At 01:29 PM 5/16/2005, Rick Karlquist wrote:
>AA6DX - Mark said:
> > OK .. that's interesting .. If you would Rick, please `splain more on the
> > separate amps causing probs w/mutual coupling ... I have ran a contest
>
>In arrays, you don't want equal power to each antenna.
>You want equal current, or 1:2:1 current, or whatever.
>Typically the rear element in an array has an impedance
>near zero and often negative. You merely have to allow
>current to flow, but not deliver much power. Thus you
>waste that amplifier (a 500W amplifier putting out 25W).
>If the antenna impedance is negative, then the "amplifier"
>has to be an active load.
>
>Instead of one simple phasing network, you need a 500 W tuner
>for each amplifier, plus a low level phase network at
>the inputs of the amplifiers.
Looking towards the future (and the SGC is only a step in that direction),
one envisions that the input side is digital. I've been seeing proposals
for "polar modulation" in wireless applications (essentially like the Kahn
EER scheme used by SGC), and it's a natural fit to doing the phasing in a
DSP. You'd send data to the amplifier with the instantaneous phase and
amplitude needed. The amplitude goes to the power supply, the phase goes
to the DDS (or whatever). The DSP also does the needed predistortion, etc.
>The SGC amplifiers are only good for 1 band (I think a lot
>of people missed that). That makes sense for a multi TX
>effort like a DX pedition. Maybe it makes sense for a multi
>multi contest station (which needs multiple amplifiers anyway).
>
>No mention of tuners. All class E amplifiers I am familiar
>with do not suffer mismatches well. So figure that into the
>cost as well, unless your antenna is really well matched.
There are (ahem) a few practical implementation details. One of the
stickier is that high efficiencies sometimes imply high Q in the output
circuit (depends on the actual class of amplifier), which may make it
trickier to "instantaneously" alter the phase of the signal being
amplified. Proabably not too tough, in the ham case.. I don't have the
time to figure out what the bandwidth of that phase varying carrier
frequency would be for SSB 3kHz voice, but it's probably not more than 20
kHz. 20 kHz out of 14 MHz is around 1 part in 1000, so as long as the
output Q is less than, say, 100, you should be ok. Since they claim it's
14.0-14.35 MHz, the Q can't be higher than 40.
As Rick points out, though, these high efficiency amps tend to be VERY
picky about their load impedance. However, I can foresee a fairly
sophisticted design that has a tunable network that serves both to store
the energy for the amplifier and to match the active element
impedance. This is decidely in the future.
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|