[GCARC] Comet CHA-250HD antenna comments

[Kenneth Bozarth]

I strongly concur with Tony. Having designed high power RF transformers
ever since ferrite became popular back in the early 70s, there is no
getting away from some loss. But this can be mitigated by choosing the
correct material and using enough turns of wire to reduce flux density. If
a transformer needs a heat sink, then it should have been designed with a
larger or more appropriate core, or lower power rating. Regarding the loss
- a good transformer can have as much as 0.2dB loss, and it will get hot
but should not get hot enough to damage the core. Don't be discouraged by
0.2dB loss, as the guy at the other end cannot hear the difference in your
signal. The good news is, you get wider bandwidth with this loss. 0.2dB is
about 5% of your power. Running 1kW will result in about 50W of loss,
requiring a large core, depending on duty cycle you run. If the matching
"box" is not at least 4 inches square, the core is too small.

On Wed, Nov 15, 2023 at 11:23 AM Tony Starr <tstarr1450 at gmail.com> wrote:

> Jeff,
>
> All that Charlie,  or anyone else considering this antenna, needs to know
> about it, is contained in this very honest paragraph on the DX Engineering
> website"
>
> *This improved Comet CHA-250HD features a new solid whip replacing the top
> aluminum section for greater flexibility and less strain on the lower
> sections in high winds. The “magic” behind the CHA-250HD is the transformer
> matching section. The transformer on the original CHA-250B had smooth
> sides. This improved design features a heat sink to dissipate the heat
> created inside the transformer from the RF that enters the power feeding
> section rather than transmitted as RF. This is the compromise needed to
> create a broad-band, low SWR, multi-band HF antenna withinimal visual
> impact.*
>
> Basically, it comes down to this:  Antenna "matching transformers" should
> not feature heatsinks or cooling fins.  If they do, it is a dead giveaway
> that they are HIGHLY in-efficient.  Unless you have lots of power ( and
> money ) to waste, there are many other vertical antenna choices that you
> can make which will put more RF in the air, and keep more money in your
> pocket. My favorite choice, which I own, would be the Hustler 5BTV, but
> there are many others, all of which will perform better than the Comet
> CHA-250 series.
>
> Tony K3TS
>
>
> On Wed, Nov 15, 2023 at 8:56 AM Jeff One <djgrath1 at gmail.com> wrote:
>
> > All,
> >
> > Charlie Wahl, KC2STO is asking for some comments on purchasing a Comet
> > CHA-250HD HF antenna. Please respond directly to him at 856-656-6122.
> >
> >
> >
> > Thanks
> >
> >
> >
> > D. Jeffrey Garth
> >
> > 621 Greenbriar Drive
> >
> > Williamstown, NJ 08094-1149
> >
> > 73, WB2ZBN
> >
> > Club Publications
> >
> > www.w2mmd.org
> >
> >
> >
> > "Choose a job you love, and you will never have to work a day in your
> life"
> > - Confucius
> >
> >
> >
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