[TowerTalk] common mode chokes, baluns and multiband doublets

[Jim Brown]

On 12/12/2017 12:21 AM, Máximo EA1DDO_HK1H wrote:
> Hi Jim,
>
>
> <On the basis of
> <that work, I do not believe that it is practical to effectively choke
> <any badly matched antenna at any power level above 100W, and even at
> <100W it's easy to fry a choke.
>
>
> So, I understand you are saying to use proper balun transformer for badly matched antennas above 100w.

Hi Maximo,

No, I'm not. Here's the problem.  "All-band" antennas fed with high 
impedance (usually 450 ohms) open wire line depend on the relatively low 
loss of those lines with high mismatch to "work." But these antennas 
have a low impedance on some bands, a high impedance on others. They are 
rarely matched on more than one or two bands. AND -- the only effective 
common mode chokes we can build from coax or 2-wire line have impedances 
in the range of 50 - 105 ohms.  Antennas with very low feedpoint Z (less 
than 25 ohms) or very high feedpoint Z (more than about 200 ohms) on any 
given band will have very high SWR, and any choke built for some other 
Zo will likely fry.

> And at same time, to use choke only on small unmatched antennas.
>
> Am I right?

No. What I'm saying is that if it's single wire that we want to use on 
all bands, we're very lucky to get a good match on only one band.  
Unless you're running low power, any choke that DOES ANYTHING (kill RX 
noise) will FRY.

> For example, to use a well sized Guanella balun (symmetric, twin toroids)

Because the antenna is high Z on some band(s) and low Z on others, that 
Guanella will provide a reasonable match on one or more bands but a 
terrible match on others. One size does not fit all!

> , at feed point, then choke the coax if necessary at the radio shack entrance.

A choke in the shack does NOT kill RX noise. It must be at the feedpoint 
(up in the air, where the feedline connects to the horizontal wires).

These "all band" random wires and off-center-fed wires were a good idea 
30 years ago when there was very little local noise, but they are a 
TERRIBLE idea for today, with every home filled with 20-30 noise sources.

73, Jim K9YC