Hi Gerald,
You've hit the nail on the head on why I asked the question. Every time I took
the stripline cover off and put it back on I got different results. In fact, I
have a suspician the original designer used the cooling chimney as part of the
tuning circuit becuase it is half metal and sticks down from the top by about
1/2 inch. I suppose he moved that up and down to get a good match, then brazed
it in place.
I do have a signal generator and return loss meter that is accurate up to 1 Ghz
so this is what I was attempting to use.. Thanks for the tips, I'll give it
another stab (now that it's easy to switch between the two tubes.)
thanks,
Paul
----- Original Message -----
From: TexasRF@aol.com
To: david kirkby <david.kirkby@onetel.net>, kg7hf@comcast.net
Cc: amps@contesting.com
Sent: Tue, 24 Jan 2012 03:08:10 -0000 (UTC)
Subject: Re: [Amps] 432 MHz stripline
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
frequency.
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
level.
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 lengths.
Then the third method is to simply tune the exciter frequency while
watching for an improvement in power output. Many transceivers will cover the
430 to 450 MHz range and that could be enough to "find" resonance.
73,
Gerald K5GW
In a message dated 1/23/2012 8:46:07 P.M. Central Standard Time,
david.kirkby@onetel.net writes:
On 24
January 2012 02:30, Paul Decker <kg7hf@comcast.net> 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
dip.
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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