Would a 744MHz filter help?

  • At present I am using an single output unmodified LNB. I use it with 13 Volts for narrow band and 18 Volts for DATV. So my question is, if I built a Interdigital filter based at 744 MHz with a bandwidth of +/- 6Mhz to cover NB and WB would it actually make a difference with respect to SNR to me listening on NB?

    I do not know if typically sampling at 1Mhz in SDR console sets the bandwidth or the total noise hitting the sdr receiver.

    Any enlightenment appreciated.


  • Hi, my opinion is that if you use an RTL you will see no difference. If so, most important is to adjust the gain of it in order to archive max SNR and maximum dynamic range. If you mix it to a lower ham band I think using a BPF you gain 3 dB of the noise of the opposite sideband, that has no signals in our case. You lose some dBs of the filter but you have gain at the SNR.

  • Thank you all for your comments.

    At the moment I am trying to focus a bit more on the NB section, age is sometimes getting the best of my hearing and I seem to be struggling with some signals on the satellite when either very week signals or sometimes with what seems high audio processing.

    Perhaps better headphones are required, or, perhaps better understanding of SDR console. I was running the software with minimal gain on the RTL thinking it would help reduce noise. Last night I ran it with auto gain and found not only did the signal meter give 59 + signals but with some signals the audio was clearer as it reduced the background noise, a bit like FM quieting.

    I think as mentioned I need some other headphones rather than Hi-fi ones, some with a shaped audio response for communications.

    The down side to running auto gain was the changing spectral colours when two stations where close to each other.

    But I will build a filter and give it a try see if it helps reduce any noise especially for the WB section.

    Again many thanks, keep playing and learning.


  • Hello Viliam; I do run it at 1MHz, I have tried a few others with no noticable difference to my ears. I just wish to get away from scribbles of callsigns and locations on bits of paper as I go and then constantly correcting things as I realise I heard wrong. I will get there, eventually.:)


  • Just a bit of an update whilst I decide on what is the best way to proceed!

    Over the previous days I managed to mill out of a solid block of aluminium, the basic cavity for the ID filter block. Used some aluminium bar stock for the rods and fixed most of it together.

    I am now contemplating how I attach connectors to the aluminium rods. I was thinking of trying to nickel and then copper plate the bottom sections of the rods so I could solder to them, not sure if that would work and i really do not want to be messing with acids.

    So looking for some simple ideas at present?

    Ideas gladly welcome.


  • If you are worried about corrosion then I am not sure it will be a whole lot better with Aluminium. Better to seal it to avoid water ingress and run sufficient power to avoid condensation. Brass rod is cheap and readily available. Easy to machine too.


  • Well I solved it almost as it was, I do not have enough brass rod to make the filter, the rods are 8mm'ish! But I did have a bit of 10mm rod which I drilled out and sweated/pushed/bashed small sleeves over the base of the aluminium, this allowed me to use the blowtorch to solder the connectors to the sleeves.

    To say I am pleased with the results is an understatement, it may, or may not improve things on WB, nothing noted on NB as the filter is wider then the NB transponder, but a simple alignment and test on the speccy An shows it does what it says it should.

    10dB per div vertical and 10MHz per div Horizontal.

    The marker at 744MHz


  • I did a simple comparative test with the MiniTiouner express I have, waiting for a time with just the beacon active and it improved the MER by 0.5/0.6 dB. The MER changing from 8.1 to 8.6/8.7. This is with a 1metre offset dish and standard unmodified LNB and POTY feed, I did have better at one point without a rain cover, but it gets it back to the same area again.

    Not a great increase for the effort I know, but still happy with the filter at this end. I guess this small gain is due to the reduction in bandwidth to 12 MHz from the 22+ Mhz bandwidth of the tuner front end at these frequencies?


  • Just a thought about aluminum.

    In the 60's you had to use silver plated everything on 2M, at least that is what we were taught by the old boys and the RSGB VHF/UHF manual. I bought into this but eventually discovered it was mostly hearsay and at best only marginal under some circumstances.

    So aluminum, just go for it !

    Dave G4IUG Ex G8EAO

    Dave Cawley | Ex G8EAO & G6ANG/T |
    (when you had to have a separate TV license !)

  • I have an aluminium filter that was designed for above 23cms that I retuned, I was surprised it was aluminium as it was copper coated.

    Aerial Facilities, as was, built their band pass filter systems from aluminium, some would have copper clad board top and bottom but the sides stayed aluminium. I used to work for them and build them many years ago.

    I did a bit of web reading before I started and apparently the issue is not the necessary aluminium which will oxidise very quickly and protect it self to an extent but the fasteners that can be used in construction, Keeping away from stainless and some other metals seems to be the key, Zinc plated basic steel appears OK with aluminium.

    But all in all it was easier to get hold of aluminium in block form, it is relativity easy to machine if not a little sticky on the tools, so my little milling machine could cope OK. The issue was how to solder connectors. I would love to be able to Nickel and then copper plate things like this but for the odd job it is not worth it. But what I have tried seems to work OK, time will tell.

    I also use a retuned Block filter at 2.4GHz that was intended for LTE gear, that is also aluminium, but is silver coated, now that would cost a bit to do, but RF losses at the higher frequencies would probably be worth while for the manufacturer.

  • @PA3YFM

    Yes i use the same Filter in front of my EDUP EP-AB003 used as a driver stage ( removed input switch and connected SMA input via cable direct to splitting point at input of the Amps) wich gives 17dB Gain - assuming the Amp is liner no worries

    but if not - to filter in front of the antenna after a real PA you wont use it - it only takes 20dBm !!

  • DG8ABG I referred to a 740 MHz SAW filter, not a 2.4 GHz one ... (?)

    The 740 MHz filter is handy when you go down in two steps from 10489.5 to 432.5 MHz:

    10489.5 - 9750 (LNB LO) = 739.5 [SAW-FILTER] - 307 MHz (2nd LO) = 432.5 MHz