L-Band patch antenna with a dish?

[JamesWest]

I bought one of these L Band patch antennas and it works great. Using it with a SDRDuo and SDRPlay.

l-band

I was going to see if I would use it with one of the old sat TV dishes I have to maybe increase the gain a bit but my understanding is pointing it at the dish reverses the circular polarity which is not a good thing.

But on the web I have seen photos of patch antennas mounted on C and Ku Band dishes without any text to explain what I am looking at.

Does any body know if these patch antennas can be used in conjunction with a dish?

If you are thinking of getting one of these antennas for 50 bucks I think it is a good deal. Needs bias-T.

I would get on the roof and experiment but it is so hot and humid outside it is really not doable.

Thanks.

 

[thebaldgeek]

Oddly enough, the RTLSDRv1 L-Band patch work better with a dish... they are smaller.


From bottom to top.
v1 patch.
v2 patch.
7 turn RHCP helix prototype 1.
7 turn RCHP helix prototype 2.
(Not shown is a 4 turn LHCP for mounting on dishes - also 3D printed).

Here is my v1 hitching a ride on my C-band dish feed:


The v2 patch is too big to get the whole thing at the focus. You could change the focus of the dish of course, push it in and thus make it worse for C-Band, but better for the L-Band v2 patch size focus.

Here is the best way I have found to check and set the focus of your dish



Just do be careful, the focal point can get super hot and be sure to pull the foil off before mounting your gear.
I took this shot when the dish was in shade.

Regarding the patch antenna RHCP vs pointing at a dish were it becomes LHCP.... You get a win-lose.
You lose some signal by having the wrong 'twist', but if you have a big enough dish, the signal gain overcomes that 'twist' loss and you end up with an overall gain.
90cm dish (DirectTV) is about a wash. They are not big enough to overcome the wrong twist loss.
Offset or prime focus does not make any difference, it just changes the aiming.
With a patch antenna and wide focus on about a 3 foot dish you should not need tracking to keep the signal peaked over the satellite orbit, but if you get up in size (like on my 6 foot dish in the photo), then even on L-Band you will need some sort of tracking on the dish to keep the signal peaked.

Most of all, just experiment and have fun!

 

[JamesWest]

With a patch antenna and wide focus on about a 3 foot dish you should not need tracking to keep the signal peaked over the satellite orbit,

thank you for taking the time for a detailed response. very helpful.

i have a 1.5 meter (4.11 foot) C Band sat TV dish I might experiment with now. my patch is the newer V2 in the grey case. it actually works well but I just wanted to see what would happen gain wise. i was not going to be using this dish for anything else so i was just going to mount the patch right were the old C Band feedhorn is. I think my next step is to decode voice which I have read needs to be as strong a signal as possible.

 

[thebaldgeek]

Here is the size difference between the v1 and v2 patch antennas so you can get an idea of how the focal point will need to be opened up.



The black and grey case v2 antennas are identical.
The guts are also the same.
Not sure if you have opened yours up yet, if not or if you don't want to.. .here is what you will find....
From the front.



And from the back...



The USB can power the LNA if you don't have a Bias-T but you will need to drill a hole in the bottom of the case.
Best to use a Bias-T and keep the case waterproof if you are going to be putting it on a dish outside.

Your 1.5m dish will give a nice bump in signal for sure.
And yes, voice decoding needs a really strong steady signal to hold up all the way through the call.
I mean, if you have the dish and arms etc, just stick the patch at the focus and see what the waterfall shows.
You will soon see by moving it around if you need to tweak the focus or not.

Cheers.

 

[prcguy]

Let me talk about "twist loss" as mentioned above. That would be cross polarization isolation and it should be massive, like 25dB or more between polarities. Its true that a single reflection on a circular polarized antenna will swap polarities, so if you need RHCP then you must use a LHCP antenna at the feedpoint of a prime focus reflector to end up with RHCP. If you use a RHCP patch antenna the so called or improperly called twist loss should be 25dB or more and basically will not work. Unless the patch antenna was not circular polarized to start with and some have very poor circularity.

When looking at an antenna to use at the focal point of a dish some basic rules are the antenna pattern of that antenna should be such that it illuminates the dish with very little spill over and you want the radiation pattern of the feed to be about 10dB down right a the edge of the dish. That will properly illuminate the dish with minimal spillover and give you a decent pattern on the dish. If you have too much spillover the overall gain will be reduced and more importantly the feed antenna will be looking over the dish edges to the ground, which is hot (high noise level) and this will raise the noise temperature of the antenna system and degrade reception.

Bottom line is you can't just stick any old thing in front of a dish and use it as a feed, it needs to meet certain criteria to play nice with the dish.

 

[JamesWest]

Let me talk about "twist loss" as mentioned above. That would be cross polarization isolation and it should be massive, like 25dB or more between polarities. Its true that a single reflection on a circular polarized antenna will swap polarities, so if you need RHCP then you must use a LHCP antenna at the feedpoint of a prime focus reflector to end up with RHCP. If you use a RHCP patch antenna the so called or improperly called twist loss should be 25dB or more and basically will not work. Unless the patch antenna was not circular polarized to start with and some have very poor circularity.

When looking at an antenna to use at the focal point of a dish some basic rules are the antenna pattern of that antenna should be such that it illuminates the dish with very little spill over and you want the radiation pattern of the feed to be about 10dB down right a the edge of the dish. That will properly illuminate the dish with minimal spillover and give you a decent pattern on the dish. If you have too much spillover the overall gain will be reduced and more importantly the feed antenna will be looking over the dish edges to the ground, which is hot (high noise level) and this will raise the noise temperature of the antenna system and degrade reception.

Bottom line is you can't just stick any old thing in front of a dish and use it as a feed, it needs to meet certain criteria to play nice with the dish.

thank you for this info it is helpful. I think the reality is since I already have a dish I will take the time to turn it around and point it at Inmarsat POR and use a notebook computer to watch the waterfall and just spend some time to see what happens. It might be a total bust. I will try to post some screenshots assuming I don't fall off the roof. 555

 

[thebaldgeek]

thank you for this info it is helpful. I think the reality is since I already have a dish I will take the time to turn it around and point it at Inmarsat POR and use a notebook computer to watch the waterfall and just spend some time to see what happens. It might be a total bust. I will try to post some screenshots assuming I don't fall off the roof. 555

That's the spirit! Just give it a go.
Don't let perfect get in the way of good.
Experiment and explore.