BATC Forum

I'm going to make a MJW patch feed for my 1.2M dish and I notice the design calls out 1mm copper or brass plate.

I was wondering if I should make the rear larger plate out of 1.5mm material for greater mechanical strength ?

Probably need to leave the driven element at 1mm so as not to de-tune it.

Any comments ?

Edit - just re read the CQ ATV 263 article and was wondering if the updated version with the little copper feed horn would be more appropriate for my 1.2M dish (F/D approx 0.6), would also avoid the need to cannibalise a Rocket LNB for the lens.

73 Tim

I formed the backplate from 1.2mm copper sheet and the patch from 1mm, both found in the offcuts bin of my local engineering supplier. This gives a very solid and rigid structure. I believe the critical dimension is the 3mm gap between them. I used 3mm shims to pack the gap and toolmakers clamps to ensure it was evenly spaced all round.
73 Shaun.

G4WIM wrote: ↑

Thu Apr 11, 2019 5:00 pm

I'm going to make a MJW patch feed for my 1.2M dish and I notice the design calls out 1mm copper or brass plate.

I was wondering if I should make the rear larger plate out of 1.5mm material for greater mechanical strength ?

Depends on what you plan to do with it. Support heavy weights - e.g. a perch for Red Kites? Make it as thick as you like, or use Titanium.

Probably need to leave the driven element at 1mm so as not to de-tune it.

Any comments ?

1mm is fine for strength, you could go bigger, it is not that critical

Edit - just re read the CQ ATV 263 article and was wondering if the updated version with the little copper feed horn would be more appropriate for my 1.2M dish (F/D approx 0.6), would also avoid the need to cannibalise a Rocket LNB for the lens.

Rocket LNBs are very expensive, but they do what you need. You can try both methods and experiment perhaps?

73 Tim

thanks for the replies guys - all makes good sense.

I think I may try a hybrid design - ie follow the original design but instead of the rocket lens try a 'plumbers horn' as described in the update.

This raises the question of dimensions of the horn, position and how it attaches to the 22mm pipe etc - it's a bit hard to understand exactly the dimensions as listed in the update.

As an aside I'm working on a LNB PLL solution which would use the internal xtal for TV so giving best phase noise, but switches over to an external GPS disciplined source when the applied voltage to the LNB switches polarization. So hopefully giving the best of both worlds . I had noticed a bout a 1dB of MER degradation on the Octagon when it was phase locked.

Had to stop work on this approach as my Octagon quad stopped working - interestingly the 6V regulator went short circuit.

73 Tim

G4WIM wrote: ↑

Thu Apr 11, 2019 7:11 pm

As an aside I'm working on a LNB PLL solution which would use the internal xtal for TV so giving best phase noise,

My St Time 0.1ppm TCXO fitted internally, gives better phase noise than the old crystal.

Dave

Hi Dave,

couple of questions - you say the reference you use gives better phase nise performance, was wondering how you measured this and what if any effect it had on the overall rx performance ?

Secondly, I recall ready about this somewhere, but can't find the thread - can you provide a link to a supplier ? As I recall the cost was about £40 ??

Back to the original question about copper thickness - I'm probably going to make the rear plate out of 1.5 or even 2mm copper such that it doesn't deform from the weight / stress of feed coax attached to it.

Regards Tim

Just been trying to source some copper sheet on ebay to make this dual band feed.

Seems as though the options are 0.9 or 1.2mm thickness but no 1mm sheet offered.

So I was wondering what others have used and if 0.9mm or 1.2mm would be suitable for the driven patch which should be 1mm thick.

73 Tim

The back really doesn't matter - 3mm if you plan to hang large coax off it. The patch will slowly go out of tune, you can bend it back but 3mm copper plate is not so flexible.

thanks again re back plate thickness, however do you think either 0.9 or 1.2 would be ok for the driven patch, ie being 0.1 thinner or 0.2mm thicker and not detuning it ?

Yes - it will be fine