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G4DGU Skobelev Derived Feed

Chris Bartram G4DGU
 

Hello John,

The G4DGU Skoboloff dual flare works very nicely .
Thanks for that nice comment!

However, being the pedant I am :-) the Skobelev derived feed which I used to make is a dual-mode feed which in which two circular waveguide propagation modes are excited in such a way as to ensure that they cancel at the mouth of the horn. This results in the sidelobe generating  currents which would otherwise be induced in the rim of the horn being (effectively) cancelled. Thus improving the efficiency of the dish/feed combination.

The horn works on the same principle as the better known W2IMU dual-mode feed, but the waveguide flare which results in the generation of the higher-order mode in that design has been replaced by a step change in the 'guide diameter. The step does much the same thing, but is easier to make.

The feed was designed as a part of the 10GHz transverter system I designed and sold back a decade or so ago. It was specifically designed to work well with TV offset dishes, and I have recently been using one modified for WG17 input with a 1m dish as I stagger back onto 10GHz EME.  I'm sorry to say that they are no longer available. I'm trying to make a bit more space in my life to do some microwaving ... However, if someone were to be interested in making the horn under some form of licence, and can convince me that they are capable of doing it properly, I'd be happy to talk.

73

Chris G4DGU


Re: Dish choices for 10GHz.....

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


Re: EME, data modes and stuff like that

Andy G4JNT
 

They're going to be really pushed to do much better that Q65.   It's so close to Shannon you'll need a huge increase in computing power to do much better.
Q65, and LDPC codes in general,  are interesting in one sense.   The theory behind them has been around since the 1960s, but it's only really become practical with GHz clock-rate 32 and 64 bit processors.



On Sat, 19 Jun 2021 at 19:05, Neil Smith G4DBN <neil@...> wrote:
Q65 will be popular until the Next Big Thing comes along, I guess
Q65-60B or C.  Still lots of JT65C being used.  When I'm listening, I
run multiple instances of WSJT-X running each mode and attach each to a
different virtual receiver on my Elad FDM-DUO. Not set up for TX on 23cm
just now, but hope to catch you at some point soon.

Neil G4DBN








Re: EME, data modes and stuff like that

Neil Smith G4DBN
 

The nice thing about CW on microwave EME is that it is often very very slow and the range of characters is very well defined. You know what you are expecting, and it is only going to be callsigns, RRRs, OOOs and so on.  Hey, perhaps we should be recommending running QRO EME on a 3m+ dish for folks learning CW!

Me, I'm entirely promiscuous and will work anybody on any mode. Even FT8 over FM rainscatter on 10 GHz. Oh yes.

Of course the only REAL datamode is RTTY, as any right-thinking operator will attest.

RYRYRYRYRYRYRYRYRYRYRYRYRYRY

Neil G4DBN

On 19/06/2021 18:52, Nicholas Shaxted wrote:

Haaaaarumph

 

Use of CW hone active listening skills, concentration levels and manual dexterity (sending with one hand and writing with the other)

 

Never miss out on the chance to learn a new skill, you find that you have to use it in different circumstances one day.

 

Nick – G4OGI

 



Re: EME, data modes and stuff like that

Dave G6HEF
 

Hi Nick, I know, I really don't need convincing. Having taken part in several VHF NFDs and many UKAC nights and hearing someone break out the key to complete an otherwise impossible QSO I do get it. One day I'll take up the challenge and perhaps EME will force my hand, literally.

I used to be a brewer for a while, and also wrote quite a bit about beer. I still drink beer (too much, some would say) and would like to think I know a bit about it. I liken the CW vrs digi modes to the cask vrs keg, sparkler vrs non-sparkler, is BrewDog still craft beer arguments that occur. It's a spectator sport and doesn't need much to poke the contestants! 

