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Loop Antenna Notes...updated Dec. 2012

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 square vertical loop on my condo 3rd floor patio in the Dominican Republic and am quite pleased with the performance (over 140 countries worked in 18 months!) on 12-40m, although my tuned 11m vertical for 10m was 2 "S units" stronger on 10m than the loop (I had the height advantage on my vertical as well as the loop of 40 ft and the ocean was only 120 feet away which both helped me!).

I do recommend not going any shorter than 3/4 size as my results on 1/2 size loops were mostly poor (that's 3/4 size of your lowest desired band of operation) on most of my outings.

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. 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.

Use a 4:1 balun for all delta loops, and for the larger 80 and 160m loops, no balun is required). I do however recommend an air choke balun on all loops (See note 1 below) to eliminate stray RF in the shack and hopefully keep your XYL 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 1/4 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).

Say you don't have the room for a full wave antenna then shorten it to 75 percent size simply by taking the 140 feet figure above times .75 which equals 105 feet. As I mentioned before, I never go shorter than 75 percent on the lowest band of operation because of previous poor results. I found my reduced size loops not only easier to install but worked DX like crazy!

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 for omni-directional multi-band capability and a little gain to boot! Not bad from just some wire!

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 DX performance is better on a delta with vertical polarization. To date my favorite portable vertical loops are full or 3/4 WL (See note 4 and 5. I must add that I was impressed with the stations I worked on 80m where the loop was 1/2WL).
Have fun experimenting with loop antennas!

 

Suggested loop configerations:

1 WL Phone Band

 

 

Configuration and Feet/Side

 

10 Meters

36'

9'

Square

6'x12'

Rectangle

12'

Triangle

12 Meters

41'

10'

Square

7'x14'

Rectangle

14'

Triangle

15 Meters

47'

12'

Square

9'x15'

Rectangle

16'

Triangle

17 Meters

56'

14'

Square

10'x18'

Rectangle

19'

Triangle

20 Meters

71'

18'

Square

12'x24'

Rectangle

24'

Triangle

30 Meters

105'

26'

Square

18'x35'

Rectangle

35'

Triangle

40 Meters

140'

35'

Square

25'x45'

Rectangle

47'

Triangle

80 Meters

258'

65'

Square

46'x83'

Rectangle

86'

Triangle

160 Meters

544'

136'

Square

95'x177'

Rectangle

181'

Triangle

NOTES:

 

 

 

 

 

 

 

A. I read somewhere that for 160M use, do not use fractional WL (ie- only use 1WL, 2WL etc.)

B. For rectangular loops I used 30 percent rule (don't reduce short sides by more than 30 percent). I like to keep it at 20 percent maximum for best results.

 


1. Ugly Choke Balun

2. 80 Meter Loop

3. 40 thru 10 Meter Loop

4. Easy Vertical Loop

5. Sloping Delta Loop

 



  


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