Dish choices for 10GHz.....


Gareth G4XAT
 

I've two dish solutions to pursue for 10GHz, a 65cm offset and a 60cm Prime focus.
I know 'back in the day' that a 'Penny Feed' was a popular solution on prime focus but which would be best now I'm wondering?
For the launcher I'm using SMA to WG16 with a small horn (for added gain?) or I imagine a 22mm Copper to SMA creation may be possible. I can design and fab/3-D print a bracket to mount that on the Offset dish arm, added benefit is that the TX and RX strips can be fitted directly underneath the arm so very short feeder runs involved.
Equally, the green dish is a lot lighter (alloy not steel) and easier to mount/aim but I'd need to feed from behind the dish. I've seen the G3PHO dual mode feed and a 'shepherds crook' adaptation that may work for me too. I also have a 12" piece of copper WG16 that may help.
Guidance and suggestions as to best route forwards please....I suspect that there may be a degree of 6 of one and 1/2 a doz. of the other involved, but thought I'd ask anyway.
I've also deigned a replacement LNB mount for a Sky Mini-dish, then another to mount a Titanium C band LNB on the same thing. If anyone would like the .stl files, just ask....
Pictures show....
Thanks
Gareth


John Fell
 

Gareth,
The 65cm offset would offer the higher gain but requires the feed to match its F/d ( probably in the 0.75 region)  for efficient operation. Figs usually quoted are around 1.75dB better due to the removal of the blockage by the feed structure .

The G4DGU Skoboloff dual flare works very nicely .

The Penny or Cutler feed is designed for much lower F/d (0.25 ) , prime focus dishes , where the beamwidth is approaching 180 degrees .As such it is more 1/2 wave dipole like .

If you are doing RX/TX on a single feed , then pick an LNB that has a neck diameter/length that will allow interface to 22mm copper water pipe  .You can use a rotary pipe cutter to cut through the diecast aluminium , leaving enough neck on the horn and LNB input to allow subsequent transitioning to pipe sections and flange or SMA guide probe .It depends on you using an SMA relay or a flanged Waveguide 3 or 4 port switch .

Using the LNB horn will provide a good illumination of the offset dish , if it is of the near standard domestic Sat TV style .

Polish the 60cm diameter dish and after filling in the hole in the middle use it as a Wok or a barby pit if steel .

73
John
G0API

On Sat, 19 Jun 2021 at 14:54, Gareth G4XAT via groups.io <g4xat=ntlworld.com@groups.io> wrote:
I've two dish solutions to pursue for 10GHz, a 65cm offset and a 60cm Prime focus.
I know 'back in the day' that a 'Penny Feed' was a popular solution on prime focus but which would be best now I'm wondering?
For the launcher I'm using SMA to WG16 with a small horn (for added gain?) or I imagine a 22mm Copper to SMA creation may be possible. I can design and fab/3-D print a bracket to mount that on the Offset dish arm, added benefit is that the TX and RX strips can be fitted directly underneath the arm so very short feeder runs involved.
Equally, the green dish is a lot lighter (alloy not steel) and easier to mount/aim but I'd need to feed from behind the dish. I've seen the G3PHO dual mode feed and a 'shepherds crook' adaptation that may work for me too. I also have a 12" piece of copper WG16 that may help.
Guidance and suggestions as to best route forwards please....I suspect that there may be a degree of 6 of one and 1/2 a doz. of the other involved, but thought I'd ask anyway.
I've also deigned a replacement LNB mount for a Sky Mini-dish, then another to mount a Titanium C band LNB on the same thing. If anyone would like the .stl files, just ask....
Pictures show....
Thanks
Gareth


Clive Elliott GW4MBS
 

The green dish looks very familiar, not just the colour, but the mounting bracket & the small hole at the rear. I used to have one of these, the dish was quite deep. It originally came with the mount for a microphone to enable ornithologists to record the sounds of nature.

I never used it on 3cm, but used it with success on 9cm with a beer can polaplexer. I also used on what was called 5cm which is now called 6cm with a Brasso tin polaplexer. Yes I know far from ideal, but dishes were hard to obtain & comical by today's standards but this was 50 years ago.
--
Clive GW4MBS (ex-G8ADP)
Pottering on 6m - 3cm in a valley in IO71XW where any QSO is a triumph of optimism over geography!


mw1fgq
 

A Grampian parabolic reflector as used by the Beeb where one was attached to the Pavilion at Lords for many years, I wasn't a sound man but I recall being told they kept it in preference to more modern microphones to avoid changing the distinctive sound from the wicket.
https://worldradiohistory.com/hd2/IDX-Audio/Archive-Studio-Sound-IDX/IDX/70s/Studio-Sound-1970-05-OCR-Page-0016.pdf

John
MW1FGQ


Neil Smith G4DBN
 
Edited

The target is to get an edge taper of about 12dB to get the best balance between efficiency and overspill. Everything depends on the focal length to diameter ratio of your dish.  In general, an offset dish like a Gibertini will have the axis just about at the bottom rim of the dish. If that's the case, then the feedhorn should be aiming at the middle of the oval of the dis. Then you need to think about the angle from the axis of the feed to the top, bottom, left and right edges of the offset.  They should be just about equal. OK, the top edge is a bit further away than the centre.

On a very deep PF dish, with f/d 0.25, the distance to the edge is exactly twice the distance to the centre, so there is an inbuilt 6dB "space taper", and your feed actually needs a much wider angle than would be suggested by the angle.

