Sunday, September 9, 2007

New Antenna!!


My wife spent the day visiting a friend, and took the girls with her, which left me about 12 blissful hours on a weekend to do whatever I wanted to do. I'd be intending to install a new HF antenna for some time, so this was the best and obvious time to do this.

I put out a call asking my amateur radio friends to come by and help. Bojan agreed, which was great. He's a bright guy, very creative when it comes to technology. If you're ever looking for someone who can turn ideas into reality, give him a call.

The antenna I chose was a Comet CHA250B. This particular antenna is unique in that despite being vertically-polarized it has no ground radials, and it will also tune all bands from 80m to 6m. The coax feeds into a large black cylinder which acts as a matching network. This particular antenna has been alternately praised and vilified on review sites like eHam. People are either giving it a 0/5 or a 5/5. There doesn't seem to be a lot of rhyme or reason to this; although it's interesting to note that people giving it a 0/5 tend to compare it to a dipole which might be unfair.

For me, I was willing to try it out. Considering that my comparison point was my aforementioned $2 "Speaker-wire Special" I suppose anything would seem better. The real challenge in the antenna install was not so much the outside work, it was deciding how to get the coax up to the roof from my shack. Doing so entailed slithering around in my attic, which is a complete mess after a roof replacement earlier this year. There were also a few setbacks, such as a broken lag bolt and a too-small bracket on another antenna (my HDTV antenna) which is now needs to be adapted to the new larger mast.

In the end we got the Comet installed, and two extra runs of coax. I fudged up a temporary VHF/UHF antenna (until I can get a replacement lag bolt and the new mast installed) by slapping a magnetic mount down onto a metal attic vent. I may also sell the VHF/UHF antenna I had on Craigslist and get something else because it's a 6m/2m/75cm tribander and the Comet now covers 6m.

As expected the Comet performs better than the homebrew. I'm getting signals where there were no signals before, I don't have RF-in-the-shack problems like I did before, and the antenna is clearly quieter (lower noise floor) than the homebrew. It tunes a lot of bands, so all in all I'm happy with it. I'm not quiet sure why some people have panned it so hard on eHam. I suspect that it may have to do with unfair comparisons to dipoles, but also it might be the install makes a difference. The Comet's instructions call for it to be installed 32 feet off the ground. Perhaps if it's not installed at that height the matching network doesn't work?

10 comments:

KI4WLR said...

Great post on your new antenna, glad you included a photo, it looks great.

73, KI4WLR
http://www.ki4wlr.com

Tom said...

In determining how well an antenna works or doesn't work, it must be compared to something. A dipole is the usual standard of comparison.

There was extensive review of this antenna published in CQ magazine when it first came out (http://www.cometantenna.com/pdf_review/Comet-CHA250B-Review.pdf). The review is slightly skewed because the comparison antennas used were not always the same. A 2 element Yagi @45' was used for 10, 15 and 20. A rotatable dipole @45' was used for 30 and 40 . And an 82' long dipole @ 35' was used for 75/80. You can see the results for yourself.

My overall impression is that it works about as well as a middle of the road mobile antenna. Whether you think this is worth $450.00 is up to the individual buyer.

I think the wide divergence in reviews is caused because some folks who are antenna challenged are thrilled to get into Europe with 100 watts with the antenna. This, of course, is no great accomplishment when you consider QRPers do this all of the time with even less efficient antennas. On the other hand, the antenna purists refuse to acknowledge that an elevated vertical antenna without resonant radials can get out of the back yard. In their minds, this antenna is simply contrary to the law of physics so it automatically rates a 0/5. These same comments were made about ground independent antennas such as Cushcraft R5s and R7s when they first came out. They have "matured" into the R8 and Hy Gain AV 640 and, in my opinion , do a very nice job.

Bottom line: It's all in the eye and ear of the beholder.

Bernard P. Fife said...

I also have a Comet CHA250B. I installed on an eave mount at 25 feet and it works great. I live in a townhouse development and wanted a good performing multi band low profile antenna for HF. The Comet has exceeded my expectations. It outperforms my G5RV Jr., Cushcraft R-6000, Cushcraft MA5V and my Gap Titan.

I also noticed the very negative reviews from the Great Britain group. After reading their reviews it appeared to me they have a gripe with the Comet Company and just wanted to lower the rating of the CHA250B. Some of them admitted they do not own the antenna. I did not consider their reviews as being valid.

I'm glad I bought the Comet and would recommend it to anyone living in a development with restricted space.

73,
K3DLB

Bob W5WSS said...

