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How to amp a dual-band setup?

Discussion in 'Hardware' started by Stephen Sprunk, Feb 4, 2019.

  1. Stephen Sprunk

    Stephen Sprunk New Member

    I have a working 1090+978 system that is feeding to ADSBx (and FA). Here's my hardware setup today:

    Dual-band antenna (outside at top of chimney)
    10ft coax (N)
    Lightning arrestor (N)
    25ft coax (N to SMA) to wiring closet indoors
    FA dual-band filter (SMA)
    Splitter cable (SMA)
    FA blue stick (for 1090) and FA orange stick (for 978)
    RPi 3B+

    (At first I put the FA filter between the splitter and the orange stick, but moving it before the splitter improved my 1090 counts yet didn't have a noticeable effect on range. That confuses me...)

    I'm currently getting ~150nm at 30kft (1090 only, obviously) and ~50nm at 5kft (both 1090 and 978), which seems pretty good for a first attempt. But, like everyone, I want to improve on that--and that means amplification. There's lots of info on how to do it for 1090-only setups, but I can't figure out the best way to do it for a dual-band setup since (so far) we're relatively rare.

    I've looked at the Uputronics filter/LNAs, but for best effect I'd need to put two (one 978, one 1090) at the mast, which means a second coax run into the house (and likely replacing the first run with SMA-SMA), plus either get two bias-Ts or switch to two SDRs with bias-T built in. All that adds up to a very expensive project, which is tough to swallow given I don't know how much it will help.

    A much cheaper option would be a single wideband LNA (fronted by the FA filter I already have) in my wiring closet, powered by USB. I'm concerned about how much noise this will amplify, though, given the FA filter has a pretty wide passband; there has to be a reason the Uputronics guys used triple filters, including a SAW, right? Amplifying after the long coax run obviously isn't ideal either.

    Are there any other options that would likely have a better (or at least more certain) cost/benefit ratio?
     
  2. Luis Colunga

    Luis Colunga New Member

    Hey there,

    What kind of splitter you got? A wideband LNA before the splitter could help reducing (at least the 3dB loss) of the splitter. I would experiment in putting the LNA in front of the filter and also the LNA after the filter, and see what gets you better results. (this really depends on your environment)

    Honestly using two Uputronics filter/lna like you are describing is not exactly a good idea, if you wanted to get very high-end. It would be a LNA on the antenna, with a custom filter that allows 1090mhz and 978 mhz with little insertion loss and lots of notching the other stuff.

    The Uputronics LNA actually just has 1 SAW (or ceramic filter) after the LNA chip. In fact in my system I have an Uputronics LNA but without the filtering (wideband) as I have external filtering.

    The LNA in your closet issue IMO is not really a noise problem, but if it won't cause overload to your FA sticks, and of course it is better if you get it in front of the antenna. By the way, 10 feet of coax is not really much, what kind of cable are you using? If it is good quality the losses are really not much in 10ft.

    Have you made predictions with tools such heywhatsthat to try to figure out your "theoretical" range? It's good to start with a base of what appears to be the max performance you can get due to geographical constraints, and take it from there.

    Hope this info helps you.
     
  3. Stephen Sprunk

    Stephen Sprunk New Member

    This is the splitter I'm using: https://www.amazon.com/gp/product/B0744CQGS5/ Nothing special; if I can improve on that, I'm open to it. The coax cables (10+25ft) are LMR240.

    I thought about separate external filters, and there's lots of good ones for 1090, but I can't find anything decent for 978; that's why I'm using the dual-band FA filter right now. It was designed for 1090, but the passband is so wide that it also (by accident, I suspect) passes 978 near the low end.

    The Uputronics 1090/978 units claim to have a high-pass in front, a SAW between stages and another SAW in back, and the photos do look different from the wideband units in three places that seem to match that. I was thinking the higher-quality filtering alone would help, even before considering the amplification itself. A wideband unit would leave me still without good filtering for 978.

    According to heywhatsthat, I should have ~230mi (~200nm) at 30kft and ~90mi (78nm) at 5kft, so I'm currently getting 65-75% of what is theoretically possible. The terrain is below 1° in all directions, so my neighbors' houses are going to limit LOS more than curvature will. But I'm already picking up a TIS-B/ADS-R tower (~15mi away) that is just below the horizon and certainly not LOS, so I'm not sure how much the houses actually matter compared to the losses in the coax and splitter and the poor filtering.
     
