What's different on 10 GHz EME ?
by DJ7FJ Josef Fehrenbach
on 10 GHz EME, compared with the well known conditions using 2m or 70 EME are pointed out
in this essay.
2. Path loss - RADAR equation
Usually, the path loss is calculated using the RADAR equation.
In this basic
form of the RADAR equation, the following units have to be used:
cross section a of a target is defined in the unit of an area. s = r².p = d².(p/4)
s = r².p = d².(p/4)
If the surface of the sphere is only poor or non conducting, e.g. the moon, the cross section has to be multiplied with a corresponding reflection coefficient.
cross section of the moon can therefore be calculated as:
s = (3.5.10 ^6m)².(p/4).0,065 = 6,25.10^11 m²
So, the moon
has the same characteristic of reflection as a well conducting sphere with a cross section
of 6,25. 10^11m²
Pr = P(t) +
G(t) + G(r) + 10 log
Pr = P(t) + G(t) + G(r) + 10 logs (m²) - 20 log f (MHz) - 40 log d (km) - 103,4
path loss instead of the received power, the values P(t) and the antenna gains G(t) and
G(r) have, to be fixed to the factor 1, equal to 0 dB.
3. Antenna beams
earth, the moon covers an average cross section of 0.5º
On 10 GHz a
dish of 1mtr has a 3 dB angle of 2º. Therefore,
a 2 mtr dish has a 3 dB angle of 1º and a 4 mtr
dish only 0.5º.
frequencies, e.g. 2 m, usually 50% of the backscattered signal is reflected from a small
central area on the moon.
With RADAR echoes in the X-band, this effect is increased
Therefore, on 10 GHz the whole visible area are more or less producing
reflections. Approximately, the moon surface is a combination of a huge, number of single
reflectors. So, the echo is scattered and contains parts with different travel times.
5. Doppler and dopplersmear
doppler on 10 GHz can be more than 20 kHz. As the doppler can be calculated in advance by
different programs, this shouldn't be a problem. The dopplersmear is a SHF/EHF specific
problem on EME. Containing spectral parts from both sides of the center frequency, the
CW-signal in the RX isn't a clear single tone. It sounds a little bit similar to
rain-scatter signals, although the rain-scatter signals might be from a rougher type. But
there shouldn't be a problem for a well trained CW-operator to copy this melodious
signals. Using signal processing care has to be taken about the covered bandwidth. As a
reason for the signalsmear, the libration of the moon was assumed several times in
previous applications. The authors opinion is, that the main part of the smear is caused
by the doppler of different moon areas formed by the rotations of the earth below the
As a reason for the signalsmear, the libration of the moon was assumed several times in previous applications. The authors opinion is, that the main part of the smear is caused by the doppler of different moon areas formed by the rotations of the earth below the moon.
Conditions: - The test stations are in the northern hemisphere.
- a) the moon is for the test station in
- b) The
station works at moonrise or moonset.
shows the relations between doppler and pass of the maximum frequency shift by the
Fig. Nr.3 shows the relations between doppler and pass of the maximum frequency shift by the dopplersmear.
- c) QSO between stations in North
America and Europe, both located around the same latitude.
- d)The test station uses a very big antenna. e.g. 20 m dish (3dB beamwidth ~ 0. 1°)
The test station illuminates only parts of the moon. As the antenna doesn't illuminate zones with a big
relative motion, there is only
The test station illuminates only parts of the moon. As the antenna doesn't illuminate zones with a big relative motion, there is onlya little dopplersmear, also when the moon is in culmination.
0H6DD has already seen this effect (see DUBUS 2/94). He worked with a very big dish during previous tests and found out, that almost no dopplersmear occurred.
6. Additional attenuation by the atmosphere
Often it is assumed that the atmosphere causes a
considerable additional attenuation, especially with fog or rain. This might be allright
when you work with very small elevations of the antenna, or during a heavy thunderstorm. But during tests with elevations of more than 15° also heavy
rainfalls didn't cause any disturbance. As it
was raining during most of the EME-QS0s of the author, he was able to get much experience. The reason for this effect is, that the bad
weather zones, the signal has to pass through, are hardly 1 km in distance when the
station works with a sufficient elevation.
Fig.4 shows typical values of the Faraday-Rotation.
This figure says, that there is no considerable
Faraday-Rotation on 10 GHz. It is therefore
not necessary to work with circular polarisation. Using
linear polarisation, you have to take care about the basic rotation for reflections at the
8. Antenna and feed 0.4 a VE4MA horn is useful.
0.4 a VE4MA horn is useful.
Fig.5 shows the modifications made by the author.
