This particular loop is now hoisted into its final position between 2
high beech trees, a convenient place. Top horizontal totally in
the clear, bottom horiz 6 ft above ground, each vertical leg about 10
feet or more clear of tree trunks and branches.
Loop is symetrical and rectangular 124 ft horiz x 40 ft vert, area =
4960 sq ft.
Inductance measured = 173 uH
Resonant F measured = 3065 khz
DC resistance of loop wire plus coax feed 1.5 ohms.
Loop wire 2.5 mm insulated.
Feed 75 ohm coax, about 150 ft long, loop feed point, bottom right hand
corner.
Loop resonated on 136 khz, parallel configuration 1502 pf needed.
Gain measured at receiver without antenna and antenna connected = 30 db
approx.
Gain difference measured on 136 khz between nonresonated loop and
resonated indicates 10 db advantage by resonating.
All gain figures relative using LEVEL METER W & G SPM-12. and spectrum
analyzer.
The spm-12 has an IF output at 10 khz. I built an ext product det/audio
stage. Selectivity on this model 1.74 khz, 500 hz and 25 hz, all filters
installed. This device makes a fantastic LF receiver. Considering the
big signals around on LF there is no evidence of breakthrough. Using
resonant hi-Q antennas overcomes a possible problem but as a precaution
a front end filter could be useful.
This loop now resonant on 136 khz is performing better than I expected
on receive, but what it will do on TX could be a different matter. I
also have to try and series resonate it and see what happens.
Next step is to calculate/measure the loop impedance and make a suitable
transformer
to match my rx 75 ohm input, but so far even with a mismatch I am
suitably impressed.
It is now as good as the vertical on receive but bi-directional
unfortunately, I suppose I need two of these at right angles to suit all
occassions.
At present ON6ND, ON4ZK are booming in and G6RO is also about, all
calling CQ and it appears they are not hearing each other, maybe they
need a LOOP !!
73 de Mal/G3KEV
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