Comet’s CHA-250B is an interesting antenna. It claims (and delivers) continuous broadband coverage from 3.5MHz to 57MHz with no ground radials. Comet’s website admits this is not without its compromises of course:
“If you have the space, budget and desire to erect a full size antenna system we suggest you do so… bigger is better…however…if you live in an antenna restricted area and must manage with antenna or space restrictions or you simply wish to operate incognito you will be forced to make significant antenna compromises. The CHA-250B will make the most of these circumstances!” – www.cometantenna.com
The antenna is shipped in sections with all mounting hardware required, and bolts onto a 1” to 2” diameter mast. It is an easy, 10 minute job to assemble the antenna with basic tools.
We tested this antenna mounted on a 5 metre steel pole to elevate it above the ground. A sweep of the antenna with an antenna analyser confirmed 1:1 SWR across its entire operating range. The design has been well documented (for example by Martin G8JNJ http://g8jnj.webs.com/cometcha250b.htm) with a lossy transformer dissipating heat. Although the numbers are all there we were interested in getting some first hand experience. We compared CHA-250B signal strengths on 40m with a dipole (apex approx 7 metres AGL) and on 20m with a TH6 3 element beam at 10 metres AGL (not a fair comparison!!). We used an FT1000MP MkV (200W PEP). Propagation was typical for this time of year.
Results – 80m
First up we had a surprise – tuning into an 80m net we called in on the Comet and received good signal reports (59) from stations in southern NSW and in VIC. Good range (1000km)! Our 80m horizontal is tuned for the DX portion of the band and a pretty ugly match on the net frequency, so we didn’t risk stressing the FT1000’s tuner. Comet wins this one thanks to its broadband nature.
Results – 40m
40m gifted us with an opening to Spain, with a couple of stations calling CQ through monoband beams with lots of power. We had success replying on the 40m dipole with a few repeats but the Comet could not be heard when we switched to it. 40M dipole wins by an unknown margin.
The biggest difference between antennas was found talking to a VK2UW approx 400km north of us. Chris had a double bazooka antenna pointing our way and our dipole was facing him. 59 report on the dipole that dropped to a 33 on the Comet. Being a vertical, the Comet is not suited to a short distance skywave contact due to its low angle of radiation.
We also spoke to two club members via ground wave (approx 8km away), there was not much difference between the two antennas, maybe 1 S point.
Results – 20m
No surprises here, TH6 wins. G8JNJ’s page says the CHA-250B is -3dB relative to a ¼ wave vertical. The TH6 claims 9dBi gain (this is free space gain, simulations suggest up to 12dBi depending on the takeoff angle). One would therefore expect the TH6 to return signal strengths on average 15dB (3 S points) better while having a lower noise floor due to its directionality.
We tuned into the Southern Cross net and called in, then switched between the antennas. Signal reports in both directions were typically S9+20dB with the TH6, and S7 to S9 with the Comet. The encouraging thing here was that basically all the participants could hear us on both antennas! TH6 wins the signal report obviously but the Comet is a viable antenna. In general tuning around we could sometimes hear stations on the Comet that were not audible on the TH6 due to direction. The Comet could therefore be used as a searching antenna and when a station is identified the TH6 swung around for a more effective contact. Since the Comet is broadband it will easily cover the TH6’s 3 bands (20,15,10m).
The Comet is a relatively quiet antenna, a description often stated as a benefit but what that really means is lacking in signal strength. However, the useful measure on receive is signal to noise ratio and this was on par with other antennas. Listening to stations on the Comet was no more or less legible than our other antennas. It was rare that we couldn’t hear a station on the Comet compared with our other antennas. The main disadvantage is the transformer loss on transmit.
On the plus side, the Comet CHA-250B is an omnidirectional antenna. It operates on every HF band besides 160m. No tuner is required, band changes are instant. It was very novel to be able to say “let’s try 30m” and put out a few calls without changing antennas. 80M performance surprised us with clear and easy contacts for such a short antenna. 20M showed that DX was straightforward when conditions were reasonable.
If you can’t put up a full size antenna, this is an attractive product. The broadband nature is in some respects a treat, it allows for band hopping and would lend itself to experiments in ALE (Automatic Link Establishment) if desired.
Antenna graciously provided for testing purposes by Benelec Pty Ltd