2004 KØS, Kurt N. Sterba Strange Antenna Challenge
Operated by Erik Weaver nØew and Dwayne Walker wb5plj,
Loadlock Dipole
Height About 8-Feet
Icom-706MKiiG, 100-Watts, MFJ-962D Versa Tuner III, Battery Power
May 29, 2004
Saturday afternoon we operated this loadlock dipole for about 30 minutes to an hour. We managed two contacts on 40-meters. One in Texas and the other one just a few miles south of our location in Ozark, Missouri (our only Missouri contact by the way). The band conditions were very poor. There was quite a bit of noise, usually about S7, with a lot of fading. Lightning crashes from dozens of lightning strikes across the country were pretty much constant, but at least our strange antenna was hearing them! If you wanted peace and quiet all you had to do was tune to 10-meters where nearly all the noise went away -- of course that was only because the band was closed and there was virtually no propagation there! (In other words, quiet band conditions are NOT good if it is only because you are getting very nearly zero of your signal carried by the atmosphere.)
We were able to fully load 100-watts into this antenna and tune it up quite easily from 17-meters on up. The longer bands were much more difficult to tune up and my MFJ-969 did not achieve a successful tune at the lower frequencies, whereas the MFJ-962D was able to do so. We did NOT alter the connection points where we hose clamped onto the ends of the loadlock. We just tightened them down and started calling CQ (well, after tuning up of course).
If you will take the time to slightly alter the position of the hose clamp against the loadlock, you should be able to alter the impedance of the feed point. This can be thought of along the lines of a gamma match used to feed some beam antennas. By altering the position along the fed antenna element you also change the impedance. So by moving the hose clamps up or down by several inches a person may be able to find a little better match (closer to 50-Ohms). If you have an antenna analyzer or SWR meter either of these devices may be employed to assist in de terming whether your feed point impedance is improving by being repositioned. The antenna analyzer will allow you to simply read the series resistance and reactance on the LCD display, and the SWR meter will display a lower SWR as your feed point impedance comes closer to 50-Ohms.
You can see that we simply placed a hose clamp around the end of the loadlock, inserted the breakout wire between the loadlock and the hose clamp and tightened it down. We did not scrape off any paint or anything like this, but there are quite a few spots where the paint has already been scrapped away by use.
For those of you who may not know, a loadlock is a device a truck driver uses to keep his loads from shifting inside the trailers of the big trucks traveling up and down our highways. These are two pieces of metal, one slipped into the other, and then a ratchet is engaged to press the two rubber feet against the interior walls of the trailer.
The Altoid's mint can is hanging by the two # 12 breakout wires that connect the coax to the two loadlocks. There is about 12-14 inches of breakout wire leading to each loadlock. As you can see, the two loadlocks are resting next two one another on one of the support braces of the step ladder (which is fiberglass). Really the two blue breakout wires should've been reduced in length, but this antenna didn't seem to be working too well and we deemed it wasn't worth our time to mess around with at that time.
Recall there are huge storm fronts rolling across the country and we wished to maximize our operating time and contacts. For this reason we didn't fine-tune these antennas, or experiment with such things as altering their heights and orientations. These are things we'd like to do, and we remain certain one of these years we'll have a nice weekend with clear skies as far as you can see all weekend. We just haven't experienced this yet! (In 2003 we experienced near-tornado strength winds, along with a tornado warning which ended our operations.)
This was one of the poorer strange antennas used this weekend. Partly because this display a rather erratic SWR (and therefore impedance) and partly because it settles down quite a bit in the higher frequencies. Anywhere from 17-meter through 6-meters it should work pretty well, but it is just plain squirrelly on the lower bands, like 40-meters where we were operating. This is not too surprising as each leg is about 9' - 9" in length.
Following are some more of the images of this antenna, along with its SWR chart....
This antenna should work pretty well on 11-meters (11 * (39.37/12) / 4 = about 9-foot for a 1/4 wavelength on 11-meters, which is no longer in the amateur band). If you look at the SWR chart, you'll see its best SWR occurred between 10- and 6-meters, which is also out of the amateur band. At 45.167-MHz it measured 34-Ohms resistance and with very little reactance.
The little gray boxes seen close to the 5:1 SWR line represent approximately the location of the amateur radio bands between 160- and 6-meters.
The notations written along the bottom of the SWR chart note the series resistance (Rs) and the series reactance (Xs), and sometimes the frequency at which the measurement was taken. In all cases I am using my MFJ-269 Antenna Analyzer.
Please note of the little gray boxes adjacent to the 5:1 SWR line. These indicate the approximate location of the amateur radio bands from 160- to 6-meters. This allows us to gain an impression of how each antenna shapes up across the entire HF range in which our amateur radio bands reside.