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