On Fri, 28 Oct 2005 00:15:03 -0400, TRABEM wrote:

OK, now we're making progress. I knew there had to be an explanation
for your insistence that the antenna presented a 2K impedance to the
feedline! I absolutely knew it could not be correct and you were
equally determined::

You are not off the hook yet.

Let me describe exactly what I hope to build, and you can enlighten me
regarding what the proper term is. Thanks for hanging in there, the
road was a little bumpy....

I'm thinking feedline attached to one end of the wire. The other end
of the wire is attached to the capacitor bank. The other side of the
capacitor bank is attached to the other feedline terminal. Or, stated
another way, the cap is in series with the wire and the 2 transmission
line terminals are connected to the left over cap and the unused wire
end.

Fine, but it doesn't really matter. The unloaded Q is one thing, the
loaded Q that is seriously depressed is another.

OK, but I hadn't thought this entered into the calculations of the
loop.....it is what it is and we all know it's too damn short and too
damn close to the ground to be efficient.

You have so little efficiency, that the proximity of ground hardly
matters (you already know this) - until we look at the lobe pattern,
balance, and that side of the coin.

or do I really need to look at this issue to proceed?

You do your best - you are not that far from it. The loop allows you
to end load it better than using a dipole with coils.

I've thrown together a quick model of your 5M on a side loop using
(and being generous) #1 wire. The bottom of the loop is 10M above
ground. In terms of performance relative to an isotropic antenna it
is down 60dB. It displays an impedance of:
Impedance = 0.05849 + J 10.37 ohms
An addition of a 0.2555µF capacitor draws this down to:
Impedance = 0.05851 + J 0.006073 ohms

Please note that the capacitor is perfect, no ESR whatever. In a
system with Zc = 0.0585, this antenna presents a 1.11 SWR. The half
power points of its resonance are only 600 Hz apart. Hence a Q of
100.


OK, are you telling me my receiver would need to have an (impossibly
low) input impedance of .06 ohms to work well with the antenna I've
planned?

Just compare that to the 2 Ohms you hoped for and see what happened.

OK, I'm not completely understanding the last paragraph. Have we
abandoned the .06 + j10 real loop impedance and/or are we talking only
about feeding a 2 ohm impedance loop into a 2 ohm impedance receiver

You have injected 2 Ohms into the series circuit - it is now part of
the antenna even if it inside the receiver.

(which seems to be a match made in heaven at first glance)?

Simply a slight of hand, mathematically. You have to pick a reference
you call Zc (characteristic impedance) of something, and your
resistance is wedded to the antenna. However, the reality of it is
that the native loop compared to the actual load is severely
mismatched. You don't lose 15dB gain on the stairway to heaven. Most
would parallel a loop's capacitor with a really big resistive input Z
match to preserve Q. You chose a path that is leading to grief.

Send me a sign, I sense an incoming lightening bolt::

ZAP

A series load in a circuit that is heavily laden with current requires
a very, very small value. Your switches are deadly elements even if
they are the best in the market.

That same circuit, sans series detection, also supports high voltages
courtesy of Q. At this frequency you have a world of components that
can tap them without seriously loading the circuit and killing the Q,
and the voltage, and the efficiency.

Hence a follower ACROSS the capacitors and AT the loop driving the
line back to the detector. A follower that presents 20K or even 2000K
Ohm load to the loop at 60KHz is a walk in the park. You aren't going
to recover that initial 60dB plunge from the size of the antenna, but
you don't have to grease the slide for another 15dB plummet.

Still, and all, there's always the practical reality that the design
will work out of the box. Wrist watches do it. It won't be the best,
but there's not much competition and this isn't DX we are talking
about, unless you want to work Europe's VLF (and even then you may
hear signals).

You still haven't bitten the bait on what makes the "I" and "Q"
channels useful beyond cramming them into black boxes to strip away
the only really useful angle (pun intended) they offer - phase
information. This is in fact the single most fascinating aspect of
the detectors.

73's
Richard Clark, KB7QHC