HF from Albuquerque

Hi all!

Some of you have expressed an interest in seeing the antenna I use when traveling in the RV. Today (December 1, 2005) I am in Albuquerque NM and just finished a QSO with Don KQ6FM on 14.3425. During the QSO we were broken by N6VJD in Lost Angeles who reported that I had the strongest signal (S9+40) on 20m in LA! For some reason, he was also interested in the antenna. All that being the case, this page will serve to describe and illustrate my antenna.

The antenna itself is best described as a "Hustler dipole". It is rather similar to a Buddipole antenna. The Buddipole has some advantages. The main reason I went the hustler route is that I already had quite a few Hustler parts and no Buddipole parts. Interested folks might do something similar with most any HF mobile antenna, Hustler, Buddipole, Iron Horse, screwdriver antennas of various breeds etc.

When asked what kind of antenna I'm using, I usually reply, "a Hustler dipole". The next thing I usually hear is some kind of confused noise, the message of which is, what's that? To answer the confused noise, here is a picture of it set up for 20 meters, followed by the thousand words of description that the picture is supposed to make unnecessary.

The most basic description of the antenna is that it is a shortened dipole. Whatever you know or can learn about shortened dipoles applies here. I chose the dipole configuration because I wanted to be able to use it in environments where the questionable availablity and quality of a suitable ground plane make use of a vertical problematic. The desire for effective NVIS coverage strongly militates against a vertical. I also wanted an antenna that requires a single support that would be quick and easy to assemble and disassemble in the field. Other desirements include an antenna that can be effective without an matching network.

The dipole center is one of the most interesting parts of the mechanical design of a diople. Folks building full sized dipoles are faced with the chore of connecting the two sides of their dipole to their feedline in a way that the whole thing will not fall apart or be pulled apart. In my case, I bought a center mount that is derived from a standard truckers CB mirror mount antenna. In this particular incarnation, there are mounts for two mobile antennas instead of just one. In my case, both mounts are intended to connect directly to a wire and are insulated from the rest of the mount.

There are also available fixtures that have one mount connected directly to the metal bracket and the other insulated and providing an SO-239 type connection for coax. I know folks who have had success with this kind of mount. However my intent is to have an antenna system that will provide reasonable NVIS coverage on the 75m, 60m and 40m bands. Currents that might flow on the coax and in the mast itself may well distort this desired pattern, so I chose to not use this kind of mount.

The desire for a low band NVIS antenna led me to include a balun in the design. The intent here is to prevent the coax from radiating. In many Ham applications, especially on the low bands, pattern distortion from radiating coax is not much of a problem because antenna efficiency is important and pattern control is not. In my case, I did not want radiating coax to detract from whatever high angle radiation I could get. I also am not fond of RF burns.

The antenna center, pictured to the left, is the modified truckers mirror mount clamped to the end of a five foot steel TV antenna mast. The balun is attached to the mast with tie-wraps and the two mobile antennas are installed as shown. I have another five foot TV mast hoseclamped to the ladder of my RV. Its narrow end sticks out above the RV roof level by several inches. When in use, the TV mast with the antenna fits over the small end of the hoseclamped TV mast, placing the center of the dipole fifteen feet off the ground. When the RV is in motion, fifteen feet is too high to pass under many bridges and signs, so the antenna has to be uninstalled, leaving the hoseclamped mast which extends above the RV roof less than the TV antenna!

I've used this antenna on a number of occasions. My initial contacts were on 40m and 60m with the HFPack crowd. I've used the 75m configuration to check into the Nevada section ARES net both from home and from the 2005 Douglas county Simulaed Emergecy Test. The 20m configuration was successfully used from Alpine CA county during the California QSO Party (CQP) 2005. In the case of CQP, the winds at my chosen site were too strong to consider putting up a small beam or even a full sized vertical antenna. The simple and rugged mechanical design saved my CQP activity from becoming a disaster.

There are many improvements than might be made to this design. What I have is a system built from off the shelf components that is rugged and repeatable. It breaks down into pieces that are no longer than four feet, thus easily stored and transported. Potential improvements (in no particular order) include:

Higher Q tapped coils so that I need only two of them instead of several pairs of Hustler resonators (the Buddipole already has this). This may be a bit tricky to do with coverage on 75m, 60m and 40m. The downside of this is the need to fabricate a mechanicaly robust component. Airdux by itself really doesn't cut it.

A longer "middle section" to raise the radiation resistance as well as increase the length of the high current portion of the antenna. The down side is that the mechanical strength required for longer arms comes at the cost of more mass and higher complexity (if the four foot length limitation is to be maintained). Also, the mechanical moments inherent in a longer antenna may make installing it by one person problematic. My wife worries when I go up the RV ladder with what I have now!

Capacity hats might be utilized to reduce loss and increase radiation resistance. The downside here is the added mechanical complexity, especially when setting up and tearing down the antenna or when changing bands. They also tend to be mechanicaly fragile.

Longer "stingers" can be added to increase the radiation resistance. While ten foot telescoping whips are available from several sources, the mechanical and electrical integrity of these over time is an unknown quantity. They also would require different loading inductances than are available in standard Hustler resonators, further sacrificing mechanical robustness.

Multiband operation might be obtained via Hustler spiders that allow the attachment of more than one resonator at each end. This would sacrifice some simplicty and mechanical robustness, but still seems like a reasonable idea. I've not tried it yet...

Finally, the standard Hustler resonators might be replaced with "super" Hustler resonators. These typically are higher Q than the standards and capable of withstanding higher power. This would appear to be the simplest improvement. The only issue I have is that I only have one and they are not inexpensive - especially at two per band!

Finally, a picture of my station in the RV. The bandwidth of the loaded antennas isn't very wide, especially for a solid state rig (Kenwood TS-50 in this case), so I use the dreaded matching network to increase the usable bandwidth. The rig is happy anywhere on the band but the coax losses rise faster as I move away from resonance than they would with a full sized dipole.

The MFJ antenna analyzer has proved to be an exceptionally useful instrument. It makes it possible to find the resonant frequency of an antenna, which is a good thing to know. You really can't tell where resonance is with only an SWR bridge because minimum SWR and resonance are usually not at the same frequency. My philosphy is that, given a choice, I'd rather tune an antenna for resonance (zero reactance) and then let the matching network transform the antenna impedance into the 50 ohms the rig is designed for. If impedance at resonance is anywhere between 25 and 100 ohms, I don't need the matchbox at all.