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Author Topic: Will a Balun work "backwards?"  (Read 7526 times)
K5JCJ
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« on: June 28, 2010, 12:28:34 AM »

I am trying to find an effective way to make my mobile HF installation work better and cover more bands. My setup consists of a Yaesu FT857D, and a bumper-mounted Ham-stick 80 meter antenna fed with LMR-400 back to the cab of my truck (Ford Ranger). I have recently come across a Yaesu FC-40 remote tuner and want to use it to drive the whip, but because of how difficult it is for me to get cabling from inside to outside I was thinking of mounting the FC-40 inside the cab and hooking the terminals up to a 1:1 balun to drive the coax. Realizing that this is an unusual use of a balun, do you think it will help things any, or should I just wire the coax directly to the FC-40's output terminals?

Thanks in advance and 73,
John K5JCJ
 
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AA4PB
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« Reply #1 on: June 28, 2010, 05:01:38 AM »

No need for a balun. Just wire the coax to the output terminals.

A balun is only needed when transitioning between unbalanced (like coax) and balanced (like ladder line). A balun will work either way. For example, you could use a 4:1 balun with the lower impedance side on either the input or the output (i.e. match 50 Ohms to 200 Ohms or 200 Ohms to 50 Ohms).

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KM3K
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« Reply #2 on: June 28, 2010, 05:37:26 AM »

A balun will work either way. For example, you could use a 4:1 balun with the lower impedance side on either the input or the output (i.e. match 50 Ohms to 200 Ohms or 200 Ohms to 50 Ohms).

Well, that is true if it is a 1:4 Guanella balun but not so for a 1:4 Ruthroff balun.
Quoting Jerry Sevick W2FMI (sk) from his book "Transmission-Line-Transformers" 4th-edition pg 9-15 for a 1:4 balun with the load floating..."The Ruthroff-balun only works in one direction. The high-impedance side is always the balanced side. The Guanella-balun,, on the other hand, is bilateral. It can work as well in either direction....Therefore, a Guanella-balun can easily be designed to match a 50-ohm coaxial-cable to a 12.5-ohm balanced-load."

The Guanella-balun is more commonly called a "current-balun" and the Ruthroff-balun is the "voltage-balun" per Roy W7EL's 1985 article in an ARRL antenna-book.

BTW, AA4PB wrote "4:1" and Sevick wrote "1:4"; in other places in his books, Sevick writes it as AA4PB did. The only consistency that I've found in this notation is inconsistency. Roll Eyes

73 Jerry KM3K

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AA4PB
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« Reply #3 on: June 28, 2010, 07:18:02 AM »

So if I place a Ruthroff balun in my feed line and it only works in one direction then how does the received signal get back down the line to the receiver?
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KM3K
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« Reply #4 on: June 28, 2010, 08:06:23 AM »

So if I place a Ruthroff balun in my feed line and it only works in one direction then how does the received signal get back down the line to the receiver?
1. For a Ruthroff-balun, that's a darn good question and I now recall, way back when I read W2FMI's book, asking myself the very same question but I never came back to it to work out an answer.
2. It is all the more significant in light of Sevick's book "Understanding, Building...Baluns and Ununs" pg 54 where in section 8.6 #1 he surprised me in recommending for balanced applications using the 4:1, single-core, two-wire, voltage-balun rather the two-core current-balun.
3. It is a non-issue for those who use current-baluns or have the luxury of a separate receive antenna.
4. Wouldn't it be fair to assume that the signal does get thru albeit attenuated to some unknown amount?
If someone has already supplied an answer, I don't recall seeing it or, if I did, I no longer recall reading it (a tragic but definite possibility at my age Embarrassed).
So, to accurately answer the question, testing of the voltage-balun should be done to see how much the signal level is affected in each direction for each of the ham-bands.
I could do it but I'd have to get material (cores/wire/tape), so it'd be many months before I could supply any answer and besides I'm preparing for two talks for upcoming club-meetings.
Anyway, as I wrote before...that's a darn good question....I'll put it on the to-do list.
73 Jerry KM3K
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KE4DRN
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« Reply #5 on: June 28, 2010, 05:58:22 PM »

hi,

this is good info, you don't find these questions on an FCC exam!

73 james
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AA4PB
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« Reply #6 on: June 29, 2010, 05:08:58 AM »

The Ruthroff-balun only works in one direction. The high-impedance side is always the balanced side.

In thinking about this last night, I think what we have is a terminology thing. I think by "only works in one direction" he does NOT mean that power cannot be sent into the high-impedance side and taken out of the low-impedance side. He means that you can't make the high-impedance unbalanced and the low-impedance balanced. It seems obvious to me that power can be transferred in either direction as witnessed by the fact that receive signals flow back through the balun.

