RE: [amsat-bb] TVRO Dish Feeds - Patch vs Helix

["Howard Long" <hlong@xxxxxxxxxxxxxx>]

Hello David

> For this type of dish (~1 m dia, offset feed) what are the pro's and con's
> of using a patch vs. helical feed? Both seem fairly easy to construct from
> website pictures/instructions I have seen.

Good question! Here's some pro's & con's I could thinks of...

Patch Pros...
Phase centre is very specific
Smaller sidelobes
Empirically they seem to have better performance

Patch Cons...
Dimensions critical
Narrowband

Helix Pros...
Wideband so helix dimensions less critical
Beamwidth can be varied by altering number of turns
Better for smaller illumination angles (large f/d)

Helix Cons...
Matching section dimensions are critical
Phase centre more vague
Larger sidelobes
Can be fragile

> Is the f/d ratio important for choosing between these differing feeds? (I
> have not measured the f/d for this dish so don't know what it is.) One
> website said the ratio is important for choosing the feed type, others are
> 'mum' on the subject.

When considering offset fed dishes, I find it easier to consider
illumination angles (the angle that the dish subtends at the focal point)
rather than f/D. The offset fed dish is only a small segment of a full
conventional prime fed parabolic dish, so the f/D is not really the correct
term to use. Some texts use the term 'equivalent f/D' when referring to
offset fed dishes.

In general offset fed dishes are physically limited in their illumination
angles, which will almost always be about 75 degrees. This is quite a large
'equivalent f/D' of 0.75. Note that some offset fed dishes (eg, Primestar)
are very oval so the horizontal and vertical illumintaion angles are
different, and will originally have feeds which take this into account.
Because we're circularly polarised, a circular dish (when viewed from the
focal point) will be better (technically) but and oval dish will still work
fine.

The rule of thumb for some is to get the 10dB beamwidth of the feed matching
the illumination angle. But because we're pointing up into space, the feed
will start pointing down to the earth and so ground noise from the feed's
overspill will start to become more significant. Therefore I've seen some
using 15dB beamwidths for the feed.

Ground noise is potentially one reason why patch feeds work better
empirically than helix feeds: the sidelobes tend to be a lot smaller on a
patch compared to the helix.

> Does a patch or helical feed 'fit better' with a specific type of dish -
> offset vs central placement of the active element?

Again, I find it easier to consider illumination angles. A prime fed dish
tends to have a larger illumination angle compared to an offset fed dish:

 f/D   Illumination angle
 0.30  159
 0.35  142
 0.40  128
 0.45  116
 0.50  106
 0.55   98
 0.60   91
 0.65   84
 0.70   79
 0.75   74

The oracle on this stuff is:
http://www.w1ghz.org/antbook/contents.htm

and in particular:
http://www.w1ghz.org/antbook/chap4.pdf
http://www.w1ghz.org/antbook/chap5.pdf
http://www.w1ghz.org/antbook/app-5a.pdf

There are also some theoretical and practical questions on the circularity
of patch and helix antennas. Generally the longer the helix the better the
circularity (but the narrower the beamwidth). With a patch there are some
other considerations, with the accuracy, the geometry and the feed method
affecting the circularity.

Finally, I discovered after pulling a lot of my hair out that one of the
best ways to figure out what works was to have a go as well as reading up on
the subject. The trick is to know when to close the books and get the tools
out! You'll be surprised what you can get away with. Good luck!

73 Howard G6LVB


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