Dear John, LF,
I would like especially to comment on using a one winding loop combined
with a step-up transformer:
I am using such a loop for years with continuing success. The primary
of
the transformer should have the same inductance as the loop itself. The
secondary should provide an inductive reactance of four times the
cable impedance, either at the frequency or band of interest, or on the
geometric mean frequency of a broader band, in my case 10 kHz to 500
kHz
(using a mean frequency of 71 kHz). As in any pure receiving
applications ferrites with high Al values can be used for the
transformer (6800 in my case). Concerning sensitivity, in all cases so
far the rf band noise has been the limiting factor, provided the loop
area is not too small.
I guess many loop builders overrate the formula saying that the voltage
of a loop will increase with the number of turns (which is ok when
tuning the loop). But what we really want from a receiving loop is
receiving
power (voltage across an rx input resistance),
and it is the loop area which counts in this respect, not the number of
turns. Just if the turns ratio of the transformer may become too high
for practical reasons, a two or three turn loop may be easier to be
matched to a
cable.
An untuned loop is a broadband rf source, however. Therefore I am
using a preselector (at VLF an 80 kHz low-pass filter) before the
receiver or converter, with some amplification behind. Tuning the
preselector in the
shack is much easier than a loop outside the house.
>
> And just one random thought on multi-turn receiving loops: As Jim Moritz has
> pointed out a number of times, a loop of N turns can be quite nicely replaced
> by a > loop of one turn and an N step-up transformer. A one-turn loop is
> easier to
> construct, and may provide a wider tuning range.
>
> John, W1TAG
>
73 Ha-Jo, DJ1ZB
|
