Topband: Wideband Balun...?

[Tom W8JI]

Hi Eddy,

> I am thinking of incorporating a homebrewed 4:1 balun as part of a 
> transmatch that I have here, which was designed to feed coaxial cable-fed 
> antennas only...

Why did you decide on a 4:1? Generally a tuner matches a much higher 
impedance than a 4:1 balun will safely permit, and has the most difficulty 
with low impedance loads. If you use a 4:1, it makes every load lower 
impedance.

> Is there really & truly such a thing as an air core 4:1 balun that will 
> cover the entire spectrum from 1.8- to 29.7-MHz...? I've looked & I've 
> repeatedly searched on the internet, but can not quite come up with such 
> an animal. Maybe the frequency range of such a beast is far too great for 
> any practical design...
>
> Anyone have any leads in this regard...?

Generally a 1:1 ferrite core balun has the least problem with grossly 
mismatched impedances, the widest useable frequency range, and the best 
balance for a given size and cost.

Air core would be the most restricted in impedance and bandwidth, and have 
the poorest balance over a wide range of load conditions. It also would have 
very high Q, being mostly reactive in any "choking" impedance, which means 
it could actually make balance worse under many load conditions.

Let me give an example. If I had an aircore balun with a common mode 
impedance of 5 +j500 ohms (Q=100) in series with a feedline having a common 
mode impedance of any reasonable R with a reactance of around -j250 or 
higher, the balun would make unbalance increase.

Those big air core coax baluns you see will actually make many systems have 
worse balance, because they are largely a pure reactance. They only help 
balance under certain conditions, contrary to what is often assumed.

As a general rule we do not want a high-Q balun, but something with very 
high resistance. We have to compromise that at times because of heat and 
impedance limitations, but almost always one of the very last things we want 
is an air core.

I use air core baluns on some of my antennas, but I control the common mode 
impedance to ensure the baluns actually do what I want. I never use them in 
random situations.

73 Tom