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Author Topic: 20 mertre loop  (Read 1291 times)
EI5GUB
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Posts: 54




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« on: April 26, 2012, 02:16:12 PM »

hi guys I put up my 20 metre loop last weekend at 70 feet and it is working very well and is very quiet compared to dipoles etc,I was thinking what if I put another 1 next to it and fed them in phase,can anyone please model something like this,cheers Richard
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N0ZLD
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« Reply #1 on: June 07, 2012, 09:07:11 PM »

That would be a quad..
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K4SAV
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Posts: 1840




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« Reply #2 on: June 08, 2012, 06:28:09 PM »

If you feed them in phase, you probably won't notice any difference for most spacings.  When fed in phase you won't get any more gain until the spacing get to about 3/4 wavelength (1 wavelength is better).   You can do better feeding them 180 degrees out of phase.  Then you don't need a lot of spacing.

A loop has no properties that allows it to reject noise compared to a dipole.

Jerry, K4SAV
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WB6BYU
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« Reply #3 on: June 08, 2012, 07:48:31 PM »

It depends, of course, where "next to it" is.

If there is one in front (or in back) of the other, then a driven element and reflector
makes a quad.

If one is beside the other and in the same plane (for example, of the two loops are hung
from a rope between two trees, then you get some broadside gain that depends on the
spacing.

With the centers of the loops 10m apart, you get about 1.5dB more gain, and the half-
power beamwidth narrows from 91 degrees to 56 degrees.

With centers 15m apart the gain improvement over a single loop is 2.9dB and the half-
power beamwidth is 40 degrees.

At 20m spacing the gain improvement is 3.3dB and the beamwidth is 31 degees.  At this
point the side lobes start getting stronger, so any wider spacing probably won't help
much unless you can make use of the side lobes.
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WA4FNG
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Posts: 162




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« Reply #4 on: June 09, 2012, 01:27:48 PM »

IMHO loops appear quieter because noise and signals are a little lower than a dipole. The directivity of the loop combined with a lower angle reduces the "apparent" noise. I've compared many delta loops to dipoles at my QTH and they do appear quieter, but they're not really. My latest comparison is a 17m dipole next to a 17m delta loop (vertically oriented) at about 40' high. For DX that is in the loop pattern it is better most of the time. 70' is fairly high for a delta loop. At some point they start to develop vertical lobes and lose directivity -- well, that's according to the modeling tools. I've never used one at that height. Hey, if it works - use it I say ;-)
73, Milt
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K4SAV
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Posts: 1840




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« Reply #5 on: June 09, 2012, 05:52:04 PM »

IMHO loops appear quieter because noise and signals are a little lower than a dipole.

The S meter reading of the noise on an antenna is pretty much meaningless unless you also know the S meter reading of the signals.  Signal to noise ratio is the only meaningful parameter when talking about noise.  Antennas at differing heights have different patterns.  When you have different patterns the signal to noise usually changes.  That goes for dipoles as well as loops.  Noise to an antenna is the same as a signal.  It has no way to discriminate between the two.  The only affect will be due to differing patterns.

So if you are trying to compare signal to noise on a dipole versus a loop (assuming the loop is oriented vertically and horizontally polarized),  the height of both antennas need to be such that they both have the same gain, and both need to be oriented in the same direction, and far enough away so that they don't interact.  Then their patterns will be almost identical, which of course will produce the same signal to noise ratio (or so close you probably can't measure the difference).  A locally generated noise source close to one of the antennas can skew the results.

The top wire of a loop needs to be higher than that of a dipole to have the same gain.  If you make the top wire the same height for both, the loop will probably have lower S meter reading on noise due to lower gain, which is probably why most people say the loop is lower noise.  Signals are also lower under that condition.

Jerry, K4SAV

edit:
There is one special case where a loop is quieter, in the presence of precipitation static.
« Last Edit: June 09, 2012, 05:59:52 PM by K4SAV » Logged
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