Sunday, April 01, 2007

Problem solving with the AntennaSmith

I got a chance to work with the AntennaSmith today and I used it to solve a problem I alluded to during my Seattle trip.

I had packed my little Buddipole system for that trip but had some trouble getting it tuned while I was there. The first problem was caused because I was stupid. This particular Buddipole system was purchased a very long time ago, long before Chris's latest coil design. I was using a small laminated card Chris had given me on Montserrat for all the coil and whip settings. Unfortunately, those directions are for the new coils, not the ones in my little Buddipole system. Stupid me. OK, I figured that out (after an hour's wasted efforts) and I dug out the nicely printed directions for that antenna that I had carefully packed with the unit. Lesson: use the directions that came with the unit or directions that have been proven to be correct.
Sheepishly, I begin trying to tune the antenna again. Now I'm using the correct settings (for this version of the coils) but it still isn't tuning up nicely. My buddy Henson is now wondering if I know what I'm doing. "This is usually very easy", I said unconvincingly. In the end, I gave up trying to figure it out and decided to let the tuner do the work. I would figure it out when I got home.

Today was the day I decided to figure this out. It seemed like a perfect assignment for the new analyzer. I set up the antenna for 20m in the front yard with the feed line running over to Sandy who was sitting on the stoop. "It's high", she reports. Indeed it was running high with resonance running about 500 KHz above the end of the band. OK. That explains a great deal. Now, why would this be?
I looked at the "black" coil, and it was set up properly. I then looked at the "red" coil and saw it was also set up properly. Then I spotted the rework Chris had done. Eureka!
It happened a very long time ago. I was experimenting with this system one day about a year ago and a gust of wind blew over the antenna. One coil, the "red" coil, hit the driveway pavement and shattered. I had sent this back to Chris to have him either repair it or replace it. (I was prepared to purchase a new coil but Chris was able to salvage the broken one.) This was the first time I'd used the little Buddipole system as a dipole since that accident.
For whatever reason, this coil, when tapped at the appropriate turn, was "too short". It was obvious from the AntennaSmiths display. I moved the tap back one turn. Now it was too long. I shortened the whip about 6 inches and it was just right!
The SWR display showed a nice curve with a 2:1 match over about 250 KHz of the 20m band. The Smith chart display showed that much of that was at or near resonance (with X either zero or close to zero). The interface for this feature of the analyzer is very clever: each sample point appears within the Smith chart display and the knob on the side of the unit lets you walk point-by-point through those samples. Each click of the knob moves the cursor to a new highlighted point and the corners of the display show the frequency associated with that sample, the R, and the X (including sign of X).
With this new magic formula (move the tap in a turn, shorten the whip 5 inches), I was able to set up the antenna for 15m, 17m, 20m, and 40m easily. Each time the AntennaSmith gave me a great view of 2:1 bandwidth, the real resistance, and reactance across the band.
I don't want to knock the MFJ 259B here as this trusty device has served me very well over the years. Further, I don't anticipate parting with it anytime soon (or ever!). That said, it would have been much harder to do what I did today with the MFJ 259B. The AntennaSmith's graphs, especially the Smith chart graph, were tremendous time savers. It was for this reason, the time-saving prospects, that I was interested in this unit. Today's experience working through this little problem has convinced me this was a good decision and a good purchase.
There are plenty of other things I'd like to check with this analyzer when I get some time. For example, the rotating arm kit allows you to put the Buddipole into many different configurations (horizontal dipole, vertical dipole, vee, inverted v, one arm up and one down, etc.). I would like to see how the antenna characteristics are affected by these altered configurations. Now that the Spring has finally arrived in New England, perhaps I'll be able to spend a Sunday afternoon in the near future doing just that.
In the mean time, I solved my problem. I can't wait to take my little Buddipole system on another trip!


Post a Comment

<< Home