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Re: [Amps] Ten-Tec Titan 10 Meter Input SWR

Subject: Re: [Amps] Ten-Tec Titan 10 Meter Input SWR
From: Gerald Williamson via Amps <>
Date: Sun, 11 Jun 2017 17:12:42 -0400
List-post: <>
Don, all you say is correct and good. BUT (seems there always is a but), as 
 a practical matter, the input R+j of an amplifier varies with drive level 
and  usual test equipment will not provide useful data. If you drive it with 
a  transmitter there is no way to extract R+j  data at that power level.
Without good R+j data the smith chart is unusable for this  problem.
Gerald K5GW
In a message dated 6/11/2017 1:13:19 P.M. Central Daylight Time, writes:

"True  SWR" doesn't change, but *WHERE* it is measured will vary  widely.

exception :  perfect Z load for perfect Z line where  Zload=Zcoax

You are measuring VOLTAGE inside the coaxial line and the  voltage  
varies along the length of the line like a sine  wave.

Some distances from the wrong Z load = peak voltage
Some  distances from the wrong Z load = minimum voltage
Some distances make for a  close-to-perfect input match.
1/4 wave away from the above perfect match is  a point that
your transmitter probably won't like.

In other words,  changing the coaxial length *COULD* improve
your situation, but it could  also make it worse.

The Smith Chart is your friend, or just make five  jumpers of  
1,2,4,8,16 feet with a handful of couplers and start  substituting  
jumpers between the meter and the load.   One  combination will *MATCH*  
or make a length that the transmitter  likes.   Others will make it  

This is why  it's important to measure the Z *AT* the load and not  
through an  arbitrary length of coax.


>> ##  On  a related note,  why is it, when I increase the length of 50   
>> ohm coax  from my drake amps to the xcvr, that  the
>> input swr rises ??   Its flat  with  4 ft  or less.... but rises  
>> when say 5-8 ft is used.  Thats  on all bands.
> <REPLY My understanding is that's a false  reading caused by current 
> on the outside of the coax braid.  The true SWR does not  change.

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