That coil does two jobs. The 3/4 wave vertical has nearly as nice a feed
impedance as a quarter wave, resistive and not far from 50 ohms. And
many of us have used a 40 meter vertical on 15 meters, the 3/4 wave
mode. But the radiation from the lower quarter is exactly out of phase
with that from the upper half wave and practically the same intensity so
the signal at the horizon cancels. The vertical radiation pattern of the
3/4 wave is more like 45 degrees above the horizon, half way to straight
up. When the lower part of the radiator is only 1/8th wave, it radiates
so much worse than a quarter wave that its not much of a factor at the
horizon, and the added spacing from ground of the upper half wave gets
maximum gain at the horizon. The coil aids in antenna resonating but
doesn't radiate much. I've used a short stub to the side or a coil and
both worked well. I liken it to taking the extra 1/8th wave of wire
needed to resonate at 3/4 wave and wadding it up so it doesn't radiate.
Its best if in that wadding that the conductor have equal bits parallel
but with the current in opposite directions to cancel out the radiation.
A random wad or a coil or a stub all work decently.
Otherwise the input Z of the 5/8 wave is about as obnoxious as possible,
somewhat high resistance and lots of reactance to make a bad mismatch to
50 ohm coax. The simple coil does well.
On a very short antenna like just 1/8th wave the reactance is strong and
the resistance low which makes it more obnoxious to feed and gives
tuners a fit because of the high currents and the need for lots of L or
C to correct for the reactance. Lots of series inductance to bring the
antenna to a 1/4 wave resonance, then the resistance is low so it takes
a high impedance ratio matching network involving extra circulating
currents within the tuner that tends to overheat tuner parts, coils,
capacitors, and relays.
73, Jerry, K0CQ
On 11/24/2010 12:03 PM, Mike Hyder -N4NT- wrote:
I was always intrigued with the 148 MHz automobile antennas which were
5/8 λ. They had a coil at the base which increased the electrical length
to 3/4 λ.
Mike
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