TopBand: verticals

[km1h @ juno.com]

On Thu, 14 Aug 1997 11:57:02 +0100 Peter Chadwick
<Peter.Chadwick@gpsemi.com> writes:
>>This is a long-held fallacy--that the higher impedance of the folded
>>unipole reduces the ground losses.  The impedance transformation
>>of the folded unipole also transforms the ground loss part of the
>>equation in the same way and there is no net improvement
>
>Interesting. The 'Services Textbook of Antennas and Propagation' by
>Glazier and Lamont defines radiation resistance as 'the resistive
>component of the input impedance of an aerial [antenna]'. On this 
>basis,
>the radiation resistance must go up when the impedance goes up. But if
>the ground losses are not very low, why couldn't I measure any change 
>in
>input impedance when adding radials - which you'd expect with alow
>radiation resistance? And why can't I measure any current in them? 


SNIP

You have an interesting question Peter and it is an answer I am not
qualified to give.

I too, have been under the impression that a half folded dipole ( folded
unipole) would have a feed impedence roughly 4X  of a straight 1/4 wave.
Maybe 140 Ohms ideal.

Over the years I have heard zero agreement and remain confused. 

Here is my sort of gut feeling:

If the antenna is a perfect 1/4 wave electrically and the folded portion
is the same diameter then the feed impedence is about 140-150 Ohms. This
would, under perfect gnd condx, be a standard folded diople with a mirror
image and taking into effect ground mounted vs half wave height
diferences. 

If the antenna is any other height, or if the diameters are different,
then all bets are off. 
In those cases the feed side would be acting strictly as a matching
section and under real ground condx anything is possible. It would take
instrumentation, not rhetoric, to obtain real values. 

BTW, Peter, Im glad you found your way over to this reflector. Your
inputs will be a pleasant addition. 

73....Carl   KM1H


  >
>Amongst professional antenna designers, I can find about a 50-50 split
>between those who hold that the radiation resistance goes up, and 
>those
>that hold that it doesn't!!
>
>I always felt that I never could understand radio - how I've made a
>living at it for over 30 years I don't know!
>
>73
>
>Peter G3RZP
>
>--
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