[TowerTalk] Baluns/tutorial/notes.

[Joe Subich, W4TV]

 >> 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.

Correct if one is interested in keeping the omnidirectional noise
from coupling to the antenna.

 >> 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.

There may be ... if the omnidirectional noise can couple to the
antenna in one direction, it should also be able to couple into
the receiver in the other direction (particularly if the equipment
is not properly grounded and decoupled as you have documented
elsewhere).

I'm a believer in placing a broadband 5K common mode choke on the
equipment side of the feedline(s) at the common (entry) ground.
All equipment should be bonded to the common ground point with a
separate, large conductor and not rely on the shield of the coax
for that purpose.   Even if the common ground is relatively poor
at RF, it will still be fairly low compared to choke's impedance
and result in about 30 dB in reduction of common mode noise on
the shield.

73,

    ... Joe, W4TV

On 5/22/2010 11:45 AM, Jim Brown wrote:
> 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
>
>
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