Hairpin match vs balun to match low impedance antennas

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by M0AGP, Sep 12, 2021.

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  1. M0AGP

    M0AGP Ham Member QRZ Page

    I am planning both an 80m full size elevated wire vertical and a 160m inverted L, both using a newly installed pulley about 70 feet up in a tree. The elevated radials are the main compromise: there will be only two and not diametrically opposed, and they aren’t elevated very high (one at 5 feet one at maybe 10 feet – I plan to make an RF ammeter and tune them to get about the same current in each).

    The impedance of the inverted L is expected (EZNEC) to be around 30 ohms and the vertical about 50 ohms. Both figures are higher than one might expect, and this is no doubt mainly due to the compromise radials and proximity to ground, i.e., ground losses.

    I had planned to feed the two antennas in a “fan” fashion, that is, the same feedline goes to both, and at any given frequency one will look like a very high impedance and the other like a low impedance, hence the ability to feed them at the same feedpoint.

    Focusing on the inverted L, at resonance the SWR is around 1.7. In order to get a wider “SWR bandwidth” I could use either a hairpin match (an inductor across the feedpoint) or a balun that steps the impedance down from 50 to 30 ohms. Plans for such a balun are around (e.g. Jerry Sevick’s “Transmission Line Transformer Handbook”.)

    Modelling the hairpin match and the balun in EZNEC (actually I just calculated SWR(30) from R and X – good enough?), both methods provide close to the same 2:1 SWR bandwidth – around 55kHz.

    I assume that the balun will heat up due to hysteresis loss, which I expect can be managed. The hairpin match changes the resonant frequency so retuning is needed, but I expect loss is lower than with the balun.

    But wait - thinking about feeding these guys from the same feedpoint, if I have the hairpin match on the inverted L, it will also be across the feedpoint for the 80m vertical! Then again, it will have ballpark double the reactance, so the effect on the vertical will be smaller... still not a desired outcome.

    Looking back at the balun idea, if I take the same feedpoint and connect to a balun that goes to the inverted L and also directly to the vertical, what effect will the balun have on the vertical? Anything?

    I’m hoping it will still look like a very high impedance, and this setup should work ok.

    I’m interested in answers to the questions above (including whether just calculating SWR(30) is correct for modelling a 50:30 balun), and any other thoughts you guys might have.

    Mike M0AGP (ex WB9WFJ)
  2. KI4IO

    KI4IO Ham Member QRZ Page

    For what it's worth, an approximate 165' inverted-L has a radiation resistance of about 50 ohms and can be tuned to resonance with a series cap.
    The same antenna can be tuned on 80 with an L-network. I use my 165' inverted-L from 160 to 20 meters, with custom LC networks switched in
    with relays. Over time, I've installed about 1500' of radials, 47 of them mixed length. Works very well.

    The 165' inverted-L was a favorite of 160 meter op W1BB

    Jerry W
    Warrenton, VA
    K0UO and M0AGP like this.
  3. M0AGP

    M0AGP Ham Member QRZ Page

    Hi Jerry - yes that sounds like a good setup. I know someone who's gone the "long radiator" approach on 160m and tuned it with a series cap - good to hear it works for you!

    Actually your comment gives me an idea - one approach I hadn't thought of trying is using a long radiator but then tuning it to resonance by shortening the elevated radials. It's a bit like an off-center fed vertical. Increasing the radiation resistance in this way might actually mitigate some of the ground losses as the radials will be carrying less current in them...

    I could go to over 200 feet on the radiator, for example 70 feet up and 130 feet on the down-sloping "horizontal" wire. I suppose as that wire gets closer to ground, I would again lose radiation resistance...

    Maybe there's an optimum in here! Interesting!
  4. KI4IO

    KI4IO Ham Member QRZ Page

    Howdy Michael
    Well any bump up in radiation resistance is a good thing.

    I went up 75' and out horizontal 90' before running out of space.

    I took advantage of these eMylo wireless (433MHZ) switches. Mine came in a package of 6. They're only SPDT
    relays. See:
    I used them to supply power to little DPDT relays which switch in various LC combinations for matching
    my wire on 160 - 20. I don't know what the maximum range is...but I sit here in the shack with a wall of equipment
    in front of me and have no problem changing bands - the remote coupler is about 100' away out in the yard.

    Power is fed to the coupler via a bias-tee.

    LC values were found by using a simple tuner at the base and measuring the Inductor and Capacitor values with
    my AADE meter. You can see a picture of it on my QRZ page.

    Elevated radials were a non-starter. We occasionally have visiting pets, plus the part of my yard, which would
    be available for elevated radials is rather I filled it up with radials held down with lawn staples. They've
    pretty much disappeared.

    Good luck!

