[Antennas] re:Gamma

Thu Jan 13 02:45:09 EST 2005

Well, for yagis that should be about right.
Bear in mind that a yagi has about 20 ohms radiation resistance at the
center, increasing as you move out to the ends. It means your rod is too
short.
Soooo, the gamma rod should be long enough to reach a point that presents 50
ohms. It will always present an inductive component of impedance, later to
be cancelled bi the gamma capacitor. But you should first lengthen the rod
until you reach a close to 50 ohms segment on the element. until you do,
leave the capacitor alone. You can even short it out temporarily.
Also, heed what Alan told you. Put a SWR meter on the line! In any case the
last stage of the tuning process is for minimum reflected power.
	Beware antenna work is addictive! Soo, if you plan fooling around
with antennas, kill somebody and steal his antenna impedance meter.
	I'll have a look in my old antenna book. There was at the time a
small unit called the antenna scope using a source of very low power, like a
grid dip meter, and super simple to build. It consisted of a small surplus
meter a diode and a potentiometer. That should allow you to read off the
resistance of the antenna close enough for any practical purpose. And, if
you choose not to kill anybody, buy an AutekResearch antenna impedance
meter: smaller than the MFJ, cheaper and very versatile. I have an old one
and happily married to it.
		Alex		4Z5KS