At 09:28 AM 12/18/2006, Stone, Gary R. wrote:
>ALSO - how about the unit being sold by DX Engineering - don't know the
>model number but it uses (4) 102 inch whips. It seems very pricey for
>the system but I am curious. Any homebrew plans around for using (4)
>102 inch whips? (they are easy to find and cheap).
The key is the low noise amplifier at the bottom of those whips.
There have been some good design articles around on FET active
amplifiers over the years. I think there was one in QST built inside
a piece of copper pipe (as a heat sink).
It needs to have good IMD performance because you've got a "wide
open" front end so you don't want the amplifier saturating on the
inevitable strong signals.
Another approach might be to couple the whip (or loop) to a fairly
narrow band filter, and then to the amplifier.
For instance, using a compact loop with a tuning capacitor might be a
good way to go.. they're hideously narrow band, which is normally a
problem, but not in this case, because that narrow bandedness serves
to provide the front end selectivity.. In fact, for a receive
application, where you can tolerate a fair amount of loss, you could
probably use the fact that you can "mistune" the antenna by a bit to
get the phase shifts needed for the beamforming.
And, doing the phasing at low powers is MUCH easier than at Tx power
levels. You can use small lumped networks with convenient sized
components, for instance.
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