At 05:09 PM 8/19/01 -0500, Ford Peterson wrote:
>The feedpoint impedance is modeled to be around 960 ohms or so. A simple
>transformer can do the trick to get it down to 50 ohms. However, when
>pondering about the pre-amp, I'm thinking of looking at doing the
>transformation in an active element instead of a transformer. Finding a
>high input impedance device for topband should not be a big deal. The
>amplifier can then be matched to 50 ohm output.
Ford,
Some points to consider:
In order to preserve the common mode rejection of the system, you would
need to feed each antenna with 960 ohm balanced line.
You would also need to make your switch box 950 ohm balanced. This is
usually harder to do than a straight forward 50 coax system. It is also
somewhat more difficult to maintain inter-antenna isolation with the higher
impedance.
The amplifier would require not only the usual good low noise
characteristics, but would also need to have very high common mode
rejection and real balanced inputs. All this and at least +20 dBm output at
the 1 dB compression point. Do you have something like that laying around?
Four pennants arranged radially would have at least one of them aimed
toward your TX antenna. Seems to me you would be taking a pretty good hit
in performance in at least one direction just to save three pieces of wood.
What has worked the best for me has been to position each RX antenna such
that the TX antenna is exactly behind it. In other words, arrange multiple
RX antennas radially around the TX antenna. Since a pennant can have about
300 or more feet of RG6 coax between it and the "preamp" without degrading
performance, I run separate feed lines back to the shack and do the
switching and preamp in a controlled environment. Flags, with their higher
output levels, can run much longer coax than that.
Separate feed lines also gives you the luxury of being able to fine tune
the positioning of each individual RX antenna so that you can null out
local interference or interaction from other antennas nearby.
Good Luck,
Larry - W7IUV
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