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
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 RG8 shield on a SB220 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
MFJ259B. Now see if the MFJ says 1:1 swr.
Jim VE7RF
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