[Elecraft] KAT 500 Loses Match

[Ken WA8JXM]

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)
>