[AMRadio] Folded Unipole versus Inverted L


Thomas Adams quixote2 at ix.netcom.com
Tue Jan 13 07:01:49 EST 2009


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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