Antenna ground planes for composite aircraft

On Sun, 20 Dec 2009 16:03:47 -0500, rich
wrote:

Here's the article: http://www.express-builder.com/docs/tip1/tip1.pdf
written by Bob Archer. It's the 5th paragraph down.
what he says is: "I do not recommend any antenna on the market that
has a little black box in the center of the antenna. This device is a
ferrite transformer which provides a very good VSWR and a very good
bandwidth but at the cost of being a very lossy (absorbs energy)
device.

Irregardless of the fact that we've been using them for about fifty
years as a way of going from coax to balanced inputs on TV sets. They
work just fine. However, I don't use them as I don't need to. The
center impedance of an infinitely small wire used as a dipole is 72
ohms. However, as they get fatter, the center impedance drops until
they fairly resemble a 50 ohm load with fat copper strip.


.. Also if you were planning to go with Jim
Weir of RST's designs don't bother with the ferrite beads. At these
frequencies the beads don't do anything that I could detect in the RF
lab.

Then with all due respects, your RF lab isn't very well equipped. The
beads do the same thing at RF that a "clamp-on" alternator filter
does...it does nothing for the noise at the source, but it strips off
the noise on the wire preventing it from radiating all over creation.
The ferrite beads simply act as a "noise filter" stripping off any RF
that gets reflected back down the outside of the coax outer conductor
(shield).



A good balun would work better as a dipole feed because it
balances the currents on the elements and matches the impedance at the
same time and it doesn't
absorb RF energy. My antenna designs do not need a balun because I use
a modified version of a feed called a Gamma match that feeds the
antenna at the fifty-ohm point and automatically balances the currents
on the elements."

And all a gamma match does is introduce a tuned circuit into the
mixture which by definition reduces the bandwidth of the device.

Jim