Delta Loop replaced with dipole, what a difference

Discussion in 'General Technical Questions and Answers' started by KC0MS, May 16, 2008.

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  1. KC0MS

    KC0MS Ham Member QRZ Page

    I've been fooling around with different wire antenna configurations for a couple months now for 80m, starting off with a simple dipole at 30'....graduating to a full wave sky loop at 30', then to a full wave delta loop with the apex at 85', all with (in my opinion) less than satisfactory results. Especially with DX.

    So, I took the delta loop down, and converted it into a 1/2 wave 80m dipole, made the angle at the apex as near to 90° as possible (for a good impedence match), and hung it back up at 85'.

    NOW I have an antenna that really kicks some serious butt, locally as well as with DX. Maybe it's not as efficient as a vertical for DX, but I bet it's not far behind. There is a TH7DX at the top of the tower, that (might) be acting as some sort of top hat for this V, but I think it's probably too small to do much for 80m.

    I have to agree with the 'experts' out there who say a dipole is a very good antenna if it can be gotten up high enough.

    On a side note, I have been also experimenting with a 1/2 wave "helical wound" vertical for 80m I put together, which is nothing more than a 30' wooden "stick" with 120' of 12ga copper wire wound around nearly its entire length and a 3' stainless "stinger" on top.

    With a 4' ground stake at the base of the antenna and 3, 1/2 wave radials oriented northeast, this crazy apparatus rivals the Inv V in received signal strength, but has much more "noise" coming in with the signal, which in my opinion, negates any possible advantage it may have over the V.

    It seems to load well and the MFJ-989D tuner has no trouble dealing with the over 100 ohm feedpoint impedence. I did wind a few coils of coax at the base of the antenna and at the tuner, to help minimize the effects of this mismatch, but have yet to actually transmit with it.

    In the opinions of others, is this type of antenna something worth pursuing further, or should I take it down and concentrate elsewhere with my "fooling around" (read that as experimenting) with wire antennas for the lower bands?
  2. W8JI

    W8JI Ham Member QRZ Page

    That's exactly how it should work. The gain of a perfect loop over a dipole at the same mean height is basically nothing at all. At sour heights the loop will have loss over a dipole, and at the sweetest heights less than 1 dB gain.

    The mean height of the loop is always less than a dipole if the highest support point is the same for both, and that generally means the loop will be poorer.

    What the loop does do is buy you much better SWR when using low impedance feeders on even harmonics than a dipole offers.

    So what you found is what really should happen in a fair accurate test.

    You don't have a half wave vertical. You have a 1/8th wave vertical. The fact you have 120 ft of wire coiled up in a 30 foot length doesn't mean a thing. It is a 30 foot antenna. As a matter of fact it probably has much more loss than a resonant antenna the same physical length that is lumped loaded, or that an Inverted L would have.

    It's all about amperes (current) distributed over linear (in line) spatial area. Not how much wire we pack into a certain in-line distance. The optimum use of 30 feet on 80 meters would be a single straight conductor with all the loading at the open end, like a large hat or even a single wire or multiple wires running out to "top load" it.

    Other than that the general rule is the more wire you pack into that 30 foot spatial linear volume beyond what it takes to be a resonant 1/8 wave worked against a good ground, the worse it will work.

    73 Tom
    SV4FFL likes this.
  3. K0RGR

    K0RGR Premium Subscriber QRZ Page

    Yep, even though I like loops for domestic work, they're nothing to write home about on their fundamental frequency. However, they're usually quieter on receive, too.
  4. KC0MS

    KC0MS Ham Member QRZ Page

    Really? I thought electrical length counted for something.

    How then does a 5/8 wave "Firestick" antenna work then, if it's not simply a 1/4 wave length of wire wound around a 4' or so long fiberglas stick?

    Ok, so it's to the scrap heap with the helical-wound 30' stick. Back to the books again to see what else I can throw together.

    I have considered the idea of simply hanging a long piece of wire off the same pulley that the Inv V is attached to at 85' and end feeding that (at the bottom) along with a few radials acting as a counter-poise, but haven't so far, figuring I'd get too much interference from the tower for it to be very effective.

    The wire would be about 3' out from the tower all the way down.

    Any ideas/comments on that idea?
  5. W4INF

    W4INF Ham Member QRZ Page

    Glad to hear of your success, and that you have been experimenting. This has been one area of ham radio I have really enjoyed.

