I have no numbers to give, only anecdotal comments.
During operation of VP8WWW last week we used one trapped 5-band Yagi (Mosley
Mini-33-A WARC) at 25 feet, one trapped vertical (Cushcraft(?) 80-10) with 8
radials, one shortened vertical dipole (Force 12 Sigma-40XK) and one
horizontal wire dipole cut for 80 with center suspended 2 feet below the
Yagi. The dipole was virtually useless at 23 feet and was converted to an
extra guy when the winds picked up to 50 mph on the morning of November 23.
On 40 meters the 40XK was 2 S-meter units higher than the trapped vertical.
On 20 the 40XK was about the same S-meter reading as the Mosley but with
more atmospheric noise. The 40XK was also used exclusively on 30 meters
with great success.
We were quite impressed with the apparent high efficiency of the 40XK. It
employs 86 inch per side T-bars at the top and bottom of the 15.5 foot
vertical for 40-30-20. It sits on an insulator 4 feet above the ground.
The T-bars are shortened for 17-15-12 and eliminated for 10. The feed
consists of two loading coils and hairpin coil match on 40 and 30, and only
shorting wires in place of loading coils (no hairpin) on 20 and above.
BTW, I had to return early to take care of some business. The other three
(VP8WFW, VP8WPF, VP8WMJ) are still operating, scheduled to fly out of
Falkland Islands this Saturday.
Keith
NM5G/VP8DXL
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Jim Lux
Sent: Thursday, December 02, 2004 12:17 PM
To: Towertalk
Subject: [TowerTalk] elevated short vertical dipole or quarterwave monopole?
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
_______________________________________________
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
|