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
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.
In a message dated 1/23/2012 8:46:07 P.M. Central Standard Time,
On 24 January 2012 02:30, Paul Decker <email@example.com> 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
> 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?
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.
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