40M Skywire loop antenna questions

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KD5NDQ, Feb 16, 2019.

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  1. KD5NDQ

    KD5NDQ Ham Member QRZ Page

    I'm fascinated by this article on a "skywire loop" antenna that makes it sound too good to be true, but I want to build one anyway:

    http://srgproperties.inetusanow.net/files_custom/9467_2192.pdf

    I really only want a good performer on 40m and 20m. What I am confused about is whether this thing can be fed directly with coax to my Kenwood TS-530 without a problem. The article makes it sound like it can, but as I read other pages, like this one:

    http://www.ai4ji.com/Projects/loopskywire.htm

    they explicitly state -not- to use coax. Are they both right? I have no ladder line but plenty of coax is why I ask!

    Robert
    KD5NDQ
     
  2. K8JD

    K8JD Ham Member QRZ Page

    Loop Report
    A fullwave horizontal loop mounted less than a halfwave above Ground is pretty much aimed straight up.
    They make a fine close-in antenna up to several hundred miles for NVIS performance. Strong signals better than a halfwave dipole in the short hop area in states surrounding your location.
    On harmonics there could be SOME sidelobes at low angle you need for DX, never used horizontal loops for DX but it kinda works, like working off the side of a Yagi !
    Using a rotatable fullwve loop, in the vertical plane, designed for the "DX Bands" like 20M and with a a director element, is called a "cubical quad" array and was quite popular for DXers in past years.
     
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  3. W1VT

    W1VT Ham Member QRZ Page

    It is curious that the page you reference uses the name "Loop Skywire," since the November 1985 article by W0MHS only describes a coax feed.

    You can use a coax feed if you are willing to accept a reduced efficiency on the bands that exhibit a moderate SWR. This is best determined with an SWR meter or antenna analyzer. The efficiency can be calculated using the coax loss and SWR. Typically, you will lose a couple dB, which isn't bad compared to a multiband dipole with a coax feed. The coax fed dipole can lose a couple of S units, or much more than a few dB, in very typical situations.

    I wouldn't trust computer predictions of SWR and impedance, as a Loop Skywire is typically too low and too close to nearby objects.
     
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  4. AI3V

    AI3V Ham Member QRZ Page

    Coax will be fine.

    A fullwave loop has a free space impedance of about 100 ohms, a bit less when mounted near the planet :) this should give you a vswr somewhere around 2:1, that's fine for keeping loss low in a 50 ohm coaxial transmission Line.

    You would not notice any difference using 75 ohm coaxial "tv cable"

    Don't confuse how the transmission Line is constructed (coaxial vs parallel) with the loss due to vswr on the line, vswr of course is caused by the ratio of impedance of the antenna and transmission line, and the loss in said line is mostly about how thick the wire is.


    Why the first article used a external "antenna tuner" is weird, you rig has one built in!

    You should be able to get rated plate current at the resonance dip just fine.

    As a horizontal antenna a fraction of a wavelength above ground the signal will favor high angles (short range), nor because it's a loop, but because it's horizontal and low to the ground.

    The classic partner to low horizontal is a low vertical, which will tend to favor low angles (long distance)

    Rege
     
    Last edited: Feb 16, 2019
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  5. KD5NDQ

    KD5NDQ Ham Member QRZ Page

    Great! By a low vertical would this be a wire antenna? Can you link to an example I could build? 73's!
    Bob
     
  6. KE0EYJ

    KE0EYJ XML Subscriber QRZ Page

    Here was mine. It was 5 stories up, in Seoul. Took it down. Currently running a doublet which is not working out very well, due to placement issues (worked better in another configuration).

    I ran ladder line to my loop.

     
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  7. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    The debate about how to feed a multi-band loop will go on forever.

    Many of those who take passionate sides in the debate don't understand that such a loop presents a range of impedances, not one single value on every band.

    Here's the general scoop: A loop that is resonantly operated presents an impedance in the 100 - 120 ohm range on its fundamental frequency. On each of the first few harmonics, the impedance will be in the 200-400 ohm range.

    Right off, you can see that compromises have to be made. Do you optimize the match for the fundamental band, or the harmonic ones?

    In addition, a non-resonant loop can present a range of impedances perhaps best described as "wacky." Wide-ranging and hard to predict or model.

    So, you have to ask yourself which bands you want to optimize, and what impedance range the loop is likely to present, before you can answer questions about how to feed it.

    Here's the good news: with a resonantly operated loop, it really doesn't seem to matter which method you use. People have success with both coax and open-wire line. The open-wire line in theory presents less loss on the harmonics, but that depends on having a good "balanced" tuner, or a good balun, to match the system to 50 ohms unbalanced. In addition, the loss presented by short runs of coax with a 6:1 SWR50 often isn't objectionable.

