DIY Radio Astronomy Project

[Doug]

Baz, there are some elements of truth in what you say, but only some.
Antenna polarization is a complex phenomenon, and one that can at times be difficult to grasp. Polarization is determined by the E field rather than the M field, though both might be legitimate measures. The polarization of simple dipole antennas is parallel to the conductor, such that a horizontal wire will have a horizontal E field, a vertical wire, will have a vertical E field.
So far I haven't said much different from what you already know.

Now if an antenna is made of a number of parallel elements such as a Yagi, the polarization will continue to be parallel to the plane of the elements.
But suppose we made up a crossed Yagi, ie. one that had elements both horizontal and also vertical. Then in that case we would have either and most probably an elliptical polarization or possibly a circular polarization. In either case, the E field will be rotating, and just to make life interesting; the rotation will be either clockwise or anticlockwise. The things that affect the sense of rotation are exact phase difference of each antenna set (I think) and believe it or not, relative motion between the transmitter and receiver.
That is why satellite antennas are circularly polarised because a satellite will either be coming at you or running away from you.

Jupiter: coming at you or running away, it too will have a circular to elliptical polarization pattern. Exactly why this is so? don't ask me, I haven't got a clue, unless it is due to the rotation of Jove and any significant tilt in its electromagnetic axis to its rotation (though this is probably slight).
All for now,
Cheers,
Doug

[74tuc]

Hi all,

I've been in Alice all last week - Now thats the place to view the sky!!

Polarisation?

1. Comes in 2 flavours right hand circularly and left hand circularly polarisation (RHCP & LHCP)

2. Circular polarisation is a special case of elliptical polarisation - please look-up ellipse.

3. If you do not know the sence of polarisation use a linearly (V or H) antenna you will experience a loss of 3db over a circularly polarised antenna. If you get the polarisation sense wrong (eg. use a RHCP antenna on a LHCP signal and vice versa) you will take a hit of at least 30 to 40 db ie. no copy!!

4. For 10 CM use a paraboloidal dish antenna and for 20 CM wavelength use either a dish or a helical array. Both these antennas will require some kind of driven mount

5. Helical antenna is nice to start off with. Initially wind two (one Left hand and the other Right hand). With these two helices you can get LHCP or RHCP or Vertical or Horizontal or any angle between H and V.

Jerry.

[Doug]

Barry don't confuse the helix antenna that I originally mentioned and that Jerry is now talking about with a helical antennas.
'Helical' antennas are by and large either top, centre or base loaded vertical antennas used for mobile (typically) HF/vhf/uhf coms.
All this might be confusing, but unfortunately the Helix antenna is so named because it is formed as a helix. The helical (whip) is so named because it can sometimes be simply a helically wound copper conductor formed/stretched over a fiberglass rod or similar. It will be vertically polarized with a main radiation lobe being closer to vertical, the more loading it has(the physically shorter it is compared to the wavelength it is tuned for).
This is why a 1/4wave helical whip will have less than unity gain. The true Helix antenna will as Jerry said have circular/elliptical polarization, but its direction of action is parallel to the axis of the helix. It has an attractively high gain over a standard dipole.
Just remember that even though an antenna might be called a 'helical', when looking for high gain and directivity, a helix is meant, not a mobile whip.
cheers,
Doug

[Dujon]

Barry,

The 21 Cm band equates to around 1,425 MHz (1.4 GHz). I don't think your current receiver will get within the proverbial bull's roar. To purchase a receiver which covers that frequency could be a wee bit expensive. I have an ancient (now) 'scanner' which in theory covers up to 1.3 GHz. How sensitive and selective such units are at the higher frequencies I do not know.

Incidentally, UHF is generally regarded as those frequencies between 300 MHz and 3000 MHz (3 GHz).

On a general note, the frequency bands are usually grouped in increases of one order of magnitude and start at 3 Hz.

Hang on a bit . . .

See here for the description - it's only terminology/jargon but enables you to be aware of what others are on about.

http://en.wikipedia.org/wiki/Super_high_frequency

Great to see that you are persevering with your project. Please keep us all up to date with your progress.

[Doug]

Barry you might find the following links of some value:
http://www.ggrweb.com/article/gulley.html

http://www.tuc.nrao.edu/~demerson/helixgain/helix.htm

There are many sites dealling with Helix antennas, you could snoop around for a while and pick up some good info.

These above are not designs for 21cm, but they will give you a rough idea.
Jerry raised the point that these antennas would need to be driven; at the smallish size/weight these helix antennas are, they could be mounted on the front of an OTA; driven, guided by the mount.
A receiver? well as John said they don't come cheap. Andrews have a scanner that will go that high for around $499. I think the pc radio mentioned elsewhere on ISS would give real value for money because you could graph the signal strength/bandwidth of your received signal whereas a scanner would maybe hiss and crackle, but not much more. Poor return on your investment.

cheers,
Doug

[bloodhound31]

AAARRRGGGHHHHHHH!!!!!!! Information overload!! Slow down fella's, I havent even got any equipment that can come close yet. All my astrofunds at the moment are going towards completing the dome. Once I have a place to put all the radio gear, then I can continue with the perserverance!

But really, the suggestions and advice are much appreciated lads. I have learned so much just by having you lads answer my questions and I thank you for your encouragement. I keep coming back to the pages of this post to read over and try to understand just that little bit more.

Anyway, back to the dome!

Baz.

[74tuc]

Hi Chaps,

Just to clarify things.

Helical antennas come in two flavours that are described by the mode of operation. The helix may be operated in the broadside mode or in the axial mode both are refered to as helical antennas. Normally one would clarify the subject by refering to a "broadside helix " or an "axial mode helix" . One does not construct arrays of broadside halix antennas but axial mode halices are very common and for most part when axial mode helix antennas are used they are just refered to simply as helical antennas. If a broadside helix was chosen it would be specifically refered to as a broadside helix. Each of these helicals has quite different characteristics. Axial mode halices are characterised by "highish" gain and very wide bandwidth.

Generally it is understood what one is refering to when one talks about helicals. At HF a helical antenna is further clarified as a helical whip antenna
at VHF helical generally means axial mode helical.

Regards,

Jerry

[Poita (Peter)]

So how did this turn out in the end?

[Doug]

Generally it is understood what one is refering to when one talks about helicals. At HF a helical antenna is further clarified as a helical whip antenna
at VHF helical generally means axial mode helical.

Hi Jerry, that is a very good clarification!

Doug