A priori: click on images to enlarge in new tabs.

Introduction
In Q1 2017 the Qatar Es’hail Satellite 2 (Es’hail2) is scheduled for launch and
will carry a 2.4 GHz receiver for amateur usage. The geostationary position is 25.5E.

My concept will be a single dish with dual band feed. Actually it will be a
2.4 GHz LHCP feed together with an X/Ku-band PLL LNB.

Standard/cheap broadcast dishes own a f/D around 0.6, so popular patch
antennas will over illuminate the dish, resulting in less overall efficiency.

The -10 dB opening angle of the feed should be around 80 degrees.

I read somewhere (don’t know where) that the amount of turns (N) should be N * 0.1 f/D.
Since my dish f/D = 0.6 I built a 6 turn 2400 MHz helix (v0.1) according to information
from the web. This helix was made with enameled 2 mm diam copper wire.

For whatever reason I couldn’t get a proper match, i.e. a match with return loss >30 dB@2400 MHz.

Time to model the helix to estimate interdependancy and weighting factors of
design parameters relative to e.g. gain, radiation pattern and axial ratio.

Modelling
Information + pictures to be presented soon. For the time being: below a gif which Van Gogh should envy ; -)
It’s the calculated axial ratio of helix v0.2, i.e. the difference between RHCP and LHCP (click to enlarge).


Difference in RHCP and LHCP gain of modelled v0.2 helix.

Practical design
Below some pictures of two helices recently made for this project. The first one (v0.1) was made with
enameled 2 mm diam copper wire according to ‘helix calculators’.

Initially I attributed the non optimal match to the position of the feedpoint due to mechanical
restraints. E.g. the ‘extra’ wire (‘not part of the helix’) acts as impedance transformer.

No matter what 1/4λ stub (length, impedance) I tried, measured return loss around 2400 MHz remained
suboptimal. I.e.  in the 15 – 20 dB range, while dips occured around 2350 and 2500 MHz but not 2400 MHz.
(generally builders/designers are satisfied with 15 – 20 dB return loss . . . I’m not . . . )

Anyway, v0.1 served very well as a template for the feed mechanics.

Below some pictures of prototype v0.1 (right picture a 60cm diam test dish). (click to enlarge)

Today I made a new helix (v0.2) with ‘drilled’ 2mm diam plain copper wire according to my simulations.
I suspect the enamel around the wire of v0.1 might have significant influence on 2.4 GHz,
perhaps due to skin effects or whatever.

v0.2 diameter is somewhat larger compared to v0.1, viz. 46mm vs. 42mm, but has the same pitch (27mm).

The helix is matched to 50Ω with a delibarate too broad 1/4λ long copperfoil ‘transmission line’.
After some fiddling I obtained a perfect match (RL =48 dB !!) around 2400 MHz. That’s much better!

Radiation patterns of antennas are more predictable and ‘pure’ when they are terminated
with the proper/calculated impedance. It’s reciprocal.

Identical to a transmitter which has to be terminated with the proper impedance for maximum energy transfer,
an antenna must be terminated properly to convert inserted RF-current into the wanted radiation pattern.
In other words, life is much easier and predictable when both impedances ‘match’ ; -)

Inserting my LNB in the helix current maximum detoriorated the return loss with ca. 10 dB and
the sharp dip increased somewhat in frequency. Anyway, looks promising and it needs some
fiddling to get everything perfect on 2400 MHz in front of a dish : -)

Below some pictures of v0.2 (click to enlarge).