So, for the next topic; All thermionic devices should be put in the bin...... ;)

Dave
G6HEF


Re: EME, data modes and stuff like that

Neil Smith G4DBN
 

Q65 will be popular until the Next Big Thing comes along, I guess Q65-60B or C.  Still lots of JT65C being used.  When I'm listening, I run multiple instances of WSJT-X running each mode and attach each to a different virtual receiver on my Elad FDM-DUO. Not set up for TX on 23cm just now, but hope to catch you at some point soon.

Neil G4DBN


Re: EME, data modes and stuff like that

SAM JEWELL
 

The current discussion is really about the benefits of Q65 (usually 60 minute modes) over JT65C and the now withdrawn and relatively short lived  QRA64 modes.
Whilst I rarely use CW, there are still CW only EME contests, but It is noticeable that during these contests there is still a fair amount of digital activity.
23cm EME has the highest level of activity, second only to 2m, and working well over 100 stations is possible, even with dishes below 3m in diameter.

73 de Sam, G4DDK






On 19 Jun 2021, at 18:42, Dave G6HEF <hardknottdave@...> wrote:

Thanks Neil,

OK, so it's just the CW brigade that's objecting to JT65... after all these years!

For their benefit I shall hang my head in shame and admit to having never learned CW. Didn't need it as I never had a desire to go onto HF. Maybe one day, but there is sooo many more important things to do than learn Morse.

Dave G6HEF


Re: EME, data modes and stuff like that

Nicholas Shaxted
 

Haaaaarumph

 

Use of CW hone active listening skills, concentration levels and manual dexterity (sending with one hand and writing with the other)

 

Never miss out on the chance to learn a new skill, you find that you have to use it in different circumstances one day.

 

Nick – G4OGI

 

From: UKMicrowaves@groups.io <UKMicrowaves@groups.io> On Behalf Of Dave G6HEF via groups.io
Sent: 19 June 2021 17:42
To: UKMicrowaves@groups.io
Subject: Re: [UKMicrowaves] EME, data modes and stuff like that

 

Thanks Neil,

OK, so it's just the CW brigade that's objecting to JT65... after all these years!

For their benefit I shall hang my head in shame and admit to having never learned CW. Didn't need it as I never had a desire to go onto HF. Maybe one day, but there is sooo many more important things to do than learn Morse.

Dave G6HEF


Re: EME, data modes and stuff like that

Dave G6HEF
 

Thanks Sam,

Quite correct, both an easy find now I know they are there. Now to disappear down a rabbit hole for several days!

If I don't come out alive please apologise to my family for me....

Hope to hear you off the moon one day soon.

Dave
G6HEF


Re: EME, data modes and stuff like that

SAM JEWELL
 

The long established, monthly, 432MHz and up EME newsletter is an essential. It is an excellent digest of activity on 432MHz and up. As well as summarising activity there are activity calendars and (usually) a technical section.
HB9Q EME chat is where you find out who is on, what they are doing and arrange skeds or just discuss activity with others of a like-mind. 
You can find either by Googling.

73 de Sam, G4DDK






On 19 Jun 2021, at 17:45, Dave G6HEF <hardknottdave@...> wrote:



Hi Sam,

No, both new to me, I shall hunt them out.

Thanks


Re: EME, data modes and stuff like that

Dave G6HEF
 

Thanks Neil,

OK, so it's just the CW brigade that's objecting to JT65... after all these years!

For their benefit I shall hang my head in shame and admit to having never learned CW. Didn't need it as I never had a desire to go onto HF. Maybe one day, but there is sooo many more important things to do than learn Morse.

Dave G6HEF


Re: EME, data modes and stuff like that

Neil Smith G4DBN
 

http://mailman.pe1itr.com/pipermail/moon-net/ for the archives.

Ooooh, wash your mouth out with soap!  There is a serious Luddite element out there who will tell you that CW is the only EME mode....

Loads of fun to be had with CW or digi. Watch HB9Q to get a flavour for what is the current mode du jour. Lots of seriously nice folks out there who will help with skeds and tests and advice.