If somehow I had a transmitter that had a 200 Ohm balanced output, I could use a Ruthroff-balun to convert that to 50 Ohms unbalanced by connecting the transmitter output to the balanced side of the balun.

The other concern with the Ruthroff-balun (voltage balun) is that to work well it needs to be operated fairly close to the design impedances. For example, if I had a 1:4 balun designed to match 50 Ohms unbalanced to 200 Ohms balanced it wouldn't work very well if I tried to use it to match 500 Ohms to 2000 Ohms. Working well means low loss and good balance on the balanced side.

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KM3K
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« Reply #7 on: June 29, 2010, 09:29:17 AM »

In thinking about this last night, I think what we have is a terminology thing.
I think by "only works in one direction" he does NOT mean that power cannot be sent into the high-impedance side and taken out of the low-impedance side.
He means that you can't make the high-impedance unbalanced and the low-impedance balanced.
It seems obvious to me that power can be transferred in either direction as witnessed by the fact that receive signals flow back through the balun.
If somehow I had a transmitter that had a 200 Ohm balanced output, I could use a Ruthroff-balun to convert that to 50 Ohms unbalanced by connecting the transmitter output to the balanced side of the balun.
The other concern with the Ruthroff-balun (voltage balun) is that to work well it needs to be operated fairly close to the design impedances.
For example, if I had a 1:4 balun designed to match 50 Ohms unbalanced to 200 Ohms balanced it wouldn't work very well if I tried to use it to match 500 Ohms to 2000 Ohms. Working well means low loss and good balance on the balanced side.

Well, there are a few things to comment about:

1. Yes, AA4PB, I think you have a good grasp of what Sevick means about "only works in one direction".
As before, the question remains, "What is the difference in insertion-loss between the two directions?" and, as I wrote yesterday, insertion-loss tests need to be done.

2. Now about your sentence above "If somehow....Ruthroff-balun...."; I don't know how well that would work; one ought to expect an efficiency (insertion-loss) problem; in normal-use (say an unbalanced 50-ohm to balanced 200-ohm), the Ruthroff-balun makes use of a delayed voltage; how that works out in the reverse direction, I'm not yet comfortable working out an analysis on paper, which is why I'd want to see some insertion-loss data or do my own tests.
Realistically, to do balanced 200-ohm to unbalanced 50-ohm, IMHO the safe and best way is to use a two-core current-balun (Guanella).

3. Now for the paragraph "The other concern with the Ruthroff-balun....", your statements there are perspective.
For completeness sake, we can state that any balun, whether it is current (Guanella) or voltage (Ruthroff), has a transmission-line, which has a characteristic-impedance. The best frequency-response and efficiency, meaning low insertion-loss  (especially for high-freq), is had by keeping in mind some facts:
3a. For a 1:1 balun, the balun should be driven (source) and loaded with impedances as close to the transmission-line's characteristic-impedance as possible.
This mean that for a 50-ohm system, our line when wound on the core should be a 50-ohm line.
3b. For a 4:1 balun, the source-impedance should be one-half the characteristic-impedance and the load-impedance should be twice the characteristic-impedance.
For a 50-ohm to 200-ohm system, the line when wound on the core should be a 100-ohm line.

Just one other item separate from anything above...
When the day comes for me to use a balun, my first "go-to" will be:
a) for 1:1, a current-balun (Guanella),
b) for 1:4, a two-core current-balun (Guanella).
These choices are based on my wanting something to work the best without hassle.

73 Jerry KM3K  FN10je
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K6LHA
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« Reply #8 on: June 30, 2010, 12:10:04 PM »

The Ruthroff-balun only works in one direction. The high-impedance side is always the balanced side.

In thinking about this last night, I think what we have is a terminology thing.

I'm still trying to come to grips with the terminology of differentiating between "current balun" and "voltage balun."   Grin

Quote
I think by "only works in one direction" he does NOT mean that power cannot be sent into the high-impedance side and taken out of the low-impedance side. He means that you can't make the high-impedance unbalanced and the low-impedance balanced. It seems obvious to me that power can be transferred in either direction as witnessed by the fact that receive signals flow back through the balun.

Not having read such modern-day reference works as the one mentioned, I just go through the older, acceptible-to-industry theory, especially for distributed constant arrangements instead of lumped-constant HF-centric cases.  Take "Scattering Parameters."  There's only four types for a 2-port black box.  Those of us who use them don't really care what is in the "box", just the characteristics of each S-subwhat versus frequency.

