Bill wrote:
...snip...
> Will the other antennas which are mounted lower on the tower have
> any loading effect?
The short answer? Yes. How much? Well, it's not that simple. In a short
vertical, loading coils are used to "lengthen" the element. The higher up
you go, the greater the loading coil must be. E.g. if it takes 5uH at the
base, it will take 10uH half way up, and a huge amount at the top. Top
loading is the same principle, only in reverse. At the top, the "hat" will
give you the biggest bang for the buck. Half way up, I would figure 1/2
the effect, etc. Everywhere in between? Well, if you feel compelled to
calculate it, go ahead and guess.
> I would like to feed the tower as a vertical but am concerned about
several
> things.The effective height of the tower will be in excess of 1/4 wave,
but I
> believe even with the top loading probably is slightly less than 1/2
> wavelength.
Have you considered elevated radials?
> I am limited to around 110 to 130 feet on 180 degrees of the antenna base
and can > go somewhat farther on the other 180 degrees.
Radials are like anything else. You buy the best you can afford and live
with it.
...snip...
> I am aware of the need to add capacitance and have a huge variable
capacitor
> around that looks like it came from a California KW.
You will be surprised to find out just how simple that capacitor can be.
Its located at the source (50 ohms?) and the voltages are not that wild.
...snip...
> How effective would this antenna be compared to a regular 1/4 wave
veritcial
> with the same ground system?
Build it and see...
> How do you think it would play compared to a
> suspended wire slightly top loaded 1/4 wavelength long with an elevated
> ground plane of eight wire at a base height of 50 or 60 feet?
Get a modeling tool and play with the alternatives. My gut tells me that
you tower (yes there is drool on my chin right now) with elevated radials
and whatever exotic matching magic you need to do will play very well.
> What do you estimate the impedence of the shunt fed tower to be?
The simple answer? Yes, it will have a feedpoint impedance. Your system if
far too complex to even guess using sophisticated models. Build it. Get a
meter to measure it. And make a tuner at the feedpoint.
Will it play? Oh yes, it will play well.
Ford-N0FP
ford@cmgate.com
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