[Elecraft] BL2 -- Antenna Balun Usage

Wed May 24 00:12:10 EDT 2017

Even Jasik, ed., "Antenna Engineering Handbook" mentions the balun.  My 
first ham antenna (1952) was a 40 meter dipole fed with 72-ohm twinlead 
and a B&W 1:1 air-core balun and I once worked on an airborn military 
system which used a balun.

The term "unun" has a much more dubious justification, however (my opinion).

73, Bob N7XY

On 5/22/17 10:07 PM, Wes Stewart wrote:
> PLEASE ignore all of Jim's pontification.
>
> I find it curious that Terman ("Radio Engineering"), Kraus 
> ("Antennas"), Johnson ("Transmission Lines and Networks") all use the 
> "meaningless" word "balun" in their books. Clearly, these guys should 
> have consulted with Jim before doing so, because obviously they didn't 
> know what they were talking about.
>
> A transmission line transformer can be as simple as a geometric mean 
> quarter-wave line between two different impedances.  No ferrites 
> required.  A balun (pardon me, I'm with Kraus) can be a quarter-wave 
> open stub at the feedpoint of an antenna.  Collins ("Fundamentals of 
> SSB") calls this a "Bazooka-type balun", but what does Collins Radio 
> know about anything?)  Or, it could be a half-wave line connecting the 
> two halves of a dipole.  A stub balun can be both a balancing device 
> and an impedance transformer at the same time.  And it's nothing but 
> coax.  A two-wire line wound around a core might be a common-mode 
> choke, but if it's long enough and different in impedance from the 
> load, then it's an impedance transformer too.
>
> In summary, just removing the term balun from one's lexicon doesn't 
> simplify anything.
>
> And I almost forgot, that N6BV article Jim mentions is titled, "Don't 
> blow up your BALUN."
>
> Wes  N7WS
>
>
>
> On 5/22/2017 2:11 PM, Jim Brown wrote:
>> On Mon,5/22/2017 12:42 PM, Bill Leonard N0CU wrote:
>>> I am no expert when it comes to baluns
>>
>> You're not the only one. :)
>>
>> Some important comments. First, PLEASE strike the word "balun" from 
>> your vocabulary. It is a meaningless word that tells us NOTHING about 
>> the device or circuit element it is used to describe. I can think of 
>> nearly a dozen VERY different devices that are CALLED baluns. Use the 
>> word "balun" conceals what the device actually is and prevents 
>> everyone involved from understanding what it does.
>>
>> A two-wire line wound around a ferrite core forms a COMMON MODE 
>> CHOKE. It is not a "transmission line transformer," nor is it an 
>> inductor, nor is it a transformer at all! The ferrite core carries 
>> only flux due to common mode current, and loss in the choke is I 
>> squared R, where I is the common mode current and R is the resistive 
>> impedance of the common mode choke.
>>
>> Arrays of common mode chokes CAN be wired in series/parallel 
>> combinations to match circuits of differing impedance, but that 
>> device is NOT a transformer, it is an array of common mode chokes. If 
>> we want to know how this array of chokes work, we must analyze them 
>> as arrays of common mode chokes, not as a transformer.
>>
>> A transformer, is, by definition, two windings that are magnetically 
>> coupled, and the impedance transformation ratio is the square of the 
>> turns ratio. If we want to know how a transformer works, we must 
>> analyze it as a transformer. It's as simple as that. The ferrite core 
>> carries ALL of the flux, and thus all of the differential power 
>> carried by the circuit into which it is inserted.
>>
>> In general, common mode chokes do NOT affect the differential signal, 
>> but there CAN be differential mode loss in the transmission line that 
>> forms the common mode choke due to transmission line effects. For 
>> example, if the common mode choke is inserted in a badly mismatched 
>> transmission line, there can excess loss due to SWR throughout the 
>> line, both in the part of the line that forms the choke and in the 
>> rest of the line. Below UHF, virtually all loss in real transmission 
>> lines is due to I square R; if the combination of the antenna and the 
>> line places a current maxima at the choke, that segment of the line 
>> can burn a high fraction of the transmitter power, greatly reducing 
>> the transmitter power that gets to the antenna and overheating (and 
>> frying) that segment of the line. N6BV wrote an excellent 
>> applications note about this for QST several years ago, to which I 
>> contributed.
>>
>> It IS practical to model (predict) dissipation in a common mode choke 
>> using NEC. A single wire is added to the model with the geometry and 
>> physical connections of the transmission line, and the known 
>> (measured) impedance of the choke is added as a Load at the point 
>> where it is inserted in the system. NEC is then set to model with a 
>> defined transmitter output power (for example, 1,500W), and currents 
>> are computed. NEC then provides a readout of current at every point 
>> on every conductor, and the current in the choke is used to compute 
>> dissipation in the choke.
>>
>> Tutorials at k9yc.com/publish.htm show a practical method for 
>> measuring the common mode impedance of ferrite chokes, and for 
>> determining values for a parallel equivalent circuit that can provide 
>> a good first approximation of dissipation.
>>
>> 73, Jim K9YC
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Elecraft at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to n7xy at n7xy.net
>