U.S. Islands Awards Program   

Loop Antenna Notes

by “Yukon John”, KL7JR

    The following ramble is from someone who doesn’t care about SWR or resonance when it comes down to full wave, multi band large loops.  I just like to get as much wire up as high as possible and work all bands I can with an ATU.  My log books show the results!


    Full wave loops are popular on the Amateur bands because they work well, are multi-banded, inexpensive to make and somewhat easy to install. Loop antennas have always fascinated me.   Simply put, loops work well even at modest heights.  Shortened loops also work well for those who don’t have the space for full wave loops.  I use a ¾ wavelength 20m rectangular vertical loop “indoors” from my apartment and am quite pleased with the performance on 15, 17 and 20 meters (10 and 12m were always dead when I checked)!

    Common loop shapes are; round, square, rectangular or triangular (aka delta loops).  The larger the area of the loop, the better antenna you will have. The number of antenna supports required depends on the shape of your antenna.  Round is impractical for most hams because it requires many supports, and obviously a square loop or rectangular horizontal loop requires 4 or 2 for vertical polarizations.  Ladder line is probably the best way to get the signal in to your shack according to many sources, but I’ve always used coax (RG-8 or RG-8X) and sometimes without a balun (some hams recommend using a high performance/high power 1:1 or 4:1 CT balun when using coax).  I do however recommend an air choke balun on all loops (1) to eliminate stray RF and hopefully keep your neighbors off your back!  Full wave, closed loop antennas are broadband, low Q devices and exhibit a theoretical gain of approximately 2 dB over a half wave dipole.  It’s hard to believe says VE2BMC, but going from a ¼ wavelength vertical to a simple rectangular loop can lead to 3-4 “S” units improvement.  That should be an attention getter!

    The standard full-wave loop formula is 1005 divided by frequency (MHz).  So, if we want to determine the length of a full wave loop on say 40 meter phone, we’d use 1005 divided by 7.180=140 feet (see chart below).  Perhaps the hardest decision you’ll have to make is do I make it a horizontal or a vertical loop? The radiating element or wire position in relation to the ground determines polarity whether it is vertical, horizontal or a combination of both.  If the wire is parallel to the ground, it radiates horizontally. If the wire is perpendicular to the ground, it radiates a vertical wave. If the wire is slanted, it radiates waves, which have both horizontal and vertical qualities.  Since I’ve used both over the years, and I also like the performance of vertical antennas, the vertical loop has the best of both worlds for me plus it is easier to install.  In conclusion, if I could only put up one antenna, it would be a 160 meter full wave vertical loop- preferable two to cover all directions!

    HORIZONTAL MULTI BAND LOOPS
The recommended height is 40 feet or more (higher is better!) but lower heights work well too.  No matter where a horizontal loop is fed, having corners or no corners, it will always yield horizontal polarity.  Feed horizontal loops at any convenient spot.  To date my favorite horizontal loop was an 80m full wavelength beauty (2)!

    VERTICAL MULTI BAND LOOPS
Same height is recommended as above. If you have poor soil conditions, it’s best not to use vertically polarized loops.  That’s an important factor to keep in mind, but if you have the height factor (3) it may not. A vertical loop may be horizontally or vertically polarized depending on where you feed it.  Feeding at a corner or midpoint produces vertical polarization. Vertical loops are good DX antennas.  Configure as a circle, square, rectangle or triangle (delta).  The larger the area of the loop the better it will work.  Feed square and rectangular vertical loops at a corner, and triangular loops place apex up and feed at one low side corner.  Many antenna elmers note that the DX performance is better on a delta with vertical polarization.  To date my favorite portable vertical loops are ½ wavelength on 80m (4 and 5).


              
Loop Length Chart
    1 Wave Length 3/4 WL 1/2 WL
10 Meter phone 35 ft  26 ft   17.5 ft
15 Meter Phone 47 ft 35 ft  23.5 ft
20 Meter Phone 71 ft 53 ft 35.5 ft
40 Meter Phone 140 ft 105 ft 70 ft
80 Meter Phone 260 ft 195 ft 130 ft
160 Meter Phone 544 ft  408 ft 272 ft
                        
LINKS

(1)  http://www.hamuniverse.com/balun.html

(2)  http://www.hamuniverse.com/kl7jr80loop.html

(3)    http://www.hamuniverse.com/kl7jrindoorloop4010

(4)    http://www.hamuniverse.com/kl7jreasyvertloop.html

(5)    http://www.hamuniverse.com/kl7jrslopingdelta806


1 April 2010

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