21 — Antenna experiments.
2015-05-24. By Patrick.
Fig. 1. Transmitter and base of mast used in every experiment. It also show the large solenoid (L1) used in some of the experiments.
In my last blog entry (#19), I described how we came up with a system to quantify the relative signal strength coming from the transmitter using Audacity to record the audio output from a receiver. The point was to get ready for experiments involving the antenna. Since then there has been much reading on antenna theory, building, and experimenting. I'll just talk about four experiments that highlight the path we've taken.
The first two experiments (xp1 & xp2) were just done in the living-room using a small antenna mast. In xp1, we tested result of changing the inductance at the base of the antenna using a large solenoid we built (L1). We built the solenoid with taps all along its length to be able to easily change the inductance. There was definitely an increase in signal strength as we increased the inductance. I'm not sure exactly how much of an increase we had because I had not adjusted the gain as well as I had thought and we had some waveform clipping in audacity. In xp2, we changed the antenna length and measured an linear increase in audio power.
These experiments were done in the near field with a very short antenna. The longest antenna tested was less than 3 m and that probably will be about the size we can manage when we're running in the woods and having to set it up quickly. The first set of antennas will be for use in small areas (about 1.5 km maximum linear distance) where speed and ease of setup will be more important than radiation as long as we can hear the signal over 400 to 500 m. We don't need to hear the signal over the entire area for the initial trials.
In the third experiment (xp3), we took it out side. We found a relatively straight road, set up the transmitter, took a waypoint with the GPS, and drove down the road. Kelly would drive a bit down the road and pull to the side. Then I would jump out, run to the back of the car where the receiver was plugged into the laptop with Audacity running, take a waypoint, and record some audio. And repeat. We had the 3 m antenna with no inductance at the base. The results were a bit disappointing. We stopped around 520 m from the transmitter and could still hear it faintly. We then stopped around 800 m away and could only hear noise. On the way back we stopped a few times to listen but without recording any audio. We first heard the transmitter at about 550 m away. This isn't great but it's enough for the small area in which we will do our first practices and may be plenty for the Maker Space / Enrichment Club at the middle school.
In the third experiment (xp4), we wanted to see if we could replace the large solenoid with a small toroid. Kelly was busy so I did the experiment by myself and stayed close to the house. Instead of using Audacity, I changed the knob on the receiver so that it had marked ticks. I set up the transmitter next to the house, drove away, stopped, took a waypoint, and recorded the lowest setting on the gain knob for which I could clearly hear the signal -- repeated until no signal was heard. Then I would come back to the transmitter, change its configuration, and drive away again. The experiment had 5 parts. One part used only the 3 m antenna, one part used a 47 uH toroid inductor (L2) we had just wound, and three parts used the large solenoid (L1) with different taps used to bracket the inductance of the toroid and to also test a much larger inductance. This time, only the highest inductance tested (183 uH) was heard beyond 250 m from the house. I'm not sure what was different yet.
So here we are. We would like to be able to hear the signal 400 to 500 m from the transmitter. We may already be there but we're not quite sure because two of the experiments gave inconsistent results. We can probably hear it that far with the large solenoid but it's too clumsy to run in the woods with several of these. We tested one toroid at 47 uH and it was not enough. So we need to repeat some of the experiments and probably test some other toroids. Hopefully we'll have system ready soon. I can't wait to get into the woods.
Fig. 2. Audio gain as measured by Audacity as a function of distance in xp3. The 'only noise' line shows the furthest measurement for which audio was recorded. On the return trip we started to hear the signal at green line 'furthest heard' about 550 m away.
Fig. 3. Recording setup at the back of the car. The audio from the receiver was split so that it could be easily monitored with earphones.
Fig. 4. Transmitter setup during xp3.