Any shape loop will work – octagon, pentagon, etc. The larger the area or aperture inside the loop the better, A circle has the largest area but is impractical.A circle has 1dbd gain over a square. Most people use a square but if you only have 3 supports you can shape it like a triangle. Triangle loops are called Delta loops.A square loop has 1dbd gain over an equal lateral triangle loop. If you use a triangle shape try to make each leg an equal length as this gives the largest inside aperture or area.
Don’t use a Balun on this Antenna! On a horizontally oriented loop you can feed a corner, center of a side or anywhere it is unimportant.
If you know you will be using a Loop, Dipole, Zepp, etc. on Multiple Bands and you want the most efficient performance of the antenna system you will always get less feedline loss if you use Open wire 450 – 600 Ohm window/ladder line.
If you feed the Full wave Loop antenna direct with a single piece of coax you can only adjust add/prune the antenna till the VSWR gets down to about 1.7:1 at resonance , so you will probably want to use a tuner if you want to cover an entire HF band . And Yes the tuner in the radio will work fine as long as you are not using a linear amplifier. The impedance of a Full Wave Loop antenna is theoretically in the vicinity of 100 ohms. That is why I use a 75 ohm “Q section” to provide a perfect match to the 50 ohm coax. My Loop antenna is flat 1:1 VSWR around 3.875 Mhz and rises to about 2:1 SWR at 3.75 and 4 Mhz . So I cover the entire 75 meter phone band without using a tuner.
Here is How I Do It.
IMPORTANT – Connect the shield of a “Short Piece” of 75 ohm coax to one end of the loop antenna , Connect the center of the 75 ohm coax to the other end of the loop antenna. The length of the 75 ohm coax is most important. It will be used as a Q section.
To determine the length to use, Use this formula “Length in feet = 246 x VF / Freq in Mhz” VF = the Velocity Factor of the coax. So multiply 246 by the Velocity Factor of the coax , Then divide by the frequency .
If you use RG59 or RG11 with a Velocity Factor of 0.66 , For the 75 meter band the Piece of 75 ohm coax should be about 42 feet long , It doesn’t need to be exact . RG59 and RG11 coax usually have a Velocity Factor of 0.66 , Foam dielectric coax such as RG6 might have a Velocity Factor of 0.81 or more.
Put a PL259 coax connector on the unused end of the 75 ohm coax, Screw a PL258 double female coax connector onto this PL259 coax connector. Then connect 50 ohm coax to the other side of the PL258 double female coax connector, Use any length 50 ohm coax needed to reach your Transceiver.
This Antenna system can be fed as a Top Loaded Vertical fed against ground for use on a lower frequency band than the loop is resonant on.
This antenna system can be operated as a vertical antenna with top-hat loading on all band as well. This is accomplished by simply keeping the feed-line run from the antenna to the shack as vertical as possible and clear of objects.
Both deed-line coneductors are then tied together and the antenna is fed against good ground.
This method allow excellent performance of the 40 meter loop on 160 meters.
When constructing the loop connect and solder the coaxial feed-line ends directly to the loop wire ends.
Don’t do anything else. Baluns or choke coils at the feed point are not to be used. They are unnecessary.
The feed arrangement for operating the loop as vertical antenna is show in the picture behind.
Dr.Ace – WH2T is very fond of the Full Wave Horizontal Loop Antennas, Another Great HF Antenna is the Double Extended Zepp. You may also want to consider that type antenna if you have the real estate. 73 de WH2T, Ace
Article originally available at http://www.bloomington.in.us/~wh2t/ant.htm