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I'm trying to receive weather satellite images (NOAA and Meteor), transmitted at 137-138MHz.
Problem is, periodically there is a powerful 144.8MHz ARPS transmitter nearby, which completely overwhelms the weak satellite signal.
To get good reception from the NOAAs i need about 60db of gain, while the ARPS signal is already very strong without any gain at all.
So, i need some sort of a notch/bandstop filter to exclude 144.8MHz without interfering much with 137 MHz. That is, with less than 12 MHz of bandwidth.
The question is - what sort of filter can do that, and what to look for?
I looked at passive filters, but most calculators give me impossibly small part values, i.e. http://www.wa4dsy.net/cgi-bin/lc_filter3?FilterResponse=Bandstop&poles=6&CF=144.8&cfunits=MHZ&cutoff=13&funits=MHZ&Z=50
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A helical resonator would be perfect for this type of application.
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I'm trying to receive weather satellite images (NOAA and Meteor), transmitted at 137-138MHz.
Problem is, periodically there is a powerful 144.8MHz ARPS transmitter nearby, which completely overwhelms the weak satellite signal.
To get good reception from the NOAAs i need about 60db of gain, while the ARPS signal is already very strong without any gain at all.
What kind of receiver are you using? A reasonable quality one should not have a problem with this, or are you using a wide band amplifier in front of it?
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How about the: AOR ABF128SMA
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I'm also starting to build an antenna for weather satellites. I bought 8mm copper tube and some RG58 cable. I'm looking at some antenna variants right now. Which blueprint did you use?
I have a limited experience in building passive VHF filters. I also find Chebyshev and Butterworth filters to have impractical capacitor/inductor values so I made my filters by adapting this design: http://yu1lm.qrpradio.com/2m%20BP%20FILTER-YU1LM.pdf
But I'm afraid it won't do much in your case since the frequencies are too close...
BTW does anybody know if the filter design from the pdf has an official name? I haven't seen it anywhere else but I haven't searched for it either. -
A SAW filter, though they can be hard to source in small quantities.
http://www.taisaw.com/en/product.php?pc=1
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A SAW filter, though they can be hard to source in small quantities.
http://www.taisaw.com/en/product.php?pc=1
SAW filter was my first thought too, Golledge may be able to help for samples as they resell these parts. They did with me, and I've bought about 50ku from them since. -
A SAW filter, though they can be hard to source in small quantities.
http://www.taisaw.com/en/product.php?pc=1
SAW filter was my first thought too, Golledge may be able to help for samples as they resell these parts. They did with me, and I've bought about 50ku from them since.
Oh yes, another vote for Golledge! Very helpful and surprisingly low cost when getting into 1k quantities. -
I'm also starting to build an antenna for weather satellites. I bought 8mm copper tube and some RG58 cable. I'm looking at some antenna variants right now. Which blueprint did you use?
I have a limited experience in building passive VHF filters. I also find Chebyshev and Butterworth filters to have impractical capacitor/inductor values so I made my filters by adapting this design: http://yu1lm.qrpradio.com/2m%20BP%20FILTER-YU1LM.pdf
But I'm afraid it won't do much in your case since the frequencies are too close...
BTW does anybody know if the filter design from the pdf has an official name? I haven't seen it anywhere else but I haven't searched for it either.
I have some experience with that design from a while ago. You need to hand tune it, and while that can be done without a VNA or Spec An + Tracking Generator, it will be hard work. -
Looks like the bandpass doesn't have to be all that tight that a cavity is required. Just ordinary lumped filters or stub filters can work. There are a lot of articles about this.
http://www.rtl-sdr.com/building-a-diy-137-mhz-band-pass-filter/
A simple stub or off the shelf lumped parts won't cut it I'm afraid. On that example just quoted, 144.8 is barely 6 or 7dB down. It'll be fine for attenuating FM broadcast, and that was what it was designed for, that's a relatively simple engineering problem.
Some years ago I went down this path, although this was to notch 152MHz pagers from 145MHz receivers, so it's a similar engineering problem. If you go the lumped parts way you will have to wind and tune your own coils to get enough Q. You also need to take care in layout to stop leakage across the stages. You won't get anywhere near enough rejection that close to the passband with off the shelf lumped parts.
A helical filter is a solution but again it'll need tuning.
If you can find a SAW filter with 50 ohm native impedance, you can lay it out on a simple board that you can fab yourself and not have to do any tuning. If the SAW filter is not 50 ohm, all bets are off, you will need to design appropriate L matches with off the shelf lumped components for input and output which in itself isn't hard. The problem comes in finding that your carefully calculated parts give appalling passband loss, usually because of misleading manufacturer data, but also to some degree because of parasitics. In that case you have to do some work with the VNA and a Smith Chart. You can do it empirically but it'll take a while with two L matches, although typically SAW filters are reciprocal devices, so you'd change pairs of parts at the same time doing it that way. -
Some pics of SAW breakouts fabricated on double sided FR4, bottom side is solid ground plane. ISTR a couple of these go up to 1090MHz, some are about 868MHz and one is around 435MHz.
Some have L/C L networks on input and output, and some have just a cap and/or 0R resistor which are natively 50 ohm, but I used the same board.
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Did you actually try Golledge? That was exactly how I started, bought a few samples for a few $ each. They don't always have every part in stock, but they keep a fair few. It's absolutely worth a try. If you want me to, I can ask, I just placed another large order with them last week. They'll probably send me a few gratis if it's anything like the last few times I've asked for samples.
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I'm trying to receive weather satellite images (NOAA and Meteor), transmitted at 137-138MHz.
Problem is, periodically there is a powerful 144.8MHz ARPS transmitter nearby, which completely overwhelms the weak satellite signal.
To get good reception from the NOAAs i need about 60db of gain, while the ARPS signal is already very strong without any gain at all.
So, i need some sort of a notch/bandstop filter to exclude 144.8MHz without interfering much with 137 MHz. That is, with less than 12 MHz of bandwidth.
The question is - what sort of filter can do that, and what to look for?
I looked at passive filters, but most calculators give me impossibly small part values, i.e. http://www.wa4dsy.net/cgi-bin/lc_filter3?FilterResponse=Bandstop&poles=6&CF=144.8&cfunits=MHZ&cutoff=13&funits=MHZ&Z=50
60dB of gain is a massive figure.
Where is this gain?
At the antenna?
At the input to the receiver?
What kind of receiver?
Is it tuned in any way?
If you are using a scanner, they have virtually no input selectivity.
Because of this, they usually have a high first IF to stop "image" interference.
High,unfortunately goes with wide bandwidth,so suppression of a strong signal 7MHz is pretty much minimal.
The second IF is where all the selectivity is,but by the time you get there, you may already have intermodulation products within the IF passband.
Simple receivers,like "Direct Conversion" types are even worse,as they rely heavily on input selectivity, along with phase cancellation techniques---these can be easily "trumped" by a very strong signal.
I don't know a lot about the simple "dongle" type SDRs, but it seems likely that they have similar problems.
I would suggest a tuned converter at the antenna, with an IF at HF going down the coax to a receiver for an appropriate band.
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Simple receivers,like "Direct Conversion" types are even worse,as they rely heavily on input selectivity, along with phase cancellation techniques---these can be easily "trumped" by a very strong signal.
Teheheh he said "trumped" ...
Vy 73
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Simple receivers,like "Direct Conversion" types are even worse,as they rely heavily on input selectivity, along with phase cancellation techniques---these can be easily "trumped" by a very strong signal.
I don't know a lot about the simple "dongle" type SDRs, but it seems likely that they have similar problems.
Dongle type SDRs are terrible in this respect; so are GPS and WiFi direct conversion receivers. They all have horrible overload characteristics do to high gain before their first selective stage.
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Simple receivers,like "Direct Conversion" types are even worse,as they rely heavily on input selectivity, along with phase cancellation techniques---these can be easily "trumped" by a very strong signal.
