# UPLINK - PATCH VS HELIX - what's better

• G0MJW Yes, the same goniometry / math is also valid for offset dishes. An offset dish is nothing else than a prime focus dish but then another part of the parabola. The higher you get into the parabola, the larger the f/D. Although the illumination may look optically 'squint', the effective surface of an offset dish is equal to a 'prime focus dish' with the diameter of the width of the offset dish, see below.

Yes, I know this for a long time since I first came across the concept in the 1980s. What I don't know is if the curvature calculation still holds when working out the angle - I could work it out but... in my dish the curvature it rather shallow so it matters not.

• G0MJW (I know that you know, but the picture was merely intended for other readers ; -)

Unlike science, the radio amateur community doesn't have a 'memory', so reinventing wheels and making 'new discoveries' is a 'mandatory part' of our hobby ; -)

• PA3FYM and G0MJW : thanks so much for your excellent explanations.

and as G0MJW said: I will just build the helix and have some beer instead of trying to understand it's math.

Kurt, DJ0ABR

• After having some Beer and Obstler I finished the Helix and did some comparisons in different configurations, here are the results:

TX-signal (received with a separate dish):

Poty: +25dB

Helix: +28dB

Transponder Noise:

Poty: 6.1dB

Helix: 4.3dB

DVB-S2 MER (measured with Minitiouner):

Poty: 7.8dB

Helix: 6.6dB

As expected with the TV offset dish, the TX signal of the Helix is almost 3dB stronger.

However, the helix shades the LNB by 1 or 2 dB, which is not relevant for SSB, but represents a significant attenuation for a DATV signal.

So what to do? Building a PA with 3 dB more power? Or buy a larger dish to compensate for the loss of MER? Or just living with these results the way they are. I think I'll do the latter.

vy 73, Kurt, DJ0ABR

• So what to do? Building a PA with 3 dB more power? Or buy a larger dish to compensate for the loss of MER? Or just living with these results the way they are. I think I'll do the latter.

I personally prefer the solution with two different antennas if space isn’t a problem it’s more flexible and can be used for other purposes as well.