Fun with Arecibo

Fun with Arecibo.

'Howling at the moon!'

Over the weekend, radio amateurs had a chance to participate in an unusual experiment involving moon-bounce signals. Now, ordinary radio setups are usually woefully inadequate for such communication, but for this experiment, the Arecibo Radio Telescope was used to beam high power signals at the moon, strong enough to be reeceived on the Earth with fairly modest equipment. Nicolas, M1HOG, a member of SADARS, joined in the mayhem and has sent me a few photos from his group's experiments.

I should mention that 70Cms was the band chosen for the experiment and that a transmitter at KP4AO Arecibo with a 400Watt PA was available. On 70Cms, Arecibo has around 60Db of gain. Now, since 400W= +56Dbm into a 50R system, plus the 60Db of antenna 'gain' the maximum signal ERP possible at the Arecibo dish would be around +116Dbm, on the Uplink to Oscar-Zero.

Not AMSAT-O-Zero I hasten to add! Perhaps GOD-O-Zero, or Allah-Oh-Zero, or even Zen-O-Zero. Have fun, don't be so lazy... think up your own! Still, I digress.

My Dbm-Volts-Watts table tops out at 53Dbm-99.9V-200W, so I'll leave it as a little experiment for you to work out the ERP in Watts.

Nick said....
We had a go EME. After quite a bit of amusement setting up our yagi we started to pick up SSB, Very weak indeed, but there. heard a few call signs and Over, Over all the time! He certainly had a pileup - very strange hearing SSB from the Moon! Then we heard CW, which was a bit clearer. Tom did try to work them but no luck. Fun evening though.









	Howling at the moon, SADARS / M1HOG style.

I like the novel antenna stand comprising of a traffic cone with two house-bricks perched on top.

OK, I do confess to having an interest in what 60Db over 400W actually represents in terms of ERP (effective radiated power). I did this by hand, yeah, with pencil and paper, because I couldn't find a ready reckoner that ran to megawatts! The actual ERP possible (+116Dbm) at the dish, using 400Watts of drive (but ignoring feedline and system losses) is around 400Megawatts ERP... (but see Domenico's notes... below). That's a lotta watts! The reality is that the power transmitted was probably around half of that figure. At lunchtime today I received an email containing the following information. It had been snipped and forwarded from the AMSAT-BB which I rarely ever read. It very conveniently allowed me to check the accuracy of my simple calculations.

Here's what Domenico I8CVS, a 'real' expert had to say.

Although the Arecibo group, callsign KP4AO, say the signal can be heard with a small hand-held Yagi pointed at the moon, Domenico I8CVS reckons a 15 dBi Yagi is required. It will be interesting to see what people achieve in practice. What is the smallest 432 MHz antenna that can produce a detectable signal from Arecibo? Domenico I8CVS has posted these calculations on the AMSAT bulletin board:

The specifications of the Arecibo Observatory Amateur Radio Club for the 432 MHz Moon Bounce test on April 16-17 and 18 are as follows:

Dish diameter: 1000 foot equivalent to 305 meters
Antenna gain: 60 dBi
Tx power: 400 W
Tx Frequency: 432.045 MHz

Since the given ERP is 243,902,443 Million Watts (see below) and since 60 dB is equivalent to 1000000 (one Million) time in power it comes out that the power reaching the feed of the dish is: 243902443 / 1000000 = 243 watt.
The rest of the power 400-243 = 157 watt is lost in the feed line.

At 432 MHz a dish with a diameter of 305 meters has a -3dB points main lobe angle equal to:
Theta = Lambda / Diameter = 0.69 / 305 = 0.00227 rad.

The above lobe of the dish at an average distance of 380000 km light up a small circular surface S over the moon wich diameter is: D = 380000 x 0.00227 = 865 km

The surface area S = (3.14 x 865^2) / 4 = 5.88 x 10^11 square meters

All the radiated power of 243 watt by the dish is now collected over the above S area.

The reflectivity of the moon at 432 MHz is the 7% so that the power scattered back isotropically by the moon is ( 243/100 ) x 7 = 17 watt

It is like to say that the power reflected back by the moon is 17 watt feeding an isoptropic antenna or 17 watt EIRP or +12 dBW EIRP radiated isotropically by the moon.

