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Author Topic: Vertical Loop Feedpoint  (Read 2074 times)
VK5DO
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Posts: 95




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« on: July 02, 2010, 12:27:05 AM »

I've installed a full wavelength 40m loop in a vertical configuration between two tall supports I have.  At the moment I'm feeding it with open wire to my SG-237 tuner.

The feedpoint at present is half way up one of the vertical sides.  This should be giving me horizontal polarization.

I'd be interested to hear peoples thoughts on the respective benefits of a vertical loop polarized as I have (horizontally) or if I was to feed it midway across the lower horizontal wire thereby getting vertical polarization.  Also I've read about feeding them from a corner.  Is that just more just for convenience?

Thanks in advance.

Dene
VK5DO


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K4SAV
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Posts: 1850




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« Reply #1 on: July 02, 2010, 06:37:47 AM »

You have that backwards.  Feeding it in the center of a vertical side gives vertical polarization, and feeding it in the center of the bottom wire, or top wire, gives horizontal polarization.  Vertical polarization will be very omni-directional (within maybe about 3 dB).  Using vertical polarization the gain will be low but with a very low take-off angle, and a null overhead.  Horizontal polarization will give a much higher peak gain but at a higher take-off angle, and no null overhead unless the antenna is about 80 to 90 ft high. It will also have nulls in the plane of the loop in the azimuth directions.   The best choice, in my opinion, depends on how high the antenna is and whether you want maximum gain in a particular direction or best coverage.

Jerry, K4SAV
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K4SAV
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« Reply #2 on: July 02, 2010, 07:24:42 AM »

I forgot to comment on feeding it at a corner (or close to a corner). 

I assume you are using this antenna on multiple bands (since you are feeding it with twin lead).  There is a problem with that on the higher bands.  Feeding the antenna either on a side or at the bottom, on some bands above 18 MHz most of the signal will be directed up at a very high angle.  Feeding it at a corner will allow some low angle radiation on some bands (although it won't be very good on 12 meters, and really lousy on 10 meters). 

Feeding it on a side about 25% above the bottom will give pretty good performance on all bands 30 thru 10 meters.  That would be my choice for multi-band use.  That assumes the antenna is square and not rectangular.

Jerry, K4SAV
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W8JI
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« Reply #3 on: July 03, 2010, 05:08:59 AM »

Things are supposed to be upside down in Austrailia, not turned 90 degrees!

K4SAV is right. You now how vertical polarization. Feeding the middle of the bottom would be horizontal.
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VE3WMB
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Posts: 289




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« Reply #4 on: July 05, 2010, 05:29:53 PM »

A couple of considerations .. the height of the base wire above ground and the soil conductivity.

If your location is rocky or sandy (i.e relatively poor conductivity) you may be better off to use horizontal polarization as vertical polarization may result in additional losses from
the poorly conductive soil.  If the base of the loop is up quite high then you should still have a
fairly low radiation angle with horizontal polarization and the losses with be a lot less.
A horizontally polarized loop should also be much quieter on RX as most noise is vertically polarized.

In his book, ON4UN strongly suggests staying away from vertically polarized antennas in locations with poor soil conductivity.

As someone else stated .. feeding the loop on a vertical side will yield vertical polarization not horizontal.

Cheers

Michael VE3WMB
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VK5DO
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« Reply #5 on: July 05, 2010, 11:04:28 PM »

Ok.  Thanks for the answers.  Actually I read around a bit more and figured out my initial mistake only about ten minutes after posting the above message but it was good of you guys to confirm it. 

Some more info and your thoughts if you please.....  The loop is rectangular.  It's now fed dead centre of the bottom wire.  The top wire is about 9metres above the ground and the bottom wire about 5.  Not ideal but I'll give it a try for a while. 

I'll be interested to see if it performs as good as the exact same wire loop but in a horizontal configuration (cloud warmer) which worked verk well with the whole antenna suspended flat at about 9 metres.

Thanks,

Dene
VK5DO
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K4SAV
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« Reply #6 on: July 06, 2010, 06:52:41 AM »

The loop is rectangular.  It's now fed dead centre of the bottom wire.  The top wire is about 9metres above the ground and the bottom wire about 5.

That will perform completely different from a square loop on the higher bands.
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