[Lowfer] RE: Worthwhile going to Shielded loop?

[John Andrews]

Alberto,

Here's a quick calculation. Everybody is free to jump on me for my
simplifications:

Start with a loop that can be resonated with 975 pF at 136 kHz. Let's say
that the unloaded Q of the loop is 300. This gives an impedance at 136 kHz
of 4 +j1200 ohms.

If you connect that to a perfect 6:1 transformer, the impedance looking back
into the "1" side will be 1/36 of the original, or 0.11 +j33.3 ohms.

Your transmission line will be very short in wavelength, so its principal
effect will be shunt capacitance. 15 meters of RG-58 would represent about
1.5 nF. While the line will not be lossless with that 0.11 ohm source
impedance, let's neglect that for the moment.

The capacitance needed for parallel resonance would be 35.1 nF. Subtracting
the 1.5 nF from the line, you wind up with 33.6 nF. The resulting resistive
impedance would be about 10k ohms, which would be quite handy to feed to a
preamp.

The major problem as I see it is the need for a variable .0336 uF capacitor.
However, if you install another 1:6 transformer to bring things back up to
high impedance, your task is much simpler, and a JFET preamp would again be
a good choice. The cable capacitance wouldn't have much effect, as I quickly
calculate the impedance as 4.4 +j1250 ohms. Now with a parallel tuning
capacitor (across the ":6" side), you get about 360k ohms, again great for a
FET. That tuning cap would be a little less than the original 975 pF (I
calculate 936).

There WILL be some loss in the transmission line, though, due to the low
resistance at the source end. Note that this arrangement of two transformers
would be ideal for a balanced, twisted pair transmission line. Shielding of
the line might be optional. There are some interesting cables made for audio
use that have large guage twisted pair wire and a foil shield.

Interesting, eh?

John Andrews