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Help!! I have a "crackling" noise in my repeater
By Kevin Custer W3KKC
In a posting on the Repeater-Builder Yahoogroup Geoff Perry wrote:
>My name is Geoff, KC4UBU, I use a Motorola MICOR base for my repeater. For
>the past several months we have been experiencing a crackling noise in the audio. This
>does not occur when the repeater ID's, only when audio is passed. This problem
>is also intermittent, however of late it has become worse. I have tried all that
>I and local hams can come up with to cure this problem. Do you have any ideas or
>help you can lend? The only thing I am left with as far as I knowis changing the
>receiver board. This machine also utilizes an S-Com 7K controller with a PL of
>103.5 hertz and I believe the prior owner installed a Communications Specialists tone
>board. I thank you for any assistance that you may provide.
I am nearly certain I know what your problem is. I have had this situation happen several times. The answer is that something is broken or loose in, or physically near your antenna. This situation happens when transmitter power is put into the antenna and the surrounding area is lit up with the RF energy - yours or someone elses. What happens is the RF then creates tiny arcs and sparks in the broken joint and the receiver is desensitized by the wide band RF created by the sparking. There is one big test for this problem: you will never hear the noise when the receiver squelch is closed, on the ID'er or during the carrier delay (also known as the hang-in timer or squelch timer). Only when the squelch is open. And if the problem is on the tower (as opposed to inside the antenna radome) the problem will be reduced when it's dead calm, or when raining.
I'll give you some "for instances":
a) I have a repeater on a fire tower. Some years ago my repeater began to have noise only on the users, the carrier delay tail was quiet. The local hams told me there was something wrong with the receiver because the noise was only on the users. The problem turned out not to be in the repeater at all. The noise was coming from the tin roof on the fire tower that had become rusty and loose. Tearing off the tin roof completely and replacing it with a fiberglass one fixed the problem permanently. The RF was exciting the the tin and rectifying / arcing in the rusty joint overlap creating the noise, especially when the tin sheets were moved by the wind. This same characteristic is why you will find a universal ban on unjacketed feedline at commercial radio sites.
b) I have another repeater on a commercial tower. There is an electrical conduit that goes all the way to the top to supply the tower-top aircraft warning light with AC power. This conduit is old, rusty and the repeater antenna is side mounted allowing the conduit to be very close to the antenna. The only way to solve this problem is is to replace the conduit run with plastic, or use a separate transmit antenna. My solution was to use a separate transmit antenna 60 feet down the tower, all is ok now. I know the problem is not the repeater because it all works fine at a clean site, or in the winter when this site is covered in ice and the conduit is locked in place and cannot move.
You could also have a broken antenna, this is common in the multi-element colinear "stationmaster"-type designs. You cannot tell one is broken by doing a SWR test because it will not show up - if one (or more) element(s) is / are disconnected. Fiberglass antennas with internally broken elements will work fine in simplex use (albeit with reduced gain) but will not duplex because of the the arcing and sparking in the broken joint or joints inside the casing of the antenna. Broken antenna problems usually appear when the wind blows and flexes the antenna, making and breaking the internal inter-element connection. The problem could also be someone else's antenna is broken that is near yours. This situation could happen if there is a system that has no problem in itself - like a simplex dispatch system used by the local EMS or fire or even a taxicab or diaper service. They would not notice their problem (except possibly reduced range at times) but you (and anyone else nearby that is duplex) will.
Note that internally broken antennas do not affect the ability to transmit or receive, just when you are trying to do both at the same time. Unfortunately the only real way to tell if something is broken is to climb the tower and shake things individually to try to see what it is that is causing the noise. Any intermittent metal to metal contact that is inside the transmitting antennas RF field could also be the source of trouble. Guy wires, unjacketed feedline rubbing against another piece of metal, a link radio's beam antenna with loose elements, an abandoned antenna mount with a loose wind-blown flat washer, etc. are just some of many examples. Tighten everything around the antenna, leave nothing loose. Strip the tower of unneeded hardware (brackets, etc.) and feedlines (those heavy-duty hot-dipped galvanized brackets are not cheap and can be reused!).
If you do all this and still cannot find the problem, run a separate transmit antenna to get the RF power away from your receive antenna - but even that won't help if it's someone elses antenna that is broken.
