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Thread: Feeding an unbalanced antenna

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  1. #1

    Question Feeding an unbalanced antenna

    After trying a few configurations over the past week, I settled for the following antenna configuration on my deck:
    - 90' loop ( 40' x 5' ) of 14AWG stranded wire along the deck's ceiling
    - At the feed point, a 1:1 current balun connects antenna to 18' coax that runs to an auto tuner

    The antenna seems to work well on 40m and 20m and usually the tuner will match to below 2:1 SWR. I have been able to make contacts along the west coast and towards east coast running barefoot.

    That said, there are obvious flaws in how the antenna is fed. While the antenna is unbalanced, it is fed with a balun. Well, I used whatever I had in my junk box but now I am looking to improve it. Of course, the easier route is to stick a remote tuner at feed point but I want to understand what's at play here. Testing with an antenna analyzer (MFJ-259), I connected a 3' coax to the balun and analyzer. At the middle of 40m, resistance shows up at 500 ohms and SWR goes to beyond 3:1 in the red zone. Readings for 20m are the same. On 80m, the lowest SWR is 2.5 with resistance hovering around 100 ohms. When same measurements are made with an 18' coax, on 40m, the resistance hovers around 50 ohms and SWR is still in the red zone. On 80m, the SWR goes to 2 and resistance shows at around 75 ohms. So clearly the 18' coax has high losses due to high SWR. Right?

    Another test I did was connecting the auto tuner directly at the feed point and then run a coax from tuner to transmitter. The difference was night and day between this set-up and the former one of running coax to tuner from balun. A station from AZ that wasn't audible at all and was below the noise floor went up above the noise floor to 57-58. Now, even when I turned off the tuner that puts it in bypass mode, the AZ station was a good copy. That essentially means the 4:1 current balun built into the tuner improves matching significantly because in bypass mode only a 4:1 current balun is in path of the feed line.

    Questions I have are:
    1. How does a balun impact the system when both ends are unbalanced?
    2. How does a step down balun impact matching for a band/frequency that is already close to 50 ohm? For example, if the antenna presents an impedance of 100 ohms at 80m but 500 ohms at 40m then a 9:1 balun would help match 40m but what happens on 80m?

    Also, if someone can point me to good online text about baluns and impedance matching that is relatively mathematics free (if there is such a text)

    Thanks!

  2. #2

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    Just a suggestion to try. My 40 meter loop is in my attic and to get enough wire up to resonate on the low end of 40, it's folded back on itself in a "U" shape. Using a LDG 1000 pro tuner I tried feeding it with 50 ohm coax, twinlead, and open wire with marginal results. I tried baluns on one or both ends and wound a choke on one or both ends, with no improvement. I now feed it direct with 75 ohm coax, from the cable company, and it will load 6 meters to 40 meters, using the same antenna tuner, don't use baluns or chokes. It tunes the warc bands great.

  3. #3

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    A 40x5 ft loop does not look like a loop. It looks more like a folded dipole. The feedpoint impedance will be no where near 100 ohms. If you feed it in the middle of one of the long sides, it should be resonant somewhere around 10.5 MHz, with a feedpoint impedance of about 238 ohms. If you feed it in the middle of a short side, it should be resonant at about 11.1 MHz with a feedpoint impedance of about 7.5 ohms. Notice the huge difference depending on where you feed it.

    If the antenna is fed at a point where the loop is symmetrical, then the loop will be balanced. You need a good choke with this antenna. There is a possibility for large common mode currents on some frequencies.

    This antenna will not work well at 40 or 80 meters. It will look like a short circuit at those frequencies, with SWR in excess of 100 to 1. However, common mode currents (without a good choke) can modify that SWR and you may see something much lower. In that case the feedline and house wiring will be doing most of the radiating.

    None of the data you gave jives with anything obtainable with EZNEC, so I suspect there are other factors at play, like proximity to other conductors, a choke that doesn't work at the operating frequency, or hand capacity when making measurements. If fed in the center of a long side using a good choke, SWR on 3.7 MHz when measured thru 3 ft of coax should be about 970 to 1. On 7.1 MHz it should be about 167 to 1.

    Jerry, K4SAV

  4. #4

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    Sorry, I did not fully describe the antenna configuration.

    The loop (40'x5') or folded dipole is fed to +ve side of a 1:1 current balun. The -ve side of the balun is connected to a welded wire fence (on wood railings) that covers three side of the deck, runs about 50' and is 4' high. The unbalanced side of the balun is connected to a tuner with a 18' RG-8X coax. Distance between top of the fence and loop (on deck ceiling) is about 6 feet. The feed point is at a corner of the loop near the deck door such that I can run shortest possible coax to the tuner. The long side of the loop (and fence) runs North-South.

    You might ask why this configuration? Trial and error. I tried:
    - Long wire and fence as counterpoise fed directly to tuner
    - Pseudo loop, that is, dipole with two L-shaped elements and fed with 1:1 current balun
    - Loop fed with 1:1 current balun
    - And, finally what I described above.

    I cannot find a good reference design for this configuration but the antenna works really well so far on 40m and 20m. On 40m, I routinely get 59 reports from up and down the west coast all the way to San Diego in the south and Portland in the north. This morning, I made a contact in Vancouver and got a 55 report. I have also been able to make contacts towards the east coast to GA (got 59) and KY, again 40m. I haven't tried 20m much mostly because I am new to 40m and excited about working this band but on 20m, I copied a station from NH at 599+ the other day. 80m loads up really easily to under 1.5 SWR with the tuner but I haven't tried working on this band much either.

