On Mon, 11 Apr 2005 08:03:21 GMT, "Pete KE9OA"
wrote:

It should be noted that these formulas are only appoximations. Actual
sizes are dependent upon the conductivity and diameter of the
elements, and the quantity and angle of the radials. When building a
resonant antenna it's a good idea to make the elements a little long
and trim to resonance.

Very true..........I use a spectrum analyzer with a directional coupler and
trim for maximum return loww.

So do I. It gives a much better picture than a simple SWR bridge.


The feedpoint impedance of a 1/4 wave ground plane is 37 ohms when you
have
the radials at a 90 degree angle with respect to the radiator. If you have
the radials drooped at a 45 degree angle, the impedance rises to
approximately 50 ohms.
If you have a single radial drooped at a 180 degree angle with respect to
the radiator, the impedance rises to 75 ohms.


These are impedances for antennas in free space, and are practical
only if you can mount your antenna well above the ground and away from
any tall objects.

Also true, but a good starting point.

Some people like to make the subtle conditions into much greater
points than they need to be.



.....uh, what?

Both tubes and transistors use matching networks, so I don't know what
distinction you are trying to make there. Power will be reflected from
an antenna/coax mismatch -regardless- of whether you have a tube or
transistor final. And what does an ALC circuit have to do with
transmission line propogation?

This pertains to solid state amateur transceivers that don't have an
adjustable output matching network.........most of today's units have
fixed-tuned bandpass filters after the output stage. I am not referring to
transmission line propagation; I am referring to the fact that, with a
fixed-tuned output network that expects to see a 50 ohm characteristic
impedance, the ALC can fold back the power. Usually, that doesn't occur
until a VSWR of 2 to 1 is reached. My explanation is for illustrative
purposes only.

Not only that, but the bandpass filter characteristics of the output
stages can change if the load impedance changes significantly,
increasing loss and broadening the cutoff points.

Dave
"Sandbagger"
http://home.ptd.net/~n3cvj