On a flattish offset dish, that extra space attenuation is lower, but on my dish, it is still over 3dB, and it is asymmetric.

Rather than trusting published f/d numbers, I now measure the distance from the approximate phase centre of the feedhorn to the top and bottom edges of the dish and the length between those points on the dish, then repeat for the left and right edges.

I cheat and use https://www.calculator.net/triangle-calculator.html to work out the angles. On this offset dish, I get 67 degrees horizontal angle subtended at the feed and 73 degrees vertical.

Call it 70 degrees.   Now you just have to find a feed with the right angle and low sidelobes. A Pickett-Potter with an oval iris and f/d 0.6, as per the design by WA6KBL has -12dB at about 90 degrees, so it appears to over-illuminate the dish by a lot, but illumination of the top edge is about 3.5 dB down compared with the bottom edge so in fact the taper of the 0.6 version is a fair match but still over-illuminates  the dish. Jeffrey's f/d 0.8 under-illuminates my dish, so I need a horn which is about -9 dB at 70 degrees for best performance.

If you work out the sort-of F/D for my dish using the raw dimensions, it would be about f/d 0.55 one way and 0.65 the other, but in reality it is approximately what you could get if you projected a 70 degree cone from the focus on to a big parabolic dish with the bottom edge of the cone parallel to the axis, and the necessary angle of illumination to get the required edge taper is close to that of a prime focus dish with f/d 0.7

A Pickett-Potter dual-mode horn would work well, but they are not a simple machining job, needing a precision iris to match to waveguide.  The simplest approach is to use a W2IMU and feed it with a coax probe. I'm doing it slightly differently and using a conical dielspike lens in a round aluminium guide. Even then, getting the parameters right is a bit of a struggle unless you move to 21.5 mm internal diameter for the guide/feed section. Most of the other designs liek Kumar chokes, scalars and Chaparrals tend to be too wide angle for an offset dish.

Doing the calculations for a W2IMU at 21 mm internal diameter and f/d 0.7, I make the dimensions to be a 27.1 degree taper, 26 mm long, out to 47.6 mm ID, with a length of 82.4 mm.

Running an OpenEMS simulation to check the illumination pattern shows it is, but there is a bit of ripple. 9dB down at +- 35 degrees though. Needs a bit of a tweak to clean up the ripple, but it is better than 20dB down.

It would be feasible to TIG weld a bit of 2 inch OD tube to a 2 inch round bar and bore and turn an internal taper, but I'd probably just turn the whole thing from solid aluminium bar in a single piece.  Whether it would be any better than a Pickett-Potter at f/d 0.7, or a dielspike made from Rexolite 1422, I rather doubt. A W2IMU is quite a slab compared with a dielspike, and still needs waterproofing.

Neil G4DBN

On 19/06/2021 14:54, Gareth G4XAT via groups.io wrote:

I've two dish solutions to pursue for 10GHz, a 65cm offset and a 60cm Prime focus.
I know 'back in the day' that a 'Penny Feed' was a popular solution on prime focus but which would be best now I'm wondering?
For the launcher I'm using SMA to WG16 with a small horn (for added gain?) or I imagine a 22mm Copper to SMA creation may be possible. I can design and fab/3-D print a bracket to mount that on the Offset dish arm, added benefit is that the TX and RX strips can be fitted directly underneath the arm so very short feeder runs involved.
Equally, the green dish is a lot lighter (alloy not steel) and easier to mount/aim but I'd need to feed from behind the dish. I've seen the G3PHO dual mode feed and a 'shepherds crook' adaptation that may work for me too. I also have a 12" piece of copper WG16 that may help.
Guidance and suggestions as to best route forwards please....I suspect that there may be a degree of 6 of one and 1/2 a doz. of the other involved, but thought I'd ask anyway.
I've also deigned a replacement LNB mount for a Sky Mini-dish, then another to mount a Titanium C band LNB on the same thing. If anyone would like the .stl files, just ask....
Pictures show....
Thanks
Neil
http://g4dbn.uk


Gareth G4XAT
 

Thanks Neil, I'll need to read that through a few times....
Regarding the Green Grampian dish, I did wonder why it didn't look very 'RF related' with just a plastic boss in the centre. Now I know why (and its relative disadvantage compared to an offset dish) I might try an electret mic and test it on my resident blackbirds who sing a treat.
My offset dish measures 650x750 and 72mm deep, somewhere I have the alloy LNB mount that came with it so I can do the triangulation required and decide on the feed to use. Initially just for TX (RX LNB on a different dish) but later all in one.
This solution http://www.g3pho.free-online.co.uk/microwaves/horn.htm with updates applied looks good and easiest to mount to my dish with just an adapter collar needed. The dish has a very substantial rear mount/tilt assembly that could do with a lot of thinning down for /P use. Nothing the MIG and angle grinder can't sort out.
Gareth


Greg - ZL3IX
 

Hi Neil,

There's a lot of interesting theory in your post above. Since I'm new to microwaves I would like to read a bit more background to the theory described in your post. Can you recommend a good reference please?

73, Greg, ZL3IX


Mike Lavelle K6ML
 

Greg...

Try Paul Wade’s On-LIne Microwave Antenna Book  http://w1ghz.org/antbook/preface.htm  
Everything you’ll ever want to know about dishes, horns and feeds.

Mike K6ML


Greg - ZL3IX
 

Thanks, Mike. After posting my question I did a search and came across Paul's book. I agree with you, it's a great publication.