[Antennas] Coaxial Sleeve antenna ?!
Dave Kelley
[email protected]
2002年3月03日 19:12:00 -0500
At 02:16 PM 3/3/2002 -0600, you wrote:
>Demetre,
>>The dipole antenna that is formed with the coax center conductor and
>folded-back braid is just like any other dipole. Its feedpoint impedance
>depends upon how it is mounted and how high, etc. You will *probably* end
>up with an antenna which is pretty close to matching 50-ohm coax without
>using a tuner, but this would not be a direct result of making the antenna
>out of 50-ohm coax.
>>BTW, you should insulate the folded-back braid very well since it will be
>at a high r-f potential near the end. Probably the most effective way to
>insulate the antenna and prevent water from getting into it would be to
>cover all of the exposed braid with heat-shrink tubing and to put some sort
>of sealing compound around the area where the braid is folded back.
>>A choke or current balun in the feedline at the braid end of the antenna is
>a good idea as well and if common-mode current on the feedline outer braid
>is a problem even with the choke near the end of the antenna, a second one
>1/4-wavelength closer to the transmitter end will help.
One way to minimize the problem of high RF voltages at the end is to
flare out the braid a few feet from the end, creating short "radials"
that protrude away from the coax. There should be at least two,
preferably three or four, "radials" to balance the current distribution
and to limit the voltage. (The charge, and hence the electric field
strength, at the ends will be 1/N what you would get without using
"radials", where N is the number of "radials".)
The choke that George suggested is a good idea and should be
placed right up against the "radials." The choke should be much
more effective in this arrangement because it would then be located
at a lower-impedance part of the antenna.
I believe the short "radials" that you see on some of the newer "no
radial" vertical antennas are a variation on the technique described
above.
73,
Dave
--
Dave Kelley NB4J
State College, PA
Centre County