Over the past decade I designed a system with three high power
transmitters that each use a pair of tetrodes in combined operation.
These are TH628L, which run at 23 kV DC each, and produce about 1.5MW
pulsed per tube, average power is approx 180 kW per tube, at ~200 MHz.
Combiners and Splitters had limited possibilities and this is a fixed
frequency operation for driving high Q cavities for a particle
accelerator. As such, reflected power can also be horrendous at times. I
chose quadrature combiner, with a 4 port hybrid custom made by a company
in 12 inch diameter hard coax. The combiner is called a branchline
coupler. The splitter is also a hybrid, fed with 3 1/8 inch hard coax.
It uses a pair of coupled lines 90 deg long inside a box. Both of these
are 90 deg devices. If the amplifiers are running at about the same
power, with the same voltages, the phase shift through them is similar
enough. We built a custom vector phase meter that measures the real time
phase difference between the PAs. When I ramp the power up from zero,
there is approx a 10 deg phase variation in each amplifier but its
repeatable and the same in each one. Once the combined system is near
the optimal 2.1-2.8 MW total power, the phase variation vs power level
is a few degrees at any time.
Since we have VSWR all times, the length from the load (source of the
reflected power) to each tube is off by 90 deg if the lines were all cut
the same length. This would create a messy operation, as one tube would
respond differently than the other due to the phase of the reflected
power being different. To fix this, there is extra piece of line on the
output of one amplifier, so that they are both in phase with respect to
the load, but there is a similar 90 deg line on the opposite input
splitter port. This maintains the 90 deg relationship of two feeds to
the combiner, while staggering the amplifiers apart by 90 deg so that
they both respond to VSWR the same. It works well like this.
We use the screen voltage (from separate power supplies) as a knob to
adjust the gain of each PA to be the same. Could also use the ouput
loading control, but that introduces a phase change. There is a 3-1/8
inch coaxial phase trimmer, a line stretcher, after the first splitter
that follows the driver stage, in the feed to one PA input side. On the
other side there is a nearly identical line without adjustable phase. We
have run these transmitters for 6-8 years and not seen problems due to
phase variation. In fact, the curves of phase vs power falloff, of a 90
deg combiner is quite tolerant. I monitor the 4th port of the hybrid
combiner, which feeds a water cooled resistor. The power on this port
occasionally gets to 5 kW peak which is nothing when the total power is
2.5 MW.
It is important to note that our system is a fixed freq setup, where you
will likely be tuning around. I can imagine that things get more complex
when varying frequency and also maintaining phase. Using in phase
combining and Wilkinson devices will help there. If you are driving two
antennas for circular polarization, small phase difference should just
alter it to be more elliptical polarization.
You can find some images from a 2017 paper on this system here. Let me
know if you want any further info on this.
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwjE6uvZ8d76AhU9jokEHYEuCLAQFnoECBQQAQ&url=https%3A%2F%2Fwww.semanticscholar.org%2Fpaper%2FDesign%252C-Fabrication%252C-Installation-and-Operation-of-Lyles-Barkley%2Faa25180c0cc7b15ba88315673f220e915fe6cde7&usg=AOvVaw083Mcx-V0X_YruR9yfAYb2
73
John
K5PRO
Date: Thu, 13 Oct 2022 12:37:10 +0000
From: Conrad PA5Y <g0ruz@g0ruz.com>
To: "amps@contesting.com" <amps@contesting.com>
Subject: [Amps] Phase and amplitude of 2 tube amplifiers
I am going to attempt to use 2 identical monoband tube amplifiers to generate
CP but before I start I have a question. The 90 deg delay for CP comes from the
array orthogonal physical boom separation. So the amplifiers must be in phase
at the outputs.
I presume that this has been done in broadcast applications and I would like to
know, assuming everything is tuned correctly how close the phase likely to be?
I have line stretchers for the inputs, I have extremely good phase matching on
the Wilkinson splitter and 2 identical directional couplers that I have tuned
for good phase and amplitude matching. I can also measure the relative
amplitude and phase of the 2 amplifiers accurately. Small phase and amplitude
adjustments are ok but I have no idea what to expect with tube amplifiers.
This was not really planned but a 2nd amplifier became available, so I thought
why not? The tubes are becoming scarce so this way I can be a lot more gentle
on them.
Regards, Conrad PA5Y
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