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1-7 of 7 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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RE: Shortwave Antenna Tuner
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by W5USJ on May 31, 2009
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It's fairly simple to build a tuner for long wires using RF Chokes.
Here's a link to my website showing the prototype I've built. Schematic and picture of assembly included on the page.
http://www.w5usj.com/images/zmatch/l-tunerrfcs.pdf
Here's some info about the testing that was done on the prototype:
Using RF chokes in place of toroids for an L-tuner has worked out quite nicely.
Using RF chokes for RF circuits is not new. They are used successfully for the filters in RockMites. Using RF chokes for inductors in a tuner might be a new application.
For a standard of comparison I used the SLT from Hendricks/KD1JV, QRPkits.com. The SLT, for 40 thru 10 meters, is now replaced by the SLTplus for 80 thru 10 meters. Check it out, it's a really nice kit.
But, for a tuner without toroids, an L-tuner using RF choke inductors has worked out really well producing good results.
The capacitor used is a dual polcon obtained from Emtech. Total capacitance range about 20 to 500pF. The smaller 60/160 pF duals would probably work OK.
http://www.angelfire.com/electronic2/index1/160pfVarCap.html
The initial results as bench tests and a field test installation were quite good if I do say so myself...8^) The L-tuner RFC compared very well to SLT with return loss values across the bands of 30dB (1:1 SWR) in most cases. Additional inductance values probably would have produced a better match where the RL was only 20dB or so. See choke list below.
I didn't have enough switches on hand so I used an 8-position DIP switch with 6 pairs of the 8 used to short out the chokes. Fastron series 22 RF chokes used at the various switch positions are:
S1 = 10uH
S2 = 6.8uH
S3 = nu
S4 = 4.7uH
S5 = 2.2uH
S6 = nu
S7 = 1.0uH
S8 = 0.47uH
XX = 0.22uH to be added
The addition of the 0.22 would be useful in that for some matching conditions, 0.47 was sometimes too much or too little. Having one more step would probably produce a better return loss (RL) at match. Something for further testing -- no 0.22s on hand. Prototype picture and schematic available. Also, if needed, a list of Mouser part numbers for the chokes.
On a sample of 1 prototype:
Bench Test
Using the band information from the SLT for wire lengths, EZNEC was used to determine the approximate feedpoint impedances. ( only 80 thru 20 field tested so far. Bench test through 10 looks good, however.)
80 = 2325 -j2553 Ohms (the 164 ft radiator length was not field tested)
40 = 4785 -j451
30 = 1656 -j2063
20 = 439 -j1098
Using the MFJ-259B as a signal source and matching indicator, the AI1H procedures using his Geometric Resistance Load Box were used for the test. The resistaive part of the impedance values shown above and more were easily matched with the L-tuner RFC. Insertion loss measurements were generally less than 1db and often less than 0.5dB.
Field Test
A 63 ft length of wire was suspended at 25 ft from a convenient tree limb and hooked to the tuner on a table at the other end. The counterpoise was attached to the tuner and laid out on the ground. The MFT-259B was again used as the signal source and tuning indicator.
A match was easily obtained on 40. A match was also obtained on 80 but with a lower RL. Even so, the SWR was less than 1.2 and would be quite acceptable.
The radiator and counterpoise lengths were changed for 30 meters following the steps used for 40 meters. Again, a match was easily obtained with good RL. Matching on other bands was also possible with SWRs better than 1.5:1.
The process was repeated for 20 meters with comparable good results.
Summary
Based on the results obtained with the prototype, an L-tuner built using RFCs for inductors would provide good service as a QRP long wire tuner.
This is fun stuff...
Chuck, W5USJ
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RE: Shortwave Antenna Tuner
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by KF8ZN on June 1, 2009
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You'd be better off winding your own inductor, simply close wind 20 turns of #14 INSULATED house wire on a short piece of 1 1/4" pvc pipe, then cut off 1/4" wide a "slot" of insulation lengthwise (axially) steal parts from an agc fuseholder, the spring and contact, to tap it.
A 1" length of 2" pvc, slid over the coil, with a hole small enough to pass the fuseholder wire, but stop the spring, should work well as a slider.
There, it's as good as done!! =0)
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RE: Shortwave Antenna Tuner
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by KB9BVN on June 3, 2009
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Build this one:
http://www.w5usj.com/images/zmatch/l-tunerrfcs.pdf
Uses molded RF chokes.
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