> Dan/W4NTI wrote:
> I wonder if you could take the time to model a couple antenna systems
> I have here?
>
> 1. A 40 meter loop. Apex at 40' Bottom wire about 4' . Not quite a
> triangle. The bottom wire is longer and makes the shape a bit
> elongated. hi. Probably about 20 feet or so longer than it should
> be.
>
> Feed is with open wire, on the bottom wire. About 10 feet from the
> far corner point. Probably not a "perfect" setup, but does seem to
> work. Just wondering why.
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Dan earlier sent me the above, so perhaps the info would be useful to
Jeff and the others.
As to his "not a perfect setup", that is the understatement of the year.
When fed as he describes, radiation is from about 60 degrees peaking at
90. Good cloud burner.
But, I did model a 40 meter triangle, and here it is.
Each sloping leg is 48.375' long, with the bottom being 51.25'. At about
7' off the ground, and fed at
75% down from the apex, the impedance is 158.9+j.12 and a 50 ohm feed
gives you a 3.178:1 SWR. But, feed it
with a 4:1 balun, and the max SWR is 1.259 from 7 to 7.1 mHz. Now the
good news, this antenna has a TO angle of 10 degrees with useful
radiation at the -3db points of 2 degrees and 34 degrees. The Front to
Side ratio is
close to -13 db ( not dbi ) down at angles above 40 degrees. Definitely
not your SS antenna. Oh, gain is
6.24 dbi, almost equivalent to a dipole.
As a note, if you run a program such as "TRLINE", you will find you can
insert an ELECTRICAL x length of 70 ohm coax at an ELECTRICAL point y
from the antenna feed, and you have a perfect 1:1.
While I was at it, I also modeled an antenna called a "Hentenna".
Basically, it looks like this:
X Y
---------------------------
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| >| | Z
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X Y
Side X is 57' Y is 13.375' and all vertical legs ( Z ) are 17'. I
modeled this one so the top wires were at 51.25'. Impedance is
47.86-j.09 ohms when fed in the middle of the wire marked with a ">".
Pattern is beautiful. A figure 8 with max gain ALONG the axis of the
antenna a little over 3.71 dbi.
SWR at resonance (7.04) is 1.041. Less than 1.5 to 1 from 7 to 7.060.
have fun.
73
Ed
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