[TowerTalk] Metal Roof

Kevin Kidd kkbroadcastengineering at gmail.com
Thu Jan 11 21:59:36 EST 2018

In another life far, far away at a Motorola MSS, after having fought the
PIM caused by rear seat springs in a sparkling new 1988 Cadillac that were
desensing the receiver of a full duplex Motorola Pulsar IMTS VHF mobile
phone....   Nothing is surprising.

Simply walking across the roof while monitoring the desense might give an
indication of the immediate location of the problem but probably not a
reliable fix.  Bonding the individual roof panels and seams to themselves
as Grant outlined or to another highly conductive surface might solve it as

Bouncing up and down rhythmically on the back seat of the new Cadillac
while monitoring desense was the test for seat spring PMI...  Our cute
secretary refused to participate in the test.  The eventual fix was that
there were so many VHF mobile phones still being installed in Cadillac's at
the time, that GM provided a bonded set of rear seat cushions as a warranty
replacement and completely cleared the issue.

Use an open tee to inject enough signal to key the repeater plus a few dB
of quieting and then do the normal desense stuff including flexing your
roof panels.  You will probably be surprised what you hear.

Good luck,

Kevin C. Kidd, CSRE/AMD
Lawrenceburg, TN
AM Ground Systems Company  -  WD4RAT
kkidd at kkbc.com  --  866-22-RADIO -- 866-227-2346

On Thu, Jan 11, 2018 at 7:31 PM, Grant Saviers <grants2 at pacbell.net> wrote:

> How well the seam capacitance suppresses diode rectification RFI is a good
> question as is what is the likely power output of such a diode.  Unsoldered
> copper seams sound like the devil's playground.
> How to "fix" an existing roof?  The seam edge is accessible at the gutter
> on my roof so a SS screw and nut, serrated lockwashers both sides to bite
> thru the paint would make an electrical short at that point.  How that
> affects a screw a few wavelengths away at VHF/UHF is ??.  For HF it might
> help.
> PIM (passive intermod) avoidance is a big deal on cell towers. Amateur
> radio stations excel at opportunities for PIM, so everything needs to be
> considered if it is a problem.

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