Saturday, October 07, 2006

Debug methodically

My buddy Greg (NE1OB) and I took a little drive today to New Hampshire to provide ham radio demonstrations to some young people in a weekend camp. There wasn't much planning for this event (I learned of it only Wednesday) so, again, I packed too much into the car, using only half of what was brought.
There were kids wandering around everywhere and I was concerned about my antenna systems being a hazard. To minimize the possibility that someone would become entangled in the antenna, feed line, control line, or guy lines, I set up the Force-12 Sigma-5 antenna close to some large boulders, thinking that would be the safest thing to do. Perhaps it was safe, but it wasn't a good idea.
I began operating on 20m with the IC-7000 and noticed immediately that the SWR was running pretty high. This was puzzling because the Sigma-5 is a resonant antenna on all bands 20-10m. Plus, I had used this successfully on Georges Island just a few weeks ago and had no such problems. I mentioned this to one of the other hams sitting with us and he offered to allow me to use his G5RV he had hung. "No", I said, "if you find a problem you should debug it!"
Here is something that is a carry-over from my professional life: if you have managed to create a problem that is repeatable, figure it out! These little side trips and portable operations provide an opportunity for me to learn about my equipment and prepare for the big, important trips. I was determined to figure this out so if it were to happen on, say, Georges Island, Montserrat, or some other faraway place, I would know what to do.
Here is the first symptom: there is high SWR on 20m. Let's walk through what I did to debug this problem. I wondered if this problem was only on 20m so I switched bands to 17m and tried transmitting there. Again, the IC-7000 showed a high SWR. So, we quickly determine it isn't associated with one band. I have my MFJ-259B antenna analyzer in my kit so I disconnect the coax from the back of the radio and connect it to the analyzer. I quickly determine that the antenna is resonant at 13.7 MHz when switched for 20m. That's not right!
At this point, I walk out to the antenna and swap the coax. I'm virtually certain it isn't the coax, but by eliminating this possibility completely, I know that the problem has to be with the antenna. The 50 foot piece of RG-8X is quickly replaced with a 75 foot run and, indeed, the problem persists. So, I've now convinced myself something is wrong with the antenna, or how I've assembled and erected the antenna.
I look over the antenna and see nothing obviously wrong. I had been a bit careless in how I had routed the coax and control cable away from the controller box, but that seemed unlikely to be the culprit. I then glanced at those big boulders I had huddled up against. The lower part of the antenna was just inches away from one particularly red one. Gee, I wondered, do you suppose there's a bunch of iron in that one?
We moved the antenna about 15 feet away from the low rock wall, guyed the antenna in its new position, and took a few extra moments to route the coax and controller cable away from the antenna at a 45 degree angle like the manufacturer recommends. I put the MFJ analyzer on the system again and remeasured. Sure enough, now the antenna showed a great match on all bands, just as it had before.
In hindsight, my initial choice of antenna placement was pretty stupid. While it was a good idea to put the antenna in a place that was safe for this public venue, I still needed to have the antenna sufficiently far away from other objects so it would not couple with them as it did with those iron-laden boulders.
I could make the glib recommendation, "don't be stupid", but that isn't the point of this post. You will have problems, even if you aren't as stupid as me, that you'll need to resolve. When you see a problem, don't just do random stuff to see if you can make it go away; think through how you will investigate the problem so you can learn what is causing it. Take single, well considered steps to eliminate candidate pieces of equipment. Narrow what can be wrong until you are left with a small handful of things, preferably one thing, and then consider the situation again. Has this worked before? What is different from the configuration that worked well? And, especially with antennas, what items in the general vicinity might be affecting the antenna?
Because I took a disciplined approach to this problem it was resolved in minutes. And, I learned something that will prevent me from making this particularly stupid mistake again (though I reserve the right to make other, equally stupid mistakes {grin}). I hope you will follow this advice and consider every problem you encounter in the field as an opportunity to learn more about your equipment, learn more about the hobby, and prepare you for your next portable operation or personal DXpedition.


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