Full wave closed loop antenna

Below is a diagram of a closed loop antenna.I'm currently working with a full wave loop for 40m fed with a quarter wave of RG59 then a short run of RG58 into shack. Works very well on 40m .I have tuned other bands through an atu (although im not too sure of the efficiency) but have made contacts into USA on 20m and 15m. 17m is a struggle but considering this is really fed for single band use i can't expect too much. A full wave closed loop will show a reasonable feedpoint impedence on its fundimental and all harmonic frequencies so can be fed via a 4.1 balun and coax needing no atu for 40,20 and 15m.

Feeding the antenna for mono band use

There are a couple of ways you can feed this loop depending on the intended use. The formula for working out the size of the loop is 1005/frequency.The feedpoint impedence of a closed loop is in the region of 100-120 ohms at resonance so this will need some matching for 50 ohm coax.For single band operation a simple quarter wave matching section of the correct Z (in this case 75 ohm) is all thats required.The antenna is a balanced antenna so its the readers choice whether to fit a balun.

3 band no atu required

As mentioned at the top of the page this antenna can be fed for tri band use needing no atu. With a 4.1 balun at the feedpoint and a little pruning you should be able to get this antenna with a vswr of less than 1.5 on the three bands (40,20 and 15) making feeding with coax possible with minimal losses due to missmatch.You will also see a low vswr on 2m but then line losses will come into play at these frequencies.

Multiband

For multiband use some people simply feed with 450 ohm ladderline straight to the shack and use a balanced tuner. Running ladderline into the shack isn't always possible for people and may not own a true balanced atu. Another way which is probably more common place is ladderline from the antenna to just outside the shack,a 4.1 balun then a short run of coax to inside the shack.

Quarter wave matching section

To work out the impedence of the matching section of feedline needed is.. The square root of the first impedence X by the second impedence. (if using coax as the feeder this would be 50 ohm) Eample. A loop with a feedpoint impedence of 120 ohms, This needs matching to 50 ohm cable. So 120 X 50 = 6000. THE SQUARE ROOT OF THIS IS 77.4*****, So 75 ohm coax would be used. The formula for a full electrical wavelength is 983.6 / freq, X (times) VF(velocity factor) of the coax.(in my case 0.66), So 983.6 / 7.100 X 0.66 = 91.43****. This is a FULL wavelength and now must be divided by four to give the quarter wave length. 91.43/4 =22.8 feet.