On Wed, 26 Oct 2005 12:07:11 -0500, Chris W wrote:
I am reading the 19th edition of the ARRL Antenna Book. I am unclear
which polarization to use if I want 2 Helical antennas to talk to each
other. In the book it says, ". . . when two stations use helical
antennas over a non reflective path, both must use antennas with the
same polarization sense.." I find this could be interpreted in
different ways and is not clear at all. It seems logical to me that if
both antennas had the same polarization that when they faced each other
they two helices would be in opposite directions and that would seem to
be bad. Am I right?
Wind the two in the same "screw sense" (i.e., both clockwise out to
the far end, or both CCW out to the far end) and they will be same
sense.
It says the circumference can be between 1.33 wavelengths and .75
wavelengths. From the gain formula it would seem that the 1.33 figure
would give the most gain for a given length of the antenna. Other than
mechanical considerations is there any reason I wouldn't want to use the
1.33 number?
The higher gains give higher sidelobes too. For earth-earth paths
that probably isn't a major issue. For satellite work it can be
significant.
Finally, I may have missed it, but I didn't see anything that indicated
how large of a wire or tube I should use for the helix. I am guessing
this is just a mater of how much power I plan on feeding it with?
Not really, any more so than with building a dipole. A popular
material for 70cm is 1/4" copper tubing, which you can easily get in
50' lengths at the hardware store. Look for refrigerator tubing in
the plumbing supply section.
PS I would like to build 2 of these one is for 2.4ghz wireless network
and one is 432 mhz amateur band EME and or satellite work. So the
2.4ghz version will likely never see more than 100 mW but could see as
much as 1 watt. The 432 mhz version could see as much as 1000 watts,
but if I did that it would be to an array of 4 of them so I guess each
one would only see 250 watts?
True.
But for EME/Satellite work, helices are not ideal for reception.
Transmit is fine, but the sidelobe levels cause weak signal noise
problems. If the satellite has plenty of downlink power it may not be
a problem, but EME would definitely be an issue. Not to mention that
you really need switchable polarization for EME, and to do that
effectively with a helix requires two of them!
--
Chris W
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