Handiham World for 10 March 2010
Welcome to Handiham World!
Recently I had a nice visit with another operator, and what do you think we talked about?
That’s right – we talked about our stations! The conversation took a turn to receiver performance, and he mentioned that one of his radios has a much more sensitive but quieter receiver than the other. I wondered how he knew that, and he said that he had performed an “A-B” test. That’s what today’s essay is about – not receiver performance.
When we compare two pieces of equipment, we want to know which one performs the best. How many times have you heard someone make a claim on the air for one rig’s superiority over another one? Or that one antenna works better than another? Whenever I hear such claims, I wonder if they are really backed up by testing. I know that most of us will never have a test lab full of instrumentation to run tests the way they are done at the ARRL product review lab, but that doesn’t mean that we have no way to perform simple, but more meaningful testing.
Imagine these situations:
Scenario 1: I have two HF antennas in my back yard. One is a vertical and one is a dipole. The dipole is connected to my Icom IC-706M2G and the vertical is connected to my Icom IC-7200. I decide to see which antenna works best for DX, so I listen on 20 meters and I hear a European on 14.060 MHz. The station is easy to copy on the vertical antenna, and the S-meter reading shows S9. I listen on the dipole and the same station on 14.060 is only S5. Do I conclude that the vertical works better?
Scenario 2: I have two HF antennas in my back yard. One is a vertical and one is a dipole. I have both of them connected to an antenna switch, so that I can switch from the vertical to the dipole by turning the antenna switch. The antenna switch is connected to my Icom IC-7200 transceiver. I start my test by listening on the vertical. I hear a station on 14.060 MHz, and the S-meter reading is S9. Then, without changing anything on the radio, I flip the antenna switch to the dipole. I now have an S-meter reading of S9 + 10 dB. I quickly change the antenna switch back to the vertical and the signal drops back down to S9 again. Changing the switch once more to the dipole brings the signal back up. Do I conclude that the dipole works better?
In both situations, I was listening on one antenna and then the other, but the results in scenario 2 were different than those in scenario 1. What could have caused the difference?
Here is a basic rule about comparing two things: You must try to eliminate as many “variables” as possible so that you are really only comparing the two things you want to compare. This is how scientists and engineers perform tests related to theoretical concepts or engineering projects. To make this as simple as possible, let’s make up a very basic example. Let’s say we have a family argument about which sibling is taller. One brother says that he is growing faster and is taller than his brother. Of course as a parent you can easily make the two kids stand side by side and then you can easily see which one is taller. But what if one stands on his tiptoes? Or if one wears shoes and the other is barefooted? Or if one wears a hat and the other doesn’t? As a parent who needs to be fair about deciding, you will have to insist that the variables of shoes, hats, and standing flat-footed are all eliminated so that the one variable you really want to measure, which kid is tallest, will not be affected by those other things.
Getting back to our scenarios about the antennas, we see that in the first situation we are using two different antennas, which is the variable we want to test, but we are also using two different radios. The difference between one radio and the other is a variable that we are not controlling, and that could account for the results we are getting instead of the choice of antenna. Perhaps the attenuator was turned on for one radio and not for the other. Maybe the antenna tuner was not activated for one radio. It could be that the run of coax between an antenna and one of the radios was defective. Do you see all these variables?
In scenario 2, we use only one radio, and we have an “A-B” antenna switch to make it easy to change from one antenna to the other, and to do so very quickly to eliminate changing band conditions or radio settings and radio performance as variables. Furthermore, we can change the switch back and forth several times to confirm our tests. Now we have performed a more scientific test, because we have eliminated as many variables as we could, at least the easy ones, so that we could really compare just the antennas.
I am convinced that there are a lot of folks out there who are simply talking baloney when they brag about how one piece of equipment is so much better than another one. As often as not, they have never performed a real A-B test and are relying on their impressions rather than empirical evidence. Building up a habit of eliminating variables and focusing on only the thing you really want to test is at the core of successful troubleshooting when you are trying to find a problem. It is essential to making sense of how things work in ham radio, as well as in so many other parts of life.
So think to yourself, “When I test my equipment, am I really testing just one thing?”
If you can answer yes, you are well on the way to solving all of your ham radio mysteries!
For Handiham World, I’m…
Patrick Tice, [email protected]