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I'm putting up a multiband dipole in some tall trees. (Perhaps it's more correct to call it a "dipole-doublet"). The situation I'm imagining looks like this:

diagram of antenna

The legs of the antenna are each 100 feet long. They are fed in the middle by ladderline. This ladderline runs 150 feet to a 1:1 current balun (e.g. the MFJ-918). This balun is fed with 150 feet of high quality coax (LMR-400) to an antenna tuner inside the shack.

The reason for doing it this way is to avoid using an expensive remote tuner, and instead use a cheap balun, a cheap antenna tuner, and I got a good deal on the coax. :)

Unfortunately, I can't run the ladderline all the way to the shack, because the transmission line has to be buried under a lawn for about 15 feet or so.

My questions are:

  1. Are the losses inside the coax between the tuner and the balun going to be ruinously high?
  2. What will the impedance of that leg of the circuit be, assuming the tuner feedpoint is 50 ohms, and the dipole and ladderline are something realistic?
  3. Is a 1:1 current balun the right choice?
  4. Is 300 ohm ladder line the right choice? Would 400 ohms or 600 ohms be better? Does it matter?

Thanks!

UPDATE This design is flawed, because the tuner does nothing to tune the impedance mismatch at the balun. As Mike Waters notes below, my drawing is basically a G5RV, with an separate wound 1:1 balun at the ladderline feedpoint. The G5RV looks like this:

Picture of G5RV

What I want is this, the random-length multiband dipole.

enter image description here

So it appears in conclusion that I do need a remote tuner at the base of the ladderline feed, or I need to bring the ladderline into the shack. Or, an alternative would be to go with Phil Frost's suggestion and use a folded dipole and operate it at its resonances. I think I probably want wider-frequency coverage than the resonances of the folded-dipole, so it sounds like the tuner + simpler wire setup is more optimal for my situation.

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    $\begingroup$ I think you are getting off track with the edit. There's nothing about ladder line that makes it more suited to this application than coax except that it's relatively cheap and the losses are low for the price. Running ladder line all the way to the shack doesn't get you anything unless it's lower loss than the alternative. Look up the datasheet on the ladder line you'd like to use: it may not be better than LMR-400. And what do you mean by "the tuner does nothing to tune the impedance mismatch at the balun"? $\endgroup$ – Phil Frost - W8II Feb 3 '17 at 2:39
  • $\begingroup$ @PhilFrost-W8II thanks for the followup. The difference between drawing (a) and the ARRL drawing (c) isn't the use of ladderline or coax, it's the lack of a separate balun between the tuner and the antenna feedpoint ... right? Doesn't that balun create a separate impedance mismatch between the end of the coax and the ladderline in my original drawing (a)? Then the tuner is tuning the impedance at the input of the coax to 50 ohms but there may still be a ton of reflection where the balun meets the ladderline. Or am I not correctly understanding how impedance matching works? $\endgroup$ – bcattle Feb 3 '17 at 2:54
  • $\begingroup$ Yes, the coax and the ladder line aren't matched, but if you make that a 4:1 balun they will be. That said, it may be moot since it terminates into the dipole which is effectively a random impedance. The addition of another mismatch at the the coax/ladder line balun could make things better, or worse. See ham.stackexchange.com/a/7053/218 for an example of how a mismatched line plus a mismatched antenna can equal a matched system. $\endgroup$ – Phil Frost - W8II Feb 3 '17 at 3:16
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    $\begingroup$ Yeah. ham.stackexchange.com/q/7174/218 $\endgroup$ – Phil Frost - W8II Feb 3 '17 at 14:39
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    $\begingroup$ "Is 300 ohm ladder line the right choice? Would 400 ohms or 600 ohms be better? Does it matter?" Your drawing looks like 500 or 600 ohm ladder line (open-wire line) with a spacer every few feet, and would have lower losses when severely mismatched (as is going to happen on some bands). Window line is more susceptible to wild impedance changes when wet. $\endgroup$ – Mike Waters Feb 3 '17 at 19:38
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This is almost certainly not going to work well on more than one or two bands. And that's if you are "lucky" and just happened to hit the right combination of antenna length/height and balanced line length/impedance where the impedance at the 1:1 balun just happens to be close to 50 ohms.

