On Sun, 10 Apr 2005 19:58:28 GMT, "Pete KE9OA"
wrote in
:

Hey Vinnie............I can help you. The formula for a 1/4 wavelength
radiator is (234 / F) whereby F is the frequency in MHz. This will give you
the length in feet. For the radials, (237 / F) will give you that length.
Always remember...multiply the radiator length by 1.05 and that is another
way of calculating the 1/4 wavelength radials.


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.


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.


There was an article in RF Design magazine a few years back, explaining why
certain impedances are used in the RF industry.
72 ohms was the impedance that produced minimum cable losses...........50
ohms is a happy medium.


Close. For lowest loss, the optimum characteristic impedance of coax
is 76.9 ohms. 70-73 ohm coax is used as a compromise between low loss
coax and coax optimized for minimization of flashover, the latter
having an impedance of about 60 ohms.

And while many people have many different ideas as to why 50 ohm coax
is made, it is just a compromise between low-loss/low-flashover coax
of 72 ohms and coax optimized for handling power (about 30 ohms, which
is too lossy for practical transmission lines).


On a final note...........at 37 ohms, you will have a VSWR of 1.3 to
1.................at 75 ohms, you will have a VSWR of
1.5 to 1. What is the difference here? For a transmitter with a tube output
and an internal matching network, you wouldn't really see much effect. For a
typical solid state transmitter, there would be some difference between the
two antenna impedances, because the broadband solid state transmitter would
be called upon to deliver more current to the antenna. It probably wouldn't
have any effect, unless the ALC circuit was aggressive in its operation. In
this case, power foldback would occur into the 37 ohm load. Would it happen?
Probably not.


......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?


I hope this helps.

Pete





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