Cell site interference?


#1

Having recently been bit by the ADS-B bug, I have a working setup at my house. I’d like to add another site at a relative’s house, but I won’t have regular access to it, or much time to test when setup. The location has a Sprint cell tower about 500ft away, firing straight at the house. Sprint uses mostly 1900MHz and 2500-2600MHz, which are high up enough that the USB dongle I’m using shouldn’t be affected. Sprint also has service in the 800-900MHz range, and I’m concerned about that.

My planned setup is a homemade ground-plane antenna (http://www.atouk.com/wordpress/?wpdmdl=397), fed into a HABAmp (http://ava.upuaut.net/store/index.php?route=product/product&product_id=85), feeding a R820T2 USB SDR (http://smile.amazon.com/dp/B00P2UOU72).

I know that the HABAmp has a nice SAW filter in it, but I’m still wondering if I can expect problems. Any opinions are appreciated.

Justin


#2

I’ve you’re running a dongle without any amplifiers, no issue.
I have a cantenna located less than 6 feet away from a 800-900 + 1800mhz + 2300mhz lte tower and the range I get is still 200nm with a high number of position reports.


#3

Try running rtl_power to survey the spectrum and see what you get!

Most of the R820T SDRs drop off before 1800 MHz, so the high end shouldn’t cause you much trouble. FOr the lower end, I use Mini Circuits high pass filters like the SHP-1000, a 1000MHz high pass. It still lets through some energy at 950.

Let us know what you find.

bob k6rtm


#4

An image of my current receiver. WMSA4367 with cell tower.
The antenna is a cantenna, on the left of a picture, sitting on a 8 foot PVC pole

http://s10.postimg.org/loofv9tpx/image.jpg


#5

Don’t know if you’d be able to recover enough energy from the cell site to power your Pi, but it would be close…

Another approach to filtering is using quarter-wave stubs. Goggle “quarter wave stub filter” for details.

Stub filters are simple to make and tune, using your Pi and rtl_power. For each frequency you want to filter out, you need a tee in your feedline, and a stub to tune. Insert the tee in your feedline using the appropriate connectors. Put a connector on one end of a length of the same feedline cable; this will be your stub. Pick a big, nasty, interfering peak from an rtl_power scan. Now use an online calculator to calculate the quarter wavelength of this frequency. Cut your cable to a little longer than that, leaving the cut end electrically open.

Attach the stub to the tee. Re-run the rtl_power scan and see what the stub has done. Trim a little off the end of the stub and repeat – the attenuation from the stub should move up in frequency. Repeat this process until you’ve gotten results you’re happy with, or you’ve gone too far and need to make another stub that’s a bit longer. This is also why if you’re doing stubs for multiple frequencies, you start at the low end and work up…

Let us know you you do!

bob k6rtm