[TowerTalk] More balun advice

[Roger (K8RI) on TT]

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.
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk


-- 

73

Roger (K8RI)


---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus