[TowerTalk] AS-AYL-4 4 direction Low Band Receiving Antenna

[Jim Lux]

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