The military isn't chasing DX or contesting ... They need enough
radiation for adequate operation in their networks. In the example, a
resistor in the folded dipole gives them that, they don't care about the
3dB, and a much easier radio system to set up. Little different for us
"Amateurs."
73,
Fred ["Skip"] K6DGW
Sparks NV DM09dn
Washoe County
Ken WA8JXM wrote on 1/13/2024 9:16 PM:
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)
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Elecraft@mailman.qth.net
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to k6dg...@gmail.com
--
This email has been checked for viruses by AVG antivirus software.
www.avg.com
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Elecraft@mailman.qth.net
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to arch...@mail-archive.com
