For that very reason, in our camper operation, I use a end fed antenna.
The matching network is a tapped coil and variable C forming an L
network. The circuit is in a small plastic box with a SO-239 on each
end, thus it is reversible. This allows a wide range of impedance
selections, plus the L network, unlike a T network, will only resolve a
match at one value combination of L and C values.
I carry a 100 ft spool of #22 insulated hook up wire. Unroll some
estimated length, drop a half hitch around the spool and toss the spool
over a convenient limb. When ready to retrieve the antenna, turn the
feed end loose and let the spool drop to the ground. I've never
experienced a length or condition where I could not satisfactorily match
the band/frequency I wanted to use.
73
Bob, K4TAX
On 11/19/2017 10:26 PM, K9MA wrote:
On 11/19/2017 02:22, Erik Basilier wrote:
The second approach, used by MyAntennas and others seems to use a
transformer with much higher impedance ratio. One way to construct
such a
transformer would be to cascade two 9:1 units for an effective ratio of
81:1.
While I haven't tried it, it would probably be very hard to build a
pair of 9:1 transformers or a single 81:1 transformer with low enough
stray capacitance to work on the higher bands. It might work up to 40
meters, I suppose.
The big advantage of a resonant end-fed half wave is that very little
current flows in the ground system or counterpoise, so most of the
power is going into the antenna. While it is hard to match that high
impedance with a non-resonant transformer, it is very easy to match
with a simple L network. The only reason the internal ATU in the KX3,
etc., can't match it is that it doesn't have enough range. (And for
good reason.) An external L network consisting of a tapped coil and
variable capacitor will do it nicely.
73,
Scott K9MA
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