The
EFHW is a convenient way to feed an antenna when QRP on a SOTA peak.
There is very little current on the end of the wire so losses to ground are
minimized even when the end of the wire is near the ground. The
feedline can be short or even non-existent.
Impedance
at the end of a halfwave antenna is very high, typically over 3,000 ohms.
Little or no counterpoise (radial) is required, but an ATU will not handle such
a high impedance.
I
know of three ways to feed it. If one wants to use an ATU, feeding the
EFHW through a 9:1 balun should bring the impedance down into a range where the
ATU can handle it. There will be loss in both the 9:1 balun and
in the ATU.
AA5TB
pioneered a simple tuner which was later kitted by Eric, KI6J, and then copied
by many others including Hendricks, SOTABEAMS, and now QRPGUYS dot com. I did
repeated tests with several of my homebrew versions of this tuner. Loss
when properly tuned is typically about .7 watts with five watts in, which is
15%, or around .6 of a DB. This loss will be undetectable by the
receiving station. Note that the loss rises rapidly if detuned.
Parfits,
original owner of LNR, made a broad band impedance transformer. That is
still sold today by LNR, though their versions use a MUCH
larger than needed core for QRP. KE5AKL began home brewing them using a T50
core, and I have since made quite a few of them using both T50 and T87
cores. I find at QRP power levels there is very little difference in loss
between the smaller or larger core.
The
loss with this broad band impedance transformer is TWICE that of
the KI6J tuner. With six different ones that I built, it varies from
about 1 watt to as much as 1.4 watts of loss with 5 watts in. That's
about 1.3 DB loss which is detectable. On the other hand, the difference
between the two approaches is less than a DB and should not be detectable.
The convenience of the no-tune broad band impedance transformer may outweigh
the loss.
Note
that IF either matching device is NOT giving a good match, the loss rises
rapidly and can quickly be half the input power dissipated in heat. Cleaning up the
mismatch with an ATU may make the transmitter "happy," but will do
nothing to reduce the loss in the device itself.
THE DEVICES
The AA5TB matching device ... 28 turns of #26 wire wound on a T50-6 core, tapped at four turns. The center of the coax to the TX goes to the tap, while the braid goes to the closest en, four turns from the tap. A 150 pF variable capacitor (varicon) goes across the secondary, across the entire 28 turns. This is not a tuner, the tuning simply creates a resonant L/C circuit on the antenna side while being an impedance transformer. Below are three variations I have built:
The BROADBAND matching device. Three turns are wound bifilar with the 27 turn secondary. A 150pF NPO capacitor is across the three turn primary which goes to the TX. Antenna goes to the far end of the 27 turns.
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