Dave wrote:
"starman" wrote in message
Hmmm. A tank circuit between the antenna input and ground, to improve
reception? I believe you, but I don't understand how it works. Could
you
enlighten me a little? Does it resonate at the selected frequency
(chosen
by adjusting the variable cap) and thereby "select" that signal for the
input to the radio? (This is all I can figure.)
I could do that real easy. I have several variable caps of that general
size, and can pick up whatever size inductor I need. I even have the
charts
and formulas for calculating the appropriate value of inductance, just
have
to sit down with them. Thanks for the suggestion.
PS: Haven't checked out the links yet, but will do so asap.
Dave
The parallel tuned (tank) circuit causes all frequencies above and below
the tuned one, to be shunted to ground while passing the desired
frequency range to the radio's antenna input. It makes up for the lack
of good preselection in the receivers front-end. This is the main
problem when connecting a good antenna to a portable radio. Be sure to
use coils with ferrite cores for good selectivity.
Gotcha. That was the only way I could figure it. I saw something almost
similar (is that like almost pregnant?) in my Practical Antenna Hanbook (Joe
Carr) last night, but suspect it is a printing error. It was an series
inductor following behind a capacitor shunt to ground. Only that would (I
think) choke off RF and shunt it to ground. (Am I wrong?) Have been trying
to figure that one out all day. Last night I fixed my big solder gun, and
can now solder a ground wire to the grounding rod outside my bedroom window.
Hope to do that tomorrow.
I am definetly going to set this up. Have already tested several of my
small capacitors, but they do not appear stable enough for anything serious.
Just to familiarize myself with the mathematics, I have already calculated
the inductors I would need for the first one I tested. Just finished
removing a better variable cap from an old junk shortwave radio (a Luke, I
think) and am going to try to use it. May have a problem with my meter
though. May have to take the variable cap to the shop and have it tested on
a known good LCR meter (gotta get one of those. I a currently using a DMM
with limited capacitance capability.) If I do that I'll take my calculator
and notepad with me, so I can figure out and write down the values of
inductance I need. Damn I'm having fun. 
Thank you very much for this suggestion. It is much appreciated.
Dave
If you can find a large enough variable capacitor, like 10-600 pf, you
can tune the entire shortwave spectrum (3-30 Mhz) with one coil. One way
to do this is to connect the individual gangs of a multi ganged
capacitor together in parallel. However there is a practical limit to
this because the high end (fully opened) range of the cap' will also get
larger and this will prevent the circuit from tuning all the way up to
30-Mhz. That's why it's usually easier to use a standard capacitor like
10-365 pf with at least two coils and a switch. It's also best to use
coils with an adjustable ferrite core (slug) so you can fine tune the
circuit's tracking on the low end of the range. You can buy such coils
from suppliers like Amidon and Miller. They come either wound for a
particular value of inductance or without any windings, if you want to
wind your own. The ferrite core material should be the type made for HF
use, such as T-43. I used to get my coil forms from old televisions that
used vacuum tubes. These sets had several coils in the I.F. and chroma
circuits which worked well for shortwave purposes. I would remove the
windings and rewind them for the value I needed in the preselector. Let
us know how your project goes.
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