370
Item nr.
Get more out of your longwire antenna!
Production | The Netherlands, 2014.
Price was 2 euro. |
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Bands | Black, Green, Blue, Yellow, Orange, Red, 1 - 15MHz. |
Cabinet | Wood. Size 12,5x9,5cm. Weight 250gr (w/o coil). |
The conversion is done by a tuned circuit (LC tank). It is brought into oscillation at the top, where the voltage is high, by placing the feeding load P (wire antenna) parallel to the tank. It feeds the radio from the bottom, where the voltage is low, by placing the load as a small series resistor S. The optimal value of C and L can be computed using formulas at Wikipedia. But this requires to know the exact antanna impedance, which may also differ with frequency. Also, for receiving the match is far less critical than for sending, so I use a series of simple air coils, and tune to max signal with the tuning cap.
My antenna tuner has a variable cap of 40 to 1000pF, and a series of pluggable coils. By connecting the wire antenna to coil jack (1), the tuner is completely bypassed. This can be for comparison, or to have a wideband setup (no adjustment necessary) with lower signal.
Because the C can be varied over a 25:1 range (1000 to 40pF roughly), theoretically the frequency range of a single coil could vary by a 5:1 range. By experimenting, I found that the usable range is much lower.
Color | Turns | Range |
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Black | 30 | 1.0 - 2.4 MHz |
Green | 16 | 2.6 - 4.0 MHz |
Blue | 12 | 3.0 - 4.4 MHz |
Yellow | 8 | 4.2 - 8.2 MHz |
Orange | 4 | 7.0 - 11.5 MHz |
Red | 2 | 9.3 - 15.5 MHz |
Warning: If you ever play with this set, don't put the plugs in the wall socket!
I mounted the coils on old power plugs that I got for 25ct at the thrift shop. The plugs are the main cause of the 2€ expense for building this tuner.
Obtained | 1/2014 from scrap parts. |
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Condition | 7. |
Value (est.) | 1€. |
The tuner started around 2001 as a matcher for my Senfor 27Mc set. Then it only had one fixed small coil, and it used only one half of the tuning capacitor. In 2013 I decided to upgrade it to use it for MW, Trawler Band and Short Wave. I replaced the fixed coil by a socket, laid out the wiring a bit more neat, wound the coils, and replaced the connector by a 75 Ohm antenna jack.
The tuning cap appears to have two different sections, which is not strange because in a LW/MW radio, the oscillator frequency is always higher than the antenna frequency. But I measured the two sections with my Singing RC Meter and found them to measure about 450 or 500pF both. Taking a closer look at the capacitor revealed that, indeed, the smaller section has the plates closer, so the capacity can be the same. So, now it puzzles me even more, why the two sections are different; a matter of isolation requirement?