My problems with RF in the shack

[K3STX]

I thought I would share my experiences of the past two days trying to deal with RF in the shack and MAYBE I can help someone else avoid the pain and misery I have endured.

This all relates to getting everything ready for the Contest season, now that I have a 500 watt amp I want to check/replace all connections. My 40 M dipole for Sweepstakes, at 30', was not oriented quite right in the trees, so I tried to find a new branch. Long story short, my 20/40 fan dipole got caught in tree, had to YANK the coax to free it, finally got it up, then the 20 M parallel wires got twisted around the 40 M parallel wires, yada yada yada. Bottom line, after 2 hours I put it back EXACTLY where it was before I started! (There is a moral there.)

Go down to the radios. NOW I have bad RF in the shack, with 400 watts out my rig was locking up on 40 and 20 with this dipole. (RF in the shack means my rig locks on transmit even when I send a “dit” with a keyer. I remembered yanking on the coax, maybe THAT was the problem? Lower antenna, replace coax, now OK on 20 but still NG on 40. Thought "ya know, that coax was attached to an old W2AU (W2DU?) 1:1 balun, maybe I injured the BALUN too?". Saturday morning lowered it again, made a 7b turn coax choke (hanging just below balun), now 40 was just fine with 400 watts, but 20 locked up the rig!! WHATTT!!!! Go outside, looked at antenna, the 20 M wires had wrapped around the 40 M wire supports: a quick tug and it was free, went to shack and ALL IS GOOD, no RFI on 20 or 40. I guess I injured the balun, I'll replace it I guess (eventually).

Sunday morning, having all this well in mind, my brain turned to my station ground. A few nights ago, even before this mess, my rig was sending erratic CW at 500 watts (the first “dit” would become a “DAH”, then all would be OK! Weird?) Maybe I have a ground loop that is showing up at high power but no 100 watts, and this has been part of my problem? I have two ground rods outside, behind my bench I have 5 feet of aluminum flashing (my common ground point), and 4 feet of RG-213 braid going from flashing outside to ground rods. I have the tuner, 2 rigs amp, antenna switch all hooked to this, but not a common point. “I’ll fix this I say”, and out comes the soldering iron to solder all ground wires to the common point at the tuner (with a single wire to the flashing). All done, key rig and … BLAAAAAAH!!!! Rig freezes with even 100 watts!! What the hell, it was fine 20 min ago!? I undo everything the way it was originally, and STILL RF in shack and lock-up. Now THIS is strange. I unhooked cables, etc… and after 30min of cursing discovered the problem was ENTIRELY due to my keying interface cable between computer and input to my rig!!! If I MOVED the cable AWAY from the rig (in the FRONT) of my bench, then all was OK. Move back…..BLAAAAAAH!!! Solution, 3 strap-on ferrite-cores around the COM cable going into the computer. PROBLEM SOLVED!

I write all this because I have wrapped an incredible amount of wires around toroids in the past few days: the LPT1 port of computer (keys radio 2), my amp power cord, leads from keyer to rig and paddles to keyer (for both sets of paddles), my power supply power cord, my amp input, my amp output, my rig output… I am not going to undo it now, but I think all of this (besides destroying my balun) is due to RF getting into the COM1 port. Behind my desk is a MAZE of wires, and at 100 watts I never had a problem. At 500 watts it seems to be important WHERE the wires are with respect to the coax out of my rig and amp (especially the keying cables from computer).

Maybe you all know this already, but I was amazed that simply tacking one of my cables so it does not dangle with the rest made a big difference with RF in the shack. I think I have a well-grounded shack, but I guess the common mode currents were just too much. The moral of the story, to you, is to start simple (unhook EVERYTHING from your rig) if you are having RF in the shack. I have two radios, each with a keyer/paddles/computer interface all grounded together, with an amp and multiple antennas. Too complex a situation. I guess I am lucky I figured it out after all!!

paul

[NE6AO]

at the end of the day can you look yourself in the mirror without flinching ? if so ,your a good boy charlie brown.

[K3STX]

At the end of THIS day, I am still having nervous ticks. But all is still fine.

paul

[AG3Y]

Paul, one of the big "NO NOs" in broadcasting is to run audio, video, control, Power, and RF leads through the same trenches or wire harnesses. You cannot believe what a thorough mess results when you have sync buzz, RF, Audio, 60hz hum etc. etc. all in close proximity with one another ! ! !

When I worked with rack panel cabinets, I would always keep wire bundles of the various types carefully separated.

When I look behind the operating position of so many ham stations, not to mention computer positions and home entertainment centers, I see so many that are just a maze of wires of different types. Why there aren't more situations regarding hum and buzz pickup then there are, is totally beyond my understanding. Maybe there are, but the people involved don't realize how much better their setups would be if they were properly cabled.

I just got done following the adventures of an old timer SSTV operator that did a complete re-work of his station. He has been sweating out this change for a month now, and still doesn't have it back to sounding and looking as good as it did before he made the change.

No-one says that it is easy! After all I have just said, just don't come over here and take a look at my shack ! I know how it SHOULD look, but what it DOES look like just MAY be another case, entirely !

73, Jim

[K3STX]

Jim,

What started my "ground" issue is I found that when my keyer locked up, if I touched the brass paddles with my finger, my rig un-keyed! So I thought I had a ground problem. I also found if I lifted this cable or that cable things would get better (RF in the shack only at >300 watts) or worse (RF even with 100 watts).

