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1-4 of 4 messages
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Log Periodic for Receiving
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by KC8GPD on July 3, 2008
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I'm looking for designs for a log periodic. I'm looking to cover the common scanner bands. 29.7-54, 144-174, 440-470.
VHF lo would be better optimized for simplex comms, vhf-hi,uhf is all repeaters.
i would think someone somewhere has built one of these and published designs.
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RE: Log Periodic for Receiving
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by N8EKT on July 3, 2008
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What you want is available most everywhere.
It's called a VHF/UHF tv antenna.
They cover from 54-950mhz.
Other than changing the mounting bracket from horizontal to vertical, little needs to change.
I get a chuckle from these companies selling a $29.95 tv antenna for $219.95!!!!!
Are there actually people buying these things?
If so, I have some swamp land I've been trying to unload for years!!
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RE: Log Periodic for Receiving
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by WB2WIK on July 3, 2008
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I'd do that, also. A LP VHF-UHF TV antenna should work fine. They're lightweight and cost very little.
If you want to add lower frequency log cells to it for some reason (probably not needed), just extend the booms and add them.
Most "public service" stuff is repeater-oriented and vertically polarized. You can mount a TV VHF-UHF LP vertically polarized just fine if you take the standard precautions about vertically polarized beams.
WB2WIK/6
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RE: Log Periodic for Receiving
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by WB6BYU on July 4, 2008
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One problem with a TV antenna, though, is that it doesn't necessarily
provide coverage throughout the range of interest. The longest
elements are designed for 55 MHz (though they may still work as a
vertical dipole for lower frequency signals.) The basic log cell generally
is designed for 54 - 90 MHz and the third harmonic from 170 to 215
MHz (roughly). It doesn't necessarily work well outside those ranges
(though simply having that much aluminum up in the air certainly will
pick up some signals.)
UHF add-ons generally are designed to start around 512 MHz and go up
from there. They are often of the "corner reflector + yagi" type, where
some yagi elements provide gain on the lower portion of the UHF band
and a corner reflector at the high end.
While there are designs that can cover most of the range (W4RNL has some
for 100 - 1000 MHz, and others that cover 3.5 - 30 MHz which is an
even wider percentage bandwidth), for your specific application I'd
suggest an omni antenna of some sort for low band and a combination
log covering 120 - 170 and 360 - 500 MHz using the same approach
as the TV antennas do. (Harmonic operation can be achieved using
either forward-swept V elements or short elements in parallel with the
longer ones.) Of course, this presumes you are using a rotator to aim
the antenna in the desired direction for each station you are listening
to.
An alternative method of building log periodics is the "Zig-Zag" which
can be built fairly easily with wire. I built one for 250 - 500 MHz and
it went together pretty easily, though making it twice as long to reach
down to 120 MHz would require stiffer wire. W4RNL had several
articles on building log periodic antennas that would be worth reading.
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