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.


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