VK6RAP-5 Roleystone

VK6RAP
146.1 - 146.7

The FM880 Repeater

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VK6RAP 2M Roleystone Perth's main voice repeater

WARG obtained 12 Philips FM880s, a remote area two way radio base station used as a telephone link from a remote area customer to the nearest telephone access point. These rack mounted radios are a FM828 radio placed in a rack mounting box with the addition of lots of telephone interface circuitry.

The decision was made to use these FM880s and rebuild WARG's 2M repeater network. The coper box repeater had proved successful but were difficult to make. The FM880 offered to standadise WARG's 2M repeater network on one design. The intention was to design a repeater that could be swapped out if there was a fault with a given repeater. Rather than work on a faulty repeater on site that proved difficult to fix, swap it over and then repair the faulty repeater back home on the bench.

The FM880 had to have all the telephone circuit boards and interface removed. What was left was a

VHF receiver in a shielded box
VHF 80mW exciter in a shielded box
Power supply regulators and switching
VHF 25W transmitter in a shielded box

There was considerable space on the top and the bottom of the FM880 when all the telephone bits were removed. This allowed additions to be made that would enhance the repeater.

Much had been learnt over the the past 15 years with the development of the various versions of VK6RAP, so it was decided to add the following enhancements to the mark 5 VK6RAP.

True FM modulation to the transmitter for high quality audio.
Audio AGC to help level out the wide audio levels from users.
Six input active audio mixer for the various audios..eg Morse ID.
Sine wave CW ident...(sounds better)
CTCSS encode on the repeater's transmitter
Low voltage (12V) detection and warning tone on TX tail.
Link interface for two link in/out transceivers
DTMF remote control of the repeater.
DTMF remote control for users so various user functions could be added.
Time out reset beep.
Second mute to reduce flutter mute cutout on weak mobile stations.

Work began with all the telephone interface boards being removed. The various boards were made up on a type of vero board that have all the copper solder points as isolated pads rather than joined in strips. It required joining with solder bridges to make connections rather than drill out copper pads and proved to have less problems than missing pads to drill out.

Printed circuit boards were not made up as the design of these individual boards was under development, with improvements being made as repeaters became available. If these circuits were to be used in future, printed circuit boards would be the way to go as many people could be involved in construction.

There were a total of 6 boards required for each repeater and as a total of 7 repeaters were constructed it required 42 vero boards had to be made up.

The boards were...

CW ID filter and Low voltage (12V) detector.
Low voltage tone oscillator and time out reset pip logic.
Speaker amplifier and weak signal mute.
Audio automatic gain control and audio pre-emphasis.
6 channel active audio mixer.
123Hz CTCSS tone encoder.

VK6RAP 2M has some extra facilities and extra circuit boards have been added to provide these extra functions...

DTMF inhibit board to prevent DTMF being transmitted through VK6RAP.
CTCSS decoder for engineering function.
Mains failure detector and audio warning on repeater tail of mains failure.
User DTMF access for dedicated functions...eg linking.

One extra board (green) is on the top layout and it is the existing 10V low drop out regulator, transmitter switching logic, SWR protection and power output control that comes with the FM880 and was retained.

VK6RAP with top and bottom covers off

VK6RAP 2M Top view

VK6RAP 2M Bottom view

Pagers at Roleystone

During the mid 1990s the Roleystone site had two pagers installed. The pagers were connected to 2 dipoles, vertically displaced on the mast, at about the 200' (65M) position on the tower. The dipoles were less than a metre apart and this was an obvious source of problems, as each pager feeds power back into the others transmitter. From memory I don't think that the pagers were fitted with isolators which reduce the cross coupling between transmitters.

These pagers caused considerable problems for VK6RAP's receiver on 146.100 MHz. Attempts to fix the interference from the pagers to VK6RAP included adding two band pass cavity filters and a front end crystal filter, but the interference continued. CTCSS was even considered to be added to VK6RAP, as the repeater was so badly effected.

The additions of the front end filtering indicated that the interference was not with VK6RAP's receiver but issues with the pagers them self.

From memory the pager company was contacted and the antennas moved further apart and isolators fitted. The problem went away. All very time consuming.

146.1 MHz Crystal Filter

The Roleystone 2 metre repeater VK6RAP has a front end crystal filter combined with a low gain pre-amp to improve the sensitivity of the FM880 and provide added RF selectivity.

Front panel of low gain pre-amp & crystal filter.

Front end RF crystal filter used in VK6RAP 2 metres.

The crystal filter is at the 2 metre frequency of 146.1 MHz and has a loss of 4dB. The selectivity is amazingly sharp being only plus and minus 7KHz at the 3dB down points. All other frequencies are at least 20dB down. A frequency only 15KHz away is 20dB down.

The pre-amp was added to overcome the 4dB loss of the crystal filter and is placed between the aerial and the crystal filter. Adding a pre-amp to a repeater's receiver has to be done with caution as the extra gain can cause intermodulation problems. The particular pre-amp has a gain of about 8dB.

Low gain (8dB) RF pre-amp