Total posts : 45366
Sorry for interrupting this very important thread by not replying earlier. I was not able to get the use of a computer for about a week.
The reports of Part 15 AM operators, like the reports of ham radio operators, are not particularly useful for system evaluation because of the lack of field strength data. Such reports are much better than no information at all, but field strength readings are imperative for troubleshooting. As I mentioned in a previous post, I am working on constructing something to get at least some rough field strength readings. I started by looking at the design of David S. Forsman, WA7JHZ, which is linked on Weak AM’s blog. This design appears to me to be reasonable, but I decided on a different approach. I have an old Motorola AM-only car radio which I am adapting for use as field strength meter. So far, I have disconnected power from the audio amplifier and installed a headphone amplifier. This greatly reduces the current consumption from a 12 V battery, making portable operation feasible. I have wound a coil on a ferrite rod to make a pickup antenna. I will connect the output of the coil to a buffer amplifier, and the buffer will go to the antenna jack of the car radio. The loading coil inside the radio will be disconnected.
Unlike the Forsman design, I will be using a tuned antenna. There are formulas for predicting the output voltage of a tuned ferrite rod antenna for a given field strength. I have a calibrated signal generator for calibrating the receiver.
The major weakness of my concept is the output reading. At first, at least, I will be using the AVC voltage, which is available from the high side of the volume control potentiometer. The AVC voltage does not change a great deal with changes in signal stength, making the output reading coarse. I am reluctant to change the receiver circuit much because these old radios were designed for absolute minimum component cost, which makes it likely that a reflex design was used (using the IF amplifier as the first stage of the audio amplifier). Such designs don’t tolerate tinkering much. Even minor changes can cause a perfectly-functioning receiver to produce whistles and birdies, which are hard to eliminate.
In the March, 1990 issue of Wireless World, there is an article by a ham, Bryan Wells, G3MND, who had constructed an HF CFA. He reported, “I managed a contact in Western Connecticut [from England] in relatively poor conditions, using 350 W PEP to maintain a Q5 signal.” In the November, 1989 issue, Wells said, “…results were almost as good as my double zepp and an 82M circumference vertical loop…” These are examples of the kind of reports that are obtained from hams. This is to be expected, because of the lack of test equipment among amateurs. More quantitative data is needed for system evaluation.
Incidentally, The CFA was first reported to the world in the March, 1989 issue of Wireless World. March, 1989 was the same month that another scientific wonder, Cold Fusion, was disclosed.
I have tested a type 3062 B&W Air Inductor on an HP (Agilent) Q meter. It is 2″ in diameter and 10″ long. It has 160 turns of #16 wire. The inductance measured to be 326 uH, with 10 ohms of RF series resistance at 1 MHz. Wheeler’s formula, which is reasonably accurate, gives an appreciably lower inductance. Coil capacitance results in a higher measured inductance. Coil capacitance also reduces Q. The measured DC resistance is .4 ohms. This is an inductor that can be used as a Part 15 AM loading coil. The Q at 1 MHz is 205.