UTARC VHF Noise Floor

You think you've got a high noise floor listening to the radio in your neighborhood? At N5XU, we have lots of noise on the bands. This makes weak signal work, especially at VHF and higher frequencies, very difficult.

We are located in the center of the engineering/sciences quadrant of the largest research university in the country. Our campus is in the urban center of 23rd largest city in the country, one well-known for a proliferation of high-tech companies. Within a 1,000 foot radius of our club station alone, there are probably over 5,000 personal computers, workstations, and servers. We are within one mile of three Amateur Radio repeater sites on four VHF/UHF bands, a cellular/PCS repeater site, several commercial and law enforcement repeaters, and a broadcast television station. We are within three miles of the entire "downtown" area of Austin, including the state capitol and associated law enforcement agencies.

The following images were produced by AF9Y's FFT_DSP program. This software uses a sound card to perform a Fast Fourier Transform on the receiver audio to detect signal sources. It is widely used in the weak signal, and especially EME, community. These images were taken using a Yaesu FT-726R in CW mode, without narrow filters installed, but with variable bandwidth tuning as narrow as possible. We have the GaAsFET preamplifier built into the Mirage 3016 amplifier in line, with about 75 feet of RG-213/U feedline between it and the antenna, which was pointed east at the time. East is our clearest and quietest horizon. This was on the evening of 5 December 1998, a Saturday night when all the pager towers downtown and such are at their quietest. In other words, this is as good as it gets.

These images captures about 45 seconds of audio, from top to bottom. In each, you will see at least a dozen discrete signal sources inside the receiver passband, many of them seriously loud. Want to hear what these sound like? Click on the image to hear the audio.

144.000 MHz