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Re: [Amps] 432 MHz stripline

Subject: Re: [Amps] 432 MHz stripline
Date: Mon, 23 Jan 2012 22:08:10 -0500 (EST)
List-post: <">>
Hi All, not to be discouraging, but, using a GDO at UHF is going to be an  
exercise in frustration.
When the cavity cover is removed, the frequency of the resonator will  
change drastically and have no relationship with the actual cavity  frequency.
There are other methods of finding the frequency with covers in place but  
they all need to be tunable over a wide enough frequency range to find the  
resonant frequency.
The driver and a vswr meter can do that with the amplifier cold by  
connecting the drive power through a vswr meter to the amplifier output while  
cold. Then the frequency of the driver is changed looking for a small dip in  
vswr. There are enough losses in a uhf amplifier for a small indication of 
lower  vswr as the losses absorb a small amount of power at the resonant  
This is an extremely crude way to find the resonant frequency. Many times  
just knowing if resonance is too high or too low is enough to begin  
progressively moving the resonance toward the frequency of interest.
Another method is using a signal generator and return loss bridge. This  
works in the same way as using exciter power, just at a lower and safer power  
Adding the resistor to simulate the plate load impedance will make the dip  
in vswr or return loss much deeper and noticeable. But, the resistor leads 
will  add inductance and lower the cavity frequency. This can be mitigated a 
lot by  use of several parallel resistors with the minimum possible lead 
Then the third method is to simply tune the exciter frequency while  
watching for an improvement in power output. Many transceivers will cover the  
to 450 MHz range and that could be enough to "find" resonance.
Gerald K5GW
In a message dated 1/23/2012 8:46:07 P.M. Central Standard Time, writes:

On 24  January 2012 02:30, Paul Decker <> wrote:

>  Hi gang,
> A few weeks ago I asked the group about modification to my  8874 stripline
> 432 MHz amplifier to replace it with a  3cx800.
> Well I made the modifications and extended just the  very back portion of
> the stirpline.  I couldn't get the amplifier  to tune up.  Grid and plate
> current were well within spec, but i  would not output much more than 50
> watts.
> It was  mentioned to use a gdo or antenna analyzer to tune the  stripline.
>  I'm wondering what the procedure for that might  be?  How would I couple
> the gdo or antenna analyzer to the  stripline?  With or without the tube
> installed?  Power  on/off?
> Thanks,
> Paul
> kg7hf

1)  Install the tube.
2) Have the power off
3) Put the GDO near the  stripline and find the dip.
4) Gradually move the GDO further from the  stripline. The dip will get
weaker as the GDO is couples less strongly to  the anode line. But the
frequency of the dip will be more accurate, as the  anode line wont effect
the measurement as much. You want to have the  coupling as weak as possible,
although it must not be so weak that you  can't find the position of the

There might be some  advantage in putting a resistor between the chassis and
the anode of the  tube, equal to the load impedance of the tube I must admit
I've never done  that, and managed to get the above procedure to work,
though I think there  might possibly be some advantage in this, as it more
closely represents the  running conditions of the amp.

Dave,  G8WRB
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