I don't know a lot about the simple "dongle" type SDRs, but it seems likely that they have similar problems.
Dongle type SDRs are terrible in this respect; so are GPS and WiFi direct conversion receivers. They all have horrible overload characteristics do to high gain before their first selective stage.
If you buy $20 ones, yes. If you buy those with a dozen or so switched & tuned preselectors it's a completely different ball game, but front end filtering comes at a price. -
If you buy $20 ones, yes. If you buy those with a dozen or so switched & tuned preselectors it's a completely different ball game, but front end filtering comes at a price.
They are still horrible compared to having even one stage of IF filtering.
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If you buy $20 ones, yes. If you buy those with a dozen or so switched & tuned preselectors it's a completely different ball game, but front end filtering comes at a price.
They are still horrible compared to having even one stage of IF filtering.
Some of them do. -
I'm not sure if the OP is still watching but with the hardware I have on hand I'd use half of a small mobile duplexer.
~ 3dB of insertion loss at 137-138 MHz and a 64dB notch at 144.8 MHz. If a 64 dB notch isn't enough I'd say he needs a better receiver...
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Many years ago I designed a very special notch filter for use up at around 1500MHz. It was designed to be quite sharp and had very low loss right up to 6GHz.
The plots below are from a real production filter measured on a VNA and imported into Genesys. You can see how special this is when you look at how good the match is right up to 6GHz and how sharp and deep the notch is.
Divide all the numbers by 10 and it would be in the ballpark of the 144MHz band
But it would be very big if scaled to work here. It was designed on some very special PCB material and used special techniques to achieve no re-entry modes right up to nearly 8GHz.
The final image was one taken with a VNA of a real filter although this plot is not of the same filter S/N as the first two.
Check out how low the insertion loss is at 6GHz. Just 0.6dB and this includes the loss for the (very expensive) RF connectors used Also try dividing the frequency by 10 and you can see that the insertion loss at 144.7MHz would scale to be at just 0.7dB wrt a notch of -64dB at 157MHz
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An 1/4 wave open coaxial cable stub can make a fairly sharp notch filter at one frequency. Sharp enough only a few MHz away? Maybe, but I suspect more probably maybe not. A sharper fairly straightforward solution is a helical resonator.
http://satsignal.eu/wxsat/filters/HelicalNotch.htm
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An 1/4 wave open coaxial cable stub can make a fairly sharp notch filter at one frequency. Sharp enough only a few MHz away? Maybe, but I suspect more probably maybe not. A sharper fairly straightforward solution is a helical resonator.
http://satsignal.eu/wxsat/filters/HelicalNotch.htm
1/4 wave stub will be nowhere near good enough I'm afraid. Here's an example using RG58, 100MHz span, centre 160MHz, somit gives you an idea of what kinf of performance can be achieved.
DIY filters such as the helical you mention will work, but you really need the right skills and tools to build them, and the right test equipment to hand tune. Personally I usually find designs like this, both online and in print, are often difficult to reproduce: the same applies to antennas. If the designer mentions they have specifically paid attention to reproducibility from the ground up then this is a good sign. While reproducibility can have an effect on performance, that benefit usually outweighs the lack of performance from a one-off contrivance that happened to work on a particular day with the right weather.
Size aside, the huge benefits of SAW filters are their reproducibility, and that, once prototyped, there is no further tuning required. With ready-matched 50 ohm SAW filters, it's unlikely you'd need to tune them at all. The downside, in small volumes, is finding a suitable one, as typically their are a limited number of off the shelf designs. -
Do you know of a good source for a single. 137.5 MHz part with low insertion loss?
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As already mentioned several times in this thread, Golledge.
For example http://www.golledge.com/pdf/products/specs/mp07202.pdf
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I called them up and they are sending me some samples this week, but they only had a few in stock. Also bear in mind that I already purchase quite a bit from them so a punter off the street may get a different response.
I'll put one or two on a board and see how they perform on a VNA.