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
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|