|The 6 metre repeater located at Roleystone
is VK6's only 6 metre repeater, past and present. The original
callsign was VK6RSM (six metres). This was back in the days where
repeater had to have a unique callsign even if co-sited with another
Early attempts to build a 6 metre repeater, using a Philips low band 1680, did not work for a number of reasons, and with no outcome in sight, a change of direction was taken to use an Icom IC60.
The repeater was constructed way back during 1982, almost 30 years ago as of 2011 by Will VK6UU.
The original repeater was made from an Icom IC60. The transceiver was fairly easy to separate the receiver from the transmitter, as the IC60 is made up of separate modules.
Icom IC60 10 watt FM transceiver
Inside the IC60 showing the separate easy to separate sections.
Getting a 6 metre repeater licence
Radio Amateurs in Australia were not allowed to install 6 metre repeaters until several years after 2 metre repeaters were allowed, by the licensing authorities. It did take several years for 6 metre repeaters to be allowed in Australia. The first hurdle was the Federal WIA, as they had to support 6 metre repeaters, as part of their policy, before obtaining licencing approval from the P&T. As you can see from the letter below to WARG, from the WIA Executive, it was a bit like a chicken and egg situation....No 6 metre repeaters hence no interest.
Reply to WARG's request for Federal WIA to have as its policy 6 metre repeaters.
Licence application to the DOC for a 6 metre licence.
However once permission was granted WARG was determined to have a 6 metre repeater. As you can see from the date of the first letter to the WIA during September 1977, to the granting of a permit (not a licence) in March 1982, some 5 plus years. Regulation change back then moved very slowly.
Found in the archives, the response from P&T department way back in 1982
Note it was not a licence but a permit as 6m repeaters were not yet allowed.
RX - TX Isolation
The repeater offset was one megahertz, but little was know about the isolation required between receiver and transmitter. After much research and calculations an isolation figure of 70 dB was "guessed" at.
One interesting point about the dB isolation required is that you can not directly related 2 metre isolation figures to another repeater band like 6 metres. Yes there are some similarities but the figures are quite different for a variety of reasons.
The difference between receiver frequency and transmitter frequency in a repeater can be expressed as a percentage.
For example 146 MHz
If you have another frequency 14.6 MHz away from 146 MHz then this is a percentage difference of 10%. At 53 MHz 10% is only 5.3 MHz away.
So if you compare the frequency difference on 2 metres of our repeater off set of 0.6 MHz this equates to 0.4% frequency difference.
On 53 MHz, a one megahertz off set equates to a 2% frequency difference.
What this tells us, is that you need less isolation between the receiver and transmitter on 6 metres than you do on 2 metres. I read some where way back that for every doubling of frequency separation, reduces the required dB isolation by 12 dB. This may or may not be true but does appear about right as there are many factors involved to work out this figure.
As the one megahertz split on 6 metres, is percentage wise, about 5 times greater than the percentage split on 2 metres, this equates to about 30 dB less isolation required on 6 metres as compared to 2 metres....! A lot.
The simplest way to get a repeater up and running is with two aerials, one for receive and one for transmit. However once again the aerial separation figures in dB are different for 6 metres when compared to 2 metres.
40' (13m) vertical separation on 2 metres gives you 60 dB isolation
40' (13m) vertical separation on 6 metres gives you 40 dB isolation
That is 20dB less isolation on 6 metres when compared to 2 metres....!
Why is this so....?
The larger the antenna the greater the transmit and capture area, meaning the larger aerial transmits a greater signal strength and the receive aerial receives a great signal strength. It can be likened to the number of turns in a coil, the greater the number of turns the greater the magnetic field generated and the greater the magnetic field received.
Another way of looking at it is to compare the signal strength received between two radio systems, one on say 30 MHz and one on 120 MHz. If the same aerials are used , say for example dipoles, and the same power levels used, the 30 MHz receiver will receive a lot more signal than the 120 MHz receiver. I think it is 6 dB increase for every halving of the frequency, so the 30 MHz receiver will receive a signal some 12 dB stronger...
So what did the 6 metre repeater require...?
As WARG intended to use a split aerial 6 metre repeater setup, where to put the 6 metre repeater so as to achieve the maximum vertical aerial separation...? Roleystone was the obvious site, due to the larger vertical separation available on the mast (250' 80m high). Tic Hill with the smaller mast of 100' (33m) could only provide about 40' of aerial separation, but Roleystone could provide 2 to 3 times that. However when you do the calculations, this only equates to about 15dB extra dB isolation. Worth having but it was know from the outset that at least two 6 metre cavity filters would be needed, whatever vertical aerial separation could be achieved.
The 6 metre cavity filters were just scaled up versions of the tried and tested 2 metre cavity filters that WARG had constructed 70 or so of.
6 metre cavity filters
The filters were constructed as notch filters, as the dB down 1 MHz away was only about 20dB as band pass cavities, not enough for the 70dB required.
