Fw: AO-51 and GRC

Hi Alex & All,

I use the usrp_nbfm_rcv.py in examples to listen on OSCAR.
Here is a sample audio recording I made a few week ago
http://www.poes-weather.com/~patrik/AO-51/ 3.6 mb

Patrik

Hi Patrik,

Thanks for the info. It’s quite good reception you have with the TVRX.
Did you use any low noise amplifier in front of the TVRX?

Cheers

Alex

2009/12/8 Patrik T. [email protected]:

Hi Alex,

I used a 1.2 m long X-yagi and a cheap broadband TV pre-amp
http://www.vekoy.com/product_info.php?cPath=85_402&products_id=12828

EZNEC told me I should get ~14 dB Gain with this gadget, It sure works!
The boom is a PVC pipe and elements are 3.2 mm Al/Ag TIG welding rods, <
$20

An image of the antenna and txt how I made the X-Yagi is at
http://www.poes-weather.com/~patrik/AO-51/

It is tricky to follow the downlink frequency change due to doppler…
Patrik

----- Original Message -----
From: “Alexandru C.” [email protected]
To: [email protected]
Sent: Tuesday, December 08, 2009 12:02
Subject: Re: Fw: [Discuss-gnuradio] AO-51 and GRC

Hi Patrik,

Thanks again for the info. I like your home made yagi antenna for UHF.
I will try to build one in the holidays.

Implementing Doppler correction in GNU Radio receiver can be
relatively easy. I can think of three ideas with increasing complexity
and performance:

  1. Make the filter in front of the FM detector wide enough to cover
    the satellite downlink +/- Doppler shift. I think for voice FM it will
    be okay if you are a few kHz off as long as you get the whole signal
    through the filter.
  2. Active Doppler correction using satellite tracking code. There is a
    python library called pyephem that you can use to calculate satellite
    position, range and range rate, hence Doppler shift:
    http://rhodesmill.org/pyephem/
  3. Track the center frequency of the carrier using the FFT spectrum.
    For voice FM this is easy because of constant amplitude. Even when the
    signal is fading, the peak in the frequency domain will correspond to
    the center frequency as long as you have decent SNR. You can try to
    visualize it using a waterfall sink.

Cheers
Alex

2009/12/8 Patrik T. [email protected]: