Narrow bandstop filter (remove 144.8 MHz, keep 137 MHz)? - Page 1
Forum section: electronics / rf-microwave

author: Artlav
author_rank: Frequent Contributor
timestamp: March 17, 2017, 05:22:03 pm
content: 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

author: David Hess
author_rank: Super Contributor
timestamp: March 17, 2017, 06:37:57 pm
content: A helical resonator would be perfect for this type of application.

author: PA0PBZ
author_rank: Super Contributor
timestamp: March 17, 2017, 07:18:37 pm
content: Quote from: Artlav on March 17, 2017, 05:22:03 pmWhat 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?

author: Lord of nothing
author_rank: Super Contributor
timestamp: March 18, 2017, 10:35:47 am
content: How about the: AOR ABF128SMA

author: matkar
author_rank: Regular Contributor
timestamp: April 02, 2017, 11:50:11 am
content: 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.pdfBut 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.

author: chicken
author_rank: Frequent Contributor
timestamp: April 02, 2017, 07:36:13 pm
content: A SAW filter, though they can be hard to source in small quantities.http://www.taisaw.com/en/product.php?pc=1

author: Howardlong
author_rank: Super Contributor
timestamp: April 02, 2017, 09:32:10 pm
content: Quote from: chicken on April 02, 2017, 07:36:13 pmSAW 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.

author: chicken
author_rank: Frequent Contributor
timestamp: April 02, 2017, 09:46:09 pm
content: Quote from: Howardlong on April 02, 2017, 09:32:10 pmOh yes, another vote for Golledge! Very helpful and surprisingly low cost when getting into 1k quantities.

author: Howardlong
author_rank: Super Contributor
timestamp: April 03, 2017, 04:48:03 pm
content: Quote from: matkar on April 02, 2017, 11:50:11 amI 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.

author: Howardlong
author_rank: Super Contributor
timestamp: April 03, 2017, 05:07:01 pm
content: Quote from: evb149 on April 02, 2017, 10:55:50 pmA 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.

author: Howardlong
author_rank: Super Contributor
timestamp: April 03, 2017, 06:04:34 pm
content: 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.

author: Howardlong
author_rank: Super Contributor
timestamp: April 03, 2017, 09:30:01 pm
content: 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.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 04, 2017, 10:58:59 am
content: Quote from: Artlav on March 17, 2017, 05:22:03 pm60dB 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.

author: babysitter
author_rank: Frequent Contributor
timestamp: April 04, 2017, 01:04:48 pm
content: Quote from: vk6zgo on April 04, 2017, 10:58:59 amTeheheh he said "trumped" ...Vy 73

author: David Hess
author_rank: Super Contributor
timestamp: April 06, 2017, 07:20:05 pm
content: Quote from: vk6zgo on April 04, 2017, 10:58:59 amDongle 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.

author: Howardlong
author_rank: Super Contributor
timestamp: April 06, 2017, 07:25:18 pm
content: Quote from: David Hess on April 06, 2017, 07:20:05 pmIf 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.

author: David Hess
author_rank: Super Contributor
timestamp: April 06, 2017, 08:27:47 pm
content: Quote from: Howardlong on April 06, 2017, 07:25:18 pmThey are still horrible compared to having even one stage of IF filtering.

author: Howardlong
author_rank: Super Contributor
timestamp: April 06, 2017, 09:47:05 pm
content: Quote from: David Hess on April 06, 2017, 08:27:47 pmSome of them do.

author: TheSteve
author_rank: Unknown
timestamp: April 07, 2017, 07:22:25 am
content: 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...

author: G0HZU
author_rank: Super Contributor
timestamp: April 08, 2017, 12:52:17 am
content: 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 bandBut 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 usedAlso 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

author: cdev
author_rank: Super Contributor
timestamp: April 08, 2017, 11:12:41 pm
content: 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

author: Howardlong
author_rank: Super Contributor
timestamp: April 09, 2017, 11:32:11 am
content: Quote from: cdev on April 08, 2017, 11:12:41 pm1/4 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.

author: cdev
author_rank: Super Contributor
timestamp: April 09, 2017, 11:44:05 pm
content: Do you know of a good source for a single. 137.5 MHz part with low insertion loss?

author: Howardlong
author_rank: Super Contributor
timestamp: April 10, 2017, 10:30:18 am
content: As already mentioned several times in this thread, Golledge.For examplehttp://www.golledge.com/pdf/products/specs/mp07202.pdf

author: Howardlong
author_rank: Super Contributor
timestamp: April 10, 2017, 11:45:06 am
content: 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.

author: eb4fbz
author_rank: Regular Contributor
timestamp: April 10, 2017, 07:53:29 pm
content: You really need 60dB of gain? What are you using as receiver? Something with 50dB noise figure?60dB without filtering is crazy, even without the nearby APRS transmitter.

author: PA0PBZ
author_rank: Super Contributor
timestamp: April 10, 2017, 08:15:48 pm
content: Quote from: eb4fbz on April 10, 2017, 07:53:29 pmI asked the same question about 3 weeks ago, unfortunately the OP never replied.

author: cdev
author_rank: Super Contributor
timestamp: April 11, 2017, 07:31:11 pm
content: NOAA and METEOR sats are semi weak signals but a single LNA (or even no LNA) is adequate for me to receive them with occasional noise from pagers, etc. With more selectivity he probably will do fine. QFH antennae are best for that and better for RHCP (NOAA) sats than omni biconical I use but vary as far as pattern, unlike a biconical a tall skinny one has more (not less) gain at horizon but picks up more terrestrial noise too.

author: AF6LJ
author_rank: Unknown
timestamp: April 11, 2017, 11:19:09 pm
content: This looks interesting...I would go with a band reject cavity or two.At that frequency they are not hard to make if one is creative.

author: cdev
author_rank: Super Contributor
timestamp: April 11, 2017, 11:47:29 pm
content: It's not easy to get nice clean images if you live in an urban area.

author: Howardlong
author_rank: Super Contributor
timestamp: April 11, 2017, 11:50:58 pm
content: In my experience, as I alluded to earlier, practically speaking making such filters that need accurate measurement and manual tuning aren't always particularly easy to fabricate unless you have known reproducible plans, and the right skills and tools at your disposal.While there are plenty of examples in print and online, finding one that's easily reproducible is a bit of a crapshoot. Any given plans may have worked for an individual on one occasion, but are those plans documented well enough and easily reproducible?Much the same applies to antennas, by the way. I am sure this lack of reprocibility in many cases is why RF is often thiught of as voodoo!Those SAW filters arrived today, unfortunately I have some other commitments right now, but I'll be putting them on a board and measuring them over the next few days.

author: David Hess
author_rank: Super Contributor
timestamp: April 12, 2017, 12:22:17 am
content: I did the tuning on my 2 meter helical resonator using an SWR bridge and less than 5 watt transmitter. In practice with only two sections, adjusting the two piston trimmers which are not shown in the photograph is easy enough with just a receiver and RF attenuator.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 12, 2017, 01:56:26 am
content: Where I used to work there was a 1kW pager on 148.3MHz (or thereabouts) co-sited with our TV/FM Broadcasting installation.Common 1970s/80s mobile 2m Ham radios were only affected by the pager if tuned up towards 148MHz----at 147MHz & lower, there was no discernible breakthrough.It is fashionable to dismiss superheterodynes as "yesterday's technology", but they are a hell of a hard act to follow.Almost any Direct conversion receiver is just an "Envelope Detector" to a strong enough & "adjacent enough" signal.

author: Howardlong
author_rank: Super Contributor
timestamp: April 12, 2017, 06:02:01 am
content: Quote from: David Hess on April 12, 2017, 12:22:17 amWhat was the passband and stop band ripple like? How well did it match across the passband?In addition, you already have a lot of relevent skills and experience to built such a device. I am pretty sure the OP doesn't, and that was what I was trying to convey. I also doubt the OP has a 5W transmitter or SWR meter. That method is exactly the method I've used in the past, but when you take a filter you've tuned that way and put it on a VNA you realise it's not quite as good as you thought!

author: David Hess
author_rank: Super Contributor
timestamp: April 12, 2017, 09:01:44 am
content: Quote from: Howardlong on April 12, 2017, 06:02:01 amWhen I designed it, the only information I was missing was how large to make the coupling window so I made it 1/3rd the width and height which as it ends up made it massively overcoupled but this had some advantages. Loss in the center of the band was like 0.05dB, there was no measurable ripple, and it covered most of the 4 MHz wide 2 meter band. It was perfect for removing pager interference.I never had access to a VNA to test it or I would have lowered the coupling. I did come up with a design for a variable coupling window but this one worked so well that I never proceeded with it.This was actually the 2nd one I designed and built. The first helical resonator was not nearly as well constructed and used film trimmers which lowered the Q significantly preventing operation at even 5 watts because they would break down. After that I found someone selling piston trimmers at the ham radio swap meet which made all of the difference.The design process for both was interesting. I started with the fixed size of the wire for the helical elements and worked out the dimensions going backwards. The center frequencies with the trimmers were right on. The final product was definitely not an optimal design but it was and still is way more than needed to protect a receiver from out of band overload and the extremely low loss has advantages.Incidentally, this project was also how I discovered that RF coupling into a gas discharge bulb is incredibly efficient. A neon bulb inserted into the helix with 1/2 watt is way too bright to look at. If I ever build a gas laser, this is how I will do the excitation.

author: Howardlong
author_rank: Super Contributor
timestamp: April 15, 2017, 01:51:53 pm
content: Today I made up a board for the SAW filter and tested it on the VNA, these are the results.144.8MHz is 44dB down, and insertion loss between 137 and 138MHz is 1.5 to 1.6dB.There is no matching required, although I left space on the board for L matches if necessary at both input and output. I used 1nF caps in series with the input and output in place of the L match.Total time designing and making the board was an hour, fifteen minutes to populate it, then a further fifteen minutes to test it. There was no tuning to do :-)Marker 1 is 137.0MHzMarker 2 is 138.0MHzMarker 3 is 144.8MHz in the S21 plotMarker 3 is 137.5MHz in the S11 plot

author: cdev
author_rank: Super Contributor
timestamp: April 15, 2017, 03:09:01 pm
content: Very nice! You could sell that as a kit with no additional work!1090 MHz too.

author: Howardlong
author_rank: Super Contributor
timestamp: April 15, 2017, 04:06:55 pm
content: Quote from: cdev on April 15, 2017, 03:09:01 pmCoincidentally, I designed and built a 1090MHz one a few years ago already as part of a bigger TCAS system I was working on. I'd have to dig out the schematics, but ISTR that was a native 50 ohm SAW too, ut a different package.

author: Howardlong
author_rank: Super Contributor
timestamp: April 15, 2017, 06:55:53 pm
content: I haven't done VHF NOAA reception myself for many years, but ISTR it's an FM modulated carrier with an IF bandwidth of about 40kHz. That bandwidth requirement was quite a problem for commonly available VHF receivers because their hardware filters were generally designed for narrower bandwidths at VHF. Nowadays, with SDR, that turns into a software problem.(Some years ago I designed a simple and reproducible turnstile antenna for VHF NOAA reception which made it to print in one of the newstand mags over here in Blighty).