Many years ago I was shown this trick in a very impressive and dramatic way - an acquaintance had an "audio s-meter" hooked to his transmitter-hunting receiver - a very modified Radio Shack Pro-2004 scanner.   The audio s-meter circuitry was enabled by a switch on the back of the scanner housing and the output tone was coupled back into the scanner audio amplifier and speaker.   The audio s-meter let him twist the 4-element quad beam while driving and keeping his eyes on the road - he simply twisted the mast for the highest pitch (the vertical support of the beam was a pipe that went right through the roof of his truck and ended in a bearing that was bolted to the floor between the two front seats).   One fine saturday morning I recieved a phone call asking me to come over to his house, that he had figured out the problem with the repeater!   I drove over and he had a large (borrowed) Celestron telescope on a tripod, with the 'scope pointed at the repeater site.   His truck was parked near by.   I looked through the scope and saw the repeater antenna gently flexing back and forth in the almost-constant breeze at the site.   He said "keep watching" as he walked over to the truck and turned up the volume on the scanner.   I heard two amateurs chatting, along with the s-meter tone.   After a while the wind paused for a second and the antenna stood upright the tone was high pitched.   Suddenly, it flexed to the left as a gust hit it, and the tone dropped.   As it came back upright, the tone went back to it's high pitch.   I looked up at him and said "Time for Mohammed to climb the mountain...". "Yup."   (Mohammed was the name of the raised 4x4 truck that another member had that we used for hill trips... how it got the name is a long story).

As it turned out, we were able to salvage the antenna (a 22-foot long Super-Stationmaster).   The antenna was demounted, lowered to the ground, laid across four sawhorses, disassembled (note that you need over 44' of working space to do this) and each joint examined with a good 4" magnifying glass.   One badly broken joint and several cracked joints were found.   Initially we were just going to resolder everything until it was remembered that one of the club members was a professional jeweler (and not a "salesman" type of jeweler that you might meet at a shopping mall jewelry store, he was a real "repair & manufacturing" jeweler).   Silver solder conducts slightly better then regular tin-lead solder, but the key point here is that it is much stronger than regular solder.   The disadvantage is that it is a high-temperature formulation and using it properly requires a lot of experience with it - especially in a situation where high temperature can damage something adjacent, in this case the insulation in the coaxial antenna segments... you have to get on the joint, heat it up quickly, melt the silver solder, get off it in a hurry and then cool it down quickly (an experienced person knows just when to hit the joint with a cold wet rag).   He was called on the phone, and showed up at the site a couple of hours later with a quickly-tossed-together tool kit and a bag of ice.   With his expertise in silver soldering the rework job was done properly the first time, and done in just a few hours - he insisted on resoldering every joint in the bottom three-fourths of the 22-foot long antenna (the club reimbursed him for his materials - both the silver solder and the special flux are expensive.   We actually had a quorum so right there at the repeater site so we had an instant board meeting, and passed a motion authorizing the treasurer to reimburse him for his materials used in support of the repeater committe's antenna repair project (plus we filled the gas tank of his car).   We reassembled the antenna, hauled it back up and side mounted it this time, complete with a brand new factory-fresh top support.   As of this writing it's been over 15 years and no problems except the fiberglass exterior of the antenna started to shed, and once the gel coat is gone the exposed fiberglass acts like a sponge to rain.   So one 4-day weekend we took a motor home to the repeater site (and getting an 18-foot motor home with full fuel and water tanks up a 12 mile long US Forest Service dirt fire road that ends 4,500 feet higher in altitude is an experience!), set up some sawhorses under a big tarp strung from the building to the motor home, the antenna was demounted, recoated, allowed to dry overnight, the next morning a second layer was applied, allowed to dry, and a third coat that afternoon. The next day a fourth coat was applied and the antenna was put back up.   See this web page on antenna painting.   Note that the "paint" is actually a two-part epoxy mix that is loaded with UV inhibitors, then goes on top of a fiberglass substrate (the old antenna) - you need to find somebody who has worked with building / repairing fiberglass boats or Corvette cars to do it right because you only get one chance - unlike normal house paint you can't strip it off and do it over.   One option is demounting the antenna and taking it to a Corvette-specializing body shop. Note that most modern pigments used in paints and coatings are metallic based. Make sure that the body shop understands that they can't use any metallic pigments on an antenna radome!

Lastly, when Kevin says "Any intermittent metal to metal contact that is inside the transmitter's RF field could also be the source of trouble" he is not kidding.   Think outside the normal tower environment - it could be ANYTHING.