Since the surface of the moon lighed up by the dish is less then the whole surface of the moon the usual calculation procedure for the EME link considering the isotropic attenuation earth-moon-earth cannot be used here because as seen by the Arecibo dish the diameter of the moon is smaller than in reality and is large only 865 km in diameter not 3476 km as is large in reality the moon.

With this is mind we must imagine the dish of Arecibo to be an isotropic antenna with 17 watt applied to it and transmitting all around isotropically from the moon.

My ground antenna has a gain G= 15 dBi and an antenna picked up noise of 70 kelvin when looking at the cold sky

My receiving system at 432 MHz has an overall Noise Figure of 0.7 dB equivalent to 50 kelvin so that the noise floor KTB of my receiving system in SSB with a bandwidth of 2400 Hz is KTB=1,38 x 10^-23 x (50 + 70) x 2400=3.97x10^-18 watt= -174 dBW

Link Budget Calculation:

Isotropic power reflected by the moon...+12 dBW
Isotropic attenuation for 380.000 Km.. - 197 dB
-------
Power received on isotropic earth ant... - 185 dBW
Ground antenna gain...............................+ 15 dBi
-------
Power applied to ground receiver..........- 170 dBW
Noise floor of ground receiver..............- 174 dBW
-------
Received Signal to Noise ratio S/N.......+ 4 dB

By the way when KP4AO will operate on CW I can switch on the 500 Hz filter on my receiver and here I will gain in sensitivity 2400/500 = 4.8 time and 10 log 4.8 = 6.8 dB so that I gain 4 + 6.8 = 10.8 dB of overall
10

Signal to Noise ratio

If I can stake two 70 cm antennas with gain 15 dBi each I can gain about another 3 dB and I can improve the S/N ratio to 10.8 + 3 = 13.8 dB

If I can stack four 70 cm antennas with gain 15 dBi each I can gain about another 3 dB and I can improve the S/N ratio up to 13.8 + 3 = 16.8 dB a real very strong signal on CW or 16.8 - 6.8 = 10 dB in SSB
Signal to Noise ratio which is considered to be optimal for a
comfortable reception in SSB

By the way to work EME using a big dish having a lobe with an aperture angle "theta" smaller than the diameter subtended by the moon which is about 0.5 degrees i.e. 0.0087 radians is useful only for the big dish to hear better those stations using smaller dishes but the big dish to be received better by the smaller one's "must" use more power and not increase the diameter of the dish because as soon as the moon is completely resolved the power scattered back isotropically do not increase increasing the diameter of the dish.

In conclusion I believe that ground stations with an antenna gain of 15 dBi and a receiving system with an overall Noise Figure of about 1 dB can easily hear KP4AO on CW and barely in the noise on SSB.

Stations with the same receiver overall Noise Figure and antenna gain from 18 to 21 dB can hear KP4AO on CW and SSB without problems.

Stations using 100 watt or more and the above antennas ranging from 15 dBi to 21 dBi have chance to make QSO with KP4AO on CW.

The above calculation shows that it is very difficult to hear KP4AO or be heard using a small hand-held Yagi pointed at the moon.

Have fun

73 de I8CVS Domenico

OK, nobody has yet invented the isotropic antenna, except in their head, I suspect in large part because you can't feed such a beast and have it remain isotropic, but even so, that's a very weak signal indeed.

Thanks Nick, and SADARS, for the photos, and for the feedback from your experiment, and thank you Dom I8CVS, for helping to get my brain back into gear..

Just thinking, next time they go back to the moon, it would be immensely helpful if they'd take a very LARGE sheet of thin Baco-foil with them, unfold it and place it in the middle of the 'Mare Tranquillitatis' with place a few rocks on the corners, to act as a passive radio reflector. Why did we miss *that* particular opportunity? I did say LARGE. Ally is a better reflector than moondust. OK, I'm just kidding now.

Since +53Dbm=200w, in three Db increments (doubling the power at each step. we get)

and so (whew!) +116DBm (or 86DbW if you prefer ) =419,430,400Watts (into a 50R resistive load)

So a 60Db increase is an increase of one million times. A real expert like Domenico would know this of course, but I don't feel too bad about working it out the difficult way.

If you like DbW better, simply subtract 30 from all the above power levels in my simple list. In reality, feedline and system losses would have eaten considerably into that EIRP figure but I firmly believe that Domenico's calculations are in the right ballpark, certainly over 200Megawatts ERP!