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1-4 of 4 messages
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Shortwave Antenna Tuner
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by UBUNTUHAM on June 2, 2008
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I have a friend who is a shortwave listener. He is wanting an antenna tuner. I have the parts in my junk box and would like to build him one before he spends his money on one. To save space I am curious if I can substitute chokes like in the link below in place of the traditional coil? I will be using a polyvaricon capacitor from an old transistor radio.
http://www.radioshack.com/product/index.jsp?productId=2103978&cp=&sr=1&origkw=choke&kw=choke&parentPage=search
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RE: Shortwave Antenna Tuner
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by W5FYI on June 3, 2008
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It might be intriguing to build a mini-tuner based on the RS choke, if you could somehow tap the turns. Perhaps carefully scraping off the enamel, then using a slider contact to vary the inductance, like they did in the old days of crystal radio. 100µH is a bit much inductance for a tuner, unless you're interested primarily in the low frequencies, but it's certainly worth a try. Just advise your friend that the quality probably won't be the same as it would with a good commercial tuner. You might loan him your tuner too, so he can see for himself which would work best for his application.
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RE: Shortwave Antenna Tuner
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by WB6BYU on June 3, 2008
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Two problems with using RF chokes here:
First, you often want to be able to vary the inductance,
or at least switch in different values. Typically this
is done using tapped coils, or by manually tapping an air
core coil with a clip lead. You can use switched RF chokes
as a replacement if you have an appropriate range of values.
Perhaps a better way to save space is to use a powdered iron
toroid core for the coil and mount it on the back of a
rotary switch to select the taps.
The other problem is the Q and stability of the RF chokes.
In order to get a high inductance in a small space many
of the smaller chokes use small wire around a ferrite
core. This has a higher loss than an equivalent air core
coil or one wound on powdered iron instead of ferrite.
With a transmitting tuner the coil can overheat due to
losses (even with an air core coil) when the efficiency
is low. While that isn't a problem with a receive tuner,
the loss is still there.
One of the things that affects the loss of a coil is
other conductive objects in the magnetic field. One
problem with trying to make a tuner too small is that
it is very difficult to provide enough spacing between
the coil and the case, etc. to minimize. This is another
reason for using toroid cores in miniature tuners since
they have less external RF field so aren't as effected
by the case, circuit board, wiring, etc.
One thing you might try is to breadboard the tuner with
an air core coil and see how it works, then replace the
air core coil with a similar value of RF choke and
see how much difference it makes. If the tuner is
always being used with the same antenna on the same few
bands then you can choose the optimum components for
those specific combinations. This is much easier to
use than a wide-range tuner with multiple controls that
has to be reset for each band change. I often build my
transmitting antenna tuners this way: I experiment with
a simple L network until I find the values that give me
a good match for each band, then wire fixed capacitors
and coils to a big bandswitch. No more two-handed
adjustment when I change bands - I just turn the switch
and operate. Of course, I have to change the values
when I change my antenna, but for many people that doesn't
happen very often.
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RE: Shortwave Antenna Tuner
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by UBUNTUHAM on June 3, 2008
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Thanks for the great replies. I will give it a shot since I already have the parts and compare it against my tuners. I will report back with my results.
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