Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1] 2 Next   Go Down
  Print  
Author Topic: Comet CHA-250B - The Inside Story  (Read 5677 times)
KG4RUL
Member

Posts: 2727


WWW

Ignore
« on: January 24, 2008, 05:20:37 AM »

I was reading the reviews for this antenna and found the post by Iain VK5ZDB.  He had actually disassembled the "matching network" on the antenna.  Contrary to popular myth, he found that it is an elaborately constructed balun, NOT a "dummy load"!!!

Here are the results of his investigation, an interesting read:  http://www.vk5zdb.com/cha250bxii.pdf
Logged
W3LK
Member

Posts: 5639




Ignore
« Reply #1 on: January 24, 2008, 07:27:36 AM »

The net effect is the same - just like the "matching network" inside the MAX-COMM "tuner" upon which this antenna (and its Diamond clone) appear to be based.

The majority of the power is lost in heat and not radiated as RF.

73,

Lon - W3LK
Naugatuck, Connecticut
Logged

A smoking section in a restaurant makes as much sense as a peeing section in a swimming pool.
WB2WIK
Member

Posts: 20603




Ignore
« Reply #2 on: January 24, 2008, 08:01:39 AM »

Not sure what to make of the results.  The test indicates 5 to 7 dB loss (varying with frequency) when terminated in a 50 Ohm load, but the actual load impedance will be all over the place, and without radials it's still half an antenna.
Logged
N3OX
Member

Posts: 8847


WWW

Ignore
« Reply #3 on: January 24, 2008, 12:04:30 PM »

See my comments in TowerTalk forum but consider that the matching network enclosure, which appears to have a  larger surface area than a hundred watt light bulb, needs heat sink fins for an antenna rated 125W CW.

73,
Dan
Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
AA4PB
Member

Posts: 12854




Ignore
« Reply #4 on: January 24, 2008, 03:22:48 PM »

Dan, those big fins tell the story about where all the RF energy is going - heat. Seems like many these days are trying find the magic antenna that will radiate a big signal on all bands from a very small size - often from the deck or attic. There will always be someone claiming to have just such an antenna for sale :-)
Logged
W3LK
Member

Posts: 5639




Ignore
« Reply #5 on: January 25, 2008, 07:04:35 AM »

<< There will always be someone claiming to have just such an antenna for sale :-) >>

And there's a sucker born every minute to just dying buy it! <g>

73,

Lon - W3LK
Naugatuck, Connecticut
Logged

A smoking section in a restaurant makes as much sense as a peeing section in a swimming pool.
TF3JB
Member

Posts: 6




Ignore
« Reply #6 on: January 27, 2008, 01:17:48 PM »

I met a ham down in Germany in November of las year. He was very angry at the amateur radio dealer who had sold this antenna to him, for not accepting to take back (in his own words): "...this "dummy-load" of an antenna". I think there is a limit to what dealers should agree to put on the market. When the experience is as bad as with this product, it will simply be to the benefit of all (including the dealers) to pull this off the market. Of course the factory should also realize that it will make a bad name for itself - not only for this product - but for all other products which could very well be good performers...

73 de TF2JB.

 
Logged
KL7IPV
Member

Posts: 984




Ignore
« Reply #7 on: February 07, 2008, 10:17:41 PM »

They are now advertising a "NEW" model. I wonder what part of it is upgraded?
Frank
Logged
N3OX
Member

Posts: 8847


WWW

Ignore
« Reply #8 on: February 10, 2008, 03:35:27 PM »

"I wonder what part of it is upgraded? "

Epoxy encapsulation so it's harder to open?
Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
NR9R
Member

Posts: 152




Ignore
« Reply #9 on: February 17, 2008, 02:58:41 PM »

When attempting to transmit on a nonresonant antenna there are two ways one can accomplish it.  First, you can build a matching network to cancel out the reactive components and bring the feedpoint closer to 50 ohms.  Second, you can build a broad band transformer or terminating resistor that will absorb the reflected wave by converting it into heat.  

I personally don't see anything wrong with companies selling broad banded antennas such as the B&W broadbanded folded dipole or the Comet antenna which use the later approach.  Since HAM radio is a technical hobby, it is important for HAMs to be educated enough to know the pros and cons of such a vital component of the station.  Just like reduced antenna size will compromise performance, so too will such a broad banded impedence transformer.

I'm sure there is a unique set of circumstances where this antenna would be a great choice over other options.  I can't think of any at the moment though.

Anthony        

   
Logged
W3LK
Member

Posts: 5639




Ignore
« Reply #10 on: February 17, 2008, 05:20:16 PM »

<< Since HAM radio is a technical hobby, it is important for HAMs to be educated enough to know the pros and cons of such a vital component of the station.>>

The problem is, the vast majority of hams (the word is not an acronym, so it's unnecessary to put it in all caps) licensed in the last 10 years are NOT technical-minded. They actually believe the advertising hype from the manufacturers because they don't have a clue how to tell fact from fiction. Just look at the number of Extra class hams posting on eHam asking for help to make a dipole or solder on a PL-259. Sad

73,

Lon - W3LK
Naugatuck, Connecticut
Logged

A smoking section in a restaurant makes as much sense as a peeing section in a swimming pool.
W5WSS
Member

Posts: 1732




Ignore
« Reply #11 on: February 22, 2009, 05:36:51 PM »

I have a CometCha250 I am not ashamed lol. I needed a balcony vertical. I have been working with this end fed hertz and the torroid broad band 6:1 transformer.ironically enough not on a balcony but in an open pasteur anyway It exhibits loss to the vertical connection that varies with impedance. 20m has been a better experience with it anyway either rf will find the sheilding of the coaxial cable as a best path /to mostly circulating within the interior dissipating as heat so in essence the real answer is how much remains is "IT Depends" on the frequency injected into it and the associated Rr loss".in conjunction with the base height(I think they new the sheilding half of the circuit needed to be as long and straight away as possible offset by recommending an rf choke "talk about a contradiction conflict of feild strength issues lol." The measurements of which appears to be rather dissapointing. However The work I have been doing involves an elevated radial system to help improve the feild strength. Emperically the challenge at least for me is in designing a radial system that is optimal or nearly so, and attempting to measure any feild strength improvements. Personally I enjoy the challenge. My theory is that rf can be diverted before entering the 6:1 pressed onto the radial set, Therby improving the feild strength by re combining outside the feed point container in the near feild adding to the radiation. This is not meant to mean that the vertical radon is contributing more because that half of the antenna remains unchanged lossy depends etc But the radial set when tuned can be optimal contribution equalling perhaps a 3/4 of a whole antenna. radials for this antenna make sense.
Logged
W5WSS
Member

Posts: 1732




Ignore
« Reply #12 on: February 22, 2009, 05:50:15 PM »

I did not purchase the antenna..hehe
Logged
WW5AA
Member

Posts: 2086




Ignore
« Reply #13 on: February 23, 2009, 06:55:39 AM »

......and if you have one of these antennas you can work almost anyone you hear. I'm still waiting for someone to buy my Light bulb antenna. Honest for certain situations its a winner.

73 de Lindy
Logged
AA4PB
Member

Posts: 12854




Ignore
« Reply #14 on: February 23, 2009, 12:47:30 PM »

That sounds like a brilliant idea!
Logged
Pages: [1] 2 Next   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!