    A more important question is the pattern presented by such a loop. On its fundamental frequency it is often a real dog for DX, with most of the radiation going straight up. On the second harmonic, the situation gets a little better. But it is usually on the third harmonic and up such an antenna really shines for DX. This is because, as always, height relative to wavelength reduces ground reflections and ground loss. So, if you want to work DX reliably on 40m with such a loop, it needs to be BIG (160 or 80 for fundamental band) and preferably at least 40' off the ground.

    Whatever you have read, such an antenna is not omni-directional. The radiation pattern consists of a number of lobes, with the number increasing with increasing frequency. If the lobes happen to be pointed in useful directions, such an antenna can do quite well on DX. If not...

    In addition, whatever you have read, a loop is not inherently more "quiet" than other wire antennas. There are individual exceptions, but they depend on the nature of local noise sources and other peculiarities of individual installations.

    But generally a harmonically-operated horizontal loop is a good compromise antenna for DX. That's why people get so excited about them.

    For those really interested in DX, the solution is a variety of antennas. Put your loop up, but also put up a vertical. Also put up dipoles for the bands for which you can get them 1/2 wave high or higher. Some signals will come in better on the loop, some on the vertical, some on the dipoles. Moreover, it will vary from day to day which is the better antenna.

    Finally, as always, nothing beats a tower and beam for overall DX performance. But it is hard for some of us to spend that much money on a hobby, even if we could afford it without it affecting our lifestyle.
     
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  8. NH7RO

    NH7RO Ham Member QRZ Page

    Bob; if you want to work dx with a low antenna you'll be better off with a simple vertical (although it's always great to have multiple antennas to switch between for best signals, lowest noise, etc).

    Here's a link to a three-band vertical (which later became a 4-band job) I built a couple of years ago that I've been quite pleased with. It cost me under $50 to build using 14 gauge wire from Home Depot, a 2x4, couple of fiberglass poles, wood dowels and some copper pipe fashioned into radial and element connection points. This is for the higher bands starting with 20m.

    Pix start several pages in, IIRC:

    https://forums.qrz.com/index.php?threads/15-17-20m-fan-vertical-guidance.560123/

    You could also build a simpler monoband ground plane for 40m (great dx band now) with a 31' tall wire and several elevated radials (or plenty of ground radials if ground mounted).

    73,

    Jeff
     
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  9. NL7W

    NL7W Premium Subscriber QRZ Page

    Maybe my loop was an aberration, but in the 90's at Keesler AFB, I installed an equilateral triangle, 80-Meter band full-wave loop with its apex at 90-feet sloping down to 50-feet at the remaining two telephone poles. Fed near its apex at 90-feet, the transmission line consisted of two taped together equal runs of RG-62 (93-ohm coax), with their shields soldered together at both ends. The taped together runs acted as shielded 186-ohm balanced line. With the paralleled RG-62 balanced feedline run under the ham shack barracks to a 4:1 current balun, and a short 10' length of RG-213 run up into the bldg, this transmission line was a perfect match for the high loop -- and I mean perfect.

    On 80, 40 and 20 meters, this antenna was a SUPERLATIVE performer. The high loop was a BOMB on all bands. The exceptions were 12 and 30 meters where it wasn't resonant whatsoever (strangely, it presented a good match and was great on 17M). A small group of us earned the Sweepstakes Contest's "Clean Sweep" two years in a row with that antenna, to include winning and setting a Delta Division record in the early 90's at Military Rec Station, K5TYP (the competitive high-power, multi-op class). In most situations, it was equal to or better than the TA-33 that was at 40-feet.

    Those were the days. :)
     
    Last edited: Feb 16, 2019
  10. WE4E

    WE4E Premium Subscriber QRZ Page

    Use .25 wl of 75 ohm as a matching section. Works FB. I have a 160m loop that I match that way, and get all but the upper 50khz without a tuner. Also matches well on other bands, at least some portion. I just use the tuner in my amp (VL-1000) and it's characteristically narrow-minded about tuning. If that thing can match it, anything can. I used RG6 quad shield I bought at Lowes. They also carry compression-type BNC connectors, and I used those with BNC to PL259 adapters. I stick a KW into it routinely and have had it up for about a year. I don't use a balun or anything, just coax directly to the wire. I do use a common mode choke inside the shack, although that's mainly to control energy it's picking up from my 6m antenna.

    The loop is strung through trees in a blob shape somewhere between 35-55 ft or so, fed near the southwest corner. By rights, I suppose all the energy should go straight up, on the lower bands anyway, and I'm sure a lot of it does, but I've had repeatable successes working EU on 75m, EU and South Africa on 40m and Asiatic Russia and New Zealand on 20m. (I'm sure the lobes get weird on 20) The 75m DX performance surprised me. I've had reports of S9 to 15 over, so it's not like they're digging me out of the fuzz, at least not all the time. Worked a KL7 as well, and he was loud. I'm sure a lot of the success was those stations' antennas, but still.

    I wish it were neater and higher, but alas, that's my situation. Something the antenna books and NEC modelers don't account for: we don't all have ideal conditions in which to erect antennas. Some of us are thankful to be on the air.
     
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