An experiment is the best way to find out the optimum for
feed-position and position of the corrugation.
corrugation in the expected position, and fix horn and corrugation near the expected
9. Noise sources
10. Moon tracking
Using programs, e.g. from VK3UM, it is possible to
calculate the azimuth and elevation angle or the declination and rectaszension with a
sufficient accuracy. Nevertheless, it is recommended to control the alignment of the
antenna once in a minute with the moon-noise. Therefore,
a broadband noise-RX in a parallel receiver section can be used.
11. Power density in an antenna beam
In order to have a closer look at the reflections on the
moon, it is necessary to analyse the power density in the antenna beam.
Fig.6 shows the radiation pattern and the equivalent
distribution of power density.
Some explanations to this graph:
- The x-axis is the rotation angle.
The result for the reception of moon noise shows Fig.7.
The non-linear gradient in this graph is caused by the radiation pattern of the antenna.
A result of this investigation shows, that the noise temperature (noise figure) of the RX is not critical, working with a large dish.
12. Relation between antenna beam and received power
In this chapter is only taken care about the absolute
received power. The signal/noise ratio will
be considered in the following chapters.
The diagram shows that with antenna diameters smaller than
2 mtr the RX-signal behaves as commonly assumed. At
diameters from 2 to 4 mtr appears an additional loss of 1.5 dB. The cause for this is the
power density in the radiation pattern as the 3 dB points now approach the moon's rim.
What happens with the received power when stations use
13. Relation between antenna size and signal/noise ratio
When the antenna gain will be increased in lower amateur bands, the signal/noise ratio also increase. This relation changes, when the moon noise starts to add a contribution to the basic noise that is worth mentioning. On 10 GHz, system noise of approx. 100 K looks realistic. It is, e.g., consisted of 70K from the preamp and the receiver and 30K external noise received by the antenna (earth, feedloss etc.). A system temperature of 150K seams to be still normal and a temperature of 50K will be the actual lower limit, hard to reach at the moment. All three mentioned noise temperatures are used in the following examples:
On 10 GHz EME the received noise temperature of the moon has to be added to the system temperature. Using 10 GHz, the moon has a noise temperature of approx. 210 K, The received amount of moon noise depends on the size of the antenna, only very big antennas (diameter > 6mtr) receive the full noise power. The smaller the antenna, the smaller the amount of noise power that is received.
Considering the antenna beams from chapter 12 the noise
parts can be found in Fig.7.
The meaning of the used factors are:
- In the used bandwidth, the received noise from the moon has the same value as the
Tab 2 and Fig 9 show the effects on the signal/noise ratio in the correct form (signal + noise)/noise. The results are really interesting.
It can be seen at once, that already an antenna diameter of 4
mtr is the limit. Bigger antennas don't produce a better signal/noise
produce a better signal/noiseratio. On the other hand, the receiving loss using a 2 mtr dish is relative small.
14. View to the combination of antenna gain, noise, and power
Working with 20 W, a 3 mtr dish and a RX noise figure of 1 dB the own echoes can be usable heard. Using this combination as a reference, the following equipment and its effects are displayed:
An improvement in signal/noise ratio is only made when both partners use dishes with more than 6 mtr, under the condition, that both illuminate the same spot at the moon.
15. Recommended handling of computer programs, e.g. that on from VK3UM (budget calculator)
The following measures are recommends in order to adapt the results of the program at the previous explanations:
RX NF: The program adds 50K automatically. So, 50K have to be subtracted from the system noise figure that is formed. by the antenna noise (without moon) and the noise of the preamp. Then, the equivalent noise figure of the moon (see chapter 14) has to be added. This noise temperature has to be transformed in the equivalent noise figure and then insert in dB.
RX BW: The effective bandwidth of the ear/brain combination of a well-trained CW-operator corresponds with approx. 100 Hz. This is relative unaffected by the real used receiver bandwidth. Therefore you can set this value to 1OOHZ.
For gains lower than 51 or 52 dB just insert the real value of the antenna gain. With gains of more than 52 dB, you have to
consider the gain of your partners antenna.
This essay shows some differences you have to deal when moon bouncing on 10 GHz. The conditions on other SHF-bands are similar. Some of the physical approaches are not in the highest scientific exactitude. But the author is convinced that they are of sufficient accuracy for practical use. Some QS0s and noise measurements with the moon prove this.
The author thanks his QSO partners (actually 9) for the tests and the informative discussions with Charlie G3WDG, Goliardo 14BER and Jim WA7CJO. Also for the support by his partner Gregor DL2GSG and some non-licenced fellow workers, who are developing RADAR devices for industrial level measurement at the QRL. Some problems of these short-distance RADAR devices are correlated with those on 10 GHz EME. For example, the target is wider than the antenna beam most of the time and the reflections of solids, e.g. dry sand, can be compared with that from the moon surface.
More on moon temperature: About moon's temperature at l = 2.77 cm