Quote
The other concern with the Ruthroff-balun (voltage balun) is that to work well it needs to be operated fairly close to the design impedances. For example, if I had a 1:4 balun designed to match 50 Ohms unbalanced to 200 Ohms balanced it wouldn't work very well if I tried to use it to match 500 Ohms to 2000 Ohms. Working well means low loss and good balance on the balanced side.

For fun, play with an old POTS "hybrid" transformer.  It was so well-balanced over a large frequency range (over a decade) and worked with a wide range of impedances, yet deliberate "side-tone" unbalance had to be added to make the user more comfortable.   Grin

73, Len K6LHA
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KM3K
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« Reply #9 on: June 30, 2010, 02:22:49 PM »

I'm still trying to come to grips with the terminology of differentiating between "current balun" and "voltage balun."

Take "Scattering Parameters."  There's only four types for a 2-port black box.  Those of us who use them don't really care what is in the "box", just the characteristics of each S-subwhat versus frequency.

For fun, play with an old POTS "hybrid" transformer.

To K6LHA, some technical thoughts...

1. for your first comment above, here is a link to several very good articles, especially the one "Baluns: what they do and how they do it". It  is a good read slanted more towards "end results" rather than design and Sevick, in one chapter of one of his books, makes heavy use of it.
I'd like to pass along a key point that took me sometime to finally "sink-in" about baluns and that is....a balun essentially has two parts in parallel, namely a transmission-line but because it is wrapped around a toroid, those wires are doing two things at the same time and are actually inductors in parallel with the transmission-line. Those inductors are critical towards getting the transmission-line behave as a transmission-line; as Sevick puts it, they prevent conventional current flow. The standard inductance formula for a toroid inductor applies.

2. Scattering Parameters....They could be applied to baluns but I don't recall seeing it used.
I think Sevick may have alluded to briefly using network-analyzers to correlate the test-results of his home-made test-equipment but that is all.
s11 is the rho, the reflection-coefficient;
s21 the frequency-response (or if you prefer the term 'insertion-loss');
s12 the frequency-response going the other way if the device is not truly bilateral;
s22 the reflection-coefficient going the other way.
Of course, in a 50-ohm system and using a HP3577a or HP8753,
testing a 1:1 50-ohm-balun should be straight forward;
testing a 1:4 50-ohm to 200-ohm balun would be straight forward (I'd use a minimum-loss pad to terminate the 200-ohm side) if one wanted to see what happens from only the 50-ohm side but the terminating becomes more complicated if one wanted to check s12 and s22.

3. What is the frequency-response of a POTS hybrid-transformer? I have not ever seen one.

BTW, I sure would like to get my hands on the original articles by Guanella and Ruthroff, especially Ruthroff's to see what he had to say about his "magnetizing current" and his "third wire"; it is a topic that to this day eludes me in the "balun" context.

73 Jerry KM3K

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KM3K
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« Reply #10 on: June 30, 2010, 02:26:18 PM »

here is a link to several very good articles, especially the one "Baluns: what they do and how they do it".
Darn, I forgot to put in the link to W7EL; here it is....
http://www.eznec.com/miscpage.htm
73 Jerry KM3K
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KM3K
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« Reply #11 on: July 01, 2010, 06:46:46 AM »

I'd like to pass along a key point that took me sometime to finally "sink-in" about baluns and that is....a balun essentially has two parts in parallel, namely a transmission-line but because it is wrapped around a toroid, those wires are doing two things at the same time and are actually inductors in parallel with the transmission-line. Those inductors are critical towards getting the transmission-line behave as a transmission-line; as Sevick puts it, they prevent conventional current flow. The standard inductance formula for a toroid inductor applies.
I should have written".....they prevent conventional transformer current flow."
Sevick's actual wording is, "...conventional transformer currents are suppressed."
73 Jerry KM3K
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K0BG
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« Reply #12 on: July 01, 2010, 10:03:42 AM »

If you want some good honest work on baluns, and balun types, you have a couple of choices.

Certainly, Tom's, W8JI, (http://www.w8ji.com) has some of the best on the net, as does Owen, ZK1OD (http://www.vk1od.net).

I think the original poster was asking about a tapped UNUN which is a type of transmission line transformer. He also referenced a Hamstick antenna, and wanting better performance. Adding a UNUN or balun isn't going to do anything for the Q of the coil, where it's the biggest loss in a Hamstick. Adding insult, is the low mounting which causes a goodly portion of the return current to flow through the lossy surface under the vehicle, rather than through the vehicle's superstructure. Both of these factors make the input impedance very close to 50 ohms, so better matching isn't a way of increasing performance in this case.

