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4:1 balun uses

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by N5YPJ, Oct 13, 2010.

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  1. N5YPJ

    N5YPJ QRZ Moderator QRZ Page

    Just curious, are there recommended applications for a 4:1 balun besides perhaps an OCF dipole? I know this balun is found in most or maybe all commercially manufactured antenna tuners but I'm wanting to know where it's use might get better results than a 1:1.
     
  2. G3TXQ

    G3TXQ Ham Member QRZ Page

    I use a 1:4 Current Balun (4:1 in reverse) to match the 12Ω feedpoint impedance of a 5-band nest of bent dipoles to 50Ω coax:

    http://www.karinya.net/g3txq/cobweb/

    73,
    Steve G3TXQ
     
  3. W1VT

    W1VT Ham Member QRZ Page

    I've used coaxial 4:1 half wave baluns on 144, 222, 432, and 903MHz Yagi antennas. They are great for feeding T matches.
     
  4. KE5FRF

    KE5FRF Ham Member QRZ Page

    Others will chime in, and I am only going to offer you what has been said and written about many times over.

    The typical "breed" of 4:1 baluns on the market are voltage baluns (Ruthroff-balun), whereas the typical 1:1 balun is a current type (Guanella-balun).

    There are designs for a 4:1 Guanella current balun. This would be a better choice for a impedance transformation than the voltage type because the varying electric field and permeable core of a voltage balun causes hysterisis and thus resistive loss (heat). It is the same principle that causes a power supply transformer to heat up. Heat is wasted power.

    The current balun avoids this issue.

    The 4:1 baluns in most antenna tuners are voltage types because, really the only reason, they are cheaper.

    A 4:1 impedance transformation has useful applications, as you mentioned in an OCF, or any antenna that is intentionally fed at a higher impedance than the characteristic 50 ohms of coax. Folded dipoles are another example of where a 4:1 balun would be useful.

    But the 4:1 current balun would be the preferred design.

    Using a 4:1 on a multiband doublet type antenna would not be a good idea, If the antenna is resonant on any band, the feedpoint impedance would be too low to step down by a ratio of 4:1. Fifty to seventy ohms would be between 12 and 18 ohms and would present a high VSWR to the transmitter and difficulties for an antenna tuner.

    In no case that I've heard of would a dipole 1/2 wavelength or more on the lowest frequency of operation present an impedance as low as what the 4:1 balun would cause. So a 1:1 transformation would be ideal. Even on bands where the impedance is great (for instance if the antenna is near 1 full wavelength on a given band), many broad-range tuners will be able to handle it. A 4:1 balun would be useful here, but the losses in a voltage type balun would be an issue.

    In most cases that can be described, the 1:1 balun is as good or better a choice in terms of resistive loss vs. a 4:1 voltage balun.

    I'm just expressing the same details that others more knowl;edgeable than I will explain.
     
  5. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Most OCFs (the 33/66 offset variety) actually do better with a ~6:1 balun if you're using coax.

    If you're using a wide range tuner feeding balanced line, the "correct" balun would depend on your operating frequency, antenna length and feedpoint, and transmission line length, and can be almost anything. 1:1 can be a better compromise than 4:1 in lots of cases; but the tuner can make up for whatever it is most of the time.
     
  6. KE5FRF

    KE5FRF Ham Member QRZ Page

    I found a good article with a thermal image photograph of a 4:1 voltage balun with RF applied for 5 minutes. The lower permeability of the first core results in heat in the core.

    http://g8jnj.webs.com/Balun construction.pdf

    That's a pretty dramatic illustration of how the permeability of the core material at various frequencies has an effect on losses.
     
  7. G3TXQ

    G3TXQ Ham Member QRZ Page

    Heath,

    One reason a 1:1 Current Balun might be preferred to a 4:1 Voltage Balun is that its core flux (and therefore the loss) is not determined directly by the transmit voltage, whereas the 4:1 Voltage Balun's is.

    However, that advantage does not apply to the 4:1 Current Balun; just like the 4:1 Voltage Balun, its core flux is determined directly by the transmit voltage!

