At 02:48 PM 12/18/2006, Rick Karlquist wrote:
><snip>
>
>There are several thousands of ohms of capactive reactance,
>depending on the frequency. The transformer does not tune
>this out or or in any way provide a conjugate match. The
>optimum transformer ratio occurs when the resistive load
>presented to the antenna equals the magnitude of the capacitive
>reactance. However, you can be off by a factor of 3 or 4
>either way with only a few dB of lost sensitivity. The loss
>of sensitivity is not a real loss in what you can receive,
>it just means you will need a few more dB of gain in your
>preamp. The preamp is a plain 50 ohm to 50 ohm or 75 ohm
>to 75 ohm amplifier. It can be preceded by an ordinary BCB
>rejection filter if necessary. You cannot precede the DXE
>amplifier with a filter. Another reason not to use it.
Indeed.. I was fooling with some active antennas recently, and found
that local EMI sources just hammered the amplifiers. One of the
advantages of a receive array is nulling out the noise, and if the
noise saturates the amplfier, no amount of clever phasing is going to help.
Unless you're in a quiet location (out on a field somewhere?) and
have "real good" preamps with lots and lots of headroom, I think you
definitely need some selectivity in front of the amps.
I haven't thought about it at great length, but I wonder if you could
come up with a simple passive network that could be put at the base
of a several meter long whip (or loop) with the transformer that
would provide enough selectivity to restrict you to just a ham
band. It might be that a single low Q LC might do it? (500 kHz BW at
3.5 MHz is a Q of 7), but it might be a couple sections... You need
to suppress things like BCB only a few bandwidths away (and broadband
power line noise)
(a tuned loop has plenty of selectivity, but then you have all the
tuning issues.. although, perhaps, a varactor?)
(or, for a high complexity approach.. something like the stepper
motor driven variable L in the new transceivers)
Although, a small stepper and a small variable C or L might not be
that hard to cobble up. You could certainly move it as fast as you
can "spin the dial" on your radio.
Jim, W6RMK
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