At 12:03 am ((PST)) Thu Feb 8, 2007, Fraser wrote:
Please see my post on the internals of the DE31MS loop head.
Thanks for your excellent efforts, Fraser. I shall
download the picture in a moment... got it :-)
Do you know if the TG33 MW-only loop is
single-turn plus a series inductor too?
Sorry if I appeared to ignore or contradict your
earlier post: I receive the Digest, so I'm just
playing catch-up now.
I had overlooked the possibility of inductance in
series with the loop. Which is most remiss of
me, since I was recently looking at the reference
design for a receiver chip which used the same
technique to switch the ferrite rod aerial from
MW to LW. So no excuses ;-).
As you say, not the most efficient, however...
...I happen to have been looking at several
band-pass filter designs and one suggested
a Q of 30 for commercial bobbin-cored inductors
vs 100 for hand-wound ferrite toroids - and both
gave acceptable insertion loss in an aerial
preselector.
[On the other hand, I have seen bobbin inductors
condemned as having much lower Q, down into
single figures, so I guess "it all depends" - on the
method of measurement, too :-(.]
I suspect much better than 100 is possible with
a carefully sized and wound toroid. Alternatively,
Q in the region of 1000 is possible in an air-cored
solenoid - making a good padding coil for a fixed
location but rather poor luggage.
For a tuned loop a Q of 30 is a bit low but 100 is
quite reasonable. Remember that the higher the Q,
the more often a SWL will have to re-tune the aerial
while searching HF, at high Q it becomes tricky to
peak the loop on the receiver frequency and audio
sidebands can be clipped, and at extreme Q CW
signals will ring - and the tuning drift will probably
make it unusable.
Some designs include a switched or variable
Q-spoiler resistor across the tuned circuit to make
the loop more friendly to operate.
Better to have spare performance in hand (and
then be able to discard some) than be found
wanting, but no need to chase the ultimate and
find it unusable.
BTW, for those tempted to rebuild their loop,
be mindful that the cute little low-capacitance
mosfet has no internal gate static protection,
so a permanent RL shunt across the loop
terminals in case the external loop should
ever be disconnected could be a good idea.
Regards, LenW
