Back Antennas...
Compiled by Mike Morris WA6ILQ

The antenna is the single most important part of a repeater - you have to get it right or nothing else works. Feedline is number two, and since both are on the tower they are the most expensive and difficult to get right initially, and becasue they are exposed to the worst that mother nature has to throw at you they can require the most expensive maintenance.
You can have the best sounding repeater on the planet, but it's both worthless and useless if the users can't get into it or hear it because you used a Ringo Ranger 15 feet off the ground and fed with RG-8, 9913 or RG-213-type feedline, or worse, RG-58-type (don't laugh, I've seen it).

Repeater owners tend to have favorite antennas - some prefer colinear antennas ("fiberglass sticks"), others prefer dipole arrays. Personally I prefer grounded dipole arrays as colinear antenas are more prone to duplex noise (see the article titled "Help!! I have a crackling noise in my repeater" by Kevin Custer W3KKC elsewhere on the Antenna page).

Add to the flexing damage the fact that most common fiberglass verticals just snap off when you combine two to three inches of ice and 60-80 MPH winter winds. Dipole arrays have their own problems, fortunately many are easily cured. Some manufacturers do not use lock washers with the antennas, 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 vibration and shaking works the bolts loose, and they become a regular maintenance item. When the crackling noise starts to show up someone has to climb the tower and tighten everything. The solution to that problem is obvious - when you are assembling the antenna the first time, use stainless steel bolts with stainless steel split lock washers and stainless steel nylock nuts (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 any fastening hardware you use out in the weather is stainless and that any brackets or other hardware is hot-dip-heavy-galvanized. And if you ding the galvanizing as you mount the antenna you need to have a spray can of cold-gal in your tool bag.

Properly assembled and mounted dipole arrays don't have the duplex noise problem but can have phasing harness degredation if you are not careful about waterproofing - and don't trust the manufacturers assembly, especially at the "Y" points. Many manufacturers use a hard plastic cast housing for the protection of the splices, and frequently have a problem with the bonding of the coax to the housings. As soon as the bond cracks you get moisture ingress into the splices, and it takes very little moisture to corrode a harness splice. And once you have corrosion the creeping green glop acts like a diode when under RF power and you get mixes and intermod. So use LOTS of non‑acetic‑acid‑based RTV around the splice housings. I repeat - NON‑ACID‑based RTV. Many of the common RTVs have an acid base. This IS the voice of experience with some Permatex products...

Phasing harness degredation - from water ingress, old UV-damaged coax, or physically damaged coax is best explained by this:

Picture four loop dipoles side mounted on a steel mast at the top of a tower. Suddenly one day you notice that the coverage has gone crazy, and it's most noticeable on weak (i.e. handheld) signals. When you check the antenna with a SWR bridge you discover that you still have a 1:1 SWR. Phasing between the individual dipoles in the array is the key concept here. The RF is fed to the dipoles by means of a cable harness of critical-length coaxial lines that accomplish both the phasing of the dipoles and RF power division. There is nothing magic about the harness, it's just basic RF physics: The top two elements are paired into one cable, and the lower two elements are paired into a second cable, and then those two pairs are paired together and fed from a 50 ohm feedline. The horizontal pattern is determined by the positioning of the elements (i.e. is a single vertical column, or in two columns 180 degrees around the mast, or one facing each 90 degree increment. The VERTICAL pattern of the antenna is determined partly by the RF phase relationship between the dipoles. When the antenna is new, the phase relationships are perfect and the RF goes where you want it - out towards the horizontal plane of the horizon, or with a sligh down-tilt (if you ordered it that way). As the coax in the harness ages you will find that moisture can infiltrate the coax and connectors unless your waterproofing is absolutely perfect. The weak spots are where the coax meets the connectors and where the molded cover on the "Y" junctions meets the coax. For some reason the coax and the mold frequently do not bond properly and water gets in. The stress from ice loading makes situation worse. People have cut the splice cases open and found water inside and seriously waterlogged coax and corroded splices. And the coax in the harness is usually that ultra-rare 35 ohm stuff. You start with Scotch-kote and 3M 130C tape as you assemble the antenna and go from there. Once moisture gets into it deteriorates the coax dielectric (which changes the velocity) and corrodes the center conductor and braid. This slow velocity change will cause the phasing to change. Since the apparent gain of the antenna is determined by the vertical pattern (and it is determined by the phasing between elements) the pattern of the antenna will change, sometimes gradually, sometimes drastically. A SWR reading will detect an impedance shift but won't diagnose a pattern shift as a perfect SWR does you no good when your RF power is going straight up in the air or straight down into the ground! So if you use a dipole array then you want to be a hyper-picky and an really obnoxious perfectionist about the phasing harness, the waterproofing of the splices and of the cable connectors on it. And if you are looking at a new antenna you might want to look at the ones that have the harness inside the support / mounting tube.
Moisture problems can be difficult to diagnose since common antenna test equipment (like the antenna analyzers made by AEA, MFJ and others) use very low power (microwatts to milliwatts), and many moisture problems show up only with the full transmit power from the repeater transmitter.