    So back to the question about how is this configuration working and how does the balun handle two unbalanced sides? How can improve this antenna other than the obvious solution of putting a remote antenna tuner right at the feed point?

    Thanks for the responses.

  5. #5

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    Thanks for the added description. No that's not a loop or a folded dipole. It doesn't have a name, and you won't find any references to this type antenna. You could get an approximation of its performance by modeling it in EZNEC. I tried but could not get close to your numbers, which means I am still missing some of the details.

    But I can guess. Since there has to be a wire from the fence to the loop above (open or closed loop?) and the feedpoint is on that wire, that 6 ft long vertical wire will look like a top loaded vertical on 80 meters. It should be omni-directional on that band. On the higher bands it is likely to radiate mostly from the horizontal portion of the long wires, so I would expect to see mostly broadside (to the long wires) radiation on 40 meters. On the higher bands, it's likely that the best direction will be at some weird angle to the long wires.

    A current mode 1 to1 balun (another name for a choke) is made to handle unbalanced antennas. If it has high impedance at the frequencies your are operating at, it should work well for this application.

    It will be difficult to determine just how well it is working without any other antenna to compare it to, but if you are happy with it, enjoy. I can't even determine what the loss in the coax might be. My model says about 3.6 dB on 80 and 3.3 dB on 40, but my modeled impedance doesn't agree at all with your measurements, so the coax loss may be totally wrong. Your experiments with the remote tuner, also suggests that the coax loss may be very high even though it is very short. My gain numbers show about -6.9 dBi on 80 and -5.4 dBi on 40, but those could be in error also.

    It's difficult to recommend improvements for this antenna since I know that I don't have all the details, but if the loop on the ceiling is a closed loop, try breaking the loop on one side of the feedpoint so that it is open. That should offer some improvement in SWR and gain.

    If you want to supply a detailed drawing with dimensions, I will improve my model and tell you the results.

    Jerry, K4SAV

  6. #6

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    Quote Originally Posted by K4SAV View Post
    But I can guess. Since there has to be a wire from the fence to the loop above (open or closed loop?)
    Closed loop. I started with an open loop but found that closed worked better.

    If you want to supply a detailed drawing with dimensions, I will improve my model and tell you the results.
    Jerry, K4SAV
    Jerry, here is a diagram:
    https://picasaweb.google.com/1003746...70848440615234

    I was wondering if a 9:1 unun would work better for 40m and 20m given that impedance is around 500 ohms on those two bands. But, I am not sure what would it do to 80m. How do I estimate loss on that 18' coax?

    Thanks!

  7. #7
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    Default

    Looks like a BIG capacitor antenna.
    73.....JD, FISTS #3853,cc 455,SKCC # 1395,tribune #12,
    Official US Taxpayer

  8. #8

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    Quote Originally Posted by K8JD View Post
    Looks like a BIG capacitor antenna.
    Where is the inductor? The tuner? Doesn't have a big enough coil.

  9. #9

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    Sounds to me like two random length conductors which aren't similar and thus yield an unbalanced load.

    The bands have been pretty good lately, so anyone can work anywhere with anything!

    But I certainly can't complain about that. Propagation is finally getting a bit better here in what ought to be Cycle 24.

    I heard 9K2MQ on 20m while I was switched to my 80m inverted vee, which is about 30 dB down from a 20m antenna.

    I love propagation!

  10. #10

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    Thanks for the diagram. I wasn't off too much. I still can't get anything close to what you are measuring, unless you are giving me the magnitude of the impedance and not the resistance. With the bottom of the deck at 4 ft, here is the EZNEC data. All the impedance numbers and SWR are for 3 ft of coax to the antenna. The coax loss and gain numbers are for 18 ft of coax to the antenna. Tuner loss is not included.

    3.5 MHz, Z = 3.2 - j137, Z mag = 137, SWR = 67, coax loss = 3.0 dB, gain = -6.5 dBi
    4.0 MHz, Z = 4.9 - j59, Zmag = 59, SWR = 19.4, coax loss = 1.5 dB, gain = -5.2 dBi
    7.0 MHz, Z = 101- j927, Z mag = 923, SWR = 149, coax loss = 4.8 dB, gain = -5.9 dBi
    7.3 MHz, Z = 57- j681, Zmag = 583, SWR = 136, coax loss = 4.8 dB, gain = -5.7 dBi
    14.0 MHz, Z = 470+ j174, Zmag = 501, SWR = 10.7, coax loss = 1.2 dB, gain = 2.4 dBi
    14.35 MHz, Z = 398 - j516, Zmag = 651, SWR = 21.1, coax loss = 2.3 dB, gain = 1.0 dBi

    As you can see the coax loss is very high, especially for only 18 ft of coax. That is because of the very high SWR on the coax, plus the gain is low because the antenna is very low to the ground. Adding a 4 to 1 balun doesn't help (even if the balun happened to operate properly at those impedances).

    It's a weird antenna. You would have to be very lucky for it to perform well over a wide frequency range. I tried several configurations using roughly the same wires in the same space but got nothing better. For comparison, a 40 meter half wave dipole at ceiling height should beat it by about 11 dB on 40 meters. An 80 meter dipole at the same height would beat it by about 13.7 dB. I realize you don't have room for either of those antennas. (Do you? How did you implement a long wire?) Those are comparisons of maximum gain numbers for the two antennas and doesn't include the fact that they don't both have maximum gains at the same angles, so the comparison is incomplete (and in the case of 80 meters somewhat misleading for low angle signals). A 20 meter dipole on the ceiling would beat this antenna by about 3.5 to 4.8 dB on 20 meters, and you do have room for that.

    Jerry, K4SAV

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