Review:
Superloop III Antenna by
RadioWorks
(Now called the Superloop 80)
by Dick Stevens, N1RCT
(Originally appeared in Nov. '96 issue of the Digital Journal)
Like many operators with small lots, a G5RV was about all I can fit for the 80 and 40 meter bands. The G5RV is 102 feet long and has a 34 foot section of twinlead followed by coax into the shack, possibly with some sort of RF choke on the coax. The ends are typically supported by ropes up in the trees. An 80 meter dipole would be about 134 feet long.
My tiny village lot ( .22 acres) is great for taxes but limited in antenna potential and zoning laws prevent real towers. I had an 80 meter horizontal loop for a year, but was not able to support 4 corners at any height above about 22 feet. I was able to get a G5RV up at 57 feet and running north/south and it was a much improved performer so the old loop was retired.
In an effort to improve on my G5RV for 80/40 meters (contesting mostly), I searched many antenna books and catalogs. I really needed something that would go on the same supports as the G5RV (as it was at the highest possible height on my lot) and was dual band. I tried the Van Gordon 80/40 stacked dipole for a while but had constant tangle problems and the performance seemed no better than the more versatile G5RV. I then tried the RadioWorks "Superloop III" designed by Jim, W4FTU, and refined over the years, although the basic "trick" involved goes way back.
PHYSICAL VARIATIONS
The standard arrangement is shown in Fig. 1. It looks like an inverted delta loop and is 112 feet across the top. It fit on the same ropes as my G5RV used and the coax even started at about the same point in space. The wire is heavy 14 gauge copper. If your space doesnt quite allow this, the top corner insulators can be moved to shorten the 112 foot dimension; also additional insulators can be added to the diagonal wires to make a rectangular shape and raise the bottom balun up in the air more. I also added 6 feet of wire to move the resonant freq closer to the band bottoms for digital work. The loop can also be mounted upside down and slanted if you only have a single support available. As with all loops, the area enclosed is important and so is the average height; the standard inverted delta shape is a very good compromise.
ELECTRICAL CHARACTERISTICS
The "trick" to the Superloop is the 30' length of ladderline hanging down from the center insulator. This length has been tuned so that appears to be a open-circuit stub on 40 meters; thus the antenna becomes two full-wave wires (at 40 meters) and is commonly referred to as the Bi-Square antenna. On 80 meters, it appears to be a short and the antenna becomes a single wave vertical loop. This happens automatically and no switching is involved. A special balun is provided which gives a match between the 50 ohm coax lead-in and the higher resistance of the loop. For best matching, a 1/2 wavelength coax is recommended (e.g. 99' of RG-8X); however mine is about 70 feet into my diff-T tuner and the SWR < 2 points are 3495 to 3787 but the short coax gives a minimum on 40 of 2.05 at 7090 KHz. If you need to run without a tuner, close attention to the coax length will help. The balun is the typical ferrite rod in a PVC pipe with foaming urethane inside. This has the effect of heat insulating; mine works fine on 500 RTTY watts contesting, but real high power may be a problem on RTTY; but those guys all have beams, right?
OPERATING RESULTS
I have used this to make perhaps 500 contacts on 80 meters and 1000 on 40 meters in 7 months; the furthest being New Zealand on 40 and Moscow on 80 meters, mostly at 500 watts (and all the reports were 599, hi). The diagonal wires make it partially a vertical antenna with a nice reduction in polarization QSB. In the SARTG this year, I had the highest number of 80 meter contacts from NA. From W6/G0AZT in California on 40 meters, I got the comment "You would not believe how loud you are out here". All-in-all, I think it doubled my contacts on 80/40 over the G5RV. RITTY helped on the reception, but they had to copy my CQ first. The Superloop tunes up fine on the 20,15,10 bands but I have not used it there much. Antenna, ropes, and coax will run you about $US 135. RadioWorks advertizes in CQ and QST and have an interesting catalog. (Since this was published, I have received several requests for instructions on how to build a home brew version. I don't think that would be too ethical for me to provide and besides, it will cost about the same to homebrew with new parts and it uses a special matching balun of unknown (to me) parameters.)
73 de Dick