In article ,
Gary Schafer wrote:
The coil in the center of a marine antenna that is designed to be used
on 2 mhz is indeed a choke at anything above 7 mhz or so. It is not
intended to be such but that is what it is when the antenna is used
above its intended range. All of the antenna above the coil is
electrically disconnected from the lower part of the antenna. You
could physically remove that upper portion and notice little if any
difference at higher frequencies.
That is the reason that type of antenna is not used on any system that
operates above 4 mhz. A straight whip (no coil involved) is the only
thing that will work satisfactorily in a multi band system. (trap
antenna being the exception)
Those old antennas with the loading coil in them perform much better
on 2 mhz than the straight whip antenna of the same physical length
but they are very poor on the higher bands as part of the antenna is
not there electrically. It is then a very short antenna at the higher
frequencies.
Regards
Gary
On Thu, 20 Nov 2003 21:37:05 -0500 (EST), (Ron
Thornton) wrote:
The coil in the center of an antenna is not a choke it is just an
inductor that electrically lengthens the antenna. This method is used
well above 2-4 mhz in mobile radio. It is not used much above VHF
because a full wave length is already relatively small.
Ron
Just a few comments here on this thread.......
Gary is right on in his assesment of the operation of the end feed
loaded antenna. The coil does indeed act as a choke on frequencies
higher than 4XResonant. In the "Old Days" we used Morad 2800 and 3600
loaded whips with 15 to 35 ft of wire below them as antennas in the
MF Frequency Range. They were just great below 5 Mhz, and not half
bad above that as the coil only choked off the upper 102" whip and
the coil itself, and left the bottom 10ft of antenna and 35 Ft of
wire as an antenna for the HF Ranges above 5 Mhz. This 45 Ft of
antenna did just fine as an antenna in those ranges as it would be
tuned (Fixed tuned antenna tuner era) as 3/4 Wavelength at these higher
frequencies. today's autotuners have a similar tuning firmware routine
that adds output Capacitance untill it finds the 3/4 Wavelength point.
They don't do as good of job as a Fixed tuned tuner due to the step size
and the sense circuits, but they will get close, and be fairly effecent.
The trick to make this type of antenna work is to get some wire under
the antenna, so that you have an effective antenna length once the coil
chokes off, as the frequency rises. Most of the North Pacific Fishing
Fleet uses this type of antenna system for MF, and HF, comms.
Bruce in alaska
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