Introduction

For many years radio amateurs have made contact across continental distances at both VHF and UHF by bouncing signals off the moon. This moonbounce, or EME (earth-moon-earth) as itís sometimes known, is a demanding technique: since the signal path is long at some 750,000 km, the received signal strengths are low. So amateurs normally use all the transmitter output power they can legally employ to feed large arrays, with sensitive receivers and narrow modes of operation - usually morse, occasional SSB. All in all itís a challenging technique.

Reading about amateur radio moonbounce led me to consider whether it would be possible to detect TV signals from distant transmitters by using the same principle. My conclusion is that it indeed may be possible to do this, and experiments are now being undertaken to see whether itís feasible in practice. The present article compares the radio amateur situation with the problems a TVDXer faces, and introduces aspects of the topic special to DX-TV.

Time Window

The basic situation is shown in Fig. 1. It will be appreciated first that the moon must be above the horizon at both the transmitter and receiver for the method to work! This puts limits on what signals can be received at any specific time If the intention is to try to pick up signals from a particular region, the time window must be first calculated.

Radio amateurs usually employ rotatable transmitter/receiver aerial systems, and it can therefore follow the path of the moon while itís above the horizon. Their time window for communication to a given region may thus be several hours wide. Unfortunately, while a DX-TV enthusiast may have a rotatable receiving aerial, the transmitting aerial is fixed, with its power beamed towards the local horizon. It follows that the only time when the moon will have any signals to reflect is when it is just rising or setting at the transmitter location. So the TV-DXer has a much narrower time window to receive signals from a particular region.

We can estimate the duration of this time window: if the transmitter aerial has a vertical beam width of say 10 degrees, then, since the earth rotates through 360 degrees in 24 hours, the moon will pass through the beam in approximately 40 minutes. The exact time to attempt reception can be calculated on a home computer, using a knowledge of spherical trigonometry coupled with the orbit of the moon. You may however, like the writer, prefer to visualize the situation with the aid of a humble globe and an orange, using the moon rise/set tables given in reference 1.

Fig 1: Signal path for moonbounce reception.

Ham Transmissions

Radio amateurs have achieved moonbounce contacts in several bands, namely 144 MHz, 435 MHz and, for region 2 amateurs in the Americas, 220 MHz. In the Americas amateurs can use transmitting powers of around 1kW to feed the aerial system. The aerial systems employed are usually stacked Yagi arrays: Iíve read of arrays consisting of 24 13 element Yagis and 8 element Yagis for 144 MHz use. If a 13 element yagi has a typical gain of some 11 dBd (dB with respect to a half-wave dipole) and an 18 element yagi has a gain of some 13dBd, then these large level arrays have gains of approximately 25 and 22dBd respectively. Aerial phasing losses would reduce the gain figures by 1-2 dB. Nevertheless the effective radiated power (transmitter power x directional aerial gain) of these amateur systems is of the order of 100kW at 144 MHz, which is comparable to the ERP of a typical main Band I/III TV transmitter.

What could be detected?

Because of the low signal-to-noise ratio we can, as with amateur radio practice, detect signals only when using a narrow-band receiving system. This means that the only aspect of the TV signal that could be detected is the field scan modulation - the field (video buzz).

As the 50 Hz buzz of a 625 line system carries no identification, it would be more convincing in Europe to detect 525-line transmitters with their 60 Hz field buzz. Since there is very little by way of 525 line broadcasting in Europe (only some low-power AFRTS outlets), the signals would have to come from the Americas By the same reasoning, American enthusiasts would try to detect 50 Hz video buzz received in the opposite direction.

Channels to Try

Two channels have advantages, channel A2 (55.25 MHz vision) in band I and channel A7 (175.25 MHz vision) in band III. As these vision carrier frequencies are the same for channels E3 and E5 respectively, itís possible to align the moonbounce system by taking advantage of meteor scatter or aircraft bounce propagation from distant European transmitters.

There is a disadvantage in using channel A2 in that multi-hop sporadic E or F2 propagation occasionally brings American signals to Europe on this channel. Care would have to be taken to check the directions of any chA2 signals to avoid ambiguity. This might be done by seeing if the signal fades when the receiver array is not pointed at the moon.

As well as these alignment advantages, the use of the lowest channels in each band maximizes the power feed to the receiver, thus giving the best signal-to-noise ratio. This is because the effective collecting area of an aerial is proportional to its gain multiplied by wavelength squared - so for a given aerial with a fixed number of elements and thus gain, the collecting area is greatest with the longest wavelength.

The Signal-to-noise Problem

We can obviously detect these TV carriers only when their received power exceeds their background noise. This background noise has several components: receiver noise, galaxy and universe radiation (the cosmic background), solar radiation, and unwanted signals picked up due to receiver imperfections, i.e. nonĖperfect IF passband performance, non-zero image rejection, and cross-modulation caused by strong out-of-band signals overloading the RF and mixer stages.

