w8jitom@postoffice.worldnet.att.net wrote:
> > Anyway, I had a half baked idea that occurred to me when I previously used
> > folded feed antennas. (I didn't use them for improved efficiency, but that
> > is another story) I realize that the ground losses are the same for a
> > folded antenna as for a monopole of the same height when connected to the
> > same ground.
>
> That statement is precisely correct. Nice way to word it.
Tom,
This thread has been very interesting. While it may be perplexing and
irritating to some it is very thought provoking to me. I have used a
variety of antennas on 160...dipole at about 70 feet, inverted L, 1/4
wave slopers, etc. Despite the fact that I do believe that for best
DXing I should put up a bunch of base-insulated Rohn 25 and 130 or so
1/4 wave ground wires, I have neither the inclination nor spousal
approval to put up a full sized 1/4 wave vertical. One tower is what
I'm allowed. It is a 68' self supporting Heights aluminum with usually
a bunch of hf yagis on top. Bar none, the best antenna that I have been
able to come up with, given these restrictions, is the so-called folded
unipole (folded umbrella) as described by the late John Haerle, WB5IIR,
in his very interesting little book called The Easy Way.
My version consisted of 4 drop wires attached near the top of the 68'
tower and pulled out about half-way down to form the umbrella frame and
then coming back together again at the bottom of the tower where they
were fed abainst the ground system. The wires were a little over 70'
long. I used an L network to match the thing and the results were
acceptable with only 8 1/4 wave radials. Since it works as well or
better than anything else I have tried, I'm inclined to keep using it.
I do remember, however, that Haerle said in the book, and I would be
very interested in reading your comments on this:
"First (and quite significant) is the fact that it is a higher-impedance
antenna. Typical impedance at resonance is in the 50/150 ohm ball
park. A 1/8 wavelength folded unipole would have an impedance on the
order of 50 ohms, as compared to less than 10 ohms for a conventional
vertical 1/8 wavelength high. Note: It should be added that the
feedpoint impedance of the folded unipole increases as the square of the
number of "folds", or added parallel conductors."
Is Haerle right so far? He then says:
"In addition to the higher feedpoint impedance, the folded unipole
design distributes the current up the vertical more evenly, rather than
concentrating it near the ground, where absorption in the ground would
cause more losses and lower efficiency....thus, the folded unipole
offers dual benefits: 1. Higher input impedance, which is easier to
feed and/or match. 2. Higher efficiency because of fewer ground
losses...so much so that the antenna will perform respectably with no
radials......although, of course, a good radial system will add
measurably to its performance."
I would imagine, on the basis of your earlier comments about short
verticals will have something to say about point 2, at least.
Let me hasten to add that while I am intellectually curious about the
design, I'm pretty much stuck with it as a "best antenna that I can put
up given the circumstances" so I would also be interested in any
suggested improvements to my own installation.
73,
Ken K4XL
grimm@alison.sbc.edu
--
FAQ on WWW: http://www.contesting.com/topband.html
Submissions: topband@contesting.com
Administrative requests: topband-REQUEST@contesting.com
Problems: owner-topband@contesting.com
|