370
Item nr.


Gerard Regenboog Antenna Tuner

Get more out of your longwire antenna!


Data for Gerard Regenboog
ProductionThe Netherlands, 2014.
Price was 2 euro.
BandsBlack, Green, Blue, Yellow, Orange, Red, 1 - 15MHz.
CabinetWood. Size 12,5x9,5cm. Weight 250gr (w/o coil).

The Design

A longwire antenna consists of a single wire of length 10 to 20 meters, hung outside. It can be connected directly to an old tube radio, because these radios were built for longwire antenna's, but modern receivers, such as the Philips D2999 are different. Modern receivers have antenna inputs of low impedance, which means that the antenna signal should have low voltage and high current, while the wire antenna gives high voltages at low currents. The antenna tuner matches a longwire antenna to a low impedance radio.

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.

ColorTurnsRange
Black301.0 - 2.4 MHz
Green162.6 - 4.0 MHz
Blue123.0 - 4.4 MHz
Yellow84.2 - 8.2 MHz
Orange47.0 - 11.5 MHz
Red29.3 - 15.5 MHz
Coils were made of construction wire and wound on a can of WD40 with a diameter of 54mm. I increased the number of turns by a factor of two: 4, 8, 16, 30, using a different color for each coil. The blue coil is extra for the 80m band. This makes my coil tray look like a rainbow, or Regenboog in my language.

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.


Obtained1/2014 from scrap parts.
Condition7.
Value (est.)1€.

This Object

The tuner works satisfactorily for the 80m and 40m amateur bands. With the appropriate coil and tuning, the signal is 1 to 2 S points higher than in the direct setup.

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?


Part of Gerard's Radio Corner.
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