Hey now you caught this "B Engineer's" attention:
>There was a book by W. O. Brown of Marconi, probably long out of print now.
>He had a whole chapter on parasitics, and one technique for high power
>( 50kW and up!) that often worked was something like a 200 ohm rod resistor,
>attached at one end to the plate and just sticking out in the cabinet, with
>the other end unconnected. It looked like a lossy open circuit quarter wave
>stub because of the size - around 10 inches long. It was a bit critical as
>to where it went. Point is that the techniques in tx's at this level are a
>bit different to our power levels. K5PRO is the guy to comment on that.
>At those power levels, it's all a bit scary!
>73
>Peter G3RZP
>------------------------------
>Has anyone tried lining the input or output cavities of PAs with the
>"lossy" material used in anechioc chambers to suppress RF reflections?
>The effect is to reduce the "Q" of the cavity, so that instead of
>reducing the series "Q", as applied in nichrome wire transmission leads,
>etc.(and which might reduce power slightly by introducing series loss in
>the tuned circuit), the overall cavity "Q" is reduced, thus greatly
>damping any oscillatory tendencies. If this works, the reduction in "Q"
>is free/lossless.
>
>This technique is highly effective is stabilising pre-amps with a
>tendency to oscillate.
>Ian ZS6BTE
>------------------------------
>From: Peter Chadwick <Peter_Chadwick@mitel.com>
>Subject: RE: [AMPS] parasites prove it?
>
>Ian says:
>
> >Has anyone tried lining the input or output cavities of PAs with
>the
>>"lossy" material used in anechioc chambers to suppress RF reflections?
>
>I suspect few people with HF amplifiers have PA compartments big enough to
>be a cavity in the VHF region where parasitics occur. I believe that Jon,
>KE9NA, felt that he may have had a problem of that sort.
>
>generally, you need a fair amount of the stuff at longer wavelengths.
-----------------------------
I would love to see the W.O. Brown book. There seems to have been a lot of
BROWN's writing RF power over the years. George Brown authored THEORY AND
APPLICATION OF RF HEATING, at RCA, in 1947. William C. Brown went silent
key recently; at Raytheon he was a major proponent of RF power transmission
via microwave beams from space.
As for Ian's comment about lining cavities, this has been done selectively
in big amplifiers. The perception that this reduction in Q is lossless is
wrong. It will dissipate fundamental frequency power if it is located where
those fields see the resistive losses in the material. To make it frequency
selective, it is put in strips, or in locations of parasitic frequency
field maxima, where the normal fields are minima. Other tricks such as
making the wall material slotted so that current flow across the slots are
stopped, but lenghtwise currents are not. Remember that a cavity is usually
analogous to an LC tank wrapped around the active device. Think of the
walls as L and the end plates or stubs as capacitances and you'll get the
idea. You have to water cool or air cool the absorber material if it is
exposed to the fundamental frequency RF power too. But it can break up a
high Q microwave parasite, if it is caused from some waveguide mode
excitation in a big cavity. The bigger HF, FM, TV and particle accelerator
amplifier boxes are prone to such problems, due to the size, and that the
tubes may have gain into the VHF/UHF region. The rod resistor technique is
still used in some commercial transmitter boxes, Continental Electronics
has always been a proponent of this trick in their big rigs. Seems to work.
In 1983, while troubleshooting a persistant 190 MHz parasite in a 4CX3500A
cavity amplifier tuned to 100 MHz, I improved a patented device that my
company used, to trap out the second harmonic in the PA. Its listed as US
Patent # 4,334,203. Check the IBM patent searcher at
http://www.patents.ibm.com/details?pn=US04334203__&language=en
The improvement entailed using Nichrome wire instead of the copper strap,
to lower the Q of the trap, and to make it broadbanded enough to suppress
the parasitic voltage on the line, and stabilize the cavity. 190 MHz
parasitic was not far from the second harmonic, and could be actually
triggered by the F2. The use of nichrome wire in the suppressor didn't seem
patentable, but it sure worked. Before that, I had tried tuned traps with
globar elements connected to dissipate the parasitic power.
With the new suppressor, the parasitic never started up, so it was a
"chicken and egg" problem, the suppressor was actually parasitic birth
control.
John
K5PRO
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