| Motto: ... in a case there will be some D-class punk barking 100 meter nearby me using 750 watts, probably I'll fuck him up. Sure, I'll be not alone ... I feel obnoxious to be compared with OK1UHU, VCB, UGA, I have the same feeling, as when I built up a house all my life and now there are moved some stinky gipsies to this my house by our authorities ... Martin, OK1RR, 9.12.2004 |
Almost every ham, especially one of those, interested in shortwave QRP, knows the need of a simple, small
and well-performing antenna tuner unit. I compared many designs and those performance characteristics
and then I chose a simple L-match. It can accomodate most of random-wire antennas and is best suited
for long wire halfwave end fed antennas, which are my favorities.
Almost all materials are from a scrapyard - my much frequented supply of ham radio materials, components
and gadgets. What do you need to build such a tuner? As a first, tuning capacitor. Use one from the old,
tube era radios. Those have suitable capacitance (1x or 2x cca 500pF) and appropriate spacing between sheets,
remember, the high-impedance excitation of halfwave antennas pulls the voltage up. I don't recommend
the tuning caps with polyester dielectrics, they can't withstand even a little higher power. The inductor is
wound on two ferrite toroides, czech ferrite material N01P, outer diamater 22mm, inner 13mm, height 8mm each.
The single toroide has AL=11nH/turn2. Glue two of them together with cyanacrylate glue,
as seen at the pictures. The winding has first ten sections all the same, 2 turns
of 0.5mm copper, PVC insulated wire each. The last two sections have 4 turns each.
Next, you need some rotary switch to switch the inductor tapings. My one is robbed from a wreck of
an old, transistor electrocardiograph, it is single-pole, 12-toggle. More, you need two single-pole, dual toggle switches for
changing the HiZ or LoZ transformation scheme and another one for joining the two setions of the capacitor
together (twice the capacity, important at low bands). Last, two HF connectors - use the ones, which are common
for your hamshack. I use the PL-239s, because they can serve as a terminals for a banana plugs, too.
Important in field conditions!
Put all together according this scheme and view. You should have an antenna tuner
for 7 to 28 MHz (limited possibilities in 80m band, too) now. To be sure it's well done,
we have to test it. Put the metal alloy resistor (non-inductive), say 1200 ohm (not critical), rated for about 5W, to the
antenna side of the tuner. Play with the tuner, observing SWR meter of your rig.
You should achieve SWR reading less than 1.5 for each band. If so, your tuner is ready.
Enjoy the shortwave QRP world!