We need to arrange that the receiverís noise figure is low and that its other imperfections are minimized by the judicious use of front-end filters. The effect of the sun is made negligible by conducting the experiment after the sun has set at the receiving location so as to eliminate any solar radiation picked up by the aerial side lobes. Also, by choosing the time of the month to minimize reflected solar radiation, we are left with the cosmic background as the ultimate background noise.

In order to access its importance, we need to consider the power available to the receiver for a moonbounce signal and then compare this with the cosmic background.

Received Signal Strengths

To calculate the power of a signal thatís been reflected by the moon, a modification of the radar equation is used, as follows:

Pr = Pt x (A/4piR2) x (1/4piR2) x (Gwavelength2/4pi)

where Pr is the at the receiver, Pt is the transmitterís effective radiated power, A is the moonís equivalent cross-sectional area, R is the distance between the moon and the earth, G is the receiving aerialís directional gain, and _ the wavelength. Note that Gwavelength2/4pi is the receiving aerialís collecting area.

Now for a sphere of diameter d and having a perfectly conducting surface, A is pid2/4 if d is much greater than the wavelength. Since the moon has a non-conducting surface that's rough on a scale roughly equal to a wavelength, A = A (perfect sphere) x Ls, where Ls is the scattering loss. For the moon Ls is 0.05 in the frequency range 50-200 MHz (see reference 2), ie. the reflected power is 0.05 times the incident power. Taking R to be 384,000 km and d 3,476 km, the values of Pr shown in Table 1 are obtained with the receiving aerials listed in Table 2.

For comparison, the signal strength for a good TV picture is roughly 1mV r.m.s at a tuner's 75 ohm input, a power of some -80dBW.

A 24 13-element Yagi array at 175.25 MHz provides equal gain to a radio amateur array at 144 MHz - but you'd probably need the resources of a club to afford it. The 55.25 MHz and 175.25 MHz quad aerial arrays are proposed by the writer for a DXer working alone.

Cosmic Noise

Radio astronomers have found the strength of the cosmic noise background depends on both frequency and the direction in the sky (see table 3). The highest background power comes from the galactic equator - the Milky Way - so it's desirable to carry out experimental reception when the moon is not in line of sight of the Milky Way. The figures quoted in Table 3 (from reference 3) for the strength of this noise are spectral powers: this means that the total power of the receiver possesses is the spectral power multiplied by the bandwidth. For instance, a receiver with a 1 KHz bandwidth would receive a power of -163dBW at 55.25 MHz from an aerial pointing at the galactic pole, the corresponding power at 144 MHz being -174dBW. A comparison with the estimated wanted signal power for amateur reception at 144 MHz, -178dBW, shows why moonbounce requires narrow-band operation - any wideband signal would be lost in the cosmic noise.

A comparison of the expected TV strengths for channel A2 (-181 dBW) and channel A7 (-186dBW) with the background noise levels of -163dBW and -174dBW respectively with a 1 KHz bandwidth indicates that TV-DX for a solo operator is a more difficult proposition than amateur radio communication done with club resources. A study of the World Radio TV Handbook however shows that there are at least three channel A2 system M transmitters in the Americas with transmitter E.R.P.S in excess of 100 kW - one is quoted as 900 kW - while there are ten channel A7 transmitters with E.R.P.S in excess of 150 kW, four being over 300 kW. This means that the estimates of wanted power listed in table 1 may be somewhat pessimistic, in which case there's a reasonable chance that a solo TV-DXer will be able to detect these signals.

Table 1

Frequency (MHz) Power at receiver (dBW)
55.25 -181
145 -178
175.25 -186 (a)
175.25 -180 (b)

Table 2

Channel Freq (MHz) Aerial gain (dBi) Array
A3/E3 55.25 14 4 3-el Yagi
Radio Amateur 144 25 24 13-el yagi
A7/E5 175.25 19 (a) 4 11-el Yagi
A7/E5 175.25 25 (b) 24 13-el Yagi

Table 3

Frequency (MHz) Power dBW/Hz Direction
55.25 -193 Galactic pole
55.25 -185 Galactic equator
175.25 -204 Galactic pole
175.25 -193 Galactic equator

EME Links

DXTV via moonbounce (EME) propagation by Tony Mann and Ian Roberts.

UHF DX TV via (EME) propagation by Todd Emslie

References

(1)Astronomical Almanac, H.M.S.O annually
(2) M.I Skolnik, Introduction to Radar Systems, McGraw-Hill. Page 604, 1962 edition.
(3) J.D. Kraus, Radio Astronomy, McGraw-Hill. Page 237, 1966 edition.

Copyright © 1984 Dr. S. C. Giess



HOME

Links

Articles