helicity on vertical dipoles
 

Hi,

I'm considering a vertical dipole array, one each in 3 corners of a
triangle around the outside of my apartment wall. I'm on the lower
floor, of course. Each vertical dipole will be disguised as a
stereotypical wooden birdhouse that you find at the craft store,
supported by wooden poles with wire cores.

The reason I am considering the vertical dipole is so that I do not
have to bother with a radial system, I would rather hide the grounding
system inside the pole support.

Only disadvantage of the scheme is that my roof is only 25 feet high,
and that the bottom of a vertical dipole should be 10 feet off the
ground. Under normal circumstances a 40 meter vertical dipole at
one quarter wavelength on the side wold be impossible. 30 would be
extremely difficult, 20 just might be doable.

What I would like to try to do is shorten the elements of each dipole
using helical winding rather than straight length wire. That way I
can have the active elements closer to the roof or even a little
above.

Question: to what extent can I shorten the winding while still
retaining good signal apeture? about 50 %? More? And what would I
need to do (if anything) to compensate for this?

Thanks in advance,

The Eternal Squire

The Eternal Squire wrote:
Question: to what extent can I shorten the winding while still
retaining good signal apeture? about 50 %? More? And what would I
need to do (if anything) to compensate for this?

75m mobile antennas are shortened by as much as 88%. Of course,
they are not very efficient. But a 50% shortening should result
in an acceptable amount of loss. However, hi-Q loading coils are
more efficient than helical windings. Tell everyone that the
loading coils are varmint guards (snakes, squirrels) for your
"birdhouses". A top hat could be disguised as bird perches and
would raise the efficiency of your antenna elements.

On another note, peak performance from a vertical array requires
phasing/matching at the feedpoints. Roy, W7EL, has a computer
program that will calculate the lengths of the phasing sections
for you - available at www.eznec.com.
--
73, Cecil http://www.qsl.net/w5dxp



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The Eternal Squire wrote:
Question: to what extent can I shorten the winding while still
retaining good signal apeture? about 50 %? More? And what would I
need to do (if anything) to compensate for this?

75m mobile antennas are shortened by as much as 88%. Of course,
they are not very efficient. But a 50% shortening should result
in an acceptable amount of loss. However, hi-Q loading coils are
more efficient than helical windings. Tell everyone that the
loading coils are varmint guards (snakes, squirrels) for your
"birdhouses". A top hat could be disguised as bird perches and
would raise the efficiency of your antenna elements.

On another note, peak performance from a vertical array requires
phasing/matching at the feedpoints. Roy, W7EL, has a computer
program that will calculate the lengths of the phasing sections
for you - available at www.eznec.com.
--
73, Cecil http://www.qsl.net/w5dxp



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-----== Over 100,000 Newsgroups - 19 Different Servers! =-----

The Eternal Squire wrote:
"I`m considering a vertical dipole array, one each in 3 corners of a
triangle---."

ON4UN has a section on "Triangular Arrays" beginning on page 11-48 of
"Low-Band DXing". Check the rest of the book. Use vertical polarization
if you have good soil conductivity or adequate radials. Otherwise,
horizontal polarization may work better. Avoid inductive loading of
short verticals. Instead use capacitive hats. The book is a winner.

Best regards, Richard JHarrison, KB5WZI

The Eternal Squire wrote:
"I`m considering a vertical dipole array, one each in 3 corners of a
triangle---."

ON4UN has a section on "Triangular Arrays" beginning on page 11-48 of
"Low-Band DXing". Check the rest of the book. Use vertical polarization
if you have good soil conductivity or adequate radials. Otherwise,
horizontal polarization may work better. Avoid inductive loading of
short verticals. Instead use capacitive hats. The book is a winner.

Best regards, Richard JHarrison, KB5WZI

"The Eternal Squire" wrote
I'm considering a vertical dipole array, one each in 3 corners of a
triangle around the outside of my apartment wall. I'm on the lower
floor, of course. Each vertical dipole will be disguised as a
stereotypical wooden birdhouse that you find at the craft store,
supported by wooden poles with wire cores.

I see some surprised birds in the picture.....;-)

Jack

"The Eternal Squire" wrote
I'm considering a vertical dipole array, one each in 3 corners of a
triangle around the outside of my apartment wall. I'm on the lower
floor, of course. Each vertical dipole will be disguised as a
stereotypical wooden birdhouse that you find at the craft store,
supported by wooden poles with wire cores.

I see some surprised birds in the picture.....;-)

Jack