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RIF In Shack On Some Bands - HELP

Oct 8th 2018, 15:22

VK4CCV

Joined: Aug 30th 2010, 01:09
Total Topics: 0
Total Posts: 0
I have just erected a Alpha Antenna 60’ J-Pole Sr, which is up at this moment around 20’.

I feed that from the shack with LMR-240 where the Coax length would be around 30’. There is no Choke being used.

The house is a two story type, where floor level is around 12’ above ground.

Grounding System is an Earth Stick directly below the shack window. ½” Earth Wire comes into the shack and is also around 12’ in length. This goes directly to the Tuner then onto my FTDX5000.
I use a SCU-17 which I use for the digital modes.

When I apply power to the antenna on 20 Meters, the SWR is just under 1.5 without using the Tuner.

I am noticing that when I use the digital modes on 20 Meters and a couple of others where I am only running 50 Watts, the SCU-17 will lock up.

I obviously have RF in the shack on some bands, but deciding what to do about it, I am unsure.

When I place all the cables (USB Input, Output to Rig, RS232 Cable) in a closed fist, the issue continues.

But, if I just touch the outside housing of the SCU-17 the problem goes away.

OK…… the big question, what approach do I take to resolve the issue?

Pete VK4CCV
Oct 10th 2018, 18:39

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
A common issue with J poles is unwanted ground or shield currents. Touching the SCU-17 creates a capacitive path that changes those currents.

The typical solution is to choke off all the currents going to the rig and computer by wrapping the cords through a large toroid. You often need 10 to 15 turns to be effective on the low bands. If the cords are thin you can wrap several through the same core. If you want to operate a number of bands it is usually less hassle to just choke off all the cords, though you may find a clever single cord solution if you just operate a single band.

Zak Lau W1VT
ARRL Senior Lab Engineer
Nov 18th 2018, 16:48

WA8NVW

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Pete, in addition to the toroid, it might also help if you ground the housing of the Yaesu SCU-17 to your station ground. Looking at photos on line, this interface has no visibly marked ground screw, so you'll probably have to tuck a #20 wire under one of the screws on the DB9 serial connector. Those are SUPPOSED to be at signal ground inside the case, via the PC board plating.

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