Hello, Thanks for posting your comments I found them quite useful. I currently reside in an apartment so I want to do some hilltopping with an Icom 718, an LDG automatic tuner and a reasonably(a subjective term) working vertical. I am shopping for a vertical radome/or a vertical dipole and the cha250b is tempting. I honestly am not concerned with this antennas low gain attributable to end fed zepps-minus a ground radial system....Yet this antenna IS expensive and when compared to a vertical rigid dipole probably falls well below 2.14dbd. I estimate that most of the gain curve should fall below 0 dbd throughout the target bandpasses,except for maybe 10m-12m where the Comets radome is reasonably long. Which I can accept as a trade-off. However an Imax 2000 is a center fed vertical even though the connector is located at the bottom, with adjustibility..that should work better on 10,12,15,17,20. I am not sure either antenna will work very well on the lower bands both falling well below 0-dbd (not intended to mean they do not radiate). Care should be excersized when comparing horizontal dipoles to vertical dipoles the intersect point depends on height above ground mostly for the horizontal antennas the verticals will need alot less height to be dx-able I would choose the vertical dipole. Not to be confused with a ground mounted radial dependent vertical antenna.(Marconi). The Vertical dipole is truely ground independent,can be pressed into multi-band service with a balun and Flatline....yes FLATLINE and one of these new LDG tuners The result should be good based on the fact that regardless of maximum gain figures the antenna will radiate mostly rather than convert energy to heat. With an antenna that can be considerably lower than a horizontal dipole. As a valid comparator, the Vert.Dipole described above can present its radiation concentrations at a low toa from a quarter wave base heigth above ground, an acheivable height for the average ham. I would choose the 17m band for the quarter wave formula base height apx.14' Do all this on a hilltop and have fun

MYLES said...

Myles VK6ZRY
It's happened again someone has mounted a 50 OHM dummy load and stuck an element on it.
I bet you a sheep station (one million acres) it has no ferrite balun in the base, like the Cushcraft R series antennas. They use a 16:1 balun and traps on the top element.

Kwac said...

Myles, could I please have the benefit of your advice - I live in a single-storey house on a 300 sq. metre plot.

XYL is antenna-phobic. 5-line power line runs along one wall at a height of 12 metres.

Instead of an antenna like this, what would you recommend?

bestonline323 said...

The CHA-250Bx vertical from Comet is a decent vertical antenna for 20m and up. In fact, it works very well on 15m as I have worked most of S.America on it using only 100W. With a tuner you can go down to 160m but don't expect much. It is a surprisingly good receive antenna and will pick up everything from LW to UHF and I've had great success on MW/BCB piping it into a Misek phasing box with an aircore loop (gotta pad down the Comet about 10dB here for it to fly).
I have mine up 20' fed with LMR-400 and have a substantial ground sytem. I like the looks and lightness of the thing but am a bit perplexed as to why just a light wind storm in February put kinks into the top two sections. Comet replaced them free of charge and only required that I submit a pic of the antenna as proof.
The older models are prone to bending, especially if you have it on a mast that might whip the thing when exposed to 40+ mph gusts.
Don't get your hopes up for 40 and 80 meters although it will get out and you can make contacts. It doesn't compete even with a very low height dipole on these lower bands though there are times when it might receive better.
The antenna is not really worth the money for a location where you have wire or rotatable dipoles as alternatives.
If Comet optimized the CHA250 for 40m and higher, beefed it up for more power, and sold it for around $325....it would be a real winner.

Cheers,
Ben
Antenna Design

Monty P Marshall said...

Hi all, I have just purchased a Comet CHA 250bx (not sure what the x means)and your reviews have helped. I have a two story house, limited space ect, so this seemed to be a good option. Does anyone have pics of this mounted on a roof top with your grounding solutions? I would be interested in how it has been secured. How many of you use guy wires? How many of you use tuners, and what kind? I am new to the hobby and have just passed the Canadian test last week, so I am currently waiting for the mail to arrive with my call sign. Any advice would be apreciated!
regards Mark

ka4rur said...

Great post on your new antenna, I have had my antenna for a few years it is the CH-250BZ, I switch from the Comet HVU-8 another antenna that they said would not work, I have talked all over the world with both antenna's .. work the Turk and Caicos islands at 5 watt's with no problems with the Ch-250 .. mounted my antenna on the side of the garage at 25ft works great ... 73's to all

fred/ka4rur

azmi said...

Hi, nice antenna. Good description on the project too. Thanks for sharing.

73, 9W2AQ
http://www.maltronix.com