Hue and others,
Thanks much for the information.
I am fortunate because I have an old air variable capacitor, 40-250pf,
that is a vernier drive. (Who knows what radio I tore apart as a kid
to get it :-).
What I am thinking, in general terms is fewer loops on the coil, and
switch in just 1 fixed capacitor, so I can go from low MW to high MW.
I already have to switch the frequency on the radio so hitting a
switch or 2 on the loop really doesn't bother me.
Thanks also for the tips on the 20 gauge stranded wire. I can get 18
gauge stranded for cheap, and I'll try that before going to the litz wire.
I'll let you all know how the project goes.
--- In firstname.lastname@example.org, "Hue Miller" <kargo_cult@...> wrote:
> > --- Jeff Welty <weltyj@...> wrote:
> > > Supposing you wanted to get the most selectivity out
> > > of a loop for the
> I think you are more concerned with the loop selectivity than
> you need to be. Remember that unless this is the tuned circuit
> for your single-tuned circuit regen receeiver, you're going to
> be doing two-handed tuning. Do you really want to have the
> tuned circuit so sharp that you have to precisely adjust it
> every few kHz? That would really make tuning across the band
> a major chore. But if you do go for maximum selectivity, you might
> also consider putting a "Q-spoiler" pot across the coil, so you
> can broaden the selectivity when just tuning around.
> If you want maximum selectivity, without going to exotic
> measures, you might sonsider adding regeneration You could
> probably have both a Q-spoiler and regeneration. If you're
> clever about it, maybe in only one selectivity control.
> I know some people have achieved astronomical seeming Q
> figures with carefully built air loops. I would (will ) probably
> take the easy way, using a ferrite core, but maybe with the
> actual winding wound on a tube or maybe air spaced off the
> ferrite bar.
> A very small tuing cap with switched in, range capacitors, i
> think you would quickly get tired of using this antenna.
> I have seen a couple d-c receiver circuits for ham use that
> had very high C tuned circuit in front end, like 10,000 pF
> on 3.5 MHz, but this makes your tuning range very very
> small and also lower impedance, so lower voltage output.
> As for poly caps vs. air variable, i have never actually
> measured the "dissipation factor" but i have tried the poly
> style on a shortwave crystal radio and i remember being able
> to definitely note a downward effect on selectivity when
> compared to your standard air variables. There is not yet
> a shortage on air variables, not for receiving style ones
> anyway, so that shouldn't be a consideration, i mean finding
> one. Also, most of the poly caps limit what you can connect
> to their often strange diameter or shaft length; air variables
> almost always have a 1/4 inch diameter shaft so you can
> couple a vernier drive to it. Or if you don't want to go to that
> length, some variables scrapped out of equipment, have a
> built in reduction drive of close to 3:1 ratio, which makes your
> tuning easier. ( BTW, i recall a fellow electronics student
> years back, always referred to air variable capacitors as
> "veritable capacitors". ) Sometimes you can find likely donors
> for both capacitors and nice long loop bar antennas in readios
> from thrift and charity stores, or yard sales, but you maybe
> kinda have to be aware of whether the radio has some collectible
> value - like early transistor radios, particularly ones with the
> Conelrad marking at 640 and 1240. (Not that all these are
> collectible. ) I have seen a pretty good supply of variable caps
> at hamfests and antique radio swapmeets. Good luck with
> your project. -Hue Miller