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Balun or Tuner?
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by KI6DYR on June 26, 2006
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I am trying to eek every last bit of signal strength out of a mariginal installation and my newbie-ness causes me to ask questions.
What is the better approach: ladder line direct to a MFJ-941e with some jumper installed in some fashion not clear in the manual, or ladder to external balun feeding 50 feet of RG-8 into the shack?
I thought that I was getting about 2 S units better receive when running the ladder line into the house, but -- connected to the external balun with a 2 foot patch cord to the MFJ. No 50 foot run of RG-8. Can't be sure. The band ws up and down.
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RE: Balun or Tuner?
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by N3OX on June 27, 2006
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The jumper wire goes from the "wire" binding post to one of the balun binding posts. It doesn't matter which one but I always connect it to the one straight up from it.
For multiband-dipole fed with ladder line use, it's best to bring it all the way into the shack as long as you're not laying along many things along the way. There can be a good deal of loss in the coax due to the high SWR. There's typically a high SWR on the ladder line too, but since the ladder line loss is so much lower to start with, the loss with high SWR is also lower.
The ladderline-->balun-->coax thing only works well if the impedance seen at the balun end of the ladder line is close to N times 50 ohms where the balun is N:1. A 4:1 balun transforms 200 ohms to 50 ohms, and that's pretty much all it's for.
Even the balun in the tuner is mostly pointless, because the tuner is there to match the arbitrary impedance seen at the line end to 50 ohms. Taking the impedance at the line end and transforming it by some value which isn't even going to be 4:1 doesn't make a lot of sense. (the balun doesn't work right extremely far away from 200ohms:50ohms. also, imagine you've got 40 ohms on the line side... your balun will knock that down closer to 10! no good!)
Anyway, there are a ton of subtleties about using multiband dipoles fed with ladderline and a tuner. Do a search on google, or eHam forums, or rec.radio.amateur.antenna for sometimes good, sometimes scary reading about the subject. Despite the pointlessness of the in-tuner balun, it often works just fine, and will tend to work much better than the balun at the end of 50 feet of coax.
Personally, my favorite approach is to put the TUNER fifty feet away from the rig and drop the ladderline straight into the tuner. I think a remote tuner is the best of both worlds for a multiband installation... you get the convenience of coax to the shack and the efficiency of direct ladderline into the tuner. I made my MFJ tuner remote controlled : http://www.n3ox.net/projects/servo
Before this, I used just 25 feet of coax in between my random wire antenna and the tuner, and signals on some bands were hugely lower. I am, despite the pointlessness, using the internal 4:1 balun in the tuner.
You'll do better runnning the ladderline directly into the tuner, at any rate, remote or not. The remote balun and 50 feet of coax will not work very well as an all-band solution.
73,
Dan
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RE: Balun or Tuner?
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by KI6DYR on June 27, 2006
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An excellent reply!
The antenna that I'm using is a Cobra S dipole: http://www.k1jek.com/index.html The web site is specific in that it says that the antenna requires a balun and a tuner. Being a new ham of course I'm clueless. Okay. Not completely. My wife laughs at the name Elmer while I embrace himn :)
Quote:
Do I Need a Tuner?
All Cobras require a tuner. In most installations, the auto-tuner built into your transceiver should provide sufficient tuning range. Because the Cobra presents a balanced load, we recommend installing a 4:1 current-style balun at the station end of the feedline (many external tuners provide a built-in balun).
So if I copy correct, you recommend running the ladder line all the way into the shack and then into the MFJ with jumper? My only concern is that I have computers that are always on. Will radiation from the line get into things?
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RE: Balun or Tuner?
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by N3OX on June 27, 2006
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Well, this is a sticky subject. I say this because it's impossible to talk much about what the best matching/balancing setup is for the Cobra without knowing what the feedpoint impedance is on each band.
A warning as well: I ended up writing a lot of stuff. A lot of it is put together from what I understand about some smart guys who talk about baluns a lot. I might be wrong. They might be wrong. Read what you can find and make your own decisions but know that there's a lot of folklore, marketing and misinformation passed around, both for noble and shady reasons.
The manufacturer recommends a 4:1 current balun. This sounds like a reasonable suggestion at first, right? The antenna is a balanced one, so we don't want to
connect it directly to the distinctly unbalanced coax connection on the rig.