Roleystone could have worked with the cavity filters as band pass, with the 55 dB vertical aerial separation, plus the 20 dB band pass cavity, giving the required greater than 70dB isolation, but using the cavity filters as notch filters may allow the repeater to operate into a single antenna, if better than 70 dB notch depth could be achieved.
After much fiddling with the design, a notch depth better than 70 dB was achieved.
This then allowed either Roleystone or Tic Hill to be used as the 6 metre repeater site. It was decided to go with Tic Hill. Two 6 metre dipoles were constructed and placed on the tower at Tic Hill.
The two 6 metre dipoles at Tic Hill
With the two 6 metre notch cavities in circuit and the two 6 metre dipoles spaced about 40' there was no desensing. However the noise level at Tic Hill is high compared to 2 metres. Large power lines due North of the site were causing a moderate amount of noise.
The repeater never work well from the site due to the level of power line noise and after a a couple of years was moved to Roleystone and operated into a single antenna.
The IC60 6 metre repeater suffered an unidentified interference for a number of years. Often distorted music and voice could be heard triggering the repeater. The interference was noisy and difficult to pin down.
Eventually Tony VK6YAG found the problem. It was mixing in the receiver with a AM broadcast station on 990 KHz....! The 6 metre repeater uses 1 MHz separation and the 990 KHz broadcast station is close enough to 1 MHz. This 1 MHz signal when mixed with the transmitter's 53.8 MHz produces the classic plus and minus mixing products of 54.8 Mhz and 52.8 MHz......The repeaters input frequency. The actual frequency is of course 52.210 MHz, but close enough to hold open the repeater's receiver once the transmitter comes on.
The problem is really a too wide IF bandwidth in the receiver. The IC60 receiver is designed for up to 10 KHz deviation, hence a signal 10 KHz away, as the mixing products between the AM broadcast station and the repeater's transmitter produced, was within the receiver's IF response and hence held open the mute.
The problem was fixed by replacing the receiver with one that had a narrower IF crystal filter.
The re-built 6 metre repeater
The IC60 repeater had worked but with the problem of the interference. Tony VK6YAG did a complete re-build with a Kimberly 101 receiver and an FM828 E band exciter and PA. The basic housing remained as shown below but Tony placed the receiver and exciter in their own RF proof boxes.
The duplexer was also changed from a 2 cavity filter design to a 4 cavity filter design.
VK6RAP 6 metre repeater
VK6RAP 6 metre repeater's FM receiver container
VK6RAP 6 metre receiver container
The new receiver was placed inside a coper screened box, with feed through capacitors for the in outs, and hinged so it could be folded down on top of the control board shown at the bottom left.
VK6RAP 6 metre repeater's FM receiver
VK6RAP 6 metre repeater FM transmitter
VK6RAP 6 metre FM828 exciter
VK6RAP's 6 metre 25 watt power amplifier
The PA is a FM828 E Band mobile which has an output of 25 watts. These PA's are very good but are not intended for extended operation, such as in a repeater. Extra heat sinking was achieved by bolting the PA to the rear of the aluminium housing, and the PA has worked without any problems.
VK6RAP's repeater control & CW ID board...The VK6CU old design.
VK6RAP on the FM test set.
|The new 4 cavity 6 metre duplexer built by
The original cavity duplexer, built by Will VK6UU, only had 2 cavity filters, one in the receiver input and one in the transmitter's output. The duplexer was originally intended to work with a split antenna system and was marginal when used with only one antenna. The 2 cavity filter duplexer managed about 70dB isolation.
Tony built a new duplexer with 4 cavity filters and this provided better than 120dB isolation and improved out of band interference.
The filters are modified low band cavity filters, with new longer outer tubes, and new longer inner plungers. They are 100mm (4") in diameter and 1.7 metres high.
Note the can of coke for scale.
The original 6 metre antenna was a dipole with a coax coil balun. The antenna was changed in 2010 to a folded dipole and place a little further out from the tower, a little over 1/4 of a wavelength out. This should optimise the radiation pattern towards the North.
Original 6 metre dipole
The new 6 metre folded dipole
Coax fed Folded Dipole
This type of dipole antenna is seen everywhere in the commercial World but not used much on Amateur repeater sites. It is a 300 Ohm dipole fed with coax through the inside of the aluminium tube and matched from the 50 Ohms coax to the 300 Ohms dipole by a 1/4 wavelength of 125 Ohm coax. These antennas are DC grounded for lightning protection and offer a wide frequency bandwidth, with a low SWR.
Unbalance to Balance
Interesting point about these antennas is that they do not have an unbalance to balance mechanism from the coax feed to the folded dipole. The effects of not having this is coax radiation, which can distort the radiation pattern. However by placing the coax inside the dipole, this minimises coax radiation and hence distortion of the radiation pattern.