UPLINK - PATCH VS HELIX - what's better

There are many roads to Rome. It's important that your uplink antenna illuminates the dish properly and when you use circular polarization it's LHCP (in front of a dish).

I made helices, and they work fine, but from a mechanical perspective and simplicity I prefer the patch. For a variant of the K3TZ patch look here.

Depends on how the patch is designed. Standard broadcast offset dishes have f/D of 0.6 (otherwise each dish needs his own LNB ; -) , which equals to 90° -10 dB beam width (width of the offset dish taken as 'size'). Patch in the link has -10 dB = ca. 105°. So it over illuminates a bit, but that does not matter as it's used to transmit only.

To illuminate a dish with a helix properly a rule of thumb is that you need one (1) winding per f/D-unit. So f/D = 0.6 needs 6 windings. With smaller f/D's less windings are needed, however, with less windings the axial ratio becomes worse.

I made a dual band feed with a helix, it works. However, from a mechanical (stiffness, size) perspective the patch is my favourite.

... my old two band helix, over 20 years on the roof, worked and worked... year for year .... also first test on QO-100 without problems ...

changed last weekend to a three band feed ..

i like helix feeds ...

DD4YR Haha : -) Of course, I forgot .. feelings are facts : -)

Robert, is the picture a 23/13cm feed ?

Yes,

now I added a 10 GHz Feed horn ..

and a weather resistant box in poly propylen ....

for the next 15+ years ....

Provided the plastic will not detoriate due to UV, it looks so firm I think it lasts forever! ; -)

Instead of onethousands words - just a picture

old RX-dish from AO-40 G3RUH-patchfeed. +

with 100mW at the dish: 539 in CW seems that its working...

Rolf, picture is too small. Can you upload a bigger one?

Looks like the dish sold by G3RUH:

More about it here:

http://www.jrmiller.demon.co.uk/products/s_ant.html

Looks like a Chinese wok to me ; -) (no pun or offense intended)

One issue that I have been thinking about with helix feeds is blockage from the helix structure in front of the feed pipe.

I have a simple feed here with 22mm water pipe protruding a few mm from the reflector of a 2.25turn G3RUH feed. The feeds works great on my big dish, but have been testing it today on a 76cm prime focus dish with approx 0.4 f/D.

Two tests so far, measuring the spectrum and MER of the WB beacon. Both tests show significant degradation when the helix is present (I unsoldered it from the feed while the feed was mounted in the dish, changing nothing else). The helix's reflector plate was left in place.

For marginal systems, particularly on the WB transponder, this would suggest that while simple to construct, a helix feed may be inferior to a patch. For the NB transponder I doubt if there is any discernible difference (from blockage) as the transponder noise floor is the limiting factor for most of us.

Upper screenshots are with helix present, lower two are with it removed. Spectrum is at 5dB/div.

There wasn't any measurable difference in beacon MER with the helix terminated with 50 ohms, or left open circuit.

There could be advantage in moving the open end of the feed further into the helix to reduce blockage, but as I understand it that would move the phase centres of the 2.4 and 10GHz feeds apart, leading to loss in gain for one of them. So some tradeoff may be possible.

Lots to experiment with these days!

73

Charlie

Nice experiment!

It could well be that the helix acts as a kind of 'lens' so that te dish is under illuminated and you de facto have a smaller dish (?)

I just confirmed the measurements using Sun noise. With helix fitted sun noise was 4.5dB, without helix present it was 5.8dB. Correcting for S+N/N, this equates according to my sums to a drop of 1.9dB in system receive performance caused by the helix.

73

Charlie

G3WDG Can you post a picture of your contraption Charlie?