Unless its something like a log periodic, I think anything with those
frequency/SWR specs can only do it with high resistive losses. It
reminds me of the HF dipole B&W makes for the military: it is designed
to appear to have a low SWR by having 3 db of resistive losses. The
military considered that an acceptable design. Remember, antenna
losses affect TX and RX, so that's 6 db system loss.
Ken WA8JXM
On 1/13/2024 2:37 PM, Walter Underwood wrote:
The C HA-50HD is rated for 250 W SSB or 75 W for digital modes. It gets a flat
SWR with resistive losses in the transformer. That is why the transformer has
heat sink fins. A KPA500 is almost certainly overheating the transformer and
causing failures. I would back off the power to match the ratings. For SSB with
compression, I would run less than 250 W, maybe 150 W.
It is possible that overheating has caused permanent damage to the transformer.
One detailed set of tests showed at least 75% of transmitter power being turned
into heat in the transformer.
Here is what Comet says about that antenna.
CHA-250HD (New version of the CHA-250B)
The Comet CHA-250HD is the revised CHA-250B.
The top section of aluminum has been replaced with a solid whip for greater
flexibility and less strain on the lower sections in high winds. A uniquely
designed broadband vertical requiring NO GROUND RADIALS.
This is EXTREMELY easy to assemble, requires no tuning or adjustments and VSWR
is under 1.6:1 continuously from 3.5MHz – 57MHz!
It is not a 1/4 wave vertical, but an end-fed long wire type antenna with a
matching transformer in a vertical position.
Broadband:
TX 3.5 – 57MHz
RX 2.0 – 90MHz
VSWR: 1.6:1 or less
Max Power: 250W SSB 75W Digital modes/FT8, etc…
CHA-250HD Transformer Section
This is a the “magic” behind the CHA-250HD, the transformer matching section.
The transformer on the original CHA-250 had smooth sides. The current version
has a heat sink to dissipate heat created inside the transformer.
Yes, some of the RF that enters the power feeding section is turned into heat
rather than transmitted as RF, but that is one of the compromises needed to
create a broad-band, low SWR, multi-band HF antenna with minimal visual impact.
https://cometantenna.com/amateur-radio/base-antennas/multi-band-hfvhfuhf-2/
wunder
K6WRU
Walter Underwood
CM87wj
http://observer.wunderwood.org/ (my blog)
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