Mt Herbert Repeater Trip

(November 4th 2006)

Purpose
To replace the dipole antenna with the collinear that was removed in March due to noise problems.
To replace the old feeder with a new length of LDF4-50 foam heliax cable.

Summary
The expedition team consisted of Mike ZL3TMB, Barrie ZL3TNQ, Rory ZL3HB, Rob ZL3RX, Alastair ZL3ARC, Trevor ZL3ADZ, and Greg ZL3IX. Thanks to Mike, Rory and Barrie for transport. Thanks to Jeremy ZL3RT for testing at Tait Electronics once the work was complete.

Thanks to Dudley ZL3GG for making protection boards for the solar panels before the trip.

Both the objectives listed above were successfully completed. The repeater now opens with a signal of -10 dBm transmitted into a dipole antenna located at the Tait premises. This figure compares with a figure of +2 dBm that was measured just prior to the expedition.

Some of the team also managed to start the old generator stored at the site, so this was left at the top rather than being returned for maintenance, as had been the plan.

Work Carried Out.
The expedition arrived at the site at 09h30, and was divided into two teams. Greg and Barrie did the antenna work on the pole, while the others concentrated on installing the new feeder.

First the old scaffold pipes, chain clamps, and support bracket were removed from the pole. This was necessary so that the scaffold pipes could be replaced with non-conducting fibreglass tubes. Greg's hypothesis was that the conducting loop consisting of the scaffold pipes, chain clamps and mounting bracket, was responsible for the noise problems previously experienced with the collinear antenna in place. (see previous report).

Once the hardware was on the ground, the scaffold pipes were removed from the chain clamps. This was not an easy exercise, as the bolts were severely rusted, and had to be freed with WD40. One of the bolts sheared off and had to be replaced. Unfortunately they could not all be replaced, as we didn't take enough spares.

There was also a little consternation when we found that the new fibreglass support tubes were more like 55 mm in diameter than 51mm. This necessitated the U bolts having to be opened out, and the 6mm stainless steel mounting plate re-drilled. Fortunately the party had taken a man-sized drill up the mountain, and with considerable effort by Rory, the new holes were drilled.

The fibreglass tubes were assembled into the chain clamps and attached to the mounting plate, and the whole assembly was hoisted back up the pole. The chain clamps were then re-attached in their original positions. The antenna was hoisted up the pole, and the top support re-attached to its bracket.

It was then realised that the antenna was not in the best position relative to the solar panels. Any melting ice would fall right on to the panels, likely causing damage. The top support of the collinear was therefore removed from its bracket, and re-inserted the other way round. The chain clamps were rotated around the mast at the lower end. (No mean feat as they are very heavy and tend to bind into the wooden pole) This manoeuvre resulted in the antenna being placed such that any falling ice would miss the panels.

Down on the ground, the cable team determined that routing the new feeder right into the main building would not be as difficult as had been thought. It was simply necessary to dig out the anti-vermin foam in the access hole in the floor. This was carried out, and the new cable inserted from the inside of the building, and chased through to the outside inspection chamber. A trench was dug around the back of the old concrete building, and out to the base of the antenna pole. A length of alkathene tube that had been purchased for the job, was laid in the trench.

The new cable was inserted into the tube using a cable chaser, along with a length of nylon cord for the easy insertion of any future cables. Once through the tube, the free cable end was terminated with a special Andrews N-type connector. This latter operation was carried out with much discussion and debate, as the Andrews instructions were not at all clear.

The terminated end was hauled up the pole, connected to the base of the antenna, and tied to the antenna support. The joint was wrapped with two layers of self-amalgamating tape, and sprayed with two coats of Plasti-Dip. A piece of 60mm PVC tube was placed over the base of the antenna to be used as a drip ring, and sealed with silicone. All clamps were checked for tightness.

The building end of the cable was terminated with another Andrews connector, and the cable entry hole re-filled with anti-vermin foam.

Testing.
Testing was carried out by Jeremy ZL3RT. He used an HP comms test test connected to a test dipole, back at the Tait factory. The test consisted of putting the test set into duplex mode, and transmitting a weak signal via the antenna. The opening and closing point of the repeater squelch was noted, along with the strength of the signal received back at Tait. Jeremy found that the repeater squelch would open with a -10 dBm transmitted signal, and close at -12 to -13 dBm. This is about 12 dB better than had been found the previous week before the work was carried out. There were no signs of any instability of the gate opening level, nor of noise cross-coupling from transmitter to receiver.

The level of the received signal was measured at -71 dBm. This result is slightly anomalous, and needs to be investigated. The forward and reflected power at the repeater were also noted at +47 dBm, and + 30 dBm, indicating a good match to the antenna. The insertion loss of the duplexer was measured at 2 dB.

The expedition left the site at 17h00 after a most successful day.

Future Work.
On arriving at the site, it was seen that the UHF yagi used for the engineering link, had broken away from its mounting, and fallen to the ground. This will need to be replaced.
The new feeder needs to be secured with black UV resistant cable ties. Only clear ties were available.
M8 and M10 lock nuts or, alternatively nyloc nuts, need to be installed on the antenna mountings.
The power cable from the solar panels to the shack, is looking the worse for wear, and needs replacing. There should be plenty of space in the new alkathene tube for this.