To: <amps@contesting.com>
> Date: Tue, 12 May 1998 19:34:17 -0500
Hi Jon,
> >As has been previously pointed out, there is no guarantee that the series
> >cap will work on every antenna combination. With a Bird dummy load there
> >is no problem. With the average feedline to say a tribander being 100' or
> >so the flat loss in the coax will flatten out the VSWR antenna response
> >at VHF/UHF.
>
> If the output Pi-network of the SB-220 has a low-pass response as most
> output networks do, then what difference at VHF/UHF does the antenna make
> anyhow?
Absolutely no difference at all for VHF Jon. The tuning cap
looks like a very low impedance, the tank inductor and leads a
high series impedance, and the loading cap a virtual dead short.
Been there, measured that.
The load, however, can upset the cart at HF near and below the
operating frequency and lower all the way to VLF.
It's a VERY simple job to break out a VHF signal source, drive the
output port backwards, and measure RF voltages through the tank.
It's only a tad more complex to add a few thousand ohm resistance,
and drive the anode terminal of the cold tube while measuring
voltages in the tank. It would take less time to do this than has
been spent on soapboxs postulating all sorts of nonsense about
extreme VHF voltages causing arcs, and VHF oscillations being load
impedance sensitive.
I always make this type of measurement (using a sweep generator and
RF detector in a network analyzer) in the normal course of
prototyping new PA's. The voltage (transmission loss) smoothly and
rapidly decreases as the probe is moved through the anode system
towards the tuning cap. By the time the loading cap is reached,
nothing measurable is left.
The anode would have to swing tens of kilovolts at VHF and drive the
system with many amperes of current at the same time to make the ten
meter tap on the bandswitch arc at VHF.
It's bad enough science to claim a bandswitch or tuning capacitor can
arc from a VHF parasitic far removed from the operating frequency, and
worse yet to claim the VHF impedance at the output port has anything
to do with VHF instability.
For goodness sakes, common sense tells us that if the VHF energy
could freely pass through the tank, the amplifier would be a harmonic
disaster. It would never pass any sort of harmonic level testing.
>Do we really need some sort of a load? After, everyone talks
> about how the output Pi-Net works as a filter to remove the harmonics, so
> if it removes (or reflects - The S11 of the filter at the frequency of
> the harmonic energy is high) harmonics, won't it also reflect VHF/UHF
> energy?
It certainly does.
> Why worry about how the HF load looks at VHF/UHF?
Good question, but one that shouldn't even need asked..
73, Tom W8JI
w8ji.tom@MCIONE.com
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