Also, we might expect that a ladder line fed doublet would present a higher-than-50-ohm impedance on the bands of interest, which is almost always true for such an antenna as long as it's longer than a halfwave dipole at those frequencies.
So imagine we have the ideal 4:1 current balun for the multiband dipole. What does this object need to do?
1) ensures that equal currents flow in each leg of the ladder line at the output
2) provide a 4:1 impedance transformation regardless of the actual impedances at input and output.
Number 1 is very useful. It forces the ladder line to have balanced currents even if something is unbalancing it; you have one side too close to the downspout, the antenna is not quite symmetrical, maybe you had to bend one leg around a tree...
Number 2 might be moderately useful if you have a tuner that can match an impedance with a 1000 ohm Z and the antenna presents 3500 ohms.
Here's the problem. Number 2 is a fiction. No balun design does this. A 4:1 balun might make a 4:1 impedance transformation between resistive impedances between 10 and 100 ohms on the input side and 40 and 400 ohms on the output side, or something like that, but if you go too far out of the useful range, or introduce a lot of reactance, it just fails to do anything like change the impedance by a factor of 4.
It changes the impedance, and if you had a good electronics simulation program or were very handy with your complex algebra, you might figure out how, but the short story is that all the 4:1 balun will do is change the antenna feedline impedance from R1+jX1 to R2+jX2 where no combination of R1,X1,R2,and X2 is at all related by a factor of 4.
Is this a problem? Not necessarily, because sometimes it will change the impedance into the range where your tuner can handle where it couldn't before. Other times, it will do the opposite. Note that I'm speaking generally here... an antenna designer *could* come up with an antenna where a "4:1 balun" does something useful on all intended frequencies, but dividing by 4 wouldn't be what it was doing.
For a random antenna fed with a random line, a 4:1 balun makes a random transformation which may or may not be useful, so maybe we can just say "take it or leave it" as long as the balun doesn't waste a lot of power in the process.
So let's go back to number 1. Balance. Balanced currents in the feedline sound great, right? I think so, and you're right to think so.
But here's the really, really insidious part. I'm repeating this hearsay, which I shouldn't be doing, but it seems to have been hashed out by some smart guys.
Some 4:1 "current balun" designs are not current baluns at all!
It says "current balun" on the box, on the tuner, in the reviews, and was designed by another very smart guy, but sometimes people make mistakes. Unfortunately, it seems that one of those mistakes has managed to get itself manufactured and sold all over hamdom. The single core 4:1 current balun isn't.
It will transform 50 ohms resistive to 200 ohms resistive. It will not ensure equal currents at the output. Calling it a current balun is a falsehood. From what I know, this is the "current balun" that is in all the tuners that come with a "current balun".
There is such a thing as a true 4:1 current balun. It needs two cores, and is basically two 1:1 current baluns with their inputs in parallel and their outputs in series. I think a couple of companies who make expensive hefty baluns do actually produce them, but they're not your $19.99 special, partially because it's more expensive to use two cores than one.
However, once again, there are caveats to their operation.
How is it that you force two currents to be equal at the output of the balun? You need to set up a situation where the current out of one terminal is exactly balanced by the current coming in the other. This is pretty easy to do if the thing between the two terminals has a pretty low impedance... it's easy to get the current to flow from terminal to terminal through a low impedance, even if there are other ways it could go (the big one that causes trouble is when currents flow on the coax shield... then the current at one terminal equals the current on the coax plus the current on the other terminal). If the thing between the terminals (the antenna+feedline) is a very very high impedance, though, and the balun has insufficient *common mode* impedance, the path of least resistance for the RF currents at one of the terminals may not be into one leg of the ladderline, but rather down the outside of the coax.
So... here we have this "4:1 balun" which, unless purchased for $80 from DX Engineering or something is not a current balun. It's not giving a 4:1 impedance transformation ratio. Even if you did spend the $80 on a superbalun, on those bands where your antenna is high impedance, even it is not going to do it's balancing job very well.
What's the 4:1 balun for? Makes us feel better because we're supposed to have a balun.
- - - - - -
Now, the practical, get you on the air without burning up a lot of power heating parts of your antenna system advice:
A ferrite core or cores wound as any sort of 4:1 balun, voltage, false current, or true current is not going to tend to be an especially lossy device at HF. So, sticking one in line doesn't necessarily cause you harm unless the strange impedance transformation it's actually doing knocks the impedance of the balun+feedline+antenna outside of the range your tuner can handle.
