Ten-Tec R4030 goes berserk on 30 meters

[K5BJS]

I've had a Ten-Tec R4030 for a few weeks now.  My antenna is an indoor magnetic loop using an MFJ-936B loop tuner.

The radio transmits fine with the loop on 40m, but it goes berserk on 30m.  The radio transmits fine with a dummy load on both bands.

I've added MFJ-915 RF isolators at both ends of the coax, and snap-together toroids on the power cord and the key.  Suggestions?

[WB6BYU]

What do you mean by "berserk"?  Is it foaming at the mouth?  Is its hair
standing on end?  Does it have a mean glint in its eyes"

[K5BJS]

I thought berserk was a standard technical term.   

On 40m, the side tone, the power meter on the radio, and forward power at the loop are steady.

On 30m, the side tone sounds nasty, the power meter on the radio jumps up and down, and forward power at the loop is very low.

I should have mentioned that the radio behaves itself on 30m when the loop is out of resonance.  The problem occurs when I peak the loop (maximum forward power, minimum reflected power, maximum loop current).

[WB6BYU]

In that case it sounds like RF feedback.  How far is the loop from the radio?
How long is the connecting cable?

I've had similar issues where a radio acts differently on some bands.  For example,
when I was at college a ham on 40m wiped out all the stereos, but my 80m signal
wasn't a problem.  Turned out that 30' was a standard length for stereo speaker
cables, and they acted like a dipole on 40m.  My Ten-Tec Argonaut has some quirk
that makes the panel lamps flash when it gets too much 40m RF.

So I'd start by trying to reduce the amount of RF picked up by the radio.  This
might mean moving it further from the antenna itself, shielding (or at least testing)
any wires connected directly to it, changing the coax length and/or adding a
choke coil of some sort, improving the decoupling of the feedline from the antenna,
etc.

For example, how long is your key lead?  Is it shielded?  One quick test is to make
a shorted plug and stick it in the jack to key the rig in place of the normal key:  if
that makes the transmitter stable, then you likely have RF pickup on your key lead.
(This can either be due to pickup directly from the antenna, or RF flowing back on
the outside of the coax.)

If you find a cable or connection that makes a difference, it may be that adding a
bypass capacitor across the jack inside the rig is sufficient to eliminate the problem.
But it will take some experimenting to see where the problem is coming from.

[K5BJS]

How long is the connecting cable?

25 feet of LMR-240.  What is a good length for 30m & 40m?

How far is the loop from the radio?

25 feet at the moment.   

For example, how long is your key lead?  Is it shielded?

About 4.5 feet, unshielded, although it sports a ferrite now.

[WB6BYU]

25' is pretty close to a quarter wave on 30m.  That can easily have a high
impedance at the far end - meaning high RF voltage at the radio.

Try adding another 25' of coax if you have it.  Otherwise find 25' of hookup
wire and connect it to the rig chassis, then drape it around the room to
make a ground radial.  See if that makes a difference.

[K5BJS]

Try adding another 25' of coax if you have it.

I have a 20' coax I can try.  Should have a barrel connector somewhere.

[K5BJS]

One quick test is to make a shorted plug and stick it in the jack to key the rig in place of the normal key

That worked!   

[WB6BYU]

Then it sounds like using shielded cable for the key (or connecting a bypass capacitor
across the key jack) might solve your problem.

[WV9K]

Basically the same problem with my 4020.

Bad case of RF feedback using a wall wart (also inside long wire through a MFJ-949D with counterpoise).  Not an issue with batteries nor external 12VDC.

[K5BJS]

Then it sounds like using shielded cable for the key (or connecting a bypass capacitor across the key jack) might solve your problem.

So far so good with a shielded cable and a couple of snap on chokes.   Thanks WB6BYU!