MX60 VHF interference

[Jerry Zeitz]

I'm installing Outback MX60 controllers at remote mountaintop VHF comm sites, and have suddenly run into interference problems. About 10 installs thus far, and the last 3 have varying levels of interference on the VHF radios when the Outbacks are operating.
These radios are normally tuned to the marine distress freq.- 156.8MHz, and this is the frequency that noises up. Other marine band frequencies are usually clean, but do occasionally noise up as well (I suspect when charging is at it's peak)
The controllers are about 5 feet from the radios, and I am pretty confident that increasing this distance would resolve the problem, but would prefer a quicker/cheaper fix. I was wondering if anyone here has experienced anything similar, and if so, has anyone attempted a faraday cage screen around the controllers as a solution?

More tidbits for those who will want to know-
24v battery
radios are 24v
pos gnd
controller cases are grounded
radios and everything else onsite grounded
2 controllers per site
24 Solarex 60W panels on each controller
panels wired 48v

[Rob Beckers]

Hi Jerry,

The noise could be radiated, and getting picked up by the transceiver's antenna, but I suspect it's coming in through the power leads. My first try would be to put a choke in each power lead of the transceiver, and a small (1nF or so, not critical) ceramic capacitor across them. Closer to the transceiver is better.

Putting a cage around the the MX60 will likely not do anything, as the MX60 is already pretty well shielded. Stuff's probably coming out through its leads. So, if the above doesn't work, my next try would be to put a similarly small cap across the battery and panel terminals of the MX60.

I've been a ham radio operator since 1979 and had to 'tame' quite a bit of equipment, mostly audio, so my transmitters wouldn't cause interference...

Hope this helps!

-Rob-

[Jerry Zeitz]

Rob,
Actually, I'm fairly sure it's radiated, as these sites/installs are nearly identical and we've had many sites with no issue.
The radios are top of the line Motorola Quantars, chokes are on the power leads within inches of the chassis, and I'm sure there's some beefy filtering.
There's 2 800Ah Absolyte strings in parallel, which I would think should keep the DC bus pretty flat.
I just got my confirmation for the Outback forum, so I'll post there and see what they say. Thanks for your help, I don't have much experience down in the VHF/HF band.
It's helicopter access, so when we go, we'll go loaded for bear. I will try caps near the controller and rerouting cables, but if we have to, we will relocate.
There are actually two buildings on each mountain, a radio hut and a generator/battery hut. Mounting the controllers in the gen hut to begin with would have been a much cleaner install, but we are working with the existing infrastructure. The original design was just the radio hut, with batteries in it, and TEGs mounted inside the solar panel frame to supplement the PV.
When generator huts were added, batteries were moved there, but solar still came to a controller in the radio hut, then over to the batteries. Looking to keep cost down, customer wanted us to just replace the existing controllers. Moving them to the generator hut will require new runs from the panels at most sites as the generator hut is further from the panels than the radio hut.
Here's a pic of one of the only sites where we will be able to re-use the existing power runs. Radio hut is in the back w/door open and guy standing in front of it.

[Rob Beckers]

I see. Interesting job you got!
Looks like I'm not going to teach you anything about RF... :cool:

Can you test on the ground with a similar but simple (temporary) setup, then wave a simple antenna around to see if it's picking up anything from the MX60 and if so, from what parts there (power leads, solar leads, display). Better still, if you have a spectrum analyzer around that would give you a much clearer idea of RF noise coming out. An afternoon of testing like that would certainly be cheaper than an extra helicopter ride.

-Rob-

[Jerry Zeitz]

Rob,
Just got this from "rlongdon" on the Outback forum-
MX60 bridge switches at about 24.1kHz
-MX aux supply switches at around 32kHz
-MX Processor clocked at 12.228MHz

That would be like the 10th harmonic, wouldn't think there would be enough energy to cause a problem. Maybe it IS on the output leads.
I am putting a bench test together now with just a couple panels and will look to see just how much ripple is on the output.

