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Linked repeaters - signal quality

Some observations on the audio recovered from VK1RGI Mt Ginini with the Wollongong link active.

The audio quality is poor, with generally much worse signal to noise that experienced from the local linked repeaters, and has a prominent not very pure tone at 750Hz.

With modulation

Here is a little wave file that is a sample that you can listen to, and a time domain plot of the contents of that file.

Although this little segment only reaches a peak level of about -9dB, the peak level at other times did reach -6dB. Note that the minimum level is about -16dB, yielding a signal to noise ratio of about 10dB. Observations on the Mt Ginini repeater with local stations is that signal to noise in excess of 30dB is usual for good signals.

Without modulation

The following screen shot is an analysis of signal from VK2WI via the linked repeaters, key down, without voice modulation as heard from 438.375 (Mt Ginini ACT).

The following screen shot is from Black Mountain (ACT) repeater with input signal key down and no modulation.

The following screen shot is from Mt Ginini (ACT) repeater with input signal key down and no modulation. The effect of the CTCSS tone can be readily seen. This radio should have a high pass characteristic with cutoff frequency around 500Hz (since it has CTCSS capability), so the low frequencies will be attenuated somewhat, hence the apparently relatively low amplitude of the 123 subtone compared with its harmonics.

Test setup

All the observations were made off air using a IC-T7A and recorded using a PC sound card connected to the external speaker jack of the radio.

27/8/00

 

 


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