Single band loop antennas vs. Multi-band loop antennas?

Discussion in 'General Technical Questions and Answers' started by AG5T, Oct 28, 2008.

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  1. AG5T

    AG5T Ham Member QRZ Page

    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.
  2. AG3Y

    AG3Y Ham Member QRZ Page

    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
  3. W1VT

    W1VT Ham Member QRZ Page

    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.
  4. K0RGR

    K0RGR Premium Subscriber QRZ Page

    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.
  5. W6GS

    W6GS Ham Member QRZ Page

    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.
  6. WD5ABC

    WD5ABC Ham Member QRZ Page

    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!

    Kerry, WD5ABC
  7. WA2ZDY

    WA2ZDY Guest

    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.
  8. AG3Y

    AG3Y Ham Member QRZ Page

    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 !

    73, Jim
  9. KF6GC

    KF6GC Ham Member QRZ Page

    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
  10. K0RGR

    K0RGR Premium Subscriber QRZ Page

    If you feed it with coax, it will provide a usable SWR on 40,20, 15, and 10 meters. The internal tuner on your rig will handle the mismatches.

    If you feed it with open wire feeders and a proper tuner, it should work on all frequencies from somewhat below the design frequency, up. If you try to use your 40 meter loop on 30 meters with a coax feedline, the losses will be very substantial.

    Another, better option, would be to locate a remote automatic antenna tuner right at the feedpoint of the loop. Then, it should also work everywhere. A loop cut for 5 Mhz. should cover all bands from 80 --10, and probably 6 too.
  11. K2GSP

    K2GSP XML Subscriber QRZ Page

    Yea I have a 40 mtr loop fed with ladderline and it tunes up fine on all the higher bands, but it won't tune lower. It's a great antenna.
  12. K4HUD

    K4HUD Ham Member QRZ Page

    Long story on loop antennas: I put up a 1,050 foot horizontal loop that is 4 full wave lengths on 160 meters.
    I have an antenna tuner and started out with 450 ohm ladder line. Result was worked ok but on 40 meters there was rf all over the shack. Articles say there is no rf inside a loop antenna.
    Talked on the air to many people and read articles. The poop was that full wave loops have a feed point impedance of around 120 ohms but no one seemed to know if this was true on 2 or more full wave loops.
    I was told to get a 2:1 balun and it would work fine. It did for a while until I burned the 2000 watt balun out running 600 watts out.
    Then I was told, no must be a 4:1 balun so I tried that and burned that one out with 500 watts on rtty.
    Next read an article saying a short run of coax would work fine and I have been using that for a while. Recently my amp blew the plate loading condenser out and I replaced it with a larger one rated at 4.6kv. I was using a tuner all this time. It worked for a while and then started the same thing but blew the antenna relay in the amp.
    I have two other threads on this problem you might want to read. Connecting the dots, I think the loop antenna is the problem. Yes it works great but you see my experience.
  13. KD5ZPG

    KD5ZPG Ham Member QRZ Page

    X 2

    Works great on 20 meters without a tuner.

    Last edited: Jan 7, 2009
  14. K2GSP

    K2GSP XML Subscriber QRZ Page

    I could be wrong here, but 1050 is only two full wavelengths. I don't think the problem is the loop, but the size of your loop. Again I could be wrong on this, but everything I have read is about one full wavelength on 160, so maybe that's the problem you are having with your loop.
  15. W8JI

    W8JI Ham Member QRZ Page

    People sure spin some big tall tales about loop antennas!! The more I listen the stranger things I hear.

    The idea there is no RF inside a loop is just silly. That has to be one of the prize winning myths.

    We also can't just parallel loops and not expect interaction. We either depend on luck, or we plan it.

    The nice thing about a loop is because it is "shorted" a multiple of 1/2 wave from the feedpoint on all harmonics (it is like a very wide spaced half-wave transmission line shorted at the far end that leaks signal out from the wide spacing), that assures the feed impedance is somewhat low on all harmoncs. It makes it easy to feed a full wave loop on harmonics. It's great for that, and that is a clear advantage UNLESS we are paralleling loops!! If we parallel them, it can be a disadvantage!

    From that simple thing, we go through all sorts of sillyness in order to justify why a loop has magic. It's almost like there is a loop religion that can't stop making things up to justify the faith.

    For example, if we have two parallel loops near each other they mutually couple just like parallel dipoles would. The feedpoints interact just like any other feedpoint of the same impedance ratios would do. The radiation is electromagnetic just like any antenna, it isn't "magnetic". The noise response at a distance is a function of the directivity just like any other antenna. The noise response close in is a function of the nearfield coupling to noise sources just like any other antenna.

    They don't have a lower angle of radiation because they are a loop, they have the same polarization issues. Earth still bothers them.

    The feedpoint impedance advantage is a good thing and is of considerable value if we use it correctly. Other than that, they are just an antenna and like all antennas have varying problems and advatages depending on the layout and application.

    How many loops do I have with hundreds of acres and big tall towers? Two. They are both three feet diameter. If they were as magical as claimed I'd have dozens of them.

    73 Tom
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