On Wed, 16 Apr 2008 09:48:34 -0700, "Jim, K7JEB"
wrote:
Jouko, OH5RM wrote:
Take 20m of wire, that is half of a dipole.
Put a 1/4 electric wavelength shorted stub for 10,1MHz so that the open end
is 7,25m from center and the shorted end pointing to the tip of antenna. One
side of stub is the 20m wire itself, like in the upper picture.
I did just that with EZNEC, but with a free-space dipole. Without the
stubs, the antenna resonated at 3.6 MHz. With the stubs, that dropped
to 2.75 MHz and additional low-impedance points were noted at 6.5 and
11 MHz. The patterns at 2.75 and 6.5 had the desired dipole shape,
but the 11 MHz pattern had multiple lobes.
Jim Bromley, K7JEB
Glendale, AZ, USA
I only have EZNEC 3 and the EZNEC 4 demo.
I don't remember the dimensions, but I remember that in the
instructions, EZNEC could not make accurate measurements with parallel
wires within a certain distance like 8-10 inches.
(someone correct me here.)
holding that assumption...
I have not modeled that antenna or made one, but I had an experience
with an 80 meter dipole that may relate. I cut a wire way too long
for 80 meters, I think it was something like 140 feet or so. I use
insulated stranded copper wire so I tried folding back the ends until
I shortened the antenna to 75 meters. What I discovered was, that the
change in frequency of the antenna did not match the reduced length of
the wire. After trying for quite some time, even after measuring the
antenna, I found it physically shorter than the calculated length, but
the center frequency, which changed a little, hadn't changed
significantly as expected. My conclusion was that I basically made a
linear-loaded dipole and the total electrical length of the antenna
was basically the wire length minus a small amount for interaction
between the folded back wire and the original leg. It wasn't until I
trimmed the wire itself that I raised the frequency of the antenna.
The wire I used had the ends loosely wrapped around the main wire of
the dipole, not tightly wrapped like the turns on a hangman's noose.
It was insulated THHN stranded copper.
Let's look at the 40/80 meter antenna cut for the CW portions (3.5 & 7
MHz). Assume, for the sake of argument that the 468/f = length in
feet accounts for the velocity factor and that it is the same for the
40 meter portion as the whole wire...
In theory, the Lattin antenna should be a total of 133.7 feet long,
with 66.85 (1/2) of the total length being the 40 meter wire and 1/2
on each end being the 40 meter decoupling trap. Overall there is a
total length of 200 feet of wire which theoretically would give you 40
and 80 meters. UNLESS the RF reacts according to my experience above
which would produce an antenna which would be tuned for 7.0 and 2.3
MHz (give or take reaction)
If this is true, and even with the reaction, the 40 meter dipole would
work, but you couldn't make the Lattin antenna work with the those two
bands. I might assume that 30 and 80 meters would work as the total
length of the 30 meter portion with trap would be less than 133 feet
and the 80 meter dipole would extend past the trap as an additional
extension of wire. Presumeably, that antenna would be resonant on 30
meters, 80 meters and somewhere else, maybe close to 6 or 7 mhz (Total
length of the wire from feed to end of trap.)
-------------------------------------------------------- O -------...
3.5 MHz | 10 Mhz
__________
trap plus ? Mhz
I don't know how the diagram shows, I hope you get an understanding of
what I am saying.
Comments?
=====================
PS, after writing all that, I re-read your post more carefully. I
think your EZNEC model reflected what i said all along. The
difference being that the continuation of the 80 meter dipole element
seems to be overlooked by EZNEC.
(ok, done this time
--
73 for now
Buck, N4PGW
www.lumpuckeroo.com
"Small - broadband - efficient: pick any two."