Height of the Ionized Upper Atmosphere

William Lonc,
VE1WPL,
Astronomy and Physics Dept.,
Saint Mary's University,
Halifax, N.S., Canada
B3H 3C3;

Robert Schultz,
VE1IF,
Site 2 Box 42,
Mt. Uniacke, N.S.,
B0N 1Z0.

Introduction

    This is a really neat experiment, designed to measure the virtual height of the dominant ionized layer, known as the F2 layer, during daylight hours.

    The measurement of the height of a relatively well-defined ionized layer in our upper atmosphere could be of interest in a senior-level physics course or perhaps as a Science Fair project. Measurement of these heights is ongoing at several sites around the world; the layers are usually called the ionosphere collectively and the measuring equipment is usually called ionosondes [1]. Of the several well-known ionized layers in the upper atmosphere, the F2 layer is usually the dominant reflecting layer and is usually about 400 km above the surface of the earth between about mid-morning and mid-afternoon, local time.

    In our project, we hoped to develop a tested procedure that would use readily available amateur radio equipment [2,3], available anywhere in the world. We can virtually guarantee that the system we describe will work as advertised.

Experimental Procedure

    Although the procedure -- a time-of-flight method -- is conceptually very simple, the actual implementation is another matter; in other words, easier said than done. To make a long story short, our experiment began with trying to transmit a pulsed carrier upwards, hoping that there would be enough of a surface wave to trigger the oscilloscope so that the reflected pulse could then be seen downstream, as in radar, sonar, etc. It was soon found that the pulse was distorted significantly by bandwidth limitations in the apparatus. It also occurred to us that the relatively broad spectrum associated with a non-sinusoidal modulation would not be welcomed on the crowded amateur radio bands.

    We then attempted using the 60 Hz sine-wave from the local power-grid, but soon encountered the problem of unknown phase relationships between the power lines feeding the two sites. An additional complication was the fact that typical amateur radio sets do not pass 60 Hz very well through their audio channel.

    Finally, we settled on the idea of a two-link configuration, one link in the AM sub-band in the amateur 40-meter band [4] and the other in the simplex sub-band of the 2-meter band. In our experimental configuration, the 40-meter radiation goes mainly upwards, whereas the 2-meter radiation goes mainly along a line-of-sight propagation path. The modulation frequency is chosen high enough to fit within the pass-band of typical amateur radio equipment. was assembled and found to work well enough.

Figure 1: Conceptual diagram of the experiment. The audio oscillator OSC generates a 300 Hz sine-wave which simul-taneously modulates the two transmitters, TX-1 (AM) and TX-2 (FM). RX-1 is an AM receiver and RX-2 is FM. The sine-wave modulation is fed into the microphone inputs of the two transmitters. The FM link also serves as an inter-com.

The general features of the experiment are shown in Figure 1. Not shown are the two antennas, one at the transmitting site and the other at the receiving. Both antennas are horizontally polarized and more or less parallel, so that they function mainly with the sky-wave.

Figure 2: Modulator details. The microphone of TX-2 is held up against the speaker while sending the reference sine-wave, and used as an inter-com to facilitate adjustments during the experimental session. If the inter-com feature is not needed, then the microphone input of TX-2 also could be connected to the output of the resistive network. A matching transformer [5] T will be needed if the oscillator output impedance is higher than that of the speaker. The resistor values in the resistive dividing network are probably adequate for most combinations of amateur radio equipment.

    Some care may need to be taken to make sure that no appreciable amount of the 40-meter energy enters the microphone input of the 2-meter radio.

Basic Theory of the Experiment

    The modulated FM signal travels mainly as a line-of-sight wave, the distance between the two sites [6] being 50 km in our case. The modulation is recovered [7] from the output of RX-2 and fed into one of the channels of a 2-channel oscilloscope, and serves as the reference for the timing measurement. The horizontal sweep is triggered on this channel. In the meantime, the modulated carrier in the 40-meter band travels up to the F2 layers, is reflected, and then arrives at RX-2, where the sine-wave modulation is recovered from the receiver and displayed on the other channel of the oscilloscope.

    The time-delay is read off the oscilloscope, and the propagation time for the reference sine-wave on the line-of-sight path is calculated using a free-space value for the speed of light. The total distance is then calculated using the speed of light in air, taking into account the delay associated with the line-of-sight path, and then the virtual height calculated from using half of this final value. This would give a first order result. A trigonometric correction could then be made to account for the non-zero distance between the two sites. A further correction could -- and should -- be made for possible phase changes of the modulation envelope within the transmitters and receivers themselves [8].

    Since the F2 layer (assuming 250 km for the virtual height) would produce a final total time-delay of about 1.6 msec, the modulation frequency should be such that the reflected wave arrives within a period [9] of the modulation cycle, otherwise the interpretation of the display on the oscilloscope becomes more complicated, but not impossible.

Concluding Remarks

    We have endeavored to give only the basic information required to understand and perform the experiment in such a way that it will definitely succeed. It is left as a challenge to the reader(s) to do whatever 'fine-tuning' their knowledge of physics will suggest. More detailed information can be obtained via email from one of us (WL).

Acknowledgments: We especially thank Jack VE1ZZ for the use of a transmitter, Bob VE1VCK for some transmitter hardware, Gordon VE1PO, and Ron VE1RHW for help along the way.

Footnotes

[1] Much information is available on the internet; for example, http://eiscate.ag.rl.ac.uk/dynasonde/basics.html

[2] Our TX-1 had a nominal 5W output in the AM mode in the 40-meter band feeding a half-wave dipole; TX-2 and RX-2 were typical mobile 2-m FM transceivers connected to 10-element yagi antennas; RX-1 was comparable to any digitally-tuned short-wave receiver, connected to a half-wave dipole. However, there is no reason why older, tube-type equipment should not be satisfactory for this experiment. In general, consult your local amateur radio outlet, or Radio Shack, ARRL, etc., for information on available equipment.

[3] Needless to say, an amateur radio licence is required to operate an amateur transmitter. Information on obtaining a licence may be obtained from the places listed in the footnote above. A standard source of amateur radio information is the ARRL Handbook, published annually by ARRL (Amateur Radio Relay League); they have several web-pages; simply search for "ARRL".

[4] This band was chosen because it is usually free of competing signals during the middle of the day and the F2 layer is almost always dense enough to act as an efficient reflector. The use of higher frequencies is attractive from the 'smaller antenna' point of view, but the F2 layer may not reflect well enough at these higher frequencies. However, given the impending solar maximum in sun-spot number, the higher frequencies should not be dismissed.

[5] In our case, the audio generator had a 600-ohm output and our speaker was nominally 3 ohms. We happened to have a 5,000 ohm to 3 ohm transformer which worked well enough.

[6] It is probably advisable to keep this distance relatively small compared with the nominal height of the F2 layers. However, it is rather important to have the receiving site in a relatively radio-quiet location.

[7] This is the audio at the headphone jack or at the speaker terminals.

[8] In our experiment, each unit (transmitter and receiver) had at least a fraction of a millisecond phase change between the modulated input to the radio and the demodulated output. In some cases, the phase of the demodulated output leads the input phase.

[9] At 300 Hz, this comes out to be 3.3 msec. A much higher modulation frequency could be used to improve the precision of the measurement, but the experimenter would need to keep track of the number of cycles used up in a time-delay of about 1.6 msec.

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Updated May 4, 1999, by Ian McCarthy.