I am not a techie at all, and I only have ONE "behind the bench". How should I do this? In my mind, the only "special" cables are the RG-213 from rig to amp and amp to tuner and tuner to coax switch. To me, the rest are "non-RF bearing". Any specific advice will be appreciated (but I gotta tell ya, as things are I am not touching ANYTHING unless I have a problem).

It's a hassle, but I COMPLETELY cured my CATV TVI on 40 meters barefoot by putting the ferrite cores around my COM cable. (Maybe it was not obvious, but I NEVER had the computer turned on during my testing/analysis). So something good came of this (as far as my kids are concerned, no more interfering with the "Lizzie MacGuire Show"). It is also worth it so I can run 500 watts when needed, what a great boost. I love it and wish I did it years ago. I heard so many people saying "you don't need an amp" I actually believed it. It's like saying you don't "need" a T/R switch, you can just unplug your coax when you go from transmit to receive. Yes you can, but having the advance sure helps.

paul

[AG3Y]

Well, Paul, you gotta remember that any cable in the shack could actually be an "RF bearing" cable, if it is in close enough proximity to the antenna, antenna switches, tuner, balun itself, etc. etc. ! ! !

In other words, any other part of your setup that is exposed to RF radiation could introduce RF into the conductors that run near or even in your shack.

In the best installations that I worked in, all the cable was either run through shielded trenches, wire ladders ( yes, they are constructed pretty much as the discription would make you believe ) or cable troughs within steel rack panels.

Electrical cable ran through solid tubular conduit ( no flexable stuff ! ) and as I mentioned before, audio, control, RF and AC feeds were all kept separated by a fair distance, and/or orientation. Cable could cross over each other, but running parallel for a significant distance was highly frowned upon!

One of the easiest ways to keep RF from being re-radiated inside the shack, is to make sure that the feelines ( be they coax, or open wire or ladderline are carried away from the antenna a good quarter wavelength at a 90 degree angle. Furthermore, the antenna should be a quarter wavelength or more away from any electrical wiring that is oriented parallel to the antenna .

Since many people are living on postage-stamp sized yards, that again is a very difficult thing to accomplish, but if it can be done, many RF feedback problems will be minimised or eliminated!

Another way to keep RF feedback to a minimum is to make sure that your antennas are fed at high currrent, low voltage points. This would imply the center of a dipole, or the base of a quarter-wavelength vertical that is placed in the center of a very agressive ground-plane system of radials.

RF grounds are NOT the same as safety grounds. The best definition I can think of for a good RF ground is that the impedance is low enough as to offer little or no resistance to RF currents that are flowing through the ground system. An RF ground could be totally isolated from an electrical safety ground and still be extremely effective for its purpose!

Hope this helps. Feel free to ask any further questions that come to mind.

73, Jim

[K3STX]

This is a useful discussion. My "problem" antenna is my 40 M dipole, about 30' off the ground. It is the "balun" antenna I mentioned. The coax for this comes straight down (obviously), but then lays on my roof (the antenna is above my roof) and the coax is PARALLEL to the dipole, lying on the roof and then coming down to the shack. I would say the horizontal dipole and horizontal portion of the coax are 15-20' apart, not even CLOSE to a quarter wavelength on 40. Curiously, I have no trouble with the 20 M part of this fan 40/20 dipole.

Maybe I will experiment and bring the coax all the way to the ground and then around to the shack, maybe a longer trip but might prevent problems in the future (and cure the remaining TVI I get at 400 watts).

Thanks for the great tips, hope others are reading this stuff. These "real life" problems aren't really documented in books, it is places like here that I find solutions to problems like this.

Thanks a million, Jim.

paul

[K8AG]

Quote[/b] (AG3Y @ Oct. 15 2006,19:43)]Paul, one of the big "NO NOs" in broadcasting is to run audio, video, control, Power, and RF leads through the same trenches or wire harnesses. You cannot believe what a thorough mess results when you have sync buzz, RF, Audio, 60hz hum etc. etc. all in close proximity with one another ! ! !

Instrumentation too. I have "fixed" many control problems by simply keeping power and non power conductors well separated. I have even gone so far as to use Radio Shack optical fiber to avoid running a signal line with a power line.

Good job.

73, JP, K8AG

Amazing how good it feels to actually fix a difficult problem.

[WB2WIK]

Paul, good to hear you tackled the problem.

So, I'm sending you some RFI in a jar. Open the jar any time you want more problems.

I've almost never found "grounding" to be much help in anything. Usually just makes ground loops that create more problems. Even single point grounds haven't ever solved a problem for me.

Station engineering, including the antenna systems and the equipment, has always helped a lot. And yes, some sorts of RFI to external equipment is most easily solved with ferrites, more so than any other single cure.

When I set up my 500W "home theater" system in our family room years ago, before I had the optical interconnects between everything I had to use almost ninety (90) ferrite cores on wiring to and from the television, DVD recorder/player, home theater amplifier, satellite converter/TiVo, etc. Over the past few years, anything carrying only "signals" and not any power (which means pretty much everything except the speaker leads) has been converted over to optical interfaces and fibre optic cabling, which eliminated the need for most of that stuff.

I got tired of paying $7.99 for two ferrite snap-on chokes at Radio Shack and the like and finally went direct to a manufacturer, Steward Ferrite, for these so I could buy them for about $1.50 each instead...

But in all those cases, I never found a "ground" did anything for me.

Don't forget to look for the jar.

WB2WIK/6

[K3STX]

When I get the jar I will send it to AG3Y; he'll surely know what to do with it!!

paul