>So I'm considering loading the coax with Type 61 Ferrite beads, every
>6" or so, the idea being to make the coax "transparent" to the antenna
>field.
This is quite solid engineering, except that #61 is not a good material
for 146 MHz. #43 would work much better. Study Fair-Rite data for the
beads you want to use, selecting beads that show an impedance peak near
your operating frequency and using enough in series so that their
impedance adds to at least 1,000 ohms, and more is better. I would
spread them out by about 0.1 wavelength along the part of the coax
that's near the antenna, assuming a velocity factor of 0.97 (that is,
adjusting only for the diameter of the coax and the presence of an outer
jacket).
Note that my comment is NOT meant to address the relative usefullness of
other methods of decoupling the coax, merely to state how THIS method
can be made to work well. This method works by reducing the current in
the coax due to the antenna current to a very small value by adding a
large resistance in series with the coax.
73,
Jim Brown K9YC
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|