Everyone has a different idea of Lazy Sundays.
For Tim VK2BT, Geoff VK2AVR, Charlie VK2GMT and El Presidente (Carlo) VK2MXC, this past lazy Sunday was an opportunity to chase some interference problems. Our VHF repeaters (analog and DSTAR) have had an issue recently – when both are keyed there is audible “splatter” on the analog tail (it sounds like somebody blowing a raspberry, how rude!). A decent incoming signal will swamp this and give clear audio but the repeater tail will have raspberry on it. On DSTAR this manifests itself with a garbled “R2D2” audio if your signal is not particularly strong and one presumes on analog that if you’re a weak signal you will struggle as well.
Using a spectrum analyser we characterised the problem. Because of the broad nature of the noise it seemed like an intermodulation problem. This happens when the radio signals mix in something non-linear – anything from another radio transmitter to a rusty bolt on a tower (the metallic junction can act as a diode and produce non-linear rectification products). By isolating our equipment in the hut one by one we searched for the cause. Nothing we disconnected made any difference. Our repeaters are also well filtered, so we didn’t suspect a problem there. We proved that to be the case by connecting their combined output to a dummy load instead of the antenna. Blissful silence and no raspberries. Not us. The problem remained even when we used our secondary VHF antenna on the tower, both primary and secondary are in good condition and swapping made no difference so it’s unlikely to be an antenna issue. Our attention then turned to the tower. The signals must be radiating, mixing with something else to cause the noise and then the noise is being received on our antenna again. Climbing it at such short notice was out of the question, but we had an idea. What if we pointed a directional antenna towards it and sniffed around, could we see the distortion still?
After a quick trip to the club rooms, we arrived back on site with what we thought was overkill – a 9 element loop fed Yagi constructed by Chris VK2YY. This was affixed to a temporary mast and connected to the spectrum analyser. After receiving some funny looks from passersby we tried to detect the interference. As it turns out, we could just barely make out a difference on the analyser screen when the interference was present and the Yagi pointed directly at the top of the tower. When the Yagi was swung away from the tower we couldn’t see any spurious signal. Given the signal was so low, we could barely see it on the spectrum analyser screen this time, the radiated power of the interference is very small but enough to upset the sensitive receivers in our repeaters.
The cause? It could be a rusty bolt, it could be some other radio equipment mixing and re-radiating distortion products. It is a shared site after all. Our next step is to contact some of the other site users and see if we can find out more about their radios.