Dual Feed for Prime Focus Dish

[g7avu]

Hi All

Apologies if this has already been answered.

I have seen lots of designs on the internet for a dual feed for TX/RX but they have all been for an offset dish.

I have have a 1.7m Prime Focus dish with an f/D of 0.33. The designs online line for the offset dish's have said they could work with a prime focus dish with and f/D no lower then 0.5 which mine is below making these feeds no good.

Does anyone know of any designs for a dual feed TX/RX for a prime focus dish (1.7m f/D = 0.33) which I can make? If there are no dual feed designs I would at least want to find a design for TX into the Prime Dish, I can sort RX separately.

Thanks in advance

Bob
G7AVU

[g0mjw]

If it only for TX then a super VE4MA style feed will suffice. You will need to feed it on two ports with appropriate phasing to get the circular polarisation.

http://www.w1ghz.org/antbook/conf/VE4MA ... _feeds.pdf

http://www.ok1dfc.com/eme/emeweb.htm

Mike

[G4FRE]

The ve4ma superfeed has an inbuilt polariser. One connector gives rhcp, one gives lhcp. Tried the 13cm one I used on eme at the weekend on the narrowband transponder and it works very well.....without a dish!

Dave
G4FRE

If it only for TX then a super VE4MA style feed will suffice. You will need to feed it on two ports with appropriate phasing to get the circular polarisation.

http://www.w1ghz.org/antbook/conf/VE4MA ... _feeds.pdf

http://www.ok1dfc.com/eme/emeweb.htm

Mike

[g7ntg]

to Mike G0MJW - I built the new circular feed as your article in CQ-TV263 but did not fit the horn at the end yet. Without the horn the performance is superb on receive but when I push fitted the short horn in to the end of the waveguide the receive signal strength dropped 8dB! I use a 1.2 metre prime focus dish with a f/d of 0.4 and the focal length is set so that the focus is at the end of the waveguide. The waveguide on my previous square patch antenna protruded 7.14mm (1/4 wave) in front of the patch. the new antenna waveguide protrudes the same amount and the receive sensitivity is within 1 DB of the old one. Am I missing something here? is it the dish illumination that is different? I wonder what dish size the horn is optimal for.
Any ideas would be much appreciated.

[dh2va]

I think the horn extension in CQ-TV263 is better suited for shallow dished (f/D 0.6 as described) as it concentrates the lobe more in forward direction (like the dielectric horn). A f/D 0.4 as you have it might want to use a wider beam so leaving both the horn (or the dielectric lens) away should be better suited.

[g0mjw]

to Mike G0MJW - I built the new circular feed as your article in CQ-TV263 but did not fit the horn at the end yet. Without the horn the performance is superb on receive but when I push fitted the short horn in to the end of the waveguide the receive signal strength dropped 8dB! I use a 1.2 metre prime focus dish with a f/d of 0.4 and the focal length is set so that the focus is at the end of the waveguide. The waveguide on my previous square patch antenna protruded 7.14mm (1/4 wave) in front of the patch. the new antenna waveguide protrudes the same amount and the receive sensitivity is within 1 DB of the old one. Am I missing something here? is it the dish illumination that is different? I wonder what dish size the horn is optimal for.
Any ideas would be much appreciated.

That horn is meant for a higher F/D - e.g. 0.6-0.7. It has a gain of 11 dB and a -10dB beamwidth of 100 degrees though not a shaped pattern you get when you add all those ridges you see in LNB horns.

You will want something much smaller for 0.4 F/D. What you are seeing is under-illumination of the dish. The good news is just the waveguide alone is close as you found - so you did not really need to make the second patch. You might gain in efficiency to 65% or so turning the horn to about 27mm diameter. The judgement you need to make is around noise which will be lower for a larger horn. The satellite is so strong anyway with a 1.2m dish you are probably transponder noise floor limited, certainly on narrow-band.

The S-band patch has a gain of about 9 dB, so you will under illuminate a bit. At least the neighbours wifi will continue to work.

Mike