Single band loop antennas vs. Multi-band loop antennas?
I am rebuilding a 40-meter loop antenna - the antenna is in the shape of a triangle with two points at the top of 37' telescoping masts and the third point of the triangle in a tree in the front yard. The wires slope down to the tree and stop about 8 feet above ground. The antenna has a balun near the top of one of the masts, a section of 75 ohm coax is in place between the balun and the 50 ohm coax to the station.
I would also like to put up a 20 meter loop, either triangle, circle or rectangle shape. How far away from the 40 meter loop should the 20 meter loop be so that the two antennas don't cause interference with each other? Can I put the 20 meter loop inside the 40 meter loop? What happens if I put one leg of the 20 meter loop along the same point as the 40 meter loop going across the top from mast to mast? I would think some kind of coupling.
I know that you can build fan dipoles for multiple bands. Is there a way to build multiple loops from one feedpoint?? (I know that quads are actually several loops, but I'm wanting to build a simpler one wire loop antenna. Any suggestions will be appreciate. Thanks.
Martin G. Blaise, AG5T
I once had a commercial cubical quad that had three loops for 20,15,and 10 meters. The three loops were on the same plane, fed at the same point with a coax cable into a balun. There was a bit of interaction to get the loops tuned, but there certainly was no degradation in gain or F/B performance that I could tell. I could easily null out almost any station that I tuned in by turning the antenna away from the incoming signal, and the antenna was always at least a couple of S-Units stronger than some dipoles that I had mounted a short distance away from the quad.
You should have very good results with your loops, although if you use a balanced tuner and open wire or ladder line, you would be able to probably do as well with a single loop for the lowest frequency you intended to use and that tuner.
Good luck, and keep us informed. 73, Jim
I've never seen it done, but it might be interesting to see if a pair of traps could be used to keep 20M out of the 40M loop. The inductance should allow you to shrink the size of the 40M loop.
There's really no need for a separate 20 meter loop, unless you intend to take advantage of it's directivity some how. On 20, your 40 meter loop will begin to exhibit pronounced lobes, which may not be in directions you desire.
I'm not sure about nesting the 20 meter loop inside the 40 meter loop. It is often done to feed multiband quad antennas, so I suspect it will work, but I am not sure what kind of interactions you will see.
Your 40 meter loop, fed with coax, should exhibit a reasonable SWR at all harmonics of the 40 meter frequency. Line losses on the coax should be minimal.
Most likely, the impedance of your loop will be closer to 50 ohms than 100, due to the proximity of the one point to ground.
The loop should exhibit some directivity, at right angles to the loop, and should favor the direction of the slope to some extent.
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It sounds like you're describing a horizontal loop to me. If that's the case, I'd suggest just building the one, feeding it with parallel line to a balanced tuner and use it on anything above the frequency you cut it for.
What you're saying will work, sorta, but having the 75 ohm matching section is going to foul it up on other bands so you'll need a tuner. The better way would be to just use the existing loop, feed it with twin lead or ladder line, use a good balanced tuner and use it on all bands. Unlike a dipole, a loop works on odd AND even multiples of the fundamental frequency.
The way you have it set up is fine for a single band loop without needing a tuner but if you feed it with twin lead and use the balanced tuner (or tuner with a balun) you can use it on all bands without needing to put up a separate loop. Give it a shot, a 40m loop was the best wire antenna I ever had!
You can reduce the size of the loop to a 30 meter loop, then use a trap opposite the feed point, try 17 uH / 12 - 16 pF. Use vertical polarisation and you may angle the loop slightly (no more than 30 degrees). You get a very similar lobe for both 40 and 20 with very low angle radiation in both. Initially I shaped it as a 5 m high x 10 meter wide but I have changed this to a delta with each side 10 m long. The trap is 2.5 from one bottom corner and the feed is 2.5 from the other bottom corner.
Originally Posted by w1vt
The big advantage is that by using the trap you will keep a certain amount of local radiation enabling ragchewing.
Enjoy, 73, ZL2ML
I should know this, but I'm having a brain freeze.
i know the loop will ork well on odd and even harmonics, but what about in between. A loop in the attic and fed with balanced line will have a lot of interaction, so I was thinking of just sending my brother-in-law up there with a random hunk of copper and saying "run it whereever it's clear."
Any thoughts are appreciated.
Chris, I am amazed that you would be stuck on an antenna question! I have an answer, but I'm not sure if I am correct or not, so if I am wrong, I hope someone can correct me.
I believe that a full wave loop works on all the harmonics for this reason. Think of a sine-wave. Where do the voltage nodes happen? Where does the voltage cross over the Zero potential points? Wouldn't it be where the antenna is 1, 2, 3, etc. wavelengths long !
At other frequencies, the loop would not be of the proper dimension to allow for a voltage node, and cancellation would result from the instantaneous values trying to be at the same point at the same time.
Isn't that the way it would be ? I know you are a very scholarly person. Tell me if I am right or wrong !
use the same one for all the higher bands
you can use the same loop for all the higher bands you don't need extra loops
Originally Posted by ag5t