[Amps] Ten-Tec Titan 10 Meter Input SWR

[Larry Benko]

Gerald,

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.

73,
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
>   
>   
>   
>   
>   
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> In a message dated 6/11/2017 1:13:19 P.M. Central Daylight Time,
> donroden at hiwaay.net 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
> worse.
>
> This is why  it's important to measure the Z *AT* the load and not
> through an  arbitrary length of coax.
>
> Don W4DNR
>
>
>
>>> ##  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
> flowing
>> on the outside of the coax braid.  The true SWR does not  change.
>
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