1420 MHz test transmitter

[worldbestpilot]

Hi there,

for upcoming tests of my dish and its signal chain I intend to use a small home brew test transmitter, mounted on top of a 10 m pole, some 40 m from the telescope.

This small battery powered device will emit on 1420 MHz; either CW or pulses (kind of "pulsar simulator").

Circuit diagram is attached.

The transmitter itself is a LO from Mini Circuits.

Using such a stable RF source, optimising the dish signal chain should be easier than using the sun as a signal source.

Maybe such a device is useful for you as well.

Cheers

Volker

[Marko Cebokli]

Beware that 40m on 21cm is at the Fraunhofer limit for a 2m dish. If your dish is bigger than 2m, you will need to go farther away., to make meaningful measurements of the dish parameters.

 

The formula for minimum distance is two times dish diameter squared divided by wavelength, for 1/16 wavelength phase error (plus minus 1/32).

 

Also, if you set your focus with a relatively close source, you will set it too far out. Using the thin lens equation, say your focal length is one meter, and your source is 40m away - that is 40 focal lengths. This wil cause your focal point to be 1/40 focal length farther out, that is 2.5cm.

 

BTW, when seeking focus, the best way is to go for deep nulls betwen main and first side lobe (remember the Airy disk and diffraction rings).

Deep nulls also tell you that your dish is in good shape - again, think of "star testing" an optical telescope.

 

Marko Cebokli

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[Wolfgang Herrmann]

Hi Volker,

There are two things which you might want to consider:

With a 3.8m dish and 21cm wavelength you are still in near field conditions at 40m distance. Due to this, optimizing the dish will not give you correct results. You would need to be at least some 150m away.

Then, using the test transmitter might give you problems: Putting aside the question of legality of transmitting in the radio astronomy band, the additional trouble is that the minicircuits has high harmonics. The first harmonic (2840 MHz) is within the band of air traffic control radar systems.

Cheers

Wolfgang

 

 

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[worldbestpilot]

Hi Wolfgang,

thanks for the comments !

I guess that I forgot to mention that this small Tx will not be used for geometric alignment of the dish - this will have been done before.

Will only be used for signal chain optimisation - pure electronics stuff.

Emitting in the 1420 MHz protected waveband is clearly an issue, however:

(i)    Emitted power is very limited.

(ii)   The Tx antenna will be shielded, so that it is only transmitting within a narrow horizontal cone towards the dish and not omnidirectionally.

(iii)   Behind the dish there is a very dense bush; absorbing lots or even all that energy.

(iv)   Within a few tens of kilometers no one is operating / listening to 1420 MHz.

(v)    Harmonics with this low power will not even harm the closest ATC equipment (some 50 km North); also see (i) above.

(vi)   Emission direction is South (nearest airport in this direction is beyond the Alps).

I know that this is a grey area, but it will be used for a few hours only and at extremely low power.

Cheers

Volker

[worldbestpilot]

Hi Marko,

thanks and please see my comments to Wolfgang.

Coming from the "Optical community" I am well aware of these nulling features and will surely consider them before the Tx will be used.

Cheers

Volker

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