[Elecraft] High Current only on 20 Meters

[Jim Brown]

Adrian,

Power factor is a quantity associated with mains power (and similar 
power distribution systems). In the old days (when I went through EE), 
it was, indeed, related to the phase angle.

With RF systems, it's the load that the antenna presents to the 
feedline, and it's different at every frequency. It's primary 
significance is how it looks to the feedline, which in turn affects loss 
in the line and, after being transformed by the line, whether the 
transmitter can put power into it. The function of an antenna tuner is 
to transform whatever that impedance is to something the 1) makes the 
output stage "happy", and 2) that the output stage can supply power to 
without stress on the output stage that could cause destructive failure.

When "happy" has been achieved, the transmitter is delivering power to 
the line, but if the match between antenna and line is poor, not much of 
that power may get to the antenna, but be turned into heat in the 
transmission line.

Remember I said that VSWR in a system is set by the load (the antenna) 
and is decreased by the loss in the line. If the line is long enough and 
mismatch is great enough, the VSWR eventually ends up a 1:1. I've used 
this example as an extreme case: a 1,000 ft spool of RG58 with a 10K ohm 
load would look like a perfect 50 ohm load to a transmitter at 28 MHz, 
the SWR would read 1:1 at the transmitter end of the line, and loss in 
the feedline would be huge.

73, Jim K9YC

On 3/4/2020 9:57 PM, Adrian wrote:
> Jim,  yes in you example ' Rs + j Xs)' I was referring to Rs, which I though
> was the antenna resistance, and jXs the capacitive or Inductive reactance,
> of which does not
> consume any power, but effects the power factor (phase shift) acting on the
> real antenna resistance. Which part did I misquote or misunderstand Jim, or
> is Rs not the antenna resistance
> under the conditions of measurement.  ?
> 
> Adrian ... vk4tux
> 
> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Jim Brown
> Sent: Thursday, 5 March 2020 2:32 PM
> To: elecraft at mailman.qth.net
> Subject: Re: [Elecraft] High Current only on 20 Meters
> 
> On 3/4/2020 8:15 PM, Adrian wrote:
>> When I say radiation resistance I include the small copper resistance
> also, which is negligible on this heavy copper wire delta loop.
> 
> An important part of my post was about using the right words to describe
> physical reality. Radiation resistance is a characteristic of an antenna,
> and can be used to compute antenna efficiency. That's NOT what you're
> measuring. You are measuring feedpoint impedance (assuming you can connect
> at the feedpoint AND that your measurement setup doesn't change the
> impedance).
> 
> So please call it what it is -- the feedpoint impedance, which your analzyer
> probably reports as Rs + j Xs). :)
> 
> 73, Jim K9YC
> 
> 
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