  4. Luis Colunga

    Luis Colunga New Member

    In your case definitely I would buy a proper splitter. Honestly that Y doesn't look right. I would buy on eBay a Minicircuits splitter (new, new old stock, or used). Avoid the ones from China.

    From the antenna you got 25feet and then to the dongles 10 feet?

    There's a FA filter that is sold as a dual band solution, as the passband is so wide as you mention. You got that one? I am not sure if it is different fron the traditional blue filter.

    Where are you reading this about the Uputronics? I have spoke with the designer (Anthony) and it only had one filter. My "wideband" LNA was actually a 1090Mhz but as he didn't had stock he was very kind to remove the SAW filter and short where the SAW should be located.

    The rtl sdr 1090 mhz is triple filtered, but if you really got money basically the better solution is just to put a cavity filter in front of the LNA. You are right from 978 Mhz there is not many options, you could buy a 978 SAW filter and put it on a board.

    Your neighbor houses will definitely limit you. Specially with weak signals, you are probably picking up that TIS-B tower because is probably very high powered. The lmr 240 loss at 25 feet is like 2dB, which is not that bad.

    Filtering wise you are not that bad, unless you got high powered towers very close or a very specific case of interference. In a ROI point of view I would definitely change the splitter, that should bring improvements.
     
  5. Stephen Sprunk

    Stephen Sprunk New Member

    I'm looking at Minicircuits splitters now, but there's dozens of models; anything in particular I should be looking for to narrow it down? When I had the long coax directly to the FA blue stick, the range was only slightly better than now (honestly, far less difference than I'd expected for a minimum 3dB loss), so the Y cable splitter doesn't seem to be hurting me much. Still, if it's not an expensive fix, why not?

    Everything is connected in the order listed in the first post.

    My FA filter is the round blue one. They were originally marked as 1090 only, but they started marking them as dual band after folks discovered they passed 978 too; the model number is the same. OTOH, that probably means it's passing a lot of noise too. I get no 978 traffic at all on the orange stick without it, though, so filtering obviously isn't optional, but I'd like something actually designed for 978 rather than something that accidentally works because the design was flawed.

    You're right on the Uputronics; I must have gotten confused while I was flipping back and forth. I just ordered a RTL-SDR 1090 amp and will have it up tonight to see how much difference it makes. Since he doesn't have a 978 model (yet), how tough would it be to get a second 1090 and replace the three filter chips with 978 ones? Or is the wideband amp plus the FA filter really going to be enough better to justify the cost/effort?
     
  6. Luis Colunga

    Luis Colunga New Member

    For the Minicircuits, you can just look at a splitter rated to work between 978 Mhz and 1090 Mhz, for example the one I have is rated for 900 - 2000 Mhz. Right now with the current setup, you don't get only insertion loss, but reflections and no port isolation.

    Modifying the RTL SDR 1090 amp could get tough. There is some guides written by abcd1234 that suggests a way for checking for strong signals. I think you should check out that first to see what you are fighting with. I would probably just go for the wideband amp after the antenna and the FA filter you can put it inside without issues. But check the guide by abcd1234 because maybe you get really bad performance with the filter after the LNA, which is my case here but I have strong FM stations and a TV station in the neighborhood.
     
  7. Stephen Sprunk

    Stephen Sprunk New Member

    I've got the 1090 amp in. I'm still working on finding the best gain setting; at the moment I'm seeing ~20% higher position and aircraft counts than my best day previously. I'm also picking up a lot more TIS-B/ADS-R traffic than before, but it comes and goes. I think that's just the nature of the beast, like me only picking up FIS-B occasionally on 978.

    I'll get the better splitter and wideband amp next week; thanks for the advice. Not sure when I'll be able to move all that to the mast, though, since it means redoing all the coax; any guess how much more that would help?
     
  8. Luis Colunga

    Luis Colunga New Member

    Hard to give a guess, but I think the splitter would probably be the priority.
     
  9. DIAZA

    DIAZA New Member

    I no longer have a dual band setup but had one beofore the hurricane hit Florida a few years back. Had the same Uptronics LNA you mention. My solution was diplexers. My 1090 had an FA filter and the 978 had a mini circuits filter if I recall correctly. You always want Filter —> LNA since you do not want to amplify unwanted signals. You can find some diplexers meant for dish/directv that can do the job. I had one coax run into the house and then split the signal again inside. It did it the job. My setup was. Filter —> Diplexer —> LNA —> single coax run —> Diplexer —> Dongles. The diplexers will provide a bit of filtering but will also introduce some more loss to the set up.