Jack,
For what its worth, I ran EZNEC on a 40m dipole at 17 feet. I think you said
yours was less than 20 feet up. For a flat top dipole, I got a resonant
impedance of 23 Ohms. When I dropped the ends 15 degrees, the impedance
dropped to 17 Ohms. Sort of in line with what you are getting. Also, with
multiple dipoles the impedance will be less, because you can not completely
ignore the other radiators. Maximum gain was in the straight up direction.
Gain was about 6db down at 25 degrees elevation. On 15 meters, you have 4
lobes offset about 45 degrees from ends/broadside. Gain was about 3db down
at 30 degrees elevation.

Why don't you post your complete configuration. Height at feedpoint, and
length and end point height for the three radiators.

Tam/WB2TT
"Jack Twilley" wrote in message
...
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The weekend was beautiful, so I was able to do some outside work.

I measured the antenna's impedance over 40m, then shortened the 40m
legs by two feet each. I then measured the antenna's impedance again,
and came up with numbers more closely resembling what I'd expect:

Frequency Resistance Reactance Impedance
7.00 14 6 15.2
7.05 18 15 23.4
7.10 18 21 27.7
7.15 21 20 29.0
7.20 24 27 36.1
7.25 28 36 45.6
7.30 28 43 51.3

These measurements were done using a slightly different method. This
time, I set the noise bridge for R=50 X=0 with the power off, then
tuned the drive on my receiver for maximum noise before powering on
the noise bridge and finding the null. The noise bridge manual is not
as detailed as I would like, and it's not clear what the "proper"
method is, but this produces reasonable results.

The impedance is well within a 2:1 match throughout the voice segment,
and a 3:1 match across the entire band.

In other news, I happened to tune across 20m for the last five minutes
of the Virginia QSO Party, and I made two contacts (NC4S and N4NW).
I feel better about the performance of my antenna on that band, but I
really hope that it's because the tuning somehow helped, and not that
contesters are the only ones willing to dig into the noise.

So it seems like I'm on the right track. I am going to try
lengthening the antenna by six inches and doing another round of
testing before moving on to the second set of legs, unless someone
- From the newsgroup pipes up with a correction to my methodology.

Jack.
- --
Jack Twilley
jmt at twilley dot org
http colon slash slash www dot twilley dot org slash tilde jmt slash
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"Tam" == t-tammaru Tam writes:

Tam Jack, For what its worth, I ran EZNEC on a 40m dipole at 17
Tam feet. I think you said yours was less than 20 feet up. For a flat
Tam top dipole, I got a resonant impedance of 23 Ohms. When I dropped
Tam the ends 15 degrees, the impedance dropped to 17 Ohms. Sort of in
Tam line with what you are getting. Also, with multiple dipoles the
Tam impedance will be less, because you can not completely ignore the
Tam other radiators. Maximum gain was in the straight up direction.
Tam Gain was about 6db down at 25 degrees elevation. On 15 meters,
Tam you have 4 lobes offset about 45 degrees from
Tam ends/broadside. Gain was about 3db down at 30 degrees elevation.

That doesn't look so bad at all.

Tam Why don't you post your complete configuration. Height at
Tam feedpoint, and length and end point height for the three
Tam radiators.

Over the next couple of days, I'll take some careful measurements of
the antenna, its supports, and the distance of everything from the
back of the house. The house was built in the 1940s and was
constructed with wood and concrete with a stucco finish.

Jack.
- --
Jack Twilley
jmt at twilley dot org
http colon slash slash www dot twilley dot org slash tilde jmt slash
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"Jack" == Jack Twilley writes:

Jack Over the next couple of days, I'll take some careful
Jack measurements of the antenna, its supports, and the distance of
Jack everything from the back of the house. The house was built in
Jack the 1940s and was constructed with wood and concrete with a
Jack stucco finish.

http://www.twilley.org/~jmt/antenna/dipole.html

The only thing I'm missing because I didn't think to measure it was
how long the individual dipole legs are. However, they started off as
the normal lengths for a full-length 40m dipole and inverted vees for
20m and 10m, as mentioned in the page. Tomorrow I'll go out and
measure the wires and update the text and the relevant drawing with
that information.

Tam, if you do model this antenna, I'd really appreciate it if you
shared the model with me as I've always wanted to learn more about
nec4 and friends but never really had the ability to get past the
punch-card mindset of the input files, and having my own antenna
modeled would probably do it.

Jack.
- --
Jack Twilley
jmt at twilley dot org
http colon slash slash www dot twilley dot org slash tilde jmt slash
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