    M0AGP likes this.
  5. M0AGP

    M0AGP Ham Member QRZ Page

    Very clever - thanks Jerry! I may use something like this when I someday build my own antenna switch or remote coupler.

    But back on the above "idea", when I do the calculation, what happens when I lengthen the "horizontal" (drooping) wire is that R goes down, not up!

    This is the opposite of what I naively thought would happen... In reality if I shorten the horizontal wire, then R goes up - If I set to 50 feet and lengthen each radial out to about 168 feet, then I get 47 ohms for R. But in reality I am fooling myself if I think this is good - I am just increasing the ground loss and making my antenna closer to a dummy load!

    If I check what is happening to R with a perfect ground, it turns out it is going the opposite direction.
    Here are the numbers:

    "Length of horiz" is the length of the drooping wire, and "length radial" is the radial length I need to get to about 1.84 MHz resonant frequency.

    So as I lengthen the horizontal wire, R goes down, not up, which is counterintuitive compared to an OCFD...

    But the right hand column is the result for a perfect ground - in this column R is going up, as we would expect with an OCFD.

    So I think adjusting the radials longer and longer is just getting a higher coupling into the lossy dirt, and I dump more and more energy into the ground.

    My conclusion: for this particular structure inverted L antenna, lengthening radials is a terrible way of matching it!

    What I could do is lengthen the horizontal wire and shorten the radials, so that the "perfect ground" value for R is higher, thereby decreasing my ground loss.

    However, this would then give me a more severe matching situation to deal with.

    But at least it makes this antenna less of a dummy load and more of an actual radiator!
  6. M0AGP

    M0AGP Ham Member QRZ Page

    OK - one last post on this idea...

    Weirdly if I keep lengthening the drooping horizontal wire, the R value starts increasing again, and the "perfect ground R" also keeps increasing.

    Here is the continuation of the above table:


    So it looks like as I length the horizontal wire and get more current away from the ground, things get better and I get to a point where I don't really even need a matching network - 40 ohms!

    Great, right? But wait - I may have increased the efficiency of the antenna by getting current away from the ground, but now I have a mainly horizontal antenna...

    The radiation pattern is a cloud burner with max radiation straight up!

    Going back where I started with 130 foot radials and a 70 foot sloping horizontal-ish wire, it looks like this:
    Not a DX flamethrower, but something with a better shot at some DX...

    Lessons learned...

    Attached Files:

  7. KI4IO

    KI4IO Ham Member QRZ Page

    Too bad we can't just stick a meter in the ground and measure ground losses. When Jerry Sevick did his famous
    tests on ultra-short verticals, he laid down 115 radials of .4 wavelength and assumed the ground losses were zero
    or near zero. He could then measure the accurate Z of short antennas without having to add in some unknown
    ground loss. Smart. He was then able to reduce the number of radials and understand the increase in ground
    losses as the number of radials were reduced.

    My last hardware store trip had me buying another 500' roll of wire which I promptly stapled into the back yard
    which is not mowed....only weed-whacked a few times a year. I did not see my resonance frequency change so I
    must be sneaking up on having a good ground. It's that age old tussle between the good R and the bad R.

    I don't have the space, etc., for elevated radials and confess to being a mere spectator in discussions regarding
    their efficacy (to use a COVID term).

    May the good R be with you....

    M0AGP likes this.
  8. K0UO

    K0UO Platinum Subscriber Platinum Subscriber QRZ Page

    165' inverted-L with just a few elevated quarter wave radials and a series capacitor really performs well/ if you can get the vertical part up pretty high.

    73 from,
    The K0UO " Rhombic Antenna Farm" 2 miles of wire in the Air & On the daily
    M0AGP likes this.
  9. M0AGP

    M0AGP Ham Member QRZ Page

    Jerry -fully agree on measuring ground characteristics- I will do this at some point, just a bit time poor these days.

    Off topic, but that is an awesome QRZ page you have! And I didn’t see anything “ugly” on that page - homebrew stuff has a beauty all its own. Super impressive all the stuff you have built. I want to try the “binaural CW” thing some day as well.

    73 es tnx
  10. W1VT

    W1VT Ham Member QRZ Page

    I tried to come up with a single coax feed for my 80M vertical and 160 M Inverted-L but after a few attempts, I ended up running separate coaxes to each feed. I used matching transformers and adjusted the wire lengths to resonance.
    Despite the low height of 36 ft for the Inverted-L I've worked 100 countries on 160 CW and twice that on 80 CW. Running my 40ft top loaded vertical on FT8 I seem to be able to work into Europe as easily as most stations--the only ones who do better are contest stations like K1AR.

    You should be able to get out great with a height of 70 ft!

    Zak W1VT
    Last edited: Sep 14, 2021
    M0AGP likes this.

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