[TowerTalk] Baluns/tutorial/notes.

[Jim Brown]

On Sat, 22 May 2010 07:11:05 -0700, Jim Thomson wrote:


>## Interesting. Since abt 1980.. I have been running all coax down the
>Inside of the tower, since the tower's were wide enough to climb up
>the inside, and it was easy to bring coax up.  Dunno if that would help
>the noise pick up issue..or not?   

Good question. Most of us do that too. As I see it, the coax inside the tower is 
at the potential of the tower, and MAY couple to it. The tower will act as an 
antenna. A choke at the feedpoint (that is, very close to the antenna terminals, 
after it has emerged from inside the tower) prevents any common mode current from 
coupling to the antenna. 

With this new crank up, coax will
>be on the outside.  OK,  if the ants are say down low to start with,
>say 30'-50', why wouldn't any noise be directly induced/picked up by
>the 'low' ants? 

Noise WILL be picked up by the antenna, but antenna directivity usually reduces 
that. Noise coupled from the feedline is added to that. Think of it as filling in 
the nulls of the pattern. Noise on the antenna is also usually less than noise on 
the feedline because it's further from the source (i.e., in your neighbor's living 
room or kitchen). 

>Each torroid could  also be wound
>slightly differently, and/or with different mix torroids,  to create
>a broadband  [or at least broader] effect.  

Yes. The DOD guys figured that out in 1966, but didn't publish it to the outside 
world. I also figured it out and DID publish it around 2005. 

> I'm just re-inventing the wheel. 

That's what happens when knowledge is not shared. :)

>## this is amazing info. 

It's entirely logical from fundamental circuit analysis. See the development of 
this in my power point on Coax Chokes. Again, the DOD guys figured that out -- 
that's why they wanted those chokes and why they hit on 5K. 

http://audiosystemsgroup.com/publish.htm 

>But the 3-5 k  has to go at the feedpoint,
>and not in the form of a line isolator at the entrance to the shack? 

That's right, because the feedline acts like any other antenna. Choking it at the 
shack simply puts an open circuit at that point, but depending on its length, that 
can make it a very effective RX antenna, which then couples to the main antenna.  

>These line isolator's appear to be just a bead balun..but with coax 
>connector's on both ends.  

I haven't opened any of them up, but probably yes. Some may be wound chokes too. 
I'm sure that W8JI has figured that out. 

>  Put another way, is their any benefit to 
>installing a 5 K choke balun at the entrance to shack... [for noise
>elimination] ?  

Probably not for noise isolation, but it is definitely a good thing to kill 
interaction with other antennas, especially wire and some verticals. See my 
discussion on using chokes as the equivalent of "egg insulators" in the feedline. 

>I think a line isolator would solve residual problems,
>mainly from TX being induced into the feed line, if ant pointed at the
>station.   I have never used one...so don't know. 

If they are wound with low loss coax with suitable power handling, there is NO 
downside.    

>>Has anybody actually toggled between 2 x identical yagi's 

>Not an easy test setup. :) 

>###  plenty of folks out there with stacks., multiple ants,
>anything that has the DE close to the tower, to swap out a
>balun, would be the easiest to do a A/B test.

Yes, but the antennas are usually at different heights, which makes a comparison 
of questionable value. >

>>will they handle 2.5 to 3 kw...with high SWR ???    After line loss,
>>I want to see at least 2250 w at the ant feed point.

>## maybe the limit is 1.5 kw in the usa...but it's 2250 w pep...
>after feedline loss... in Canada.  750w on cw / fm... and 800w
>CXR on AM [4 kw pep with 125% pos mod] .. and 750w on 30m
>band.  There are several countries who have even higher limits. 
>If you want ther balun to be reliable.. and handle 1.5 kw.... then
>build it to handle double that. 

Like I said, the coaxial chokes I've described have a lot of headroom. The key is 
to keep the choking Z high. 

>## I'm referring to running it down the coax..from top to bottom
>of the tower. 

I understand. See discussion of coax inside tower, above. Also, many of us use 
antennas other than aluminum arrays on towers. I presently have six wires high in 
trees, only two antennas on a tower. 

>##  I will try one of these 5K  baluns this fall.  I have enough spare vac
>relays,  I might be able to toggle  between the outputs of 2 x baluns..
>with just a pair of spdt vac relays.... and also use 2 coax feedlines.   I would
>like to be able to do a A/B test on it.

By all means do that if you like, but I can say with confidence that 1) there is 
no downside to using a choke if you keep the choking Z high and use suitable coax; 
2) they don't cost much to build (much less than anything you can buy); and 3) in 
addition to my own experience, I've had considerable feedback from very good 
engineers, mostly in the contesting community, confirming that these very high Z 
chokes can make a big dent in RX noise. They also can help with RF in the shack 
and other forms of RFI. 

73,

Jim Brown K9YC