Four examples:
1) I heard once about a strange source of duplex noise - it turned out to be the stainless steel needle bearing inside a anemometer head at the repeater site.   The noise only happened when the wind blew and the repeater owner had been all over the tower several times looking for the problem.   One day he was on the tower and happened to look down at the building and noticed the anemometer / weather vane assembly attached to the building.   The mental "light bulb" went on and he called down to one of the ground crew to grab a ladder and climb up and hold the anemometer cups still while the repeater was keyed.   No noise!   The crew member let go of the cups and let the anemometer spin up to speed: Lots of noise!   The cure: the anemometer head was taken down and rebuilt with a home-brew delrin (a very durable derivative of nylon) needle bearing - yes, it will require more frequent replacement, but the duplex noise is gone.   The weather vane head bearing was of the same design so it was rebuilt at the same time...

2) One system used two diplexers to allow one feedline to serve a 2m and a UHF repeater with separate 2m and UHF antennas on the tower (there are very, very few commercial grade dual band antennas, and those that exist have separate feed lines).   The back cover was a little bit loose on the top-side diplexer due to a stripped screw hole.   That's all it took.

3) A loose panel on an air conditioning unit housing on the repeater site building roof created some wind-blown intermitten noise. Replacing the missing screws and adding a split washer to each screw made all the difference. Use stainless steel hardware to avoid rust.

4) Don't even try to duplex an antenna mounted to a push-up mast.

And to repeat one major point - it may not be YOUR system / antenna that has the problem!   Nearby fiberglass antennas with internally broken elements, nearby dipole antennas with external problems, nearby loose hardware on abandoned antenna mounts, or any loose metal-to-metal contact will cause major scratchies and cracklies on (physically) nearby duplex systems.   Just disable your timeout timer and PL decoder, set up the service monitor to generate an 8db to 10db quieting carrier using its whip antenna (or have someone provide a weak signal from their house) and send someone up the tower to shake each antenna, guy wire, wiggle an air conditioning housing panel and pause any anemomenter head one by one while listening for the noise to change (a speaker-mic clipped to the jacket collar and hooked to an HT on the tower-climbers belt helps).   Look at the mounting hardware of each antenna - all it would take is a one loose wind-blown washer on an abandoned clamp.   If it's a multi-tower site you may have to go up each of the other tower(s) as well.   And once you find the problem antenna, you may have a public relations problem if the owner runs a simplex system (for example a taxi dispatch system with a wireline controlled hiltop base), because HE isn't seeing ANY problem and won't want ANY interruption to his radio system while you "fix" a nonexistent (to him) problem. And some dispatch systems (like shuttle buses at major airports) are run 24hrs x 7 days.

Folks in area with snow and ice have an additional problem: capilary action. The surface of the fiberglass becomes powdery as the gell coat deteriorates and micro-cracks develop in the fiberglass surface. These fine cracks are prone to moisture retention and when frozen, expansion, widening the crack for the next cycle of thawing and freezing. And if you are near the ocean the retained moisture in the crack will have salt in it... which speeds any corrosion and preformace deterioration. So the end result is that your fiberglass antenna can be made to last if you side mount it with a top support, but check it periodically and redo the gell coat as soon as any fibers are felt.

In short, keep in mind that when you're running a duplex system, any slight mechanical problems with the antenna, the feed-line, the connectors, or other nearby metal to metal contacting surfaces can cause noise that will affect the duplex receiver.   Bob Dengler NO6B made the observation that basically any loose metal-to-metal contact in the repeater's RF field at the site can cause trouble - it doesn't even have to be gorunded.   As a demonstration to yourself, try rubbing the shafts of two screwdrivers together near your antenna (or not so near, even 50' away) while an aproximtely 10 dB quieting signal is keying your repeater and listen to what it does to that signal... You will understand.

Some jurisdictions or site agreements require a fence around the tower base, or both the tower and the building.   Guess how many intermittent connections a wind-blown galvanized chain link fence has....   All you can do is (a) keep the RF field elevated high enough in the air to keep the fence out of it and (b) keep the fencing under good tension.   Or use a nonmetallic fence...

Last point: The above writeup only covers stuff on the tower or the roof of a building...  duplex noise can come from anywhere that you have micro-sparks caused by RF energy in a cable that has both a receiver and a transmitter on it.   Picture an antenna system that has a heliax feedline entering a building through a lightning arrester bolted into the feedthrough panel that is mounted in the building outside wall, and ends just below the cable tray above the repeater cabinet.   There is a six foot long coaxial jumper from the heliax connector to the duplexer (which is mounted in the rack cabinet).   If you use the wrong type of coaxial cable (like the stuff that has two concentric copper braids) as that jumper, you can create all kinds of duplex noise. Fortunately it won't show up until flexed, and the stuff that is inside the cabinet is pretty stable.   But why use the wrong stuff to start with?   Use solid outer conductor cabling like Superflex, or a quality coax with silver-plated braid. I KNOW that those cables doesn't make noise.   To prove it to yourself, put a piece of RG-8, RG-58, RG-213, Belden 8213, 8214, 9913, or 9914 in a duplex environment (i.e. between the duplexer and the antenna) and flex it.   You will hear the racket.

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This page originally posted on 14-Oct-2003