"Downtilt" or "beamtilt" is a frequently misunderstood term, mainly because most of the time it is used in a nondescriptive or incomplete manner. The term "downtilt" is most commonly heard and is usually used to aim the main lobe of the vertical plane radiation pattern of an antenna below the horizontal plane. "Beamtilt" is usually used when other directions are desired. I'm going to use the word "downtilt" for the rest of this discussion just to save typing.

There is mechanical downtilt and there is electrical downtilt.

Mechanical downtilt is usually accomplished by physically mounting in such a manner as to lower the angle of the signal on one side (one way is to bend the mounting tube - not the antenna tube, the mounting tube). While this will direct the main lobe of the antenna downward into the target area it also wastes a significant part of the transmitter RF power as the back half of the pattern will be directed up into the sky, wasting about half of your RF power (unless you are talking to UFOs, airplanes or spacecraft). This characteristic makes mechanical downtilt very unattractive and useful in only very limited situations.

Electrical beamtilt is usually accomplished by varying the phasing between antenna elements - it is tweaked to make the signal go down (usually) in all directions. This is extremely useful when the antenna is at a high site, and the main lobe of the signal is likely to overshoot the target audience (broadcast audience, two way radio users, even cellphone users) entirely. With electrical beamtilt the front and back lobes tilt in same direction: for example, an electrical down tilt will make both front lobe and back lobe tilt downwards. This is the property used in the above example where the signal is pointed down in all directions. And note that if you are going to mount an antenna inverted then you want to order it with up tilt (and verify that the supplier understands that inverted mounting plus uptilt equals the desired downtilt). Note that inverted antennas are usually special order as they have to put the drip holes (sometimes called weep holes) in the "wrong" end.

One of the most important things to do with a new antenna installation is to keep a logbook - note the forward power, the return power, the SWR, etc. from day one, and if you have any degredation you can compare the readings. One useful antenna measuring trick is to take a broadband mobile radio and connect it to the feedline then go up and down in frequency and note the upper and lower frequencies that create a 2:1 SWR. If something changes in the future you can sweep it again and note if they have moved. If you have access to a SiteMaster (or other analyzer) you can print two copies of the plots and put one set into the on-site notebook/logbook, and the second set into the at-home copy. These notebooks are vital for future comparison and to spot trends (like phasing harness degredation). Just remember that a common antenna analyzer does not have enough RF power to "see" the misbehavior caused by flexing or vibration (i.e. the breaking of an element on a fiberglass antenna) or by water ingress.

Contact Information:

The author can be contacted at: his-callsign // at // repeater-builder // dot // com.



This web page split from the main page 12-Nov-2011.


Layout and hand coded HTML © Copyright 1995 and date of last update by 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.