On 12 Sep 2005 14:01:59 -0700, "K7ITM" wrote:

Actually, the thing you call a detector IS a mixer. You can probably
find some references for "H-mode mixer." It's good, to be sure, but
it's inaccurate to say that it's _perfectly_ linear. (I'd LOVE to find
a practical sampler which had zero distortion...though then I'd need
amplifiers with zero distortion, too...)

As someone else pointed out, any practical antenna you have for LF is
very unlikely to be a good match to 50 ohms, and is very likely to be
quite reactive so that by the time you add components to tune it, the
bandwidth will be pretty narrow. So if you have a tuned LF antenna,
which is quite usual, the response will be quite narrow, and why would
you care about a bandpass filter? I'd recommend a loop with a tuning
arrangement at its feedpoint (variable capacitance), and an appropriate
preamp to drive a feedline back to the receiver. With that, you won't
need any filter, just a transformer going into the mixer
(converter-detector-whatever). A while back, I did some work to modify
a design you can find at
http://www.cpinternet.com/~lyle/bal-pre/bal-pre.htm, so that the
control was done as a DC current , which also fed the power to the
preamp on the same line that signals come back on. It worked out well.
But check out other antenna options from Lyle's website
(http://www.cpinternet.com/~lyle/) or others devoted to LF, too.


Thanks Tom,

I've followed lowfer technical discussions for some time now and
thanks to some recent input from a few of them, I have a much better
idea of what I need to do to get a decent antenna up.

My plan is to have a single turn or 2 turn centertapped loop, each
side being 10 to 12 feet long and the turns spaced 5 inches apart. The
conductor will be 200 A aluminum service entrance cable which I have
laying around.

The impedance of this antenna will be low and the Q should be quite
high, with lots of area, so it should drive the receiver well.

It has occurred to me that the antenna itself has a great deal of
selectivity, yet some loop users still report front end overload from
AM broadcast band and other megawatt LF rf sources.

The QSD is susceptible to harmonics also, so a very high attenuation
low pass filter is the minimum filter necessary to keep these signals
out.

Whether the tuned antenna by itself is adequate, I don't know.

But, I'm considering putting in a bandpass or low pass filter designed
to match the lower impedance loop antenna directly, so the filter
would have input and output impedances of 5 or 10 ohms.

Anyway, that's a topic for another day I suspect.

Regards and thanks for the input.

T

On Mon, 12 Sep 2005 21:31:52 -0400, TRABEM wrote:


It has occurred to me that the antenna itself has a great deal of
selectivity, yet some loop users still report front end overload from
AM broadcast band and other megawatt LF rf sources.

Look for "antenna effect". Most likely the broadcast signals are
connected directly to the pickup wiring, which works then as an
ordinary "electric" antenna.

One way would be to make the pickup loop of coaxial cable. Bend the
end of the cable back to a convenient place on the cable to form a
loop, solder the end (both centre connector and shield) to the shield
at that point. At the opposite end of the loop cut the shield and now
you have a nice electrostatic shield around the centre conductor.

Paul OH3LWR