Removing RF from Base Current Monitor?

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Removing RF from Base Current Monitor?

Post by Qs23 » Fri Jun 10, 2016 2:42 pm

Hey All,

So I've got a 10kW AM that I'm trying to install a Sicon-8 to use as our remote unit at the transmitter site. (We're currently paying through the nose for a dedicated line going out there at the moment)

To start out, I was going to wire up the device to measure the kW output of the transmitter, the building temperature and antenna base current.

I attach the building temp and there's no problem. I wire up the kW output of the transmitter, no problem. I try and wire up the antenna base current line and I get an RF shock in the process.

What I have installed to measure the base current is a Delta Electronic RF Ammeter. It's outside in the matching box, which is about 2 ft away from the base of the tower. The connector coming from the box is just a regular 2 wire cable with shield. the problem that I thought was going on was the fact that the shield wasn't connected at either end. So it's like it's not even there.

I've tried grounding the shield of the wire to the rack that houses the sicon, but we're still having issues. It's not as hot as it used to be, but I'm still noticing data errors.

Any thoughts on what else I could do?

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Re: Removing RF from Base Current Monitor?

Post by taylorengineer » Fri Jun 10, 2016 3:02 pm

Use a .01 disc cap to ground to filter out the RF. Do this at the remote control end.

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Re: Removing RF from Base Current Monitor?

Post by kkiddkkidd » Fri Jun 10, 2016 3:07 pm

If you unplug the DB9 from the meter do you still get shocked?

Is the site grounding OK? IE, is the meter cable THE site ground? Don't laugh...

I assume that the ATU mounted Delta meter indicates correctly?

How far is it from the Delta Meter to the rack where the shocks are being received?

What freq are you on?

I work on a few 1-15kw sites that are using a similar setup. RF on the metering line from the Delta have never been a problem. Most of them are using either Beldon 8451 or Cat5 twisted pair.

Looping the cable thru a toroid several times and caps across the metering line and to ground might help but I would highly recommend locating the root cause if you can.
Kevin C. Kidd CSRE/AMD
AM Ground Systems Company
KK Broadcast Engineering

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Re: Removing RF from Base Current Monitor?

Post by Deep Thought » Fri Jun 10, 2016 10:43 pm

You're going to have to put a resistor and capacitor on that sample before the Sicon will give you anything even remotely resembling a reliable base current sample. They do a really horrid job of input filtering.

You should not be getting an "RF shock" from the Delta meter remote cable. If you are the meter is not properly grounded like Kevin said. Also make 100% sure the toroid sample transformer for the meter is properly grounded.
Mark Mueller • Mueller Broadcast Design • La Grange, IL •

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Re: Removing RF from Base Current Monitor?

Post by SmokChsr » Sun Jun 19, 2016 10:31 pm

What you are seeing is most likely RF being inducted onto the Shielded pair going out to the tower.

As always good engineering practices apply.
Step 1 Make sure the Delta Toroid is properly bonded into the facility ground. This means that the torroid in mounted on at least a 2" .020 copper strap which is bonded into the ground system.
Step 2 Make sure the shield from the meter pair is grounded into the station ground at both ends of the run.
Step 3 wrap the cable arriving at the transmitter shack 10 or so times around a ferite torroid core
Step 4 Filter out RF on the shielded pair run each conductor through a choke coil after the coil place a .01 - .1uf cap to ground from each side.

Once you've done that you should be doing pretty good.

Any wire leaving an Am tower is going to pick up RF, the worse the grounding system is the worse the RF is going to get into anything conductive.

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