ConcealFab tested a multi-antenna infrastructure solution and found passive intermodulation (PIM) issues. Through over 500 tests, they developed three solutions: 1) custom fiberglass antenna mounts to replace steel brackets, 2) a proprietary coating for the steel mast to shield RF energy, and 3) a PIM Shield product to isolate antennas. These solutions ensured the fully assembled infrastructure met carrier PIM requirements. ConcealFab invested in PIM expertise to test, diagnose, and develop mitigating products for integrated antenna assemblies.

1. ConcealFab’s Passive Intermodulation Suppression System
INTRODUCTION TO PIM
Passive Intermodulation (PIM) is caused by non-linearities in "passive" components whose sources
generate mixing products of multiple (two or more) input frequencies. When stimulated, these passive
components produce a predictable set of new frequencies. These spurious emissions show up as
unwanted interference. Absolute linearity is only a mathematical ideal.
PIM is growing in importance in the wireless industry because the number of channels per antenna is
increasing as well as the amount of power per channel. Analog systems, with higher Signal to
Interference and Noise Ratios (SINR), were more tolerant to PIM than the digital systems being
deployed today. Older antennas were not constructed with PIM compliance in mind and would likely
never pass even a "blue sky" PIM test.
For PIM effects to be present there must be two or more signals within the passive device or assembly.
The stimulated passive device acts like a frequency mixer with a local oscillator (LO) and RF input.
Third order products at lower frequencies around 700 MHz are the biggest problem as they have the
most significant lower propogation losses and deeper skin depth penetration effect. Cellular systems
using digital low SNIR modulations; HSPA, CDMA, and LTE are the most susceptible to PIM effects.
Sum and difference signals are produced with product signals. As these non-linearities increase, so does
PIM amplitude. The mixed 3rd order product will be 1f1 ± 2f2 or 2f1 ± 1f2. The mixed 5rd order
product will be 2f1 ± 3f2 or 3f1 ± 2f2. The mixed 7rd order product will be 3f1 ± 4f2 or 4f1 ± 3f2. The
difference frequencies are of greatest concern and the spacing between the various intermodulation
products will always be the same as the spacing between the two primary signals.
In systems utilizing modulated signals, 3rd order products take up to three times the bandwidth of the
modulated signal signal. Similarly, for 5th order products, five times the bandwidth can be seen with
other PIM products covering a wide range of frequencies. This can also be compounded by real life
situations where three or more carriers are in proximity and when complex modulation platforms are
utilized.
This energy feeds back into the receive side of the system which raises the noise floor and can prohibit
certain receive frequencies from being used because it limits the receiver's sensitivity (de-sense) which
can result in dropped calls, limited coverage, or reduction of node capacity. This can sometimes be seen
as receive noise floor diversity imbalance. The PIM products only need to fall within the receiver's pre-
filter operating frequency range to cause issues because they are normally a full band component.
The passive components that can cause PIM include connectors (especially if they are dirty or have
contamination of any kind, even body oils that can get trapped in the elastomeric environmental seal),
braided RF signal feed-line cables, antennas (especially at higher powers), ground or power cables in
proximity to the near field of the antenna, cable to connector junctions / poor soldering, connector to
connector junctions, especially if they are made of two galvanically mismatched metals, or improperly
torqued connectors (this is a very common culprit). Using connectors with low contact resistance such
as gold or silver can help eliminate PIM product generation.
Additional external PIM sources can be unrelated to the system or the specific installation like a rusty
chain link fence, guy wires, gutters, metal flashing, exhaust pipes / vents, railings, air conditioning
units, signage, or even other equipment and antennas located nearby. Rusty or loose hardware on
nearby equipment or structures can also contribute to the PIM issue. Even, and especially, improperly

2. constructed or sized antenna concealment or mounting solutions can cause PIM as great care should be
taken with any material in proximity to the antenna's radome.
Wide temperature variations, salty or polluted air, and mechanical vibrations can all exacerbate the
issue due to the instabilities of the intermodulation level down to a microscopic level in the individual
parts that make up the system. Connectors that were disconnected while they were still powered up can
have micro-arcing which can pit the surfaces of the connector or cause chemical changes in the
material they are made of or the coatings on the connector. Surface oxides on the connector's mating
surfaces also create point source PIM issues as opposed to a distributed non-linearity.
A common technique for investigating a potential PIM problem at a site is to intentionally add
mechanical stresses by flexing or tapping on components to induce vibrations and cable flexure that
can happen with wind blowing on the components which causes the connections to fatigue. This is
different than VSWR testing where the mechanical stimuli are not applied. It should be noted here that
well matched VSWR has little or no influence on PIM. Feed-line cables should be mechanically
fastened to the antenna bracket(s) / tower to prevent movement after they have been run to the antenna
and connected to easily solve this contributing PIM factor.
Metals in the mechanical portion of the antenna under test or its mounting bracket(s) or possibly on an
antenna in proximity to the one being tested that have magnetic or para-magnetic properties like iron
etc. when in the RF current path can exhibit non-linear properties. Iron, steels (even some stainless
steels), and nickel exhibit a magnetic memory effect due to magnetic hysteresis which is non-linear and
can create PIM.
Dissimilar metals with different electrical potentials constitute a possible voltaic element. With
increased humidity or salty air they can transform into a galvanic element which acts just like a diode
non-linearity potentially producing PIM
PIM products of the fifth or seventh order fall off at 5-10 dB / product. PIM is generated by multiple
high power RF signals and is almost always found on the high-power transmit side, and while not
generated in the receive only RF path it can be introduced through coupling. The desired level of 3rd
order intermodulation products is normally around -150 dBc related to the transmit carrier level when
tested at 20 watts. Solving for which component or material is contributing to the PIM product
negatively effecting the system is extremely difficult. Care must be taken in the selection of only PIM
rated components, handling, and assembly of all aspects of a system during deployment.
Antennas not only radiate out the front through the radome but a material amount of energy radiates out
from all sides, including the back. As a guiding principal, the smaller the antenna the more energy that
radiates out the sides and back. The more electrical down-tilt being applied, the greater the design
produces more energy that radiates upward. Having more than one antenna on a mast or having them
back to back or in 120 degree sector arrangements, which are very common deployment configurations,
brings them into proximity with each other’s' near field and can create severe PIM issues at the system
level.
The focus on PIM is exceedingly relevant to the Small Cell industry as well because the antennas used
in these deployments are normally smaller and lose their front lobe "focus" as more energy is radiated
out the top, bottom, sides, and back compared to their larger Macro brethren. Co-location of carriers
necessitates antennas to be placed in close proximity, often in the same concealment, increasing the
likelihood that a PIM source will occur. While advanced planning and infrastructure design work
during the pre-deployment will help, the number of components (i.e., likely PIM sources) are too
numerous and additional solutions to address PIM must be developed. External PIM sources in the
environment are more likely in the Small Cell deployments and need to be included in site selection
and design.

3. PIM CHALLENGE: Introduction
ConcealFab Corporation was requested by a major customer to certify
that certain infrastructure solutions were not introducing PIM sources.
The infrastructure solution in question was a multi-antenna cluster
mounted to a high-strength steel mast frame covered by ConcealFab’s
ultra-RF transparent shroud. In collaboration with Kaelus,
ConcealFab quickly developed a knowledge base of how to evaluate,
diagnose and address PIM sources in a fully-assembled infrastructure
solution. Kaelus helped seed the effort with their highly-competent
engineering cadre to supply us with hand-on training to utilize their
PIM test equipment, which included their low PIM "Load" to test the
cable assemblies right up to the antenna as well as a PIM "Source" to
generate PIM to help determine the distances to known faults while
screening the infrastructure solution with their "Range to Fault"
module and PIM tester. The "Range To Fault" module enabled us to
determine if the component causing PIM issues was within the
confines of our infrastructure solution or somewhere else within the test range.
Testing was completed in accordance with IEC Standard 62037-2 which specifies the use of two +43
dBm (20W) tones as the industry standard PIM test power level. The Kaelus equipment took only a few
minutes to understand and use effectively. Low-powered systems do not have the same vulnerability as
higher-powered systems as there are normally an increase of about 2.5 to 3 dB of PIM for each dB of
power. We initially were getting test results in the -93 dBc range on antennas within our concealment,
which confirmed there were some material issues to be addressed relating to our infrastructure solution.
We first isolated the individual performance of each antenna being deployed in this project by doing a
blue-sky test on both low band ports and associated this data with each antenna in the system. There
was a surprising amount of variability between the ports and from antenna to antenna. Not that they
were worse than the limit promised by the manufacturer as they all met the -150 dBc PIM specification,
rather that some ports were in the -165 dBc range. This means that if you have a location within your
deployment that is close to passing but won't, it may be as simple as switching it out for an antenna that
performed relatively better during the blue sky test.
After initial antenna testing, we began mounting the antennae to the ConcealFab® tri-mast
infrastructure solution and running PIM tests. It was immediately apparent that this was going to be a
challenge because even attaching each antenna individually to our tri-mast mounting infrastructure
using the antenna manufacturer’s mounting bracket system had deleterious effects. It was further
complicated by the fact that five additional antennas had to be mounted to the frame. Therefore, to
adequately diagnose the situation, we had to deconstruct the full assembly to isolate and address each
potential PIM source.
PIM CHALLENGE: Trial-and-Error Testing
Our first set of trials involved isolating the impact of the steel antenna mounting brackets typically
included with packaged antennae. For Trial 1, we mounted a single antenna to a PVC rack using the
antenna manufacturer's galvanized steel antenna mounting brackets. We were getting mixed results and
elected to develop custom, non-metalic, pultruded fiberglass antenna mounting brackets to replace the
manufacturer’s galvanized steel bracket system. This had very encouraging results in the single antenna
tests on the PVC rack, producing stable PIM readings very similar to the blue sky tests.