Neil G4DBN

On 19/06/2021 17:14, Dave G6HEF wrote:

OK, so a bit of a learning exercise here.


Re: Dish choices for 10GHz.....

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


Re: marking out on metal

Neil Smith G4DBN
 

Gareth is spot on, I use a Sharpie on small stuff and a big chisel tip marker for medium stuff.

I have a pot of Dykem Red for aluminium and Dykem Blue for brass or steel where a marker doesn't fit the bill.  It comes with a brush in the lid and wipes off with Acetone.

Neil G4DBN

On 19/06/2021 18:21, Gareth G4XAT via groups.io wrote:
Permanent Blue chisel tip felt marker/sharpie much handier these days for small jobs...


Re: marking out on metal

Neil Douglas
 

Ben,

 

Marking Blue is the proper stuff, but I just use a blue chunky Sharpie Marker pen.

 

NeilD G4SHJ

 

From: UKMicrowaves@groups.io <UKMicrowaves@groups.io> On Behalf Of militaryoperator via groups.io
Sent: 19 June 2021 17:42
To: ukmicrowaves@groups.io
Subject: [UKMicrowaves] marking out on metal

 

Whats the stuff called you can paint on metal so you can see the scribe marks easily? and then wipes off with spirits or such. 

 

Ben

 

 


Re: marking out on metal

Gareth G4XAT
 

Permanent Blue chisel tip felt marker/sharpie much handier these days for small jobs...
Gareth


Re: marking out on metal

Dave
 

It’s called Marking Blue. 
73 
Dave G4GLT. 


On 19 Jun 2021, at 17:41, militaryoperator via groups.io <Military1944@...> wrote:


Whats the stuff called you can paint on metal so you can see the scribe marks easily? and then wipes off with spirits or such. 

Ben



Re: EME, data modes and stuff like that

Dave G6HEF
 

Hi Sam,

No, both new to me, I shall hunt them out.

Thanks


marking out on metal

militaryoperator
 

Whats the stuff called you can paint on metal so you can see the scribe marks easily? and then wipes off with spirits or such. 

Ben



Re: EME, data modes and stuff like that

SAM JEWELL
 

Dave,
I guess you receive the EME 432 and up newsletter and log into the HB9Q EME chat?
Both essentials.

73 de Sam, G4DDK



On 19 Jun 2021, at 17:15, Dave G6HEF <hardknottdave@...> wrote:



OK, so a bit of a learning exercise here. I am really keen to have a go at moonbounce. Indeed nearly everything I am doing at the moment is working towards that.

I have chosen 23cm as this is the band I am already very active on and it seems the most simple for a start.

I am already building a legal limit QRO capability which is going well. I burnt out one dummy load already, yup, that's a price of success!

I have both a DDK VLNA and a Kuhne LNA. I have all the QRO relays a man could ever wish for.

I am building a 3m mesh dish and have a BIG RAS Spid lump of ballast.

I also have a clear, low-noise take off that can see the moon really clearly most of the time, at least when it has risen in the UK of course.

Still have to build the dish feed and phasing stuff and order enough aluminium to finish the dish, but I think I understand that now.

The bits that is still eluding me is understanding stuff like the correct digimodes, correct operating etiquette, resources for arranging skeds etc. I thought JT65 was the mode but I'm seeing lots of people say that isn't the way to go anymore?

I'm subscribed to moon-net which I receive by email, but is there a back catalogue of messages archived somewhere? Is there a EME groups.io forum? (found couple but have nearly no content)

I almost feel like the EME community resides in the dark net somewhere... Joke, I'm told the EME community is really friendly and helpful, I just haven't found you guys yet, and I'm sure it's my incompetence rather than an attempt to hide from me, unless my reputation really does precede me.

So, any advice, pointers, either on what is wrong with my approach so far, or the bits I'm still missing would be very gratefully received.

73
Dave
G6HEF

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