ARTICLE FROM AUGUST 1997

TUNING FM RADIOS, PART 3

In the last few articles I've discussed some of the practical aspects of tuning FM radios. Now its time to get it hooked to an antenna and get on the air. Feedlines and antennas don't need to be a black art. They work according to well understood principles. I've said this many times and I'll say it again here: a few hours spent reading the ARRL Handbook or Antenna Handbook can save all of us (regardless of how experienced we think we are) a lot of grief.

I've found it to be very wise to sit down and review the basics before starting any new project. Antennas and feedlines are no exception. A little reading can save a lot of money and time for you!

That said, lets look at feedlines. My first VHF station (this is ancient history here kids) was a Gonset Communicator IIB on six meter AM. I used 300 ohm open wire TV twin lead for feed line to my home brew beam for the simple reasons: A: I couldn't afford RG-8 B: Someone gave me the open wire

Loss on this stuff was non-existent! As long as it wasn't raining that is. If you really, really want to have the lowest loss feedline, open wire still works. All the rage in 1963, I haven't seen it since and don't miss it either!

I will not bore you with the long list I have of things that don't work very well. Beldin 9913 and its problems with moisture infiltration is very high on my list of mistakes not to repeat. It is better to dwell on the things that do work.

STUFF THAT WORKS

First use "N" series connectors wherever you can. They are water proof and constant impedance. These are the only connectors that I will use outdoors. They can be up for years with NO problems. When you disconnect them, the interior is will still be bright and shiny. N connectors do not need to be wrapped with tape. They have internal rubber "O" rings that seal them very effectively.

One thing you do have to watch for is sealing of the mating female connector on the antenna. On some "N" females the center pin is not sealed in the insulating body. Just to be on the safe side, AFTER you solder the antenna connection to the center pin, seal it with urethane based glue. Never use silicon sealant that smells like vinegar. The acidic acid used as a curing agent will corrode the connector.

Repeater antennas are a subject all their own. Absolutely the worst antenna I have ever seen on a repeater is the infamous Ringo Ranger. They (and other poorly designed antennas) have two serious problems. One is RF noise, the other poor RF decoupling of the feedline.

Never forget that SWR is not the only measure of antenna performance. A low SWR only means that the antenna presents a reasonably non-reactive load to the transmitter. That is, it looks like a 50 ohm resistor to the feedline. The SWR tells you nothing about what is really important, the antennas noise level, its pattern (gain) and decoupling of the RF from the feedline.

RINGOS

Lets go back to our Ringo again. I used one on a repeater once. I had a huge desense problem, crackling on the received signals and the duplexers refused to tune properly. Finally one day I did something smart, I tried the repeater out on a dummy load. To my surprise it worked perfectly! The desense (white noise) and crackling disappeared. The duplexers tuned right up with deep notches like they were suppose to have. Simply amazing!

As fast as I could I replaced the Ringo with a Hustler G7-144. The difference was shocking. On the Hustler the receiver became as quiet as it had been on the dummy load and everything worked perfect! Inspection of the Ringo showed where the problems came from. First off, its poor grade of aluminum had corroded. All of the corroded mechanical joints in the antenna were acting like diodes, generating RF noise from the energy of the repeaters transmit signal.

Another source of RF noise (maybe the worst, there's a lesson here) is the Aluminum SO-239 connector used on the Ringo. It is not water proof and remember that tape will not make an air tight seal on any connector. Inside mine (and others I've inspected) I found a white paste of corroded aluminum. I have made Ringos work much better simply by replacing this horrid connector with a silver plated brass "N" female.

The Ringo also lacks radials, there is nothing to act as a choke to prevent RF currents from traveling on the outside of the feedline. This RF I believe is what caused the problems with the duplexers.

LOOP CO-LINEARS

Another subtle antenna problem the repeater owner can run into is exhibited by loop dipole co-linears. These are popular and rugged commercial antennas with four loop dipoles side mounted on a steel mast. Suddenly one day you notice that old 34/94's coverage has gone to hell. When you check the antenna with a SWR bridge you discover that you still have a 1:1 SWR.

Phasing is the key concept here. The RF is fed to the dipoles by means of a RF harness of lines that accomplish the phasing on the dipoles and power division. 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 this type antenna can work very well.

As the antenna ages, moisture can infiltrate the coax and connectors. Coax dielectric can begin to deteriorate. This will slowly cause the vertical pattern (and gain) of the antenna to change. A perfect SWR does you no good when your RF power is going straight up in the air! Remember that antennas are two way affairs. If the transmit pattern is bad, the receiver pattern will be bad too.

HUSTLER G7

After fighting my share of antenna problems, I rapidly came to the conclusion that simpler is better and rugged better yet. The Hustler G7-144 is about as perfect an antenna as I have ever seen. Unlike some recycled commercial antennas it is resonate on the two meter ham band. It is VERY low noise. About as good as a dummy load. It has a real honest to God "N" connector so you can properly terminate the feedline without worrying about water in the coax.

Best of all the colinear phasing sections are an integral part of the antennas structure and made of fiberglass. There is nothing here to deteriorate and have its electrical characteristics change. Mine is several years old and it works as good as the day I bought it. Cost is about $150 and well worth it.

One great advantage of this antenna is that it is at DC ground. This brings to mind the great SEITS electric nose picker incident. Like most ham clubs we tried to save money. Being short of antennas for the LINK system hub at Winfield, we tried, as an experiment, to run both the UHF hub repeater and the 145.29 repeater on one Comet dual band antenna. A mobile diplexer (note: the correct term is diplexer, not duplexer) connected the two repeaters duplexers to the single feedline.

To our pleasant surprise this arrangement worked far better than we thought it would. Coverage on both repeaters was excellent and there was no RF noise (desense) problems on either machine. All went well for few days till we had the next thunder storm. Lighting hit the Comet and well... It blew up and became a comet.

The next morning Randy Nelson, WB0VHB, went to the site and discovered the top half of the Comet antenna stuck in the ground like a spear. The bottom half was completely shredded with long thin strips of fiber glass hanging outward like the leaves of a plant. At this point Randy and I decided to get serious.

The Comet's remains went to the next hamfest for display. The 145.29 repeater went back on its hard line and a Hustler G7-144. I bought a $900 commercial UHF antenna for the Hub repeater. This monster Antenna Specialists co-linear had a lighting knob on the top the size of my fist. The body is as big around as your arm and the base a heavy aluminum casting. It was worth every penny of what it cost. Its taken a lot of lighting in the last few years and it just keeps on working.

We'll continue next month with some thoughts on feedlines.

David Metz, WA0AUQ
Comments to davemetz@muscanet.com
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