  6. K4SAV

    K4SAV Ham Member QRZ Page

    I remember your frustration at trying to get the loop to work. I think you finally discovered some opens and some shorts. Did you ever fix those? Did you ever try feeding it 15% up on one side? Maybe you just took it down.

    Your inverted vee should beat the old loop (fed at the top, after it was fixed) by about 4.5 dB at just about all angles. The old 80 meter loop fed 15% up on a side should beat the inverted vee by about 2 to 6 dB for very low angles. The inverted vee should be much better than the loop (fed on the side at the bottom) for all high angles. The break even point is about 20 degrees. The loop fed on a side should have a much better signal-to-noise ratio for DX than the inverted vee, assuming you don't have local ground wave noise.

    Your guess about an inverted vee interacting with the tower is probably correct. Every one I have ever modeled did that. That doesn't mean you can't make it work. If you have the tower tied into the radial system it might work very well. At that spacing the tower is not likely to be a reflector, but will probably act as part of the main radiator. The resonant frequency and the feedpoint impedance may not be anywhere near what you expect it to be. This will require some experimentation and/or modeling.

    Helical wound radiators are fun to experiment with and they can be very educational if you have anything to evaluate their performance against.

    Jerry, K4SAV
  7. W8JI

    W8JI Ham Member QRZ Page

    It's all BS from the advertising people.

    First, adding more wire than the amount needed to make it resonant doesn't gain you a thing. It actually just adds loss.

    Second, 1/4 wave of wire stretched out is NOT an electrical 1/4 wave of wire when coiled or wadded up in a ball. They only time it is the same is down near DC, where radiation is not involved.

    Third, a 5/8th wave antenna gets gain by the spacing of the current maxima above a large infinite groundplane. A 5/8th wave BC tower has just over 2dB gain over a 1/4 wave BC tower along the ground only when it is installed over flat level earth that is highly conductive and that ground extends out several wavelengths. If you bunched that tower up into a 1/4 wave or less of space the gain from pattern compression would vanish and you would actually have negative gain over the 1/4 wave tower.

    I agree. It is probably too close to the tower and the vertical running feedline that feeds the dipole.

    30 feet is a decent height. Why not put up an Inverted L?? 30 feet up and maybe 40 feet out. Then use the best ground you can. That would run circles around any helical and be better than most coil loaded verticals you could build. The ground system is the key.

    73 Tom
  8. KC0MS

    KC0MS Ham Member QRZ Page

    Yep, sure did. Replaced the coax, which was severely contaminated with water, with some hi-priced low loss coax. While I had it down, I rotated the entire 260' of wire around until the feed point was about 20' up from one of the corner supports.

    Even though that was the only thing that was actually altered, the antenna settings were totally different, according to the analyzer.....especially the feed-point impedence, which went from around 50 to nearly 110.

    Now, while I realize any "comparisons" are purely subjective in the absense of any real time "other antenna" to compare it with, plus the ever-changing propagation, I can only suggest that there was no significant improvement over the delta loop.

    I presently have negligible white noise around here, however the antenna IS being saturated with RF from a local BC station, which interferes with many of my readings. The big difference I noticed so far, is the ability to not only HEAR some dx with the Inv V, but able to bust some pileups and make the contact, especially on 30m of all places.

    I could not accomplish this with the Delta loop when it was center fed OR end-fed. However, the loop was considerably quieter where atmospheric noise is concerned.

    Oh goodie, MORE stuff to mess around with. This is fun, but so time-consuming. Tell me, what about simply getting a Butternut H2V or whatever it's called, for 40 & 80 mtr bands? Worth the effort and expenditure, considering what I already have here with the Inv V at 85'?

    I tried transmitting with it and it seemed to be "getting out", but no one came back to me, so the results were inconclusive, and a moot point now, as I went and dismantled it.

    Jerry, K4SAV

    'Perciate your help and information. I don't know much, but I know more than I did a month ago, hi hi.

    Karl, KCØMS
  9. KC0MS

    KC0MS Ham Member QRZ Page

    Hmmm, sounds interesting. Might as well give it a shot. Is this Inv L considered a DX antenna and would it be any better than what I already have up with the Inv V at 85'?

    Also, how is this thing fed? the lower end, I would presume. And how does it radiate, off the end, or right angles to the top radiator? And finally, wouldn't the tower interact with the vertical part of this antenna?

    I realize I could go look this stuff up, but since I have your attention here, I'm asking your opinions first.


    Karl KCØMS
    KI7MJU likes this.
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