60 Meter Antenna Test

Fixed Low Dipole vs TarHeel Mobile on Jeep

Testing by Glenn WB6W and Bill K6ACJ on October 28, 2009

reported by Glenn, WB6W:

I ran a coax from my backyard coax switch to the Tarheel. That way I could easily switch between the Tarheel (pre-tuned to 60m) and the 60m resonant dipole easily from the shack. I was also able to use the same rig (SDR3K) on both antennas, so the only differences between the two antennas were the antennas and the coax to the switch. Both antennas provided an SWR of 1.1 or better at the SDR.

Bill can expand on this description if he wants. He told me he drove to his favorite beachside parking lot, using his hybrid Hamstik/Hustler antenna and TS-480 as a receiver. Thus, we were able to compare the dipole against the Tarheel by switching the same rig between them quickly enough to minimize the effects of propagation. I initially referred to the antennas as "antenna 1" and "antenna 2" without indicating which was which so as to avoid a possible source of bias.

After alternately transmitting on antenna 1 and antenna 2, switching between them about once every two seconds, Bill reported that the usual signal level for antenna 1 was about S7 and antenna 2 was S9+5 to S9+10.

KE7CRG joined us from Henderson NV and reported that antenna 2 had a "good signal" while antenna 1 was inaudible.

Bill then made a long transmission while I switched between the two antennas and noted the received signal strength. The relative signal strength of Bill's signal was -101 dBm for antenna 1 and -93 dBm for antenna 2. Thus, antenna 2 appeared to be about 8 db better than antenna 1, which is consistent with what Bill reported on transmit.

Oh. yeah. Antenna 1 was the Tarheel and antenna 2 the resonant dipole.

Based on this, the Tarheel antenna looks to be about 16% efficient on 60m, assuming that the dipole is close to 100% efficient. Put another way, I can run up to 315 watts into the Tarheel on 60m without exceeding 50w ERP.

We then did a comparison between Bill's two 60m mobile antennas. One is a Hustler/Hamstick hybrid (96" long) and the other a 240" monstrosity with a coil made from silver plated #8 wire. One antenna is 2.5 times the length of the other, so the radiation resistance of the longer antenna should be 6.25 times that of the shorter antenna. That translates into an 8db advantage to the longer antenna. The higher Q coil of the longer antenna (Huster 60m close-wound resonator on the shorter antenna) may provide some additional advantage.

The measurements of Bills antennas were more affected by changing propagation because there was 10 or 15 minutes between measurements of the small antenna and the large antenna. As it was, the signal from the small antenna was -93 dBm and, some time later, the signal from the large antenna was -75 dBm. The difference is an 18 db advantage to the longer antenna - much more than expected. Given the long time between measurements, it seems reasonable to think that the unexpected results were influenced by changing propagation.

After some discussion, we decided to try again, this time trying to minimize the time spent changing antennas. At 6PM the large antenna signal was -80 dBm and two minutes later it was -90 dBm, for a 10 db advantage to the large antenna - much more in line with what we expected.

What we have out of all of this is that the claim we recently heard that the 50w ERP restriction on 60m can be met is questionable at best. On the other hand, I clearly can run 100w into the Tarheel on 60m and not exceed the FCC ERP limit. YMMV with a different mobile antenna and installation. As I recall, there was some discussion when the 60m band first opened that one should include in station records any rationale for why higher power into some antenna that's not a halfwave dipole will not exceed the 50w ERP limit. I now have that.

73 de Glenn