POTY lens question

[Basil]

For experimental purposes I added the stepped plastic lens that came with my Netherlands sourced POTY kit, and went from MER 12 + to no decipherable beacon signal at all... Making a vernier adjuster for the big dish has proven a Godsend, you did warn me they were critical, but my idea of critical was inadequate initially

Does this tell me / you anything useful? I experimented with the lens + / - 40 mm in 5 mm increments from the no lens focal point and still no decipherable beacon. Thanks. Dish is prime focus, maybe the lens is designed for offset dishes??

[g0mjw]

What is your F/D? The point of a lens is to match the waveguide to the focal length of the dish. Prime focus dishes tend to have low F/D, offsets higher F/D. If the f/D is 0.4 a lens intended for an F/D of 0.7 would make things worse than open waveguide. Tell us the Focal length, i.e. how far the feed is from the dish centre and the Diameter of the dish.

[Basil]

I got it off a well known microwave buff, Charlie Suckling G3WDG who tells me it is a 0.3 f/D. Adding the same lens to the same LNB and POTY on my old 1.8 PF dish of unmeasured focal length, (I just used the same distance as the original entry to the factory made waveguide), made no noticeable difference. On this dish the signal all but disappears.

Thanks, my interest is mainly academic as it seems to work OK now with a capped end to the 22mm POTY tube at the focal point, but if the lens addition attenuation suggests other tricks are potentially beneficial I am all ears.

[g0mjw]

If 0.3 F/D then you don't want a lens at all. The depth positioning will be very critical.

Mike

[Basil]

OK, thanks Mike. The dish came with an original Andrew feed, like a shepherd's crook, photos attached. it is undamaged and very sturdy. What i have done is accurately measured from the cast alloy plate the crook's waveguide goes through to the meshed feed entry. I have duplicated this distance of about 3 feet from memory, to place the mouth of the POTY tube the same distance. Accuracy was to within plus or minus a couple of mm. Is my simplistic approach OK? I have built in a goodly amount of vernier adjustment both ways, just in case. I am not sure what frequency the shepherd's crook affair was designed for, but I am assuming the feed point is the feed point whatever the frequency?

I also made the POTY holder do two things, one is obviously to hold the POTY and LNB, but also to allow me to slide the POTY out and insert a 22mm dead straight tube into the holder, that I then use to align the POTY tube holder concentric with a 23mm hole in my replacement for the shepherd's crook dish centre plate.

[PA3CRX]

I would like to advise you to (re)read my article about illuminating of parabolic dishes in CQ-TV 273. For both understanding illumination as well as optimizing the feed for your dish.
Best regards,
Chris PA3CRX

[Basil]

Thanks, I will certainly re read your article, the reminder is appreciated.