# 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.

• I have a K3TZ patch feed for 2.4GHz,

Does anyone know a way to reduce the beamwidth / increase the gain so it illuminates a 0.6FD offset fed dish with better efficiency?

I was thinking to add a single director in front of the patch. A loop ? or a disc? or a traditional pair of halfwave (x0.9) long directors as used in a crossed Yagi.

Thanks David

• Yes you can add elements. A disc or a loop, of the right size and spacing (that's where it gets tricky). You could even use a lens instead. The question is is it worth it for the amount of gain gained.

• I was thinking about a waterproof enclosure for the patch, Then it occurred to me that if I make the enclosure the correct length, a director could simply be added onto the outside or inside of the enclosure. - Think end cap over tube with disc or loop bonded on.
Shame we don't have those antenna test ranges we've organised in the past at University of Surrey in the AO-40 days.