Cecil Moore kirjutas:
andy wrote:
To get 80, i feel that i need to coil the wire around the fishing
pole.

You can do that to get 80m, but you will degrade the performance
on the other bands.


If I would get some low angle radiation on 80 then would'nt mind some
performance degradation. But NEC shows that best gain would be at 90
degrees...
Probably could get 80 another way. Ok, but I still need to find
"magical" antenna wire + feedline length combination. For that,
probably 10 to 20 metres around that fishing pole. Could I model the
vertical section as a straight (20m for example) and get same results
as if the wire was coiled. Dont want to model helixes!

Andrus

andy wrote:
Probably could get 80 another way. Ok, but I still need to find
"magical" antenna wire + feedline length combination. For that,
probably 10 to 20 metres around that fishing pole. Could I model the
vertical section as a straight (20m for example) and get same results
as if the wire was coiled.

No. You might try modeling it with a number of lumped inductive
loads distributed over the length of the antenna.

Dont want to model helixes!

Helixes make poor multi-band antennas because of self-resonance
effects. And remember, when a vertical is made longer than 5/8WL,
the take-off-angle increases possibly causing a lot of your radiated
energy to be lost in space.
--
73, Cecil http://www.qsl.net/w5dxp

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andy wrote:

If I would get some low angle radiation on 80 then would'nt mind some
performance degradation. But NEC shows that best gain would be at 90
degrees...
Probably could get 80 another way. Ok, but I still need to find
"magical" antenna wire + feedline length combination. For that,
probably 10 to 20 metres around that fishing pole. Could I model the
vertical section as a straight (20m for example) and get same results
as if the wire was coiled. Dont want to model helixes!

First, you can't just wind X meters of wire in a coil and expect it to
behave in any way like X meters of wire stretched out. It doesn't work
that way, because the field from each turn couples to all the other
turns, and the coupling is much different than for a straight wire.
Resonance will depend on the diameter of the winding, the number of
turns, the turn spacing, and the length of the winding (actually, only
two of the last three, since any two determine the third). If the turns
are uniformly spaced, a model of a straight wire of the diameter of the
coil will give you about the same pattern shape (just slightly fatter
than a dipole, ho hum), but it won't tell you anything about what the
impedance of the helix will be. You will get some useful information
from the straight-wire model, though. Make a note of the resistance the
model shows. Then when you build your antenna, measure the resistance
(which is the impedance at resonance). Divide the modeled resistance by
the measured resistance and you'll have the efficiency. It might not
make you happy.

I'm afraid you're going to have to find resonance ~shudder~ experimentally.

One additional comment. Don't worry about where NEC shows the best gain
to be, unless you're using the antenna only for receiving. If using it
for transmitting, pay attention only to what the gain is at the
elevation angles of interest for your communication goals.

Roy Lewallen, W7EL

Roy, W7EL wrote:
"First, you can`t just wind X meters of wire on a coil and expect it to
behave in any way like X meters of wire stretched out."

Of course, Roy is right. Yet, the more wire wound coonsistently on a
form, the more inductance and the lower its self resonant frequency.

My 19th edition of the "ARRL Antenna Book" offers some guidance in
winding radial-mode helical antennas based on experimental results. It
appears on page 6-26. It says that a 1/2-wavelength of wire wound with
equal spacing between turns comes close to making a resonant 1/4-wave
whip.

J.L. Davis, W5LIT (now SK) in 1949 made such an antenna to use on his
new Ford, with a surplus PE-103 powering his mobile rig. The antenna was
wound on a bamboo fishing pole. It had such a high-Q that when he
modulated,a plasma broke out at the tip of the antenna. J.L. was an
operator at KXYZ and KPRC which shared a transmitting tower. He made
lots of contacts using the fishing pole antenna.

Best regards, Richard Harrison, KB5WZI