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

Subject: Re: [Amps] Ten-Tec Titan 10 Meter Input SWR
From: Larry Benko <>
Date: Sun, 11 Jun 2017 16:33:19 -0600
List-post: <>

I have done this exact problem this many times for a high power ISM application at 13.56MHz.

1.) For example put an "L" network between the amp and the exciter.
2.) Adjust the "L" network for a 1:1 match at the exciter at full amp power.
3.) Assume the amp impedance at full power is 40-j10 (but you don't know this value) 4.) For a low pass "L" network with the shunt capacitor at the transmitter side a match occurs with L=340nH and C=112.5pF (you do not need to measure the component values) 5.) Now take the "L" network, reverse it, and measure the impedance with any VNA with the "L" network terminated in 50+j0.
6.) You will measure 40+j10 with the analyzer.
7.) The amp input impedance is the complex conjugate of the measured value or 40-j10.

The Smith Chart is not useless to solve this problem.

Larry, W0QE

On 6/11/2017 3:12 PM, Gerald Williamson via Amps wrote:
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. 73,
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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