4. Trial 2 was to galvanize the steel mast so that it would be at the same galvanic potential as the
galvanized antenna brackets that came with the antennas. The galvanized tri-mast failed outright, both
with the antenna OEM provided steel brackets and with ConcealFab’s custom-developed, low-PIM
fiberglass antenna brackets. This was initially a shocking discovery since antennas are frequently
mounted to galvanized steel towers. However, it is important to remember that this deployment
involves much smaller 2’ antennas vs. the typically larger 4’+ antennas mounted to galvanized steel
towers. As one factors in the radiated energy characteristics of smaller antennas, this discovery
becomes less surprising.
Trial 3 was to experiment with a variety of coatings to attempt to fully shield the RF energy from
reacting with the steel mast. We knew a steel solution was required as we could not get the required
structural strength using PVC or fiberglass materials to construct the sub-structure of our concealment.
It was apparent that we would have to make an across-the-board change to our coating methodologies
for all of our steel components in proximity to antennas. A new PIM Kote™ coating technology was
developed through exhaustive trial and error until we landed on a formulation that negated the impact
of the steel mast. This proprietary coating technology effectively shields the antenna’s RF energy from
the steel and allowed us to continue assembling the full structure to identify any additional PIM
sources.
Once the solutions for the mounting infrastructure were identified for the single antenna, we then had
to mount the rest of the antennas to the structure to understand how they interact with each other when
in close proximity. The answer is that there was a tremendous amount of PIM generated from the
surrounding antennas. Given the number of RF connectors, internal componentry, etc. involved with
each antenna, it wasn’t too surprising that antennas in close proximity can cause PIM. Therefore, Trial
4 involved electrically isolating each antenna from one another when they were in close mechanical
proximity. As you would imagine, numerous RF absorbing and reflecting materials were positioned in
and around the concealment and antennas with mixed results, none of which completely solved the
issue but some encouraging results were noted. Even the top of our concealment, which was steel, had
some influence on the PIM readings of the antennas mounted in the top-most position. Similarly, as the
bottom of our concealment was also made of steel, the lower-level antennas were experiencing mixed
PIM results. A wide variety of RF materials and coatings were tested in numerous arrangements,
thicknesses, and densities before we landed on the ConcealFab® PIM Shield™ solution to address
antenna isolation. A product that is custom-formed to fit over the top of any panel antenna, the PIM
Shield™ effectively addresses the RF energy emitting out the top of the antennas and interacting with
surrounding infrastructure of antennas, bringing the final infrastructure solution into full PIM
compliance.
PIM CHALLENGE: The Solution and the Making of a PIM-Mitigating Tool Chest
Over 500 individual PIM tests were completed over a three month time period to arrive at a set of PIM-
mitigating solutions. The solutions developed help ensure PIM reading of the fully-assembled
infrastructure solution fall within the required carrier tolerances. These PIM-mitigating solutions were:
1. ConcealFab’s custom-designed, low-PIM fiberglass antenna mounting brackets, 2. ConcealFab’s
PIM Kote™ proprietary coating technology for the steel substructure, and 3. ConcealFab’s PIM
Shield™, to address RF energy emitted out the top of the antennas, reducing the energy from
interacting with surrounding antennas. Exhibit A portrays the readings and configuration snapshots
taken along this journey.
As the densification of smaller antennas increases, coupled with carrier requirements to meet stringent
PIM guidelines, we expect an increasing amount of customer demand for PIM-certified assemblies in
the marketplace.

5. ConcealFab Corporation has made the necessary investment to develop an expertise in this arena, not
only in testing and diagnosing of PIM sources in a system, but in developing PIM-mitigating products
to ensure fully integrated assemblies meet carrier-dictated PIM levels.
Release 2 on September 7, 2014
Authors:
William E. Pounds (Doc) VP Engineering, ConcealFab Corporation
Jonathan Fitzhugh - CEO, ConcealFab Corporation