The problem could also be in the repeater but I doubt it. Put the repeater on a dummy load and see if you can work it locally at the site. If you have no noise on the dummy load, reconnect the feedline and move the dummy load to the far end of the feedline and try again. If you still have no noise then look at the tower or structure you are on, is it clean???
Hope this helps,
Kevin Custer W3KKC
Owner of 12 MICOR repeater stations.
Comments by Mike Morris WA6ILQ:
Kevin wrote "Fiberglass antennas with internally broken elements will work fine in simplex use (albeit with reduced gain) but will not duplex because of the the arcing and sparking in the broken joint or joints inside the casing of the antenna".
To elaborate on this, the problem is the micro-arcs caused by the high levels of RF power jumping across the micro-gaps in the cracked or broken solder joint(s) between segments in the antenna. These micro-arcs, like any spark, are broadband RF generators. Unfortunately they are also mixers that will take any RF around them (i.e. all other transmitters at the site in addition to your own) and mix them together. And because they are inside the antenna there is next to zero attenuation between the generator and a very sensitive receiver. All of this results in a very noisy receiver and a quiet carrier delay time after the squelch closes - leading people to initially believe that there is a problem in the repeater receiver.
The broken element problem in the slender fiberglass antennas is why the "Stormaster" (or "Storm-master", depending on the vintage of the literature) series of antennas was developed - picture a Stationmaster on steroids. More robust, less flex, due to a larger diameter tube with much thicker walls.
Note also that the amateur grade fiberglass antennas (think Comet, Diamond, Hustler, etc) have even thinner walls than the Stationmasters and Super Stationmasters, and will flex even more than them. I was told of a site in the midwest where one of the Hustler fiberglass antennas was mounted on a crossmember over three feet out and 200 feet up on a 300 foot commercial tower. It flexed enough that the tip of the antenna got stuck in the tower. That's a lot of flexing!
The fiberglass antenna broken element problems can easily be prevented if from day one you mount the antenna on the side of the tower, or on crossmember below the top, and use a top support made specifically for the purpose - picture a fiberglass "arm" sticking out from the tower with the end clamped onto the side of the antenna at a point 2/3 to 3/4 up from the bottom. Every manufacturer of the antennas makes and sells them. Several local repeater owners have given up on top mounting a fiberglass stick - after replacing them every 4 to 5 years they have gone to nothing but side mounts (besides the side mount cuts down the damage potential from a lightning strike - let some idiot that wants the top of the tower or the tower itself take the hit). Never expect that a fiberglass type vertical antennas will survive a direct strike, even if they have a nice robust metal tip on them.
Back to top supports - I suggest you just bite the bullet and include the top support kit as a required part of the antenna purchase, and install both the antenna and the top support at the same time. Some southern California mountains are over 6,000 feet tall with peak winds in excess of 100 MPH, and those antennas get two support arms. Several local folks have made their own supports for lower sites from broken fiberglass pole-vaulting poles or from outdoor-rated PVC pipe. However they do not last as long as something made for the job. The broken pole-vaulting poles get a thick coat (or two) of fiberglass gel heavily loaded with UV-inhibitors, and if the PVC needs a stiffener they put a half-dozen coats of Thompson's Water-Seal (or similar waterproofing) on a long dowel, slide it inside the PVC, and run a couple of stainless steel machine screws through the pipe into the wood to hold it in place. One gentleman went so far as to make a pressure chamber for sealing the wood by taking a piece of large diameter thick wall high pressure PVC pipe, gluing a cap on one end, and gluing a threaded cap fitting on the other end. He added the dry wood (with a weight at the bottom to keep it under the surface) poured in the Thompsons to cover the wood plus a few inches more, screwed the cap on (it had a tire valve mounted in it) and pressurized the tank to 80 psi for three weeks to drive the Thompsons into the wood. That wood was well preserved!
Secondly, sometimes the broken antenna can be diagnosed remotely, depending on how many elements are "disconnected". Get a receiver that has an needle-type S-meter (or a radio with an analog (needle type) VOM on the received signal strength test point) and locate it at a distance where the repeater is maybe half-scale (or use a variable attenuator in front of the receiver). Then watch the S-meter as the wind blows at the repeater site. If the break in the antenna is low enough that it disconnects a majority of the elements (i.e. a large change in antenna gain) you will see the meter "step" between two (or more) fixed levels.
