Re: [TowerTalk] elevated short vertical dipole or quarterwavemonopole?

["John Tait" <johnei7ba@eircom.net>]

Check out  "Another Way to Look at Vertical Antennas" by  Rudy Severns 
N6LF....QEX March1999  and  http://www.iol.ie/~bravo/low_band_antennae.htm
73
  John EI7BA

> Here's an interesting optimization question..
>
> Say you have a maximum height limit (call it 33 feet) and you want to put 
> up
> a vertical antenna for 40m.  You have limited space for a ground radial
> array (think suburban back yard, not 40 acres of tidal plains in a salt
> marsh).
>
>
> You could either put up a standard 1/4 wave monopole with ground wires, 
> etc.
> The ground radial array is going to be non-optimal, and moderately lossy.
> Maybe 10-20 ohms loss? Rr would be around 30-35 ohms (or pretty close, I
> would assume, depending on the diameter), so the system would be 60-70%
> efficient.
>
> Or, you could put up a loaded vertical dipole  that's 1/4 wavelength 
> long(so
> the feed point would be 15-16 feet off the ground).  Ballparking it, you'd
> need about 16 uH of loading, and some matching to get the impedance
> reasonable. Rr=13-17 ohms (1/4 that of a halfwave dipole), if Qcoil=400, X
> is about 700ohms, so Rcoil=2 ohms.  You'd have some loss in the loading
> coil, however, you wouldn't have the loss in the ground radial array.  The
> current in the short dipole would also be bigger, so IR losses in the
> element itself would be greater.  All in all, it looks like it might be in
> the same general efficiency ballpark, if not slightly better, because the
> poor ground radials really hurts the monopole.
>
> The far field (many wavelengths away) effects would be essentially the 
> same
> for both.. they're both vertically polarized, etc.  There will be a
> different lobe structure; the phase center is a different height above
> ground.
>
> I'm wondering about close in.  Loss due to RF absorbtion in the reactive
> near field of the elevated dipole in the earth for instance.
>
> The elevated dipole will also be narrower band, which has to be dealt 
> with.
>
>
> Any comments, thoughts, practical observations, etc.
>
> Jim, W6RMK


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