[Lowfer] Band conditions

[Jay Rusgrove]

Johan

The transistor I use is an LT1001A originally made by TRW and then Motorola.
It might be obsolete now, though, as we did a lifetime buy about a year ago.
There are probably some still around.

It's similar to the '5109 and '3866 but with about about .5 to 1.0 dB better
noise figure in the circuit I use with overload characteristics on par with
the others - not really an issue at LF but the amp is flat up to 30 MHz. The
amplifier has >15 dB input/output return loss at 50 ohms so it makes a nice
termination in those applications where that's important. The 1 dB
compression is +22 dBm, the 3rd order intercept runs +35 dBm and second order
intercept runs +55 dBm. Not too bad for a single ended device.

With the 100' loop I'm currently using there is no noticable compression or
intermodulation - except when the VD beacon fires up! Even then, I can
receive within 50 kHz or so of VD and not know it's there. I suspect that
within that +/-50 kHz window much of what I hear is reciprocal mixing from
the synthesizer that generates VD's signal.

I tried the Burhans preamplifier on the 100' loop (without VD on) but
overload was apparent with Loran and broadcast all over the band. The Burhans
would probably be fine with a much smaller loop but I want to be able to use
the loop down to at least 10 kHz - thus the 100' size. Since the "heavy duty"
preamplifier does not overload, I can use a step attenuator after the
preamplifier in the shack to lower the background noise to a level a few dB
or so above the receiver noise floor to remain below the receiver agc
threshold.

By keeping the receiver background noise and Lowfer signals (specifically WA
and TAG which can be fairly strong here!) out of agc range I can use Spectrum
Lab's spectrum analyzer to compare signal-to-noise ratios of various receive
antennas. With step attenuators in each antenna I can equalize the background
noise from each antenna to a certain number of dB above the receiver noise
floor and compare received signal levels on the display. Wonder if you or
anyone else has tried this? I'd be interested in hearing the results.

Steve Ratzlaff did some independant testing on the "heavy duty" preamplifier
a while back. I'll forward that e mail so you can see what he measured. I'd
be interested in hearing about any antenna/preamplifier work you've been
doing!

Jay

Johan Bodin wrote:

> Jay,
>
> you wrote:
>
> > [...] a heavy duty bipolar (50 mA idling current!)  2.5 dB noise figure
> > 50 ohm broadband fedback preamplifier.
>
> What kind of transistor are you using?
>
> I have been experimenting with a number of broadband preamp's,
> both the "classical" amp (with partly unbypassed emitter resistor plus
> collector-to-base feedback) and amplifiers with "lossless feedback"
> (called Norton amplifiers I think). I have tried a lot of transistors in
> these circuits, including 2N5109, 2N3866 etc, but I am not satisfied
> with the noise figure... I haven't tried FETs though..
>
> A really good LF preamp would allow substantial reduction of loop
> size... (for RX loops, that is :-)
>
> 73
> Johan Bodin SM6LKM
>
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