[Jerry Zeitz]

My bench test ended up being a 12v in/12v out setup from someone else's demo project that was sitting in our storage yard. It was convenient.
Using an o'scope, I first looked at the leads with input panels disconnected (sleep mode)- input leads were quiet, output leads had a 50KHz blip, about .1v p/p.
I drained the batteries enough to get the controller going, disconnected my load so I knew it was not contributing to any noise, and flipped on the pv. Outback showed 11A in, 13A out. No longer a clean 50KHz, other components mixed in so I couldn't trigger on it, but spikes were up to .6v p/p on the output, and .8v p/p on the input! As the batteries charged back up, the amplitude began to come down.

Who knows how big they get if you've got depleted batteries and full sun. Remote sites can churn out 14x this much current. Today I play with caps and toroids.

[Rob Beckers]

Hi Jerry,

As I mentioned offline there is no HF filtering in the output of the MX60. Just the switching inductors and a 2200uF cap, neither one filters out components over a few 100kHz. The square wave switching signal will have every odd harmonic in it with amplitude dropping off as frequency increases. I don't think you can see on a scope signal if there are components in the 100+ MHz region though, they only need to be -70dBm or even less to cause near-field interference on your receiver. That signal would be waaay to small with respect to the main component to see on a scope. A spectrum analyzer is the way to go with that.

Putting in chokes and caps don't have to do anything to the main (LF) components, so the scope picture won't change (you'd need huge ones to affect the 50kHz component, and those components wouldn't do anything for VHF then). What they need to do is kill that little bit of VHF energy that's part of it. What I'm trying to say is that you can't rely on a scoop for this, you need something much more sensitive. If you have a receiver around that would do, with a simple 1/4-wave piece of wire at the end of a little coax line as an antenna.

-Rob-

[Jerry Zeitz]

And you thought you weren't going to teach me anything. No need to clean up the whole thing, just that part that offends me. Never thought of it that way, but you're right.
I'll get the spec an out tomorrow and stick a piece of wire in the n conector, then wrap it around the power conductor and see what I can see.
More details and discussion at Outback's forum-http://www.outbackpower.com/forum/viewtopic.php?p=11851#11851

[Rob Beckers]

Jerry, did you find out what was causing the trouble? Found a solution? If you have a moment let us know. Not only because I'm curious, but also to get the end of the story on record in case anyone else runs into the same.

-RoB-

[Jerry Zeitz]

Haven't gotten to any of the mountains with interference, but just got back from two more mountains/installs. Had the crew do the install just as they had at the other sites, but no interference. I then set out seeing if I could create some.
The wiring to the PV panels is actually in a flexible spiral steel conduit, and found the "drain" wire tied to the pos. gnd bus near the controllers and also tied to the PV structural steel. Tried lifting this wire off gnd at the panels to simulate the conditions we believe exist at the trouble site, but this did not cause noise. Then went on to remove everything on the site from earth ground (one at a time, testing each time) and even with the entire site floating free of ground, had no interference.
Sun was bright, controllers putting out max power, I disconnected the transmission line running out to the tower mounted antenna, and connected to a spare antenna leaning against the Outbacks/wiring. Nothing.
Measured a piece of wire to one wavelength, hooked it to spectrum analyzer, and stretched it out towards the radios, nothing. Towards the antenna, nothing.
Tried grounding just the PV panel structure and nothing else, no noise.
Tried grounding just the Outbacks and nothing else, no noise.
Was able to get a handheld to break squelch if I put it right up against the controller wiring, but not consistently.
Took the cover off the MX60 and still nothing.
Attached my one wavelength wire to the neg input to the MX60, then strung it out toward the radio, nothing. Tried the same thing on the pos input, and both outputs, nothing.
If weather cooperates, I'll be on the worst site Friday. Bringing chokes, more Outbacks, conduit, spec an, megger, and enough cable to move these suckers to the generator shack if all else fails.