    There may be good reasons to use a 4:1 Current Balun rather than a 4:1 Voltage Balun - especially if you want to balance currents rather than balance voltages :) - but the 2 cores in a 4:1 Current Balun, between them carry the full differential-mode transmit voltage as a common-mode signal.

    In this respect, the 1:1 Current Balun is unique amongst all the balun types.

    73,
    Steve G3TXQ
     
  8. KE5FRF

    KE5FRF Ham Member QRZ Page

    Steve,

    What, if any, effect does frequency and core matching have on this?

    My understanding was that if the two cores in a 4:1 current balun were a matched pair that it would be more efficient over a broader frequency range than a 4:1 voltage balun.

    I've seen it stressed that the cores should be matched for the transmission line mode to work properly and for all to be in balance.

    At any rate, thanks for the information.
     
  9. G3TXQ

    G3TXQ Ham Member QRZ Page

    Heath,

    Here's a schematic of a 4:1 Voltage Balun, drawn slightly unusually to emphasise that at its heart is a common-mode choke:

    [​IMG]

    At the output of the choke (terminals 3 & 4) we generate a "copy" of the input voltage Vin. Because of the isolating properties of the choke we can consider that voltage to be "floating" and therefore we can ground terminal 3 with impunity. The effect is to force terminal 4 to -Vin, thereby generating 2*Vin between terminals 1 & 4 - the output of the balun.

    However, as the frequency increases the transmission line forming the choke becomes a significant part of a wavelength and therefore the version of Vin at 3/4 becomes delayed wrt Vin at the input, and slightly out of phase. The two "versions" of Vin then no longer add to produce 2*Vin.

    If you do the same analysis on a 4:1 Guanella balun you can see that the two "floating" versions of Vin that the two chokes develop are delayed by equal amounts providing the transmission lines are identical.

    A long way of saying that the upper frequency limit of the 4:1 Guanella Balun is higher than the 4:1 Ruthroff Balun. Obviously, keeping the TL short and compensating for the fewer windings by using a high permeability core material helps extend the bandwith of the 4:1 Ruthroff.

    Steve G3TXQ
     
  10. KE5FRF

    KE5FRF Ham Member QRZ Page

    Ah Steve, excellent explanation and another nugget for me to add to my collection of nuggets. Eventually I might actually come out with a gold brick :)

    If I may, I'm going to reword your explanation, emphasis on what I already understand explained with an example (I love examples) and then to add the part that you opened my eyes with.

    The way I have always looked at the impedance transformation of a balun is similar to the output of a power supply with both a -neg and +pos output voltage. Take a 5 volt supply with two outputs. Truth be told, -5 and +5 are "relative". An isolated load requiring a 10 volt DC supply could very well be driven by the full 10 volt differential and the common terminal could be ignored, as long as the supply circuit can handle the load current.

    In other words, just as the choke output terminals are floating, so is a DC supply on the other side of a transformer until the common terminal is tied to ground. This also "forces" the output voltages with respect to each other and they are no longer floating. What the voltage balun is essentially doing is taking what "would be" the more positive output voltage and forcing it to the same potential as the lower of the two input voltages, then taking the differential between the more negative output and the more positive input, which would now be 2*Vin, as you described.

    That is pretty clear as you trace it out.

    Now, what you described about the increase in frequency/shorter wavelength, means that the two parallel chokes in the balun have an IMPEDANCE across each coil, which shifts the output out of phase. This is sort of a counter-impedance to the impedance transformation intended by the balun, because the voltages out of phase with each other would mean mathematically less differential if the two sine waves are plotted against one another. So, I'm to surmise that outside of the frequency range that the windings permit, the output impedance would be <4:1

    I had never considered that point, that the balun itself would be "too big" at higher frequencies. An SHF balun must be a real bugger! Do they make such a beast?

    So, the end lesson here is that there IS an advantage to a 4:1 Guanella over a 4:1 Ruthroff, in that it is EASIER, or less mechanically intolerant, to build a broad-banded transformer that operates at acceptable efficiencies in the current style, whereas the voltage style requires more attention to "detail" i.e. a more permeable core is required.

    Would that be in agreement with your post? I want to understand and I think I do.
     
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