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 feet of working space to do this) and each joint examined with a good 4" high power magnifying glass. One badly broken joint plus several cracked and crystallized 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 and 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 much higher 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 coax segments that make up the antenna... you have to get on the joint, heat it up quickly, melt the silver solder, get off it in a hurry and all the while protecting the coax cable antenna elements from the heat with cold wet rags - and do it without getting any moisture in the coax segments! He was called on the phone, and showed up at the site a couple of hours later with a quickly-tossed-together tool kit, some shop rags 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 top 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. by chance we actually had a quorum 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 and fresh hot-dipped galvanized brackets.
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 now-exposed fiberglass acts like a sponge to rain. So one 4-day weekend we drove a motor home to the repeater site (and getting an 18-foot motor home with full fuel and water tanks up a US Forest Service dirt fire road that gains 4,500 feet in altitude in 12 miles of road is an experience!), then set up a big tarp strung from the tower to the repeater building to the top of the motor home and to the top of a pickup camper. The antenna was was demounted, lowered and laid across some sawhorses under the tarp. It was 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 after it dried the antenna was put back up. See this web page on antenna painting and this page with some additional notes on painting. Note that the "paint" is actually a two-part epoxy mix that is loaded with UV inhibitors (and a lot of the cheaper stuff has next to none of the inhibitors). The four coats required mixing four seperate batches as the "pot life" is the same as the drying time (the pot life is based on a catalyst reaction, not on evaporation). The fresh epoxy goes on top of a fiberglass substrate (the old antenna) - you need to find somebody who has worked with building / repairing fiberglass boats or Corvettes (and some others - Saturns and some GM minivans had fiberglass panels) to do it right because you only get one chance - unlike normal paint you can't chemically strip it off and do it over. One option is demounting the antenna and taking it to a body shop that does fiberglass boats and cars. Since the shop may never have done an antenna before you may have to point out 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! (and they may not know which ones are metallic and which ones are not - unless the particular paint product lists the contents of the pigment it may be best to go with a clear-coat). And the normal fiberglass car repair epoxy is bottom-of-the-barrel low priced and low quality, so you may have to specify that they use a high quality epoxy loaded with UV inhibitors - and you really want be sitting down when they quote the price of the materials - good epoxy is not cheap, but assuming that the antenna tube and internals are good it's much cheaper than a new antenna. And some group politics are such that funding a repair may be more acceptable that funding a replacement.
Many system owners prefer dipole arrays over fiberglass sticks due to the broken element problem mentioned above, but dipole antennas have their own problems - fortunately they are easily cured. Some manufacturers do not ship lock washers with them, and some do not use stainless steel (i.e. non-rusting) hardware. The result is that the ferrous hardware rusts and the iron oxide becomes a diode... or on windy days the tower vibration works the nuts and bolts loose, and they become a regular maintenance item. When the noise starts to show up someone has to climb the tower and tighten everything - and in consistently windy areas that may be twice to three times a year! The solution is obvious - when you are assembling the antenna the first time, inspect the hardware and if necessare upgrade it before you start the assembly. Add both split lock washers and nylock nuts to every bolt (yes, the nylocks and split washers are redundant, usually only one or the other is needed or used, think of the pair as being both belt and suspenders). And make sure that anything you use out in the weather is either stainless or hot-dip-heavy-galvanized hardware.
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.
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. And avoid diplexers with pigtails - the coax they use on most the pigtails is bottom-of-the-barrel junk. If you are going to use a diplexer in a repeater system you want to use one that has good coax connectors mounted in the housing, and silver-plated ones at that. And avoid amateur grade diplexers on the tower. Put it inside a weatherproof NEMA housing if you have to.
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 even if you have to buy all new screws and split washers for every panel.
