"starman" wrote in message
...
Dave wrote:

"starman" wrote in message
...
Dave wrote:

"starman" wrote in message


It sounds like you need a passive preselector rather than an
antenna
tuner, if your receiver can't handle the strong signals on the
antenna.
Are you hearing stations outside the bands where they are actually
located? Is there a lot of noise from many stations all over the
shortwave range.


I am picking up a few very weak signals out-of-band, and there is
only
background noise on the DX setting. The RF choke seems to have done
a
nice
job of cleaning up most of the hash and trash in the background. I
would
like to enhance the signal I am actually looking for, however.
During
periods of good propogation I have ocassionally picked up Voice of
Korea
transmissions aimed at Central and South America, and I would like
to
load
the dice in my favor if I can.

I really think you should try a passive preselector. It will reject
the
strong out of band signals which cause intermodulation products while
peaking the weak stations. The choke helps but it also reduces the
receivers sensitivity to the signals you want to hear. The preselector
does the opposite. I used a preselector with my Sony-2010 and an
inverted-L antenna. It worked great. Without the preselector the 2010
would overload and produce a lot of spurious signals throughout the
shortwave spectrum.

It's pretty easy to build a passive preselector that will cover the
range of 3-30 Mhz with two coils. The larger coil will tune the range
of
about 3-7 Mhz and the smaller one 7-30 Mhz. It's best to use ferrite
cores in the coils for the best selectivity or 'Q'. Of course you'll
also need an air variable capacitor like 10-365/pf and a simple rotary
switch to select the coils. The circuit is a parallel (tank) design
which is connected between the antenna input of the radio and ground,
along with the antenna itself.

If you're not into building, you can buy a passive preselector at:

http://www.grove-ent.com/MFJ1046.html

Here's an interesting article on antenna tuners:

http://www.nyx.net/~dgrunber/tuner.ssi



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