Your design is a little like the G5RV. That took a lot of trial and error to make it work on several bands. Later, ZS6BKW used computer modeling software to fine-tune the G5RV. You would save yourself a lot of time and effort if you modeled this in EZNEC or a similar program.

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  • $\begingroup$ Thanks Mike. Yeah I see your point, having the balun there does make this like the G5RV, which is not what I want. I want the nonresonant dipole-doublet mentioned in the Cebik paper. Thanks for the suggestion on modeling, I'll try that. If setting it up in EZNEC, would you model only the horizontal elements, or the ladder line as well? Thanks. $\endgroup$ – bcattle Feb 2 '17 at 20:40
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Times Microwave LMR-400 is pretty low loss stuff, about the same at 30 MHz as the 300 ohm ladder line sold by DX Engineering. So it's roughly the same as running ladder line all the way. Certainly many people have run setups like this and it's been fine.

Burying the coax transmission line is a good thing. You still want a balun to avoid most of the common-mode current. Burying the coax will make things even better. Bury it the whole length if you can. It doesn't need to be deep.

I would suggest one improvement: use a 4:1 balun instead, and instead of a simple dipole, use a folded dipole. This will make it possible to get a low SWR at all points for frequencies where the dipole is resonant.

If you use 75 ohm coax (RG-11 is a good choice), a 4:1 balun, 300 ohm ladder line, and a folded dipole, everything will be matched at frequencies where the dipole is resonant. Sounds like you already bought the LMR-400: that's fine because the difference between 50 and 75 ohms is not that much.

Where the dipole is 0.5, 1.5, 2.5, ... wavelengths, it will be an ideal match. Where it's 1.0, 2.0, 3.0, ... wavelengths, it will be antiresonant and you will get the worst possible match. Avoid putting one of those antiresonances on a band you want to use.

You might also try an off-center fed dipole instead of the folded dipole. It takes some fiddling to get the lengths just right, but you can get a pretty decent match to 300 ohms on most bands. I bet it's possible to get the SWR low enough to have at worst 1dB extra loss due to SWR. You will want to add another balun at the antenna feedpoint in this case, or just run the coax all the way up to the antenna and have the balun there. Coax all the way has the advantage of not having additional loss when wet.

Are the losses inside the coax between the tuner and the balun going to be ruinously high?

It will depend on the SWR, which is going to change based on frequency. In the best case you have a 1:1 SWR. Under those conditions, for 150 feet of LMR-400 at 30MHz it's about 1 dB.

If the SWR is 5:1 then the losses go up to about 2.1 dB, and at 20:1 the losses are up to 5.3 dB. It's less than ideal, but I wouldn't call that "ruinously high". Multiband antennas are always some kind of compromise.

That's just in the coax: you have another 150 feet of ladder line which is not without loss either.

What will the impedance of that leg of the circuit be, assuming the tuner feedpoint is 50 ohms, and the dipole and ladderline are something realistic?

It will depend on the frequency and length of the transmission lines. As the length of the transmission lines, the reflection coefficient magnitude will remain the same, but its phase will change. In other words, the impedance will move around a constant-SWR circle on a Smith chart.

As drawn in the question, you have two mismatches: the dipole to the ladder line, and the ladder line to the coax. As you change frequency, the feedpoint impedance of the dipole will change, and so will the electrical length of the transmission lines.

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  • $\begingroup$ Phil, thanks for the insight. You're right - I had been assuming that somehow by tuning the impedance seen by the tuner you could match the mismatch at the balun. Obviously that isn't correct. The folded dipole idea is interesting, but I'd rather run this in non-resonant mode and have wider frequency coverage. Sounds like I need a remote tuner. $\endgroup$ – bcattle Feb 3 '17 at 0:41
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    $\begingroup$ @bcattle You can run a folded dipole in non-resonant mode, just like an ordinary dipole. The difference is with an ordinary dipole, you are guaranteed to have a bad match everywhere. With a folded dipole you can have a good match in a few places, and in other places the match is at least less bad. $\endgroup$ – Phil Frost - W8II Feb 3 '17 at 2:21

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