If you have a nice symmetrical antenna with an open wire feed spaced far from other objects, it *wants* to be balanced, and a balun in the tuner will *allow* it to be so more than connecting one side of ladder line to ground.
With this nice symmetrical antenna (the Cobra does seem to qualify), you're probably not going to get feedline radiation even with a miserable balun.
If you stick coax in between the balun and the tuner though, you could drop a huge amount of signal in the coax. Don't do it. Antenna tuners with SO-239 outputs are a crime against hamkind. Same with internal rig tuners. It's OK to run a 2:1 SWR on your coax, even a 3:1, at the ANTENNA END. The problem is that if you put a super high impedance at the antenna end of the coax, the rig end of the coax is often in the matching range of the tuner. You can try a neat little experiment sometime, if you're inclined. Put a fifty foot run of coax on your tuner output and tune into it. See if you can get a match on, say 20m. After you get a match on 20m, hook a 40m dipole to the far end of the coax and see how much the SWR changes.
- - - - - -
The executive summary: Don't use coax anywhere in a multiband antenna feed unless you've thought about the losses and are willing to live with them.
4:1 baluns in a multiband antenna system are a sham that sticks around because they don't NOT work. In my book, not not working is not exactly the same as working properly... depends on your design philosophy.
If you stick the ladderline from the Cobra into the "balanced antenna" port on the MFJ with the jumper installed, it will work well. If you have "RF in the shack" a better balun probably won't be a fix; either try to make the antenna more naturally balanced, move to a truly balanced antenna tuner (most of which you have to build yourself) or get a remote tuner and get the ladderline out of the shack.
73,
Dan
N3OX
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RE: Balun or Tuner?
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by KI6DYR on June 27, 2006
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An outstanding reply! Sadly, I'm in the middle of a thunderstorm and can't go out and change stuff around. The hail might hurt as well. But it appears clear that I have loss as a result of a 4:1 VOLTAGE balun inline connected to RG-8 connected to a MFJ that also has a balun.
Now let's do this the right way.
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RE: Balun or Tuner?
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by KI6DYR on June 27, 2006
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I routed the ladder line down the house, under it, and up throughh the floor with the LMR400 runs, into the shack. At 1700 PST WWV 10mHz was 30 over. Hard to call it an improvement yet. But strong signals.
I'll be on 7279 at 1800 PST for a personal thank you.
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RE: Balun or Tuner?
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by W6OP on June 28, 2006
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I have the Cobra Senior. I have the full 100 feet of ladder line to a DxEngineering 4:1 balun and about 50 feet of LMR 400 to the shack. Works fine on every band with the auto tuner in my TS-480.
I bought it primarily for local work and to cover bands I didn't have another antenna for. Turns out it works pretty good for DX, too. Here is a website that modeled it with EZNEC. http://www.vk1od.net/cobra/ They also have graphs showing what frequencies have the greatest losses.
Pete W6OP
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RE: Balun or Tuner?
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by KI6DYR on June 28, 2006
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"Your mileage may vary" seems to apply. Given my QTH and low-level 30 foot in the air, and oriented e/w instead of n/s -- I may just be having a tough time.
But --now that I have the ladder line coming into the shack and into the tuner/balun as depicted on the web site you indicated my performance on the receive side has increased. This is exactly how the vk1od web site shows it config. In my case I was running two baluns unintentially.
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RE: Balun or Tuner?
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by N3OX on June 29, 2006
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VK1OD is the man to go to for this sort of thing, for sure.
I think he's tackled the G5RV and all sorts of other all band antennas...
His transmission line calculator is invaluable for finding out how much loss you actually have in your lines...
W6OP: I wonder what the overall loss is in your system as the LMR400+DX Engineering balun combo is going to be lower loss than a cheap balun and RG-8X... On bands where the balun-end ladderline impedance is far away from 200 ohms, the ladderline to tuner is still less lossy, but I wonder if it's approaching negligible or not... I guess we have to know the actual impedance at the balun end of the ladderline to know.
KI6DYR, I imagine your improvement is more evident on some bands than others; that's the way it was when I switched from having a coax feed to having the tuner right at the antenna feedpoint. I remember it being a difference of night and day on 17m (this was just an end-fed wire fed against my balcony railing). I couldn't hear anything on 17 before I switched feed methods.
73,
Dan
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