----- Original Message -----
From: <TexasRF@aol.com>
To: <jim.thom@telus.net>; <amps@contesting.com>
Sent: Saturday, December 07, 2013 9:30 AM
Subject: Re: [Amps] PARALLEL CAPS IN OUTPUT
Hey guys,. this is the kind of thing that can easily be modeled and derive
some factual data.
If a 50R load is connected to the amplifier output then through a strap,
wire or coax of a given length, say six inches long what happens to the
50R
is dependent on the characteristic impedance of the 6 inch piece.
Transmission line software will show what happens. At 28MHz, 6 inches og
RG213 would be about 8 degrees long. Also, the 50R load would still be 50R
at
the end of the line.
Changing from RG213 to a wire or strap would lower the velocity factor
which will shorten the line length to about 5 degrees and the impedance
would
increase to a number determined by the spacing of the wire or strap to the
underlying ground plane (chassis). A highish impedance could be on the
order
of 250 ohms for a wire that is .125" in diameter. Smaller diameter would
have higher Z and larger diameter = lower Z. Closer spacing to ground
plane
= lower Z.
Anyway, taking the 250 ohm value, the 50R would be rotated to about 61R
shunted with about +125 ohms X. To restore proper matching in this case
would
require a reduction in the load C to match 61R and cancel the +125 X.
Use of a wider strap would lower the impedance and require a smaller
change
in the load C to restore the match.
Obviously a shorter length would reduce the correction needed.
If the amplifier doesn't follow this kind of prediction then there is some
nuisance reactance present and not accounted for.
At a lower frequency such as 3.8MHz, the line length becomes so short that
a very small rotation of the R and X happens and the effect is basically
negligible.
This can also be plotted on a Smith Chart for a better visualization of
what is happening.
73,
Gerald K5GW
I agree in part Gerald but that is not what happens in the real world.
A relay is used which has its own imedance bump which far exceeds a 6" strap
at 28MHz.
Edit: I see you corrected the VF while I was typing this.
But the argument has been about the term "coax" and I maintain that it is
inmaterial what sort of shielded wire is used. The lower the C and ability
to handle the current with minimal heating the better.....period.
My 2M 3CX1000A7 stripline amp does not even use a variable Load cap which
are generally troublesome at homebrew QRO. I tap right off the line at the
sweet spot that gives me a solid ~65% efficiency with the external DowKey
transfer relay in place. Many so called "experts" said it couldnt be done
but it has been chugging along about 30 years now.
Carl
In a message dated 12/7/2013 7:22:22 A.M. Central Standard Time,
jim.thom@telus.net writes:
From: "Carl" <km1h@jeremy.mv.com>
To: "Jim Thomson" <jim.thom@telus.net>, <amps@contesting.com>
Subject: Re: [Amps] PARALLEL CAPS IN OUTPUT
With all your bluster you still dont get it. All you are using is a short
piece of shielded cable, impedance has no bearing since it is too short
to
present a meaningful reactance.
Your load cap change is due to the added C to ground of that shielded
cable.
Having it is beneficial if the cable is long or passes near other
circuitry
such as the input. Lift the RG-8 shield on a SB-220 and see what happens.
A high percentage of the ones I get thru here have cold solder joints
which
will do wonders for stability.
Carl
### heres one way to test my theory. Toss the load cap..and replace it
with a 50 ohm resistor....
then wire from top of resistor with strap, wire etc, over to a coax
connector......then on to the
MFJ-259B. Now see if the MFJ says 1:1 swr.
Jim VE7RF
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