The poster mentioned he was using LM400, when LM200 would have been just as good. The smaller coax would also allow the use of a multi-turn common mode choke, which would probably do as well for performance (less ingressed common mode noise).

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K6LHA
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« Reply #13 on: July 01, 2010, 11:58:45 AM »

I'm still trying to come to grips with the terminology of differentiating between "current balun" and "voltage balun."

Take "Scattering Parameters."  There's only four types for a 2-port black box.  Those of us who use them don't really care what is in the "box", just the characteristics of each S-subwhat versus frequency.

For fun, play with an old POTS "hybrid" transformer.

To K6LHA, some technical thoughts...

Thanks for the reply, but, as I wrote, I've not read Sevick's book so I can't comment on specifics from that book.

I use a collection of notes on wideband RF transformers that goes back to when they first appeared in industry texts/publications...were applied on the bench, in the lab, and worked.  That goes back in time like to the late 1960s.   Grin

I'm not considering baluns (in the generic form) as just toroids or structured "just so" according to some book.  The inductors can be any form, toroidal or cylindrical or whatever, air-core to magnetic-material.  BTW, in some cases, toroidal forms don't always wind up with their by-themselves inductance but get modified by coupling to other windings as a transformer.  That's where some grunt work and skull sweat come into play with basics.   Cheesy

Quote
2. Scattering Parameters....They could be applied to baluns but I don't recall seeing it used.
True enough...in amateur radio publications.  I try to avoid that in writing to amateurs who haven't worked on anything else but amateur equipment below 6m.

Quote
3. What is the frequency-response of a POTS hybrid-transformer? I have not ever seen one.
POTS = Plain Old Telephone Set/System.  There used to be one in every single telephone "set" of the analog kind.  The upper frequency range goes above 3 KHz but the core material and windings are done to deliberately cut off the audio range below (roughly) 200 Hz for the handset circuit.  You may not have seen one but I'll bet lots of folks older than around 40 have used one.   Cheesy

That (to me) associates a lot of things with the telephone infrastructure.  For example, long-lines repeaters in wired telephony.  Most of those use bilateral amplification in full-duplex mode.  Usually two separate amplifiers with a more complex hybrid transformer set-up.  Response with Type C Carrier equipment had to cover the audio range beyone 12 KHz and be quite balanced and flat (it was "high-fidelity" before that became an electronics hobby genre).  On really long lines there would be dozens of such bilateral repeaters and just one getting off-balance would not be a good thing.  <shrug>

Phil Smith's "Wonderful Diagram" (the Smith Chart) got its start prior to WWII when Phil was doing grunt work checking out physical long-lines' quality.  600 Ohm characteristic impedance on open-wire lines back then.  Enormous amounts of desk work doing calculations back when mechanical calculators were scarce due to tight-fisted bean counters.  A polar-coordinate chart would evolve using normalized quantities of impedance/reactance/admittance...and also help give birth to the Scattering Parameter concept after WWII.

One just doesn't associate Smith Charts with audio frequency ranges but that is how Phil came up with it...as he wrote in his own book on it years later.

"Hybrids" as a generic name (good old terminology thing again) were made by the millions starting before WWII.  By the evil old telephone infrastructure back when they were (not) a "greedy monopoly."  They trace their development to the diffeent "bridge" circuits used for metrology.  Signals into/out-of hybrids (or some balun hook-ups) may seem to be unilateral or "one-way" but there is always some error to make them imperfect.

Quote
BTW, I sure would like to get my hands on the original articles by Guanella and Ruthroff, especially Ruthroff's to see what he had to say about his "magnetizing current" and his "third wire"; it is a topic that to this day eludes me in the "balun" context.
Ruthroff and a colleague wrote several concise papers along with many illustrations for McGraw-Hill's Electronics biweekly magazine.  [a subscription periodical which McGraw-Hill abandoned some time ago when the electronics industry got so big that advertisers could keep periodical competitors afloat as freebie magazines as "controlled circulation" structures]  I no longer have copies of those early 1970s Electronics articles.

I've not come across any reference to "Ruthroff's third-wire" in baluns but his lower-frequency limit of winding twisted-pairs as a quarter-wavelength at the lower end does work just fine in practicality for the balanced winding in a wideband RF transformer.  As such, I used it in 1980 as a 40/41 MHz coupling for Laser instrumentation at Rocketdyne (large sub-division doing laser work).  Since then I've gone to purchased components already specified as a block as being much easier to get/buy/achieve, such as the products of Mini-Circuits and others.

73, Len K6LHA

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N2EY
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« Reply #14 on: July 01, 2010, 02:36:13 PM »

Yes
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