Can I convert a 1.2 Fibo dish from Gregorian Feed to normal offset feed point ?

[G4GUG]

Hi to all Members.

Best wishes for 2020.

Newbie question.


Can I convert a 1.2 Fibo dish from Gregorian Feed to normal offset feed point ?

https://www.satellite-calculations.com/Fibo/fibo.htm

I understand that I can't use gregorian feed for the following reasons.

1. Secondary reflector reverses polarisation.
2. Secondary reflector is to small < 4 wavelenghts at 2.4 Ghz.

After removing secondary reflector.
Is possible to find normal offset feed point for POTY feed ?

73 Mike G4GUG.

[RimaNTSS]

Mike!
Primary reflector of the Fibo 1.2m is nothing but simple offset dish. Ind it is possible to calculate exact position of it's focal point. Then if you place feed in the way that it's phase center so-locates with antenna's focal point you will get performance of such a system as usual 1.2m offset antenna has. You will need to make special LNB-holder though.
Rimants

[G4GUG]

Hi Rimants

Thanks for your reply.
Can you give me any formulae or practical tips how to proceed.
Please remember, this is my first attempt at using dish antenna.
Best wishes for 2020.

73 Mike G4GUG

[RimaNTSS]

Mike! From the measurements I made long time ago I just made this drawing for you. In addition I wish to mention that an offset angle of the main mirror is 27.3* and opening angle (angle AFB) is 86*. F/D equivalent .62.
What else... I would advice you to do the string test to the main mirror (those antennas are so old and made out of aluminum, so they are usually warped). Secondly, when you try to adopt this antenna without sub-reflector, do not do unrecoverable things such as cutting the feedarm. So, after experiments you will be able to return to dual-reflector design. BTW, the beauty of the Gregorian type antenna is having sub-reflector on.
If you need some ideas how to make special LNB-holder for your Gregorian, say so, I have some.

Fibo120Geometry.jpg (274.32 KiB) Viewed 5105 times

[G4GUG]

Hi RimaNTSS.

Thank you for advice and diagrams.
All noted.
I will setup dish with minitouner feeding existing dual DBS LNB.
To check for working operation.
Weather in UK is very wet and windy at the moment.
So... may have to wait for better conditions.
You mention custom mount, is that for POTY type feed?
Please post a diagram or photo.
If I can get POTY type feed to work on downlink.
Next move,to start thinking about uplink hardware.
Much appreciated.
73 Mike Stroud Gloucestershire.UK

[RimaNTSS]

You mention custom mount, is that for POTY type feed?
Please post a diagram or photo.

For any type of feed, Mike! Depending on the clamp or ring spacers used (some examples attached) the holder can hold C/Ku-band LNB (65/40mm) or POTY dual feed (25mm neck). Those I have made not to use on Fibo antenna without the SUB but to hold LNB in front of the SUB (instead of original Fibo feedhorn). For your needs you will need to use different sizes of supporting struts, to make sure LNB is positioned in the right point.

IMG_20191226_221254 (Large).jpg (247.06 KiB) Viewed 5091 times

[G4GUG]

Hi RimaNTSS.

Thanks again for information.
I have read all your posts to the forum.
Clearly, I am very lucky, you responded to my first question.

It can be very intimidating, as a novice, posting on a forum with so many professional microwave engineers present....

Perhaps in 2020, I can slowly unravel some of "mystique and black art of offset dish feeds !"

"Remember every long journey, starts with a first step"

So..

I will ask another rhetorical question.



If, from earlier testing with professional test eqipment ( Which I don't have )
You optimise return loss of both 2.4 and 10 Ghz of feed system pointing away from dish.
Is that what is accepted practice ?

Mike G0MJW and associates has the expertise and equipment to have already optimised the POTY feed.

Can you determine best physical position when placed in roughly correct phase point.
By then doing further return loss measurements, to sure best possible placement ?
Or you optimise, only by improving signal to noise performance ?

So.. to reiterate.

Does optimum position coincide with best return loss and signal to noise ?


73 Mike G4GUG

[g0mjw]

If you are looking for mounting hardware for a POTY type feed, I had the same issue earlier this week and found a solution in B&Q ( I was desperate).

I am sure these are available elsewhere. A 6mm drill, some M6 bolts, bit of aluminium strip plus this (all from B&Q)

Clipboard01.jpg (916.69 KiB) Viewed 5011 times

They are called Munsen rings - https://www.diy.com/departments/plumbsu ... 135_BQ.prd

I am sure they are available for a more reasonable price elsewhere. Screwfix for example. The large thread is M10 and you can buy extenders or use a long M10 bolt and pack it to the right height.

In my case the POTY feed fouled against the feed arm and needed to be a bit higher above it. My solution to this is to modify the feedarm to be lower - e.g. bending it brute force if that's practical or through adjusting it's mounting. In my case the latter as the feed arm attaches to the back bracket using 4 bolts and all it needed was a minor adjustment with a file to two of the holes in the mount.

Yours above looks a very similar arrangement but it might fit as is..

[G4GUG]

Hi Mike.G0MJW

Thanks for the information,and all the best wishes for 2020.

Maybe I need a machine shop, as well as loads of test gear.

A bit different from making HF antennas.

Can you shed light on my last question.

Reference :

Return loss and signal to noise measurements.

How do you do it ?

73 Mike G4GUG

[g0mjw]

I will ask another rhetorical question.

If, from earlier testing with professional test eqipment ( Which I don't have )
You optimise return loss of both 2.4 and 10 Ghz of feed system pointing away from dish.
Is that what is accepted practice ?

Mike G0MJW and associates has the expertise and equipment to have already optimised the POTY feed.

Can you determine best physical position when placed in roughly correct phase point.
By then doing further return loss measurements, to sure best possible placement ?
Or you optimise, only by improving signal to noise performance ?

So.. to reiterate.

Does optimum position coincide with best return loss and signal to noise ?


73 Mike G4GUG

I suppose I should not answer this as it was intended as a rhetorical question, but this is the internet and you don't get to choose if a questions get's answered or asked.

For amateur use, you can just optimise the feed in free space for best match. The 10 GHz should just work, 2.4 GHz it helps to measure it. There are several relatively cheap ways to do this. You can buy an antenna analyser or a Vector Network analyser. Not as expensive as you might think but its also possible with a simple directional coupler which you either buy or make or recover from ex-telecom equipment. The principle is exactly the same as at HF, minimise reflected power.

Search for "N Type Female RF Coaxial Directional Coupler 800-2500MHz" . They are £5 and you probably want the 40 dB model.You will also need a power detector. E.g an AD8317 based board. They are about £10. Also maybe a 10 dB SMA attenuator or two as the AD8317 maximum input is 1mW.

This combo will allow you to measure forward or reflected power, so becomes a useful power output monitor.

If you have a 23cm VSWR meter it might work well enough, it won't be accurate but minimising the VSWR will probably be OK enough.