |
Anritsu Anerica, Passive Intermodulation (PIM), Oct. 20, 2017, pp. 1-13. |
Anritsu, The PIM Source, Sep. 29, 2017, pp. 1-5. |
Johns Manville, Dynaclad® Fiber Glass-Reinforced, SBS Aluminum Foil-Surfaced, Cap or Flashing Sheet, Mar. 2016, pp. 1-2. |
William D. Wade, Development of Low PIM, Zero CTE Mesh for Deployable Communications Antennas, Lockheed Missiles and Space Company, pp. 1175-1178. |
Number | Date | Country | |
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20200204273 A1 | Jun 2020 | US |
Number | Date | Country | |
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62426673 | Nov 2016 | US |
Number | Date | Country | |
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Parent | 15786982 | Oct 2017 | US |
Child | 16803813 | US |