[Ground-station] channelized FM SDR

[KENT BRITAIN]

 Even a simile series tuned circuit to ground tuned to about 100 MHz can take a lot of the energyout of the FM band.   This is a big problem we have been seeing with indoor TV antennas.The switch to Digital greatly reduced the ERP limits from the FCC.   To compensate most of theTV antenna companies increased the gain of their amps.  Lots of IP3 problems from FM stations.

A simple series  .22uH 12 pf notch filter across the input of the amp does wonders.Not a brickwall filter, but does removes most of the FM Band energy from the amp input.
Kent WA5VJB



    On Sunday, November 1, 2020, 8:19:23 PM CST, Robert McGwier <rwmcgwier at gmail.com> wrote:  
 
 It is a real shame we can't purchase high quality FM bandstop filters. I have purchased some I hoped would work and they attenuated the FM band and much more. Next, as you well know, lots of people pretend every piece of crap they hook to the antenna terminal is a 50 Ohm resistive load at all frequencies from DC to light and doesn't crash the front end filtering etc with their decidedly not 50 Ohm resistive loads.
 Cheers,Bob

On Sun, Nov 1, 2020, 10:42 AM KENT BRITAIN <wa5vjb at flash.net> wrote:

 Hi Bob
I covered this topic at my Def Con talk on the Care and Feeding of SDR's.
A bit harder to get these days, but Radio Shack and several others sell anFM Band Notch filter   In with the rest of the TV accessories.   Very littleloss outside 80-120 MHz.
These days if you put up a broad band antenna in a urban area, about2/3rds of the RF energy it picks up will be in the FM Broadcast band.(Quite a bit of local variation in that 2/3rd's)
Amazing how many guys though they could eliminate the FM band overloadbut just telling the SDR to ignore 88-108 MHz in the software.
73 Kent WA5VJB



    On Sunday, November 1, 2020, 10:20:36 AM CST, Robert McGwier via Ground-Station <ground-station at lists.openresearch.institute> wrote:  
 
 A single strong FM transmitter anywhere near will collapse the front end.  A FM bandstop filter also degrades 2 meter low end and has insertion loss that harms the sensitivity further. 
RTLSDR is great but you get what you pay for 
Bob

On Sat, Oct 31, 2020, 2:10 PM Phil Karn via Ground-Station <ground-station at lists.openresearch.institute> wrote:


On 10/31/20 11:10 AM, Douglas Quagliana wrote:
> Hi Phil,
>    Please let me know when the code is available. I would like to try
> getting it to run off an RTLSDR dongle and try to have it identify the
> callsigns of the repeaters that it receives. 
> 73,
> Douglas KA2UPW/5
>
In principle I could use the RTL-SDR, but I haven't actually used it
myself yet. It has a rather low dynamic range, so strong local repeaters
could blank weaker signals on other channels within its bandwidth.
Getting the most out it will probably require a lot of work on a good AGC.

My favorite SDR used to be the AMSAT-UK Funcube dongle, but it has a
very limited bandwidth (192 kHz). I'm now using the Airspy R2, which has
a 10 MHz bandwidth and 12-bit sampling. This is enough to easily cover
the entire 2m band. It can cover most of the 440-450 segment, but the
actual coverage of SDR is a little less than 10 MHz because of
anti-alias filtering ahead of the A/D. But it can easily cover the 5 MHz
half of whatever segment used in your local area for repeater outputs
(i.e., either 440-445 or 445-450).

Phil



  
  
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