Posts by G0MJW

Quote from I0LYL

excellent, but let's say that for the NB transponder antennas greater than 100 cm are excessive. We say it for any new users who maybe worry, they think a big effort is required. For the NB transponder, a 40 cm antenna is sufficient for good activity. I repeat it only for new users.

Yes. If you can see the transponder noise floor there is little to be gained on receive with a larger antenna and even if you can't by as much as 10 dB you should still be able to copy easily signals at the same level as the beacon. That means a signal level about 19dB down on those reported by DD4YR above, i.e. a 20cm dish, should still be viable. I would not recommend that small but a re-purposed 45cm satellite TV dish should be sufficient.

On transmit things are different. Smaller dishes need more power. To reach beacon level I need 200mW-300mW or so to my 2.4m dish. This would imply needing 10W to a 45cm dish, but that level of power is readily available, so it's not a problem.

The reason I am using a larger dish if for digital television, which needs more power, but even then, 1.2m would be sufficient. The benefit of the larger dish here is the 6 dB reduction in power needed, around 5-10W instead of 20-40W. For DATV you need a very linear amplifier and that's more difficult, but not that difficult.

In other words don't be put off by only having a small dish!

Mike

Depending on the DRO SDR console may be able to lock to the beacon and stabilise the transponder. Try it and see.

Typically around MER 12 with nobody else on. It's 11.6 at the moment as the transponder is fairly busy, but no big 1 MHz carriers. When they come on it drops quickly well below 11.

Surely the other way around? G is rare DX. Never seen that before!

Only 3G ?

Quote from DL5LF

... perfect fit ....

... attenuation to be checked, at a first glance looking ok

What is good here is you show how you extended the feed arm so the feed will fit. Simple, neat, easy, but seems to defeat many people unfamiliar with dish antennas.

Mike

Black PLA loaded with carbon perhaps?

That depends on the amplifier response but I doubt it will be clean. If it is that's clearly good but a mixer that has no image or carrier leakthrough is a rare thing indeed. Also its advisable to check all spurious, not just ones less than 180MHz away. You might get away with a saw filter to clean things up.

It is needed because there will be noise at the image frequency. If you do not mind losses you can make an evanescent mode filter very easily. Ideally though don't use a 28 MHz first IF, use something sensible, e.g. somewhere in the 100-500MHz range. It will make filtering much easier.

http://www.g4jnt.com/EVANFILT.pdf

I didn't say it was easier, I said it was an alternative and one that could have better performance. You do not need a patch feed, you a waveguide feed with a transition usually. These can be made from 22mm pipe, mine is, with a circular to WG16 tapered transition manufactured using a hammer.

There is always the option of making a 10.5 GHz down-converter. The converted LNB route is very easy but we can do better, particularly with respect to noise figure and a sensible IF. Anyone who has a 10 GHz transverter could re-purpose it for QO100 reception. It will need re-tuning of course and possibly a new crystal but don't forget the option.

Thanks - I am seeing an MER of 11.6 to 11.7. My system is not optimised, but is a 2.4m dish. Because of the reduced FEC this is about the same margin. As summer develops and trees grow this will reduce but a useful benchmark.

Mike

It's a German thread. Anyway, yes, I am well aware of the limitations of the simulation as I had only access to the student version of CST. My copper version works fine. If it's consistently 80 MHz high I would suggest scaling the patch by 2% and see what happens. Make sure you look at it on a Smith chart though as you can't tell what's really happening otherwise.

So

So apologize for the English, but they differ in at least 3 places from the recommended design. The reflector is not as recommended. The material is not as recommended and the plug is not as recommended. I don't know about the patch thickness, but that also has a little effect. All of these deviations are perfectly acceptable, but can change the resonance slightly. The most critical is the distance of 3 mm and it is very difficult to correct it over the entire area - but this is not necessary because you can adjust it.

The POTY design at 2.4 GHz is necessarily a very narrow band to create circular polarization from a single feed on a patch with a large wavelength tube that forms a short in the middle where it shouldn't be by right. To keep the design simple, I only used one feed and not two with phases of how it should be done.

Voting is easy and necessary to get a good circle response. All you have to do is bend the corners slightly inwards or outwards, depending on the resonance you set. Only a little should be enough. This is one of the reasons why I have recommended soft copper and non-springy brass. Non-resilient brass is fine too, but higher loss. I doubt it is important with normal power inputs.

apologies for the English but these differ from the recommended design in at least 3 places. The reflector is not as recommended. The material is not as recommended and the connector is not as recommended. I don't know about the patch thickness but that also has a small effect. All of these deviations are perfectly acceptable but may change the resonance slightly. The most critical is the 3mm spacing and it's very hard to get it correct over the whole area - but you don't need to as you can tune it.

The POTY design is necessarily very narrow band at 2.4 GHz in order to create circular polarization from a single feed, on a patch with a large in terms of wavelength bit of pipe forming a short circuit in the middle where by rights it should not be. To keep the design simple I used just one feed and not two with phasing like it should be done.

Tuning is simple and needed to get good circular response. All you need to do is slightly bend the corners in or out depending which resonance you are tuning. Just a little should be enough. This is one of the reasons I recommended soft copper and not springy brass. Non springy brass is also fine but higher loss. I doubt it matters at normal power inputs.

show an image.

the RF connector grounding especially. Oh yes, do not expect not to have to adjust a little, love especially if you change materials.

(sorry for google German but I don't speak German and this is the German section)

Nice article - google translate is you friend. The POTY should be about 50% efficient with a 0.7 f/d dish so gaining 3 dB is interesting. Possibly it's not quite circular, but anyway 3 dB is worth having. The problem with a concentric helix is it impacts the receiver quite a lot, so it really depends if you are uplink limited or downlink limited. Patch feeds are optimum at around 0.5 f/D. A radical alternative is to use a POTY but extend the dish with wire mesh - assuming this is possible. This will have the effect of reducing the F/d to better match the 2.4 GHz pattern and to increase the gain at 2.4 GHz. It will have no impact at 10 GHz. This extension can use quite coarse mesh and accuracy does not need to be too great though obviously the better the better.

You don't really need a full bridge for this and could use practically any directional coupler that works at 2.4 GHz. In this case all we really need to know is the coupling factor which you can find quite easily in forward mode, putting a load on the through port and measuring what you get on the coupled port. Then just turn it round to sweep measure the reflected power and take into account said coupling factor.

Mike

You will see the TX drop, the question is by how much and is it an acceptable compromise?

The proposal is to have the LNB and Helix not together but offset from one another, e.g. the helix above the LNB. This will impact TX efficiency because the beam is no longer precisely towards the satellite but hopefully not by too much as the beam at 2.4 GHz is 4 times as wide as it is at 10 GHz.

This is a good idea and should work for smaller dishes too. The 2.4 GHz beam will not be correct, there will be an elevation error, but this will not matter very much because the 2.4 GHz beamwidth is relatively wide. It could be several degrees out before it matters. If we assume a 3 degree error with a 1m dish we would lose 3dB on TX. A 3 degree error on a 2.4m dish is much more serious, 10 dB down, but of course the distance to the feed is twice as far, so the angular offset for the same physical offset will be smaller and it will all equalise. Offsets have a real advantage as the focal length is usually longer meaning the feed is farther away from the dish and therefore the relative error less.