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Date: Sat, 7 Dec 2013 09:30:52 -0500 (EST)
List-post: <">>
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
In a message dated 12/7/2013 7:22:22 A.M. Central Standard Time, writes:

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

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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