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Tune Around! Topics For
Technicians! |
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 3/4 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 supports for
vertical polarizations. 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).
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 (See note 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 (See note 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 1/2 wavelength on 80m (See note 4 and 5).
1. http://www.hamuniverse.com/balun.html (Ugly Choke Balun)
2. http://www.hamuniverse.com/kl7jr80loop.html (80 Meter Loop) 3. http://www.hamuniverse.com/kl7jrindoorloop4010 (40 thru 10 meter loop) 4. http://www.hamuniverse.com/kl7jreasyvertloop.html (Easy Vertical Loop) 5. http://www.hamuniverse.com/kl7jrslopingdelta806 (Sloping Delta loop)
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