[AMPS] 2nd harmonic pumped parasitic

[John T. M. Lyles]

Designing a cylindrical power amplifier cavity at 100 MHz, using the 
4CX3500A tetrode series (also tried the 3000A and the 5000A tubes), I 
'picked' a geometry which supported a mode near the 2nd harmonic. 
Computer models didn't show this, as they were using pure TEM mode 
calculations, such as ladder analysis using ABCD matrix transmission 
line approximations. There was apparently enough feedback through the 
tube and gain at 210 MHz that it could come back in phase to make an 
oscillator. This essentially made a parasite which would be excited 
when driven by the 2nd harmonic. The amplifer was class C. The 
parasite wasn't always there, and when it came on strong, the 
harmonic filter would blow one of the capacitors (it was a Bird 5 kW 
shoebox style filter) inside. The parasite was not an integral freq 
of the fundamental, thats how we knew it was parasitic. We could see 
F2, and then the parasite nearby, and then there was smoke. Of couse, 
the presence of the parasite made a lot of mixed products in the PA, 
which complicated measurements.

When the transmitter was terminated in a pure 50 Ohm water cooled 
dummy load, there was no parasitic. When terminated in a harmonic 
filter, either the Bird L-C LPF network, or a stub line tuned for 2nd 
harmonic, it would become instable. The reactive termination in the 
200 MHz band was essential to make it unstable.

Internally inside the amplifier cavity, we had a patented 2nd 
harmonic suppressor.
US patent # 4334203
It was a high Q device, coupled into the electric field along the 1/2 
wave amplifier circuit. You can read the patent yourself, with a 
patent search engine such as IBM's:
http://www.patents.ibm.com/details?pn=US04334203__

I am talking about lumped and distributed ciruits such as coaxial 
cavities now, not LC discrete component circuits. But parts of the 
parasite suppression scheme are somewhat similar to what I have been 
hearing about.
It was coupled into the field of the main TEM mode, not directly 
inline with the plate connection of the tube.

Anyhow, my parasitic would eat this suppressor circuit as well. The 
solution, found after about 6 weeks of empirical testing and 
modifications, was to change the copper strap on this suppressor to a 
nichrome coil of wire. This device had lower Q, and squelched the 
parasitic, as well as continuing to trap out the F2 component in the 
tank circuit. At this time, I was convinced that Nichrome or other 
resistive alloys had merit in high power RF circuits. Also, I played 
with Carborundum resistors, and ferrite tiles and tubes. Nothing 
worked as well in my application as the wire. The nice thing with 
lower Q was that it was no where as sensitive in position as it was 
with a copper strap and a paddle.

By the way, the entire amplifier for this series was patented, as we 
developed a way to eliminate finger contacts carrying high current, 
which had to move while tuning and loading.
US Patent # 4,363,000

I am not vouching that resistive wire is needed in 3-500Z circuits, 
or that it will solve all problems with parasites. Just that it IS 
another way to skin the cat, so to speak. Also, yes, second harmonic 
can pump a nearby parasite. I have seen it, recorded it, and worked 
around it with a suppressor.

John
K5PRO

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