The method described below, of placing the design resistance across the
tube or socket, is correct when determining if your matching and tuning
are in the ballpark. It is difficult with a cavity or transmission line
circuit to get the exact impedance transformation correctly measured.
But you should see a dip or peak depending on what method you measure,
S11 or S21 with an analyzer. The tube itself supplies the right
capacitance to terminate the matching circuits and resonate on
frequency. The R part is of course from the operating tube with beam
inside. It is not easy to measure a circuit under high power except with
just VSWR and power meters.
In the amplifiers I build at work, with high power VHF cavity circuits,
it is impossible to measure the input match cold, and even worse, place
a resistor where the tube is. The tube is at least 30 cm in diameter at
the grid connector. Where does one place this tiny resistor? Impossible
to localize and find a single spot, as the tube is symmetric in the
cavity. I would need a sheet of resistance that is like the radial beam
in the tube. In this case, all I can see is a slight dip in return loss
on the input when it is cold. But with idling current (Izsac) the input
matching is terminated by the tube cathode Z and beam is present. Then
it can be seen with a network analyzer sweeping the input connector to
the cavity. Not so easy for output matching.
73
John
K5PRO
From: Vic Rosenthal K2VCO <k2vco.vic@gmail.com>
To: amps@contesting.com Subject:
Re: [Amps] antenna analyzer
The method requires that you know the input and output impedances of the
the tube (I think you are talking about tube amplifiers?) Then you put
non-inductive resistors of the appropriate values in place temporarily.
So, for example, if the plate impedance of an amplifier when operating
properly is 2000 ohms, you put a 2000 ohm resistor from plate to ground.
Then you connect the analyzer to the output of the amplifier (you might
need to bypass a relay) and adjust the amplifier tuning for lowest SWR.
1:1 means that you have matched the 50 ohm analyzer to the 2000 ohm
plate impedance. Once matched you can look at the capacity required in
the tuning and loading caps and estimate if the Q is reasonable. You do
something similar for the input circuit. You leave the tubes in place
but of course no power is applied.
On 4/5/2014 12:05 PM, Paul Decker wrote:
> Hello amp builders,
>
> I've been attempting to convert some commercial vhf and uhf
> amplifiers over to the ham bands. The gear operates in the 150 Mhz
> and the 550 Mhz range.
>
> Someone a while back said I could use one of the fancy impedance
> analyzers to tune the input and output circuits. I'm wondering if
> this really works? Before making any changes, I tested the input and
> output circuits, and the best "match" the analyzer could see was a
> 6:1.
>
> What am I typically supposed to see when using this method? Does
> this 6:1 imply resonance? Does the input and output change that much
> when voltage is applied to the tube?
>
> thanks in advance,
>
> Paul, kg7hf
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