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Static Drain Choke location

Posted: Sun Aug 28, 2016 11:03 pm
by ChuckG
AM Folded unipole, I'm installing a Kintronics Static drain choke. It should go at the ATU output, but I but can't fit it inside the ATU without moving the other components to make room.

Is there any reason I cannot install it inside the building where the transmission line enters? There is a Main/Aux contactor mounted to the wall that would provide easy access to both RF and Ground and the choke will fit inside that cage.
ATU is a T- net, two series coils and a shunt cap. No series caps anywhere between tower and transmitter. Seems to me it would drain static off just as well there as anywhere. Any reason I shouldn't put it there?

Re: Static Drain Choke location

Posted: Sun Aug 28, 2016 11:11 pm
by Deep Thought
It needs to go on the tower side of the ATU output. If not located there that shunt cap is an arc gap as far as lightning is concerned. If you just can't put it there then fashion a horn gap of some sort for the unipole lead-in at the bowl insulator and set it as close as you can without it firing under normal modulation conditions. The problem with unipoles (well, one of many) is the high operating voltage at the feedpoint.

How the heck are you getting away without having a capacitor in series with the output leg?

Re: Static Drain Choke location

Posted: Mon Aug 29, 2016 4:09 pm
by W2XJ
If you can't fit it in the existing ATU, get a separate enclosure. If you don't put it right on the fold wires you will likely destroy any bridge you might attach in the future, not to mention the risk of personal injury.

Re: Static Drain Choke location

Posted: Mon Aug 29, 2016 10:46 pm
by ChuckG
Deep Thought wrote:
How the heck are you getting away without having a capacitor in series with the output leg?
It's a single coil, triple tapped with a shunt cap to make a "T", like the old Gates units, only this one is installed in a larger housing and the coil has an adjustable slug inside the windings.
Only about 1/3 of the 400' tower is skirted, more of a three wire slant feed, IMHO. The feed point is around 120 ohms, not 50. I don't recall the reactance. It's never had an output cap that I know of.
I've bridged the transmission line at the transmitter output and it's 50 /j0...so I'm not asking any questions. :lol:

Thanks both of you for the comments. I'll either move components around or add an additional housing so I can attach @ the ATU output.

Re: Static Drain Choke location

Posted: Mon Aug 29, 2016 10:55 pm
by ChuckG
W2XJ wrote: If you don't put it right on the fold wires you will likely destroy any bridge you might attach in the future, not to mention the risk of personal injury.

I'm curious why that might happen? I've seen them almost universally mounted at the ATU output and assume there is a good reason for that, but never learned exactly what that reason was. I just assumed it was as to keep the tuning components behind the drain.

Re: Static Drain Choke location

Posted: Mon Aug 29, 2016 11:07 pm
by W2XJ
You just want it on the fold wire side of the j-plug. Otherwise if you pull the j-plug to bridge the tower there will be a high static charge looking for some place to go like through your bridge and/or your body.

Re: Static Drain Choke location

Posted: Tue Aug 30, 2016 12:22 am
by Deep Thought
W2XJ wrote:You just want it on the fold wire side of the j-plug. Otherwise if you pull the j-plug to bridge the tower there will be a high static charge looking for some place to go like through your bridge and/or your body.
Which is where you put one for a series-fed tower...it should always be the first thing the tower "sees" inside the ATU box. Same for the ground winding of tower lighting chokes for the same reason.

The static charge thing is a bit overdrawn because the same hazard exists with a series-fed tower. Skirted towers are no more or less susceptible to static buildup than any other, but series fed towers with no connection to ground at all can be even more potent. They make really good lightning detectors. :shock: