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To: <>, <>, <>
From: "Carl" <>
Date: Sat, 7 Dec 2013 10:36:29 -0500
List-post: <">>

----- Original Message ----- From: <>
To: <>; <>
Sent: Saturday, December 07, 2013 9:30 AM

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

This can also be plotted on a Smith Chart for a better visualization of
what is happening.

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.


In a message dated 12/7/2013 7:22:22 A.M. Central Standard Time, writes:

From:  "Carl" <>
To: "Jim Thomson"  <>, <>

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

Having it is beneficial if the cable is long or passes  near other
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
will do wonders for stability.


###  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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