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Ask Dr. SETI ®

Chapter 6: Technology

Birdbath vs. Meridian Transit

Dear Dr. SETI:
This weekend I installed my new 8 foot antenna in my terrace. For now, I have set the antenna over a concrete base with an equatorial mount. I may have the oportunity of motorize the mount, with one actuator in equatorial mode, and in the future with two actuators will have az and el with a bit of modification of the mount.

For the moment I have a helix feed and I listen without problems to GPS, INMARSAT and WEFAX signals.

Now I'm a bit confused about the orientation of the antenna. For the moment I will try to put it in bird-bath mode, because is the more cheap and easy.

I don't know if I can put the dish in bird-bath, because the mount only permits up to 70 of elevation. I can try to modify the mount to get to 90. In the future, I would like to mototize the mount, but I don't know if this is a good idea, or if the effort and the investment justify the results.

Iban (Spain)

The Doctor Responds:
Motorized mounts are always worth the effort and investment, but perhaps the use of only one rotor is justified. I don't know the details of your mount, Iban, but let me tell you how I handled this with my own dish. It is a modified C-band satellite TV antenna, originally on a horizon-to-horizon polar mount. It had a limited range of declination steering, set by manually adjusting a turnbuckle. It also had a chain, track, and electric motor/gearbox arrangement for remote Right Ascension adjustment over a wide range of (to track from one geostationary satellite to another). At zenith, the antenna was supposed to be pointing due South (since I'm in the northern hemisphere), with its declination turnbuckle set to point at the Clarke orbit belt.

So, here's how I modified the arrangement for meridian-transit radio astronomy: I simply rotated the dish and mount 90 degrees on their pipe. This turned the declination turnbuckle into an azimuth (RA) adjustment, and the RA rotor into an elevation rotor. I set the turnbuckle for 0 degrees offset from meridian transit (that is, so the dish runs along a true North-South line), and I use the former RA rotor to move the elevation from the Southern horizon to the North polar star (about a 135 degree range at my latitude). Now, using the chain drive to set declination, and the Earth's rotation to set RA, I can scan just about anywhere in the Northern sky, as long as I don't mind waiting 24 hours (acutally, 23 hours 56 minutes; one sidereal day) for the object of interest to rotate into view.

I did find with this arrangement that the dish was quite front-heavy at low elevation angles, and put quite a strain on the elevation (formerly RA) rotor. I ultimately had to mount a counterweight on an arm extending back from the rear of the dish, to provide better balance.

I hope you find this description useful. Good luck with your system, and congratulations on your progress to date.

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