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 ANTENNA THAT HAS THE PROBLEM! Nearby fiberglass antennas with internally broken elements will cause major static, scratchies and cracklies on ALL (physically) nearby duplex systems - but only while that system is transmitting! And nearby loose hardware on abandoned antenna mounts, nearby dipole antennas with external problems (like a rusty element clamp bolt), or any loose metal-to-metal contact can also cause major scratchies and cracklies. Just disable your timeout timer and CTCSS decoder, set up the service monitor to generate an 8dB to 10dB sinad quieting carrier using its whip antenna (or have someone provide a weak dead carrier signal from their house) and send someone up the tower to shake each antenna element, coax pigtail, 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 since the noise source may be on a different tower. And once you find the problem antenna, you may have a public relations problem if the owner of the problem antenna runs a simplex system (for example a fire department dispatch system, or 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), or police and fire systems, are run 24hrs x 7 days. And it gets even worse when the problem system is a paging system (transmit-only) - as a whole they are notoriously dirty, and most paging system owners rarely give a darn about RF cleanliness.
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 from the ultraviolet light in the sunlight, plus the flexing creates very thin cracks in the fiberglass surface. These fine cracks (sometimes called micro-cracks) are prone to moisture retention and when frozen the water expands, widening each crack for the next cycle of thawing and freezing. The taper of the antenna radome and the mechanics of the wind flexing are such that 90% of the the cracks develop in the top 2/3 of the antenna and most are in the middle 1/3. They create a situation where moisture penetrates the radome, and if the weep holes get plugged by dust, dirt, or dead critter bodies the bottom of the radome chamber can actually collect enough moisture to corrode and rust the interior elements. If you get enough moisture pooled in the chamber the next freeze can crack or burst the fiberglass at the base. This is why some models of antennas that will be used in an inverted position have to be specially ordered for inverted mounting - the inverted antenna is built with radome mounted upside down with the "weep holes" for the moisture are at the physical bottom (the original antenna top).
If you are near the ocean the retained moisture in the radome microcracks will have salt in it... which speeds any corrosion and performance deterioration. If it's salt and ice, it's even worse.
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, clean out the weep holes with a thin wire, and redo the gell coat as soon as any fibers are exposed. Some people recoat any used fiberglass antenna after they acquire it and before they mount it - just on general principles.
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 static or crackling noise that will affect the duplex receiver. Bob Dengler NO6B once made the observation to me 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 grounded. As a demonstration to yourself, climb your tower while an aproximtely 10 dB quieting signal is keying your repeater and rub the shafts of two screwdrivers together in the near field of your antenna and listen to what it does to that signal... You will instantly understand.
Some political 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 outside 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 LMR series cable with copper braid positioned up against aluminum foil... But why use the wrong stuff to start with? Use solid outer conductor cabling like Heliax or Superflex, or a quality coax like mil-spec RG-214/U with a close-weave silver-plated double 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 or 9913 or LMR-400 (in fact, any LMR series cable) in a duplex environment (i.e. between the duplexer and the antenna) and flex it. You will hear the racket - and because is is inside the cable there is no attenuation between the source and the receiver.
A comment by Eric Lemmon WB6FLY in an email:
Genuine, MIL-C-17 RG-214/U coaxial cable has double concentric silver-plated
copper shields. Several companies manufacture an RG-214 "TYPE" cable
that is very similar, but without the silver plating. As you would expect, it's
a lot cheaper than the genuine RG-214/U stuff. Such "TYPE" cable may
also have less braid coverage / braid density than the genuine cable.
The words "military specification" or "mil-spec" by themselves mean nothing, it has to
say something like "meets military specification MIL-C-17/28C" (or whatever the number
is). Genuine mil spec RG-214/U has to meet the Mil Spec Number MIL-C-17/1908.
manufacturer is going to ignore one part of the specification (the silver plating) what
is preventing him from ignoring another part (like the sunlight resistance (i.e. the
UV resistance) or the thickness of the outer jacket, or the braid density)? If
you don't see RG-214/U Mil Spec Number MIL-C-17 printed on the jacket, it's not real
RG-214/U cable. And this is true of ANY mil-spec cable, not just RG-214/U.
Be very cautious about buying any coaxial cable that has the word "TYPE" or "LIKE" on the jacket, even if the seller claims that is "military specification" cable. Saying "TYPE" or "LIKE" can allow the maker or seller to market an inferior product to unsuspecting buyers.
There is a collection of mil-spec drawings and specifications for coaxial cabling in the "Feedline, Coax, Connectors and Shielding" section of the Antenna Systems page at this web site.
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This page originally posted on 14-Oct-2003
Article text © Copyright 2004 Kevin Custer W3KKC and Mike Morris WA6ILQ
This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.