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Date: Sat, 07 Dec 2013 08:58:23 -0600
List-post: <">>
And just to hijack this further, I think it's more important to have a wide uninterrupted copper ground plane between the major components ... input / cathode/tuning cap/loading cap/ output connector than what goes on above the ground plane.

ie :  don't build sub-assemblies that require long rf runs.



re: lower v.f. should read higher v.f. My bad/sorry,
Gerald K5GW

In a message dated 12/7/2013 8:30:52 A.M. Central Standard Time, writes:

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"  <>, <>

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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