[SECC] Models

[k4sb@bellsouth.net]

> 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.
-------------
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 
---------------------------
|                    |     |
|                   >|     | Z
|                    |     |
----------------------------
           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