At 09:33 AM 1/13/2009, John King wrote:
Since the response to my previous email was not voluminous, I will
pose a more direct question for my edification.
What is the difference in the radiation pattern and angle of
radiation between the folded unipole and the inverted L?
Well, unless you can get the inverted L to at least 1/4 wavelength
up, the folded unipole is going to beat it out, hands down, as far as
a nice, low radiation angle is concerned.
Is there an advantage to having the folded unipole over the inverted L?
Depends on how you're doing it.
If the antenna is an existing tower / beam combination with a dorp
wire down the side to make it a folded unipole, depending on the
height the feedpoint impedance (i.e., the wire and ground radials)
could wind up being a LOT easier to match than an inverted L; many
times it surprises you, and it'll match coax almost directly, with
just a small series inductor and / or shunt capacitor to take care
of some reactance.
Mechanically, since you've already GOT a tower, most of the hard work
is already done... you THINK, anyway! You'll quickly find yourself
obscessively adding buried wire radials from the tower base, to get
that last little bit of efficiency!
Is the difference merely ease and convenience of feeding the tower
via folded wires as opposed to feeding it, at the base, with
extension wire attached near the top to make it an inverted L?
Unless the tower is sitting on an insulator (a mechanical pain in the
butt for amateurs!), it's gonna be sort of difficult to feed the
tower against ground, since it's already there! You'd have to rig a
gamma match rod or a shunt feed wire part way up the tower... and
getting that adjusted just right is gonna be a BIG time waster.
Besides the time... such a match is going to tend to limit the
usable bandwidth of the antenna (i.e. the frequency spread between
VSWR points of 2:1, or whatever your arbitrary point of VSWR
acceptability is).
Would the inverted L provide both low and high angles of radiation
and greater efficiency due to longer length?
Maybe.
If the L is a significantly larger part of a quarter or half
wavelength than the folded unipole, then the radiation resistance is
going to be greater in the L. However... unless the flat top portion
is AT LEAST a quarter wavelength up, the nulls caused by the confused
ground reflection waves will probably null out that advantage.
I am in the middle of an antenna project , re-establishing my
antennas since Hurricane Gustav left me with only a GAP Titan
Vertical. That is the reason for my persistent queries. Thanks and
73, John, K5PGW
Good luck with it... I'm in the same process here.
WHY is it that you ONLY seriously consider antenna work when the snow
is up to yer knees and the temperature is about 10 degrees? <<smile>>
Mr. T. W9LBB
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