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Fuel tanks and SSB counterpoise.
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 -- add a 2 before @ |
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