[TowerTalk] More balun advice

[Earl Morse]

Mea culpa.

These observations were for a transmission line transformer.  Specifically, a 4:1 200:50 ohm Guanella design with one side of the 200 ohm output grounded to make it a 4:1 unun.

The designs were very wideband.  100 kHz to as high as I could get them to go (not all TLTs are used exclusively in the ham bands).  The power levels were 10kW (not all TLTs are used with amateur power levels, though there are probably some amateur band TLTs that see more than this).  They were for a one-time development test so I built them with what I had laying around the lab.  That limited my choice of core material.  I built 4 of them with different cores, coaxial versus parallel transmission lines, etc.  One arced over and burned during testing (apparently multiple layers of shrink tube aren't as good as teflon when it comes to making your own parallel wire transmission line).

Performance was flat across 100 kHz to over 20 MHz where the mentioned resonance starts to creep in.  

Since most hams won't be striving for that extra decade of frequency below 1.8 MHz a similar design that has enough choking impedance at 1.8 MHz would probably work all the way through the HF spectrum.

I thought I had taken better notes on this but as I pore over them now I have more questions that must have seemed obvious 2 years ago when I did this.  If old age memory is anything like traumatic brain injury memory loss then old age is gonna suck. 

Earl
N8SS


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At my age, I've not kept up with the theory and forgotten much of what I used to teach from lack of use. I hope I phrase this correctly. I think your tutorial would make good reading for those entering into this discussion and label which type of balun is being described. 


The way I interpret what he wrote (with my basic understanding of the subject), is similar to what you have said about SWR. The balun does not change the characteristics of the antenna, but rather what the feed line and rig see. His remarks do not seem to apply to a choke which is all many antennas now use, such as (many, most, or all) of the Force 12 family, but rather the voltage baluns which are usually transformers, either auto, or isolation. The preferred baluns shown are the so called bead baluns (or open chokes as in your tutorial) which do not act as resonant circuits over their operating range, but rather as a fixed resistor, or inductor for common voltage, but do not affect differential voltage (the signal). Ideally the maximum and desired isolation can be around 5000 ohms for common mode voltages / signals at the desired operating frequency. Useful isolation is available over an octave, depending on what is needed. There, because of the generic use of the term, "balun", I think it becomes confusing and only applicable to a subset of baluns? 


The choke appears to be unbalanced to unbalanced, but it can offer enough common mode rejection to appear to work well as a true balanced (the antenna) to unbalanced (the feed line) balun. 


I disagree with his # of turns analysis as with the chokes more turns and increased inter turn capacity lowers the frequency of maximum isolation at the expense of higher frequency isolation, but that isolation still covers a wide range,. I believe the curves in your tutorial demonstrate this with no resonances involved within the operating range of the choke. The isolation just drops off as I understand it. 


What he says might possibly be attributed to transformer / voltage baluns which could / might have have high frequency resonances. With out attributing characteristics to a particular type of balun, it does become confusing. When a specific term like "balun" becomes generic the use and meaning can often become muddied. 


A choke is a choke, is a choke. It is not normally resonant as we use them. It has a wide frequency range of relatively high isolation that tapers off in both directions outside of its designed range as I understand it. 

73

Roger  (K8RI)


 On 11/13/2015 3:38 AM, Jim Brown wrote:

On Thu,11/12/2015 10:07 AM, Earl Morse wrote:


The first problem is that the word "balun" is used to describe at least a dozen very different things. Thus, one or more of the answers below sort of make sense for one of those things but not for the other things. 


That said, most of what is written below makes no sense to me for any of those "things." 

73, Jim K9YC


The antenna impedance won't change, it will still look like a typical dipole with or without the balun. 


The balun will act like a transformer over a fixed set of frequencies depending on core material, # of turns, and inter-winding capacitance. The high frequency limit would be determined by core material and inter-winding capacitance and the low frequency by the core material and # of turns. 


# of turns and inter-winding capactance are mutually exclusive meaning that for the improvement you get at lower frequencies by adding more turns you will lose at the higher frequencies due to the increase in inter-winding capacitance. 


A typical balun when terminated in its design load impedance would show less than 50 ohms at the low end of its frequency range, maintain 50 ohms through its usable frequency range, then go high when it hits its self resonant frequency at the high end of its usefulness. 

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