Comet Antenna, of Japan, developed a 24 foot vertical antenna for HF that requires no radials, is multi band, and I picked up one at a recent hamfest. How good is it? Reviews on the Internet are mixed which means even though some people hate it others love it. Why?
It is extremely clean looking with no top hat radials or spines or traps to break up the visual lines. It has a mysterious matching device at the base. Not mentioned in the sales brochure and almost buried in the instructions is an important note that might be a clue as to it’s love-hate reviews. This antenna must be mounted a minimum of 35 feet above ground. How many reviewers had their antennas mounted correctly at 35 feet or more? From the Comet instructions:
Aha. The secret to this antenna is to put it ridiculously high, use the coax and mast for a counterpoise, and let the coax or the mast do the radiating. It is marketed as a small yard solution awkwardly worded this way on the Comet web site:
“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!”
A 35 foot mast with a 24 antenna adds up to 59 feet tall. Does a 59 foot vertical equal incognito?
Enough of the negativism. One must get past the surprise of the height requirement and just think of this is as a 59 foot vertical antenna. The Comet CHA250B is a 59 foot vertical. How does it model in EZNEC? Will it work some DX? Our reference antenna is a full size quarter wave ground mounted vertical with lots of radials over average soil. We care most about the gain at a take off angle of 15 degrees and secondly how “full” the pattern is. That is, does it have serious nulls that would compromise operation. Here is a view of the antenna with rf current modeled at 14.1 MHz.
Let’s run through the bands and I’ll make comments starting with the toughest of all, 160 meters. A gain of 0.77 dBi on 160 is quite respectable. And there are no nulls. Nice. This should work some DX on top band. This is roughly equivalent to a full size quarter wave vertical. HOWEVER, I’d be skeptical of a model showing a 24 foot antenna having a .77 dBi gain on 160.
Next band up is 80 meters which often proves to be the hardest band when it comes to 5BDXCC. The pattern here looks good, too, with -0.21 dBi. Again, I am skeptical of the model.
Now for the esoteric band, 60 meters. Pretty good again. Very usable.
How does it do on one of the most popular bands, 40 meters? Can a person work some DX here? Yes, as long as the band has gone long, very long. This band has one of those dreaded serious nulls above 30 degrees and stateside signals will be missing. No NVIS either. A person will need a separate 40 meter antenna to work domestic stations. On the other hand interference from strong U.S. stations should not be a problem when working DX. This is the first band where the pattern is not ideal.
Moving on up through the bands, next is 30 meters, the first WARC band so far. The pattern looks good here. Good gain at 15 degrees and a nice plump pattern the rest of the way.
The big one, in fact the biggest one for working DX, the 20 meter band. How does it look? It’s not equivalent to a beam but it holds it’s own against a quarter wave vertical. It also has plenty of plumpness for higher angles, too.
Now for the upper bands beginning with the WARC band, 17 meters. Wow, even a dB of gain here. A little disappointing above 45 degrees but otherwise a usable band. Critical frequency never goes this high to allow NVIS contacts on this band, anyway.
Now we’re getting into what our grandparents called Ultra High Frequency, 21 MHz. How does it do? Fine. Like the 17 m band, we don’t have much high angle radiation but in the low angles used for DX it looks good. And it has 2 dBi gain even.
Skipping 12 meters because we assume it will behave the same as 10 meters, here is a look at the “light” band (from the phrase, “DC to light”). Ten meters is a very usable band on this antenna on the rare occasions the band is open. Rooster tails are almost ready to appear but they haven’t yet. Therefore no serious nulls. In addition we have almost 2 dBi of gain.
Overall it would be reasonable to assume this is a good multi band antenna usable for both DX and domestic contacts on almost all bands. How in the world we can erect this at 35 feet is questionable. The author’s or translator’s grasp of English is not strong as indicated by the use of the word “erectric” instead of “electric”. Could it be that the author does not have a grasp of the measurement system and he (or she) actually means 35 centimeters or something else? An attempt to find a copy of the instructions written in Japanese failed. The thought was maybe there had been a failure in the translation if we could find the original instructions. We will attempt to put this antenna up at 35 feet and see how it plays. At that height it’s hard not to play well.