Depends on what type of signal. If it was an omni-directional signal, the inverse square law will affect it a great deal and basically even near Pluto most natural TV and radio signals are undetectable. This table will clearly show that (since I doubt formatting will work, go to the link:
http://www.faqs.org/faqs/astronomy/faq/part6/section-12.html)
Table 1 Detection ranges of various EM emissions from Earth and the
Pioneer spacecraft assuming a 305 meter diameter circular
aperture receive antenna, similar to the Arecibo radio
telescope. Assuming snr = 25, twp = Br * Tr = 1,
r =
0.5, and dr = 305 meters.
-------------+--------------+-----------+--------+--------+-----------+
Source | Frequency | Bandwidth | Tsys | EIRP | Detection |
| Range | (Br) |(Kelvin)| | Range (R) |
-------------+--------------+-----------+--------+--------+-----------+
AM Radio | 530-1605 kHz | 10 kHz | 68E6 | 100 KW | 0.007 AU |
-------------+--------------+-----------+--------+--------+-----------+
FM Radio | 88-108 MHz | 150 kHz | 430 | 5 MW | 5.4 AU |
-------------+--------------+-----------+--------+--------+-----------+
UHF TV | 470-806 MHz | 6 MHz | 50 ? | 5 MW | 2.5 AU |
Picture | | | | | |
-------------+--------------+-----------+--------+--------+-----------+
UHF TV | 470-806 MHz | 0.1 Hz | 50 ? | 5 MW | 0.3 LY |
Carrier | | | | | |
-------------+--------------+-----------+--------+--------+-----------+
WSR-88D | 2.8 GHz | 0.63 MHz | 40 | 32 GW | 0.01 LY |
Weather Radar| | | | | |
-------------+--------------+-----------+--------+--------+-----------+
Arecibo | 2.380 GHz | 0.1 Hz | 40 | 22 TW | 720 LY |
S-Band (CW) | | | | | |
-------------+--------------+-----------+--------+--------+-----------+
Arecibo | 2.380 GHz | 0.1 Hz | 40 | 1 TW | 150 LY |
S-Band (CW) | | | | | |
-------------+--------------+-----------+--------+--------+-----------+
Arecibo | 2.380 GHz | 0.1 Hz | 40 | 1 GW | 5 LY |
S-Band (CW) | | | | | |
-------------+--------------+-----------+--------+--------+-----------+
Pioneer 10 | 2.295 GHz | 1.0 Hz | 40 | 1.6 kW | 120 AU |
Carrier | | | | | |
-------------+--------------+-----------+--------+--------+-----------+
Note that the frequency is very important to how far the signal can go, as well as how tightly focused the beam is.
So in answer to your question, most of our radio transmissions are only detectable out to .3 light years with the Aericibo telescope.
Now, directed signals don't fall off as quickly as omni-directional signals, bet even they are not immune to physics, and are difficult to detect as well.
