Dishes for Ka-band: what works - what doesn't?

[John]

Expand above post to see content properly

 

[Llew]

View attachment 103231

Yes, just wondered how a Ku test performed for comparison with Ka re. leakage outside the dish itself. A Ku test on a Ku optimised dish should receive little signal directly through the dish one would think.

 

[Channel Hopper]

The mesh contained inside a SMC dish of any type will determine the upper frequency of reception, but so will the process of applying it to the mould before it is pressed/heated/set/

On older, or antennas that have seen more action in coastal regions (sometimes the same occurs with incorrect cleaning methods) the metallic fabric will poke through the compound surface, which will give an idea of the 'holes' in the chicken wire and so the cut-off frequency can then be determined from the materials inside. There are other considerations though .

The feed design at higher frequencies becomes far more critical, and it is not just about focal distance, but the destructive interference at a particular point from the reflector.

If you are looking for dual, or switching polarities then you have to rely on the manuufacturer of the LNB to have given good isolation in the circuitry at the front end. It is far better to remove with mechanical filters, either the feedhorn and dish matching (there is a reason for the term 'very small aperture' ) or a 2:1 ratio waveguide and dedicated single polarity LNB.

Lastly there is the beamwidth characteristic change at higher frequencies since there is a direct correlation between increased diameter and the degree of signal received, which may not all be useful stuff (more noise than transponder).

Sometimes pointing a smaller dish at the desired satellite may be the better choice (which hurts the finger(s) to post).

 

[archive10]

I did some tests with my dishes here -

Considerations when choosing a dish for Ka band

Purely subjective, and limited in testing with the three dishes I had then at my location . Since that test, I still go back to the TD88 for most of my Ka reception, as it gives me a consistency not found on the Gibby 1.2 or especially the SMW 1.05, the latter performing especially poorly - maybe something to do with the reflector coating material used on SMW dishes, or perhaps the unusual design of the dish?

Hmm.
I now see very little reason to keep the SMW 1.05 lying around.
Too small for Ku when I have at least 6x1.2m lying around at the mo.
Too crap for Ka.
Way too small for C (and difficult to get feedhorn that doesn't block 1/5 of the dish).
Recycling site next stop?

 

[Llew]

Recycling site next stop?

Possibly, but a try at auctioning it might find a willing buyer. Back in the day, it was an excellent performer for me when set up well with an SMW LNB, then later with an Inverto BU.

 

[archive10]

Possibly, but a try at auctioning it might find a willing buyer. Back in the day, it was an excellent performer for me when set up well with an SMW LNB, then later with an Inverto BU.

I see.
I tried 28D with an Inverto Slim Quad (White) LNB, and a Black Pro C120 + SMW feedhorn, but it did very poorly.
(I believe it's on some other thread here).
Have not tried it after 28EFG came along. I also only have a 23 mm holder, so I am limited in the range of LNBs that fit without too much bodging.

Futhermore, I have, as you probably know, repurposed the polar mount, and was (until Winter set in) trying to mate it with the Laminas 1200.
I wonder what value it would have with just som plain botched-on AzEl mount...

 

[Llew]

I see.
I tried 28D with an Inverto Slim Quad (White) LNB, and a Black Pro C120 + SMW feedhorn, but it did very poorly.
(I believe it's on some other thread here).
Have not tried it after 28EFG came along. I also only have a 23 mm holder, so I am limited in the range of LNBs that fit without too much bodging.

Futhermore, I have, as you probably know, repurposed the polar mount, and was (until Winter set in) trying to mate it with the Laminas 1200.
I wonder what value it would have with just som plain botched-on AzEl mount...

Ah yes, I remember you used the SMW mount for another dish. Mine's on the Gibby 1.2 now, working very well.

Yes, might be a problem selling it without the original mount. Never seen one or indeed an SMW 1.05 at auction although they might turn up in your part of the world, as you have found.

My LNB holder was a DIY job, made with the fixed tube piece they include for fixed dish working - a perfect fit over tubular LNB arm - and a 40mm plastic water pipe clamp for the LNB.

 

[John]

Yes, just wondered how a Ku test performed for comparison with Ka re. leakage outside the dish itself. A Ku test on a Ku optimised dish should receive little signal directly through the dish one would think.

.... Did this yesterday Llew, the results left me scratching my head.
Report & pics later

 

[scopus]

.... Did this yesterday Llew, the results left me scratching my head.
Report & pics later

Theory and Practice are totally different in my experience......

 

[John]

Theory and Practice are totally different in my experience......

Yes...... your beloved was telling me that the other week lol

 

[scopus]

Oooooohh you are awful, bit I like you!

 

[John]

.... Did this yesterday Llew, the results left me scratching my head.
Report & pics later

Need to remind y'all, I'm no RF engineer or electonics guru, nothing i say should be taken as gospel as we say. Study the pic's and make your own judgements.

My good friend scopus Chris mentioned the other day about angling the RF source in a downward angle to the dish face to replicate the incoming signal from the satellite as it hits the dish inner mesh.
Good point i thought , but got same results as before.

Knowing what signal levels should read like from looking at satellites in the normal way on the Promax , i set the distances between the the RF source at the front of the dish and the receiving lnb/Promax on the back side of the dish.
To try to ensure that the pickup lnb was not being influenced by any stray radiated RF i extended the waveguide on the SSA RF device and also fitted an extension to the pickup waveguide lnb to hopefully try to ensure the applied RF was concentrated as best i could to a 50mm circle on the dish mesh.

The Ka results were the same as the other day, ie: appears to show leakage through the mesh at 21GHz which is not a total surprise.
However, on Ku, where you might have expected a lot less leak-through, was a different story. Again-adjusting the devices separation so as not to over saturate the lnb/Promax pickup and to replicate the same sort of received signal level from a normal sat transmission, the leakage seemed to be fairly evident.
The head scratching then started, it was cold and wet, decided i'd had enough, so i packed up and went back inside.
Any Pro RF engineers thoughts / suggestions most welcome.

Ps: Don't ask me why the forum software keeps flipping certain images ??, i don't know.

 

[s-band]

John, what you are trying to would be difficult at the best of times. It is quite likely that signals are getting around the dish by multiple reflections, knife edge diffraction or a combination of those and other factors. Tests like this would normally be done in an anechoic chamber and are still difficult. Really, you need a section of the dish on the wall of a screened enclosure to be able to measure the transparency of the sample. Even then it is hard to ensure that you don't have leakage around the sample.

The feed you are using is circular on Ka isn't it? Your TX is from a dipole - linear polarised. That is good in some respects as the RX feed will be insensitive to the actual angle of the linear polarisation. If you use linear on both ends you should rotate for max signal. You still won't know if it is going through or around. If your test antenna is only 10mm from the surface, it will be in the near field where effects may not be what you expect.

Using foil to confine the field might not be doing what you think either. You can get more weird effects where the foil forms a slot (as in slot antenna). Copper tape with conductive adhesive would be better but I'd be inclined to use absorber as that will minimise reflections.

 

[John]

John, what you are trying to would be difficult at the best of times. It is quite likely that signals are getting around the dish by multiple reflections, knife edge diffraction or a combination of those and other factors. Tests like this would normally be done in an anechoic chamber and are still difficult. Really, you need a section of the dish on the wall of a screened enclosure to be able to measure the transparency of the sample. Even then it is hard to ensure that you don't have leakage around the sample.

The feed you are using is circular on Ka isn't it? Your TX is from a dipole - linear polarised. That is good in some respects as the RX feed will be insensitive to the actual angle of the linear polarisation. If you use linear on both ends you should rotate for max signal. You still won't know if it is going through or around. If your test antenna is only 10mm from the surface, it will be in the near field where effects may not be what you expect.

Using foil to confine the field might not be doing what you think either. You can get more weird effects where the foil forms a slot (as in slot antenna). Copper tape with conductive adhesive would be better but I'd be inclined to use absorber as that will minimise reflections.

Cheers Graham, don't fancy cutting the dish up though lol.
Would like to continue further at some stage with a better set-up and guidance.
The Ka lnb & feed is linear.

 

[Llew]

As the front end of an LNB is able to make sense of the tiny signal picked up from a satellite, maybe it's no wonder it's able to sniff RF from a source a few inches away, despite being supposedly screened by the dish material.

No doubt as s-band suggests, there a ways in which the RF is being reflected. Also, could the dish itself be acting as a nice reflector, bouncing the signal to adjacent materials, even if they're some distace away?

 

[Channel Hopper]

The Ka results were the same as the other day, ie: appears to show leakage through the mesh at 21GHz which is not a total surprise.
However, on Ku, where you might have expected a lot less leak-through, was a different story. Again-adjusting the devices separation so as not to over saturate the lnb/Promax pickup and to replicate the same sort of received signal level from a normal sat transmission, the leakage seemed to be fairly evident.
The head scratching then started, it was cold and wet, decided i'd had enough, so i packed up and went back inside.
Any Pro RF engineers thoughts / suggestions most welcome.

The noise may not actually be at the incoming Ka/Ku frequencies, but leaking in at the lower 900-2250 range, picked up either through the cabling, or possibly at the last stage of the LNB.

Try a different downlead , one that is shorter and of better quality connectors.

 

[John]

The noise may not actually be at the incoming Ka/Ku frequencies, but leaking in at the lower 900-2250 range, picked up either through the cabling, or possibly at the last stage of the LNB.

Try a different downlead , one that is shorter and of better quality connectors.

Understood, the crude attempt with the foil was to do a bit of quick RF shielding and was wrapped around the plastic SSA device box as well in case it was radiating from there but in actual fact removing the foils made no difference at all. The only thing left un-screened at test time was the usb interconnect lead to laptop & the laptop itself. The coax link from the lnb was WF100 about one and a half metre length with Thomas & Betts Snap N Seal connectors.
When the weather perks up a bit in a month or so's time i'll re-do the experiment with the RF source SSA device/ usb lead / laptop four metres or more away from the dish set-up with a WF100 fly lead to connect to a better RF shielded waveguide & probe to see what that set-up produces.
Also ..... after i have given up doing the above i'll set up my spare 1.2 Precision solid PF with my spare Ka XMW 21GHz lnb on it looking at a designated freq on 7°E or whatever and take some readings off the Promax ..... Then afterwards, set up my spare fibre CM1.2 with the same tackle and just get some simple comparison readings. Keeps me off the streets i suppose.

 

[archive10]

Keeps me off the streets i suppose.

Possibly, but also provides the forum with a highly laudable and very interesting investigation!
Full marks for "just doing it" - even if there may be many reasons for scratching heads at the results.
They are, nevertheless, results to ponder, one way or another.

Wish I had the time (and equipment) you have - but I'll take pleasure in reading your further adventures.

 

[John]

Possibly, but also provides the forum with a highly laudable and very interesting investigation!
Full marks for "just doing it" - even if there may be many reasons for scratching heads at the results.
They are, nevertheless, results to ponder, one way or another.

Wish I had the time (and equipment) you have - but I'll take pleasure in reading your further adventures.

Cheers ST, appreciated.

 

[Channel Hopper]

Have you tried putting the transmitter from the first picture into a lead-lined box and tried again ?