On Mon, 19 Oct 1998 12:12:14 -1000 "Jim Reid" <jreid@aloha.net> writes:
>
>Aloha Big Gun HF amp users,
>
>At what power levels do you have to start worrying about
>antenna system components? All our "stuff": PL259's, N's,
>9913, LMR-400, typical commercial antennas, switches,
>even the Top Ten devices remote switches, etc. are all
>fine up to and somewhere above the 1500 watt CW,
>RTTY continuous output level. Some antenna tuners
>are rated to 3kW; at least one soon to be for sale,
>the Ameritron ATR-30 will handle a real 3 kW, or
>at least so I am told. I suppose it will have S)239 sockets,
>so maybe they are good to at least that power level.
>
>Any comments on the topic? Up to what powers will
>the usual componentry live, and at what power do you
>have to start upgrading to large hardline, perhaps
>home brew antennas, tuners, etc?
I dont trust any multiband antenna at QRO. Many commercial monobanders
will handle 5KW and it sure is simple enough to build your own gamma
match, etc for ones that cant.
>
>I calculate that at 3 kW, for example, in a 50
>ohm system, with say 2:1 vswr, the RMS voltage
>will be 548 volts; peak voltage, 1.414 times that,
>or at 775 volts, and the RF current will be about
>11 amps.
>
>Typical good RG8 size coax is rated at 4000 or 5000
>volts breakdown, and yes, as the vswr increases, this
>rating drops, so also the power handling capacity.
The flat losses also increase as you go up in frequency, independent of
the VSWR. This is usually the controlling item since RF resistive losses
build up the heat.
RG-213 can safely handle 5KW PEP ICAS up thru 20M, after that it starts
to get a bit warm. I have melted it and cheap SO-239's at 3KW at 50 MHz
while testing a customer conversion....I now use N connectors for
anything over 1500W at 6M.
For the fairly small difference in price I would go to 1/2" or larger
Heliax type cable or CATV hardline.Too many hams spend a fortune on rigs
and antennas and then shop for the cheapest feedline....never could
figure that logic out!
I use almost all 3/4" CATV line here.
>
>In a balance feed/antenna system, using, say 450 ohm
>twin line, and still 2:1 vswr (band edges, say), the voltage
>increases to an RMS of 1640 volts, and peak of 2325 volts,
>and the current, of course drops to about 3.6 amps.
Cant say, I havent used that stuff since I was a novice in 1955,
>
>Amidon High Power baluns still use the SO239 socket
>on units rated at 10 kW,
Yeah, sure !! Overpriced and overrated.....
I believe.
>It would seem the lowest lost system would be the twin line
>feed and balanced tuner, a la, old Johnson AM kW Matchbox.
The Johnson has been proven to have fairly high attenuation at certain
settings. The best system uses no tuner and an antenna matched to the
feedline. The advantage with coax is that you can just about guarantee an
installation with no TVI-RFI generating feed line radiation. It is darn
near impossible to do that with open line.
Open wire does not like sharp bends, proximity to other objects and it
sure hates to get wet or covered with ice....the latter not a KH6
problem.
73 Carl KM1H
>
>Just curious,
>
>73, Jim, KH7M
>On the Garden Island of Kauai
>
>
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>
>
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