More work over the weekend. Saturday attendees at the Bluff were Peter VK5BE, Col VK5DK and John VK5DJ.
Three main tasks:
1) Remove the UHF antennas from the fence ready for the site inspection this week.
2) Install the Mt Eckersley/Mt Gambier Clubrooms linking yagi on the Gambier electronics tower.
3) Install an additional filter in the main receiver cavity line.
So what happened?
1) UHF repeater was turned off and the antenna laid on the ground neatly against the fence, Peter guiding the pole and Col and John bravely holding the end of a rope doing the lowering.
2) Peter brought along a long ladder and has installed the yagi pointing at Mt Eckersley about 7 metres above ground level. He and Col arranged the coax in a neat but temporary fashion. Rather than cut the long length of heliax, a temporary coax line was built out of three shorter pieces with two joiners. A compromise until we know if the yagi is high enough to get through to Mt Eckersley. Once this is confirmed we’ll make the coax one length. Tests show that the link is very strong into Mt Gambier and can be heard off the back of the beam in Millicent.
3) It was suspected that there were strong signals (or noise) overloading the 2M receiver. The cavity shown to people last Friday was retuned at Millicent to 146.3MHz and installed on site. Using a return loss device it was possible to show there was an additional 10dB of sensitivity gained by inserting the additional cavity.
As an aside some of the noise from nearby transmitters can still be heard on signals into The Bluff receiver but this can’t be filtered out for obvious reasons. The overload problem is fixed. Noise is just a feature of the site – there is so much RF on the hill.
Some measurements of the cavity performance at spot frequencies:
Commercial TX on site 70MHz -45dB
Commercial FM radio on site 96.1MHz -41dB
Repeater input 146.3MHz -0.39dB
Pagers 148MHz -18dB
Commercial services 160MHz -41dB
What does 45dB down look like? It might be easier to picture that as the power is down by 31,600 times. That is a great filter!
Even 18dB down is a reduction in power of 63 times.
On Sunday Peter and Col returned to the site as the link output had suddenly dropped off and signals became noisy into the link input of the Mount Repeater. Examination showed that the relay had failed in the link. These relays are 40 years old being ex-FM828. Fortunately there was an unused relay system in the link frame so Peter with judicious use of a soldering iron, various tools and great patience, was able to do a successful swap. Well done Peter. The link is now outputting 8W and all is good back to the Mount.
We hope to have the UHF antenna back up later in the week after the site inspection although a more permanent installation onto the tower is planned.