No apology at all from here for piling on prior postings. This really
needs to drubbed into hamdom's collective consciousness.
Folks want things to be simple: no long paragraphs, no big words, no
long versions allowed, executive summaries only, no details. Some
things actually are simple that way for useful results, if you don't
cut it too fine.
Other things are complex, full of traps, riddled with opportunities
for bad results. The devil is in the details, and without the details
you can't tell what you've got.
Transformers, chokes, auto-transformers and (true) baluns are
complicated with lots of opportunities to mess them up. If you have a
simple mental concept for those, you are probably going to be had.
When you are working on those, it's a mental tiger on your workbench,
not a pussycat.
The word "balun" has been so misused it's almost worthless for a clear
discussion. These days just about any blob with a coax connector on
either end is called a "balun". And some manufacturers take full
advantage of this ignorance to put some real junk out there. Other
manufacturers put out tested designs with windings and core very
carefully designed and tested to a specific purpose. Caveat Emptor !!
Put a bifilar winding on a toroid. Depending on how you connect the
winding wires, it can be a transformer with isolated windings, a
common mode choke, an auto-transformer, or a 4:1 Ruthroff balun. They
all behave differently in one aspect or another. The specifics of the
winding and the core material and size used will have enormous effect
on the operational range and behavior of the product. Some
combinations of winding and core are completely useless at a given
frequency. Other combinations of wire and core will have excessive
loss for QRP use and will crack or burn at QRO.
There is no way to simplify that discussion and reach factual
conclusions. We just have to deal with it.
73, Guy.
On Tue, Aug 13, 2013 at 9:43 AM, JC N4IS <n4is@comcast.net> wrote:
> Yes, that's a complicated matter. The name and the function can get very
> confused if you don't know what you are doing. Any transformer can change
> the voltage from the primary to the secondary and the impedance follow the
> square of turn ratio. How you connect the transformer is an application. How
> you build the transformer is an art!
>
> For broadband RX antennas you want the transformer to be broadband. For
> isolation from the primary to the secondary you want low capacitance. An
> autotransformer could be used as BALUN, balances input and unbalanced
> output, it could be broadband, but has no isolation.
>
> One example, you take a FT140-77 core and build a primary 12 turns in one
> side and 4 turns on the other side, you have a voltage transformer but it
> will perform very bad as a BALUN, or a BALBAL or UNUN depending your
> application. However if you build 3 times 4 turns for the primary and add 4
> turns on secondary in between the primary, you can get the same voltage
> transformer but It will work as a broadband impedance transformer from 1 MHz
> to 10 MHz with no adding reactance if the load is a pure resistor or low
> inductance resistor.
>
> I did try to explain it with text, I used pictures, I posted diagrams but
> people come back to me saying the antenna is not working. When I check what
> the guy did, he was using the wrong "transformer".
>
> Jim I'm with you again, very few hams really understand it.
>
> Regards
> JCarlos
> N4IS
>
>
>
> -----Original Message-----
> From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Shoppa,
> Tim
> Sent: Tuesday, August 13, 2013 8:08 AM
> To: 'jim@audiosystemsgroup.com'; 'topband@contesting.com'
> Subject: Re: Topband: Zo of an individual CAT5 twisted pair
>
> A transformer that is connected such that it is UNbalanced on one side and
> BALanced on the other, and connected that way on purpose, is not a balun?
>
> Tim N3QE
>
> ----- Original Message -----
> From: Jim Brown [mailto:jim@audiosystemsgroup.com]
> Sent: Tuesday, August 13, 2013 03:16 AM
> To: topband@contesting.com <topband@contesting.com>
> Subject: Re: Topband: Zo of an individual CAT5 twisted pair
>
> On 8/12/2013 2:10 PM, JC N4IS wrote:
>> 50/75 BALUN
>
> Thanks for the detailed post, Carlos. BUT -- please let's use the right
> words to describe things so that people understand what you're describing
> and how it works. I strongly suspect that at least some of those things you
> are calling a "balun" are really a simple transformer
> -- that is, a primary and a secondary with magnetic coupling between them,
> and probably on a ferrite or powdered iron core. If it's a transformer,
> let's call it a transformer. Likewise, if we have a common mode choke formed
> by winding a coil of the transmission line, it is a common mode choke, not a
> "balun." Using the word "balun" confuses things, because that word is used
> to describe at least a dozen very different things that I know of.
>
> When we use the word "balun," it's a magic box that few hams really
> understand. When we use the right word, most hams have a chance of
> understanding what it does in a circuit. :)
>
> Yes, there are arrays of common mode chokes that can be used to transform
> impedance, and there are transmission line transformers of various sorts
> that can do that as well.
>
> BTW -- your discussion of phasing between elements of an RX array causes me
> to add an important post script to my advice that a perfect match is not
> required. When ANY passive network is used to produce phase shift, the
> source and termination impedances DO matter. The tricky part, though, is
> knowing what the input Z of the RX is, and if you're doing something like a
> phased array using phasing lines that end at the RX input, it might be a
> good idea to actually measure input Z and the antenna Zs with a VNA.
>
> 73, Jim K9YC
> _________________
> Topband Reflector
> _________________
> Topband Reflector
>
> _________________
> Topband Reflector
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Topband Reflector
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