SDR as tool to aim TV antenna

In preparation for installing a UHF TV antenna on my roof, I installed TV viewing software on a laptop computer so that ATSC TV broadcasts can be viewed by means of a USB tuner stick plugged into the computer. However, the viewing software can display only one channel at a time, and the signal strength indicator only displays six bars, not very good resolution. It occurred to me that even though SDR software is not designed to view television, it can display the carriers of several channels simultaneously, with very good resolution of amplitude. Toward that end, I ordered an RTL DVB-T USB stick to use with SDR# software. I expect that viewing the amplitudes of several channels at the same time, perhaps even the entire UHF band, I should be able to optimize the orientation of the antenna to receive the greatest number of channels.

I'm also interested in playing with SDR to monitor VHF police and aircraft communications, so I'll eventually mount a vertically polarized scanner antenna on top of the same mast. I have an RCA outdoor preamp with separate UHF and VHF inputs, so that should work nicely for UHF TV plus VHF communications. I wonder if there might be an issue with impedance matching, as scanners and antennas tend to have 50 ohm impedance, whereas TVs and antennas tend to have 75 ohm impedance. The output of the mast-mounted preamp is 75 ohms, so 75 ohm coax is the obvious choice there. I presume the input of DVB-T dongles is 75 ohms, but correct me if I'm wrong about that. On the inputs to the preamp, I may use 75 ohm cable from the TV antenna and 50 ohm cable from the scanner antenna, but I wonder if mismatch to the scanner antenna will be an issue.

I use a program called SDR# or SDR Sharp to tune radio signals with a DVB dongle. A quick web search will give you several pieces of software to try. I have used SDR# on Windows XP SP3,Windows 7 and Mac OSX 10.7.5.
For antenna cable, I use 50 Ohm coax connected to a Diamond discone antenna. The software in some cases compensates for the 50/75 ohm cable differences. You will have to experiment with you setup and see what works best with your computer, sound card,software and antenna/coax.

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For the dongle and receiving the 75 or 50 ohm answer is "It doesn't matter". Even a regular scanner won't care about the coax impedance. Use coax that matches the feed point impedance of your antenna... don't sweat the receiver end.

SteveBrown said:

It occurred to me that even though SDR software is not designed to view television, it can display the carriers of several channels simultaneously, with very good resolution of amplitude. Toward that end, I ordered an RTL DVB-T USB stick ...

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You might be limited to measuring one channel at a time with the RTL dongle. I think the TV channels are spaced 6 MHz apart and your dongle is limited to 2-3 MHz bandwidth.

This is a good web site for pointing your TV antenna.

TV Fool

The best way to aim a TV antenna at a digital station is to measure signal quality, not signal level. Do any of the USB dongles have parameters to lock on an ATSC TV signal and the ability to read quality in something like BER or Eb/No, Es/No, C/N or ??
prcguy

prcguy said:

The best way to aim a TV antenna at a digital station is to measure signal quality, not signal level. Do any of the USB dongles have parameters to lock on an ATSC TV signal and the ability to read quality in something like BER or Eb/No, Es/No, C/N or ??
prcguy

Click to expand...

An ATSC tuner might, but the "cheap USB TV tuners" we're all using nowadays for SDR purposes were designed for the European DVB-T system and don't work the same way at all; they won't tune ATSC signals in the same manner, oddly enough - I suppose they will tune the signal but won't do anything with it, even with the DVB-T software since it's a different technology altogether is what I mean. I think the OP means using some kind of directional antenna towards the transmitter site as you would with most any other kind of signals.

The TV Fool site is probably better for that kind of directional and aiming info I suppose, these "cheap USB TV tuners" seem to best be used for SDR purposes, but it's up to the OP I suppose. As noted, the limited 2.4 MHz bandwidth of most RTL-based sitcks using the R820T tuner means a lot of hopping around to get signals anyway.

Using a SDR dongel, software and a computer seems like over kill when it comes to point a TV antenna. I have 3 digital TVs in my house and all three have a way to measure the TV signal's quality. Since you will be using a TV to receive the station(s), why not use it to point the antenna? If you have to have a computer in the loop, use google maps to locate the TV transmitter and your home. Plot the direction in degrees and set that on your rotor.

Actually I think using one of the RTL2832 sticks for aiming an antenna even with ATSC is quite useful because of the fact that most SDR programs offer an FFT and waterfall display as a sort of spectrum analyzer. Just having visual feedback can help to troubleshoot a number of issues such as multipath and various others if one knows what to look for. In terms of using one as a scanner they work quite well for just about everything I have thrown at them including P25 digital trunked systems. No experience yet with analog systems but I get the feeling those work quite well too. Only thing is the sticks with the r820t tuner can handle stronger signals a bit better and have a wider range for their RF gain control.