FM 828 repeater information

[VK2ZIP]

Hi all,

I'm interested in information about conversion of Philips FM 828 radios for repeater use, with particular reference to

1. Getting optimum receiver sensitivity from U band radios at 438-439 MHz, and
2. optimum audio routing from RX to TX, especially when using CTCSS tones.

I think I once heard that the (or a) SA repeater group had produced a document about this, but I don't know whether that involved FM 828 radios

I would be grateful for any leads, and I will be happy to pay costs.

My main interest is UHF RX sensitivity.

Best wishes,

Lew.

[VK4ADC]

Does this info fit at least some of your requirements ???

http://www.qsl.net/zl1hk/Philips_FM828U_...tions.html

and/or

https://groups.google.com/forum/#!topic/...Fhtn12awQg

[VK2ZIP]

Hi Doug,

Thankyou for the two very helpful links. I have studied both. Assuming that Philips knew what they were doing when they developed a kit for convering the FM828 into a TTR, and I'm sure they did, the first link would indeed give me guidance on the best way to route the audio from RX to TX. I will probably need to look at the instructions with a circuit diagram and a radio in front of me in order to understand it fully.

The first link does not really address the RX tuning issue, but the second does. Indeed, the first link suggests that U-band radios were designed for 440-470 MHz, from which one might infer that there would not be too much difficulty in tuning for 438-440 MHz repeater use, noting of course that the input frequency is 5 MHz lower. Information in the second link indicates that U-band radios were designed for 450-470 MHz, and they need an extra half-turn turn added to the front-end helical filters, which accords much more with our experience building a repeater in Sydney. However, when we add a half turn to the helical filters, although they all peak quite convincingly at 434-435 MHz, we do not get the sensitivity that we would expect.

On the basis of this, I think there is another issue, and I will address that in another post under the heading of FM, Repeaters, D-STAR when I get time.

In the meantime, I wonder if there is anyone else out there who has anything to contribute on this subject? Does anyone know of a document produced by a VK5 group?

Best wishes,

Lew.

[VK4ADC]

Information in the second link indicates that U-band radios were designed for 450-470 MHz, and they need an extra half-turn turn added to the front-end helical filters, which accords much more with our experience building a repeater in Sydney. However, when we add a half turn to the helical filters, although they all peak quite convincingly at 434-435 MHz, we do not get the sensitivity that we would expect.

Lew.

Lew,

The extra half turn may have moved the frequency down but also altered the LC ratio and characteristic impedance away from the optimum design value/criteria for its environment/cavity dimensions and the coupling to either the next resonator or the in/out active device/s or terminations. It is quite some time since I looked at a UHF series FM828 so cannot recall the physical arrangements for the helical sections.

Is it possible to compress the helical resonator coil assembly slightly to increase the (1) inductance and (2) the turn-to-turn capacitance, thus lowering the frequency in lieu of the extra 1/2 turn ???
(Of course this is not possible on grooved formers....)

Also remember that receiver sensitivity is not the overriding factor in building repeater systems, effectively the duplexer separation is. A super-sensitive receiver is useless if it is being de-sensed by its own transmitter. The (common path) antenna gain can raise the effective receiver sensitivity provided the losses in the duplexer plus cables do not exceed the raw antenna gain figure. More importantly, measure the Tx-Rx port-port loss required in the duplexer to prevent de-sense and make sure your duplexer characteristic exceeds that value.

Keep in mind that it is sometimes more beneficial to run two FM828s with receiver in one active, transmitter in the other case - that provides lower RF coupling & hence better RF noise separation.

Hope my comments help

73 Doug VK4ADC

[VK5PJ]

the West Aust Repeater Group has done a heap of work with the FM-880 / FM-828 series of the years, they have moved on to new hardware but Will, vk6uu has an archive of information here

<!-- m --><a class="postlink" href="http://www.vk6uu.id.au/fm880.html">http://www.vk6uu.id.au/fm880.html</a><!-- m -->

not sure if the main WARG site has much: <!-- m --><a class="postlink" href="http://www.warg.org.au/wordpress/">http://www.warg.org.au/wordpress/</a><!-- m -->

Peter, vk5pj

[VK2ZIP]

Hi Doug and Peter,

Thankyou both for your very helpful replies.
I will answer in order.

Doug,

I take your point about how an addition (or subtraction) of turns at the top of the helical filters could, indeed almost certainly will, change the characteristic impedance of the resonator, and the coupling in/out, and this in turn may no longer be optimum. Your idea of compressing the winding to increase both L and C, to lower the resonant frequency, is a good one. I think that would probably also change the characteristic impedance but it might have less effect on the coupling, because it does not change the ratio of turns above and below the tapping point. (Actually there are taps only at the input and output - the inter-stage coupling is done via slots between the cavity sections.) Alas, Doug, your memory is good. The winding is indeed on a grooved former!

By the way, I should probably mention here that I was mistaken in an earlier post when I said that the google-groups article (Andrew, VK4CRT)suggested adding 1/2 a turn to the helical filters. It suggested that for W1 band radios (470-500 MHz), but NOT for T Band (403-420 MHz) or U Band (450-470 MHz) radios. However our experience is that 1/2 turn is needed for U Band radios, but resulting sensitivity still leaves a lot to be desired.

I agree that receiver sensitivity is only one part of the consideration in repeater design. Receiver sensitivity, transmitter power and cleanliness, diplexer isolation, and rejection of nearby signals are all strongly interacting factors. However, receiver sensitivity relative to transmitter power is very important, to ensure good performance with hand-held transceivers, which will generally have higher sensitivity than the repeater, and much lower power. So, ideally I would like to start by saying what sensitivity and power I want, and then engineer the isolation, whether by separation of antennas, filtering (diplexer) or separation/shielding of the receiver from the transmitter or indeed some combination of these. I generally leave the antenna gain out of this consideration, because it works equally for receive and transmit (i.e. capture area and gain increase in proportion to each other). (There are considerations of the maximum desirable gain of course, in relation to structural issues, lightning, and site location.)

Having said all that, I would also say that I would not go for a ridiculously high receiver sensitivity, and I would not risk putting a low-noise preamp in front of a repeater receiver. I think that would be asking for trouble. But I have run W1 FM828s in commercial service for many years (around 475 -480 MHz) and I have found them to be very happy to work with full sensitivity and 15 watts output, all in the one radio, and without detectable de-sensing. (I did have a very good diplexer with band-pass filtering.) I think the standard FM828 sensitivity specification (0.5 dB for 20 dB of noise quieting or 0.3 dB for 12 dB SINAD) is a good compromise - not too much to cause problems, but enough to perform reasonably well when accessed by hand-held radios. However, I think your point about separate transmitters and receivers is very relevant for UHF CB repeaters, where the offset is only 750 kHz, rather than the 5 MHz which we use for amateur repeaters.

------------------------------------

Peter,

Thanks for the two links. I had forgotten about the great series of articles by Will McGhie in AR /Repeater Link, and I will also revisit those. They give almost "too much information" about audio routing, so I think now have that covered. I'm not sure whether I missed something, but my impression is that there is not anything to help me on tuning of UHF FM-828 receivers on the WARG web-site.

My next step will be do some measurements on un-modified FM-828s and I may eventually report on those in a new post under FM, Repeaters, D-STAR. So far I have acquired a few W-Band units and a T-Band unit. I still need to acquire an unmodified U-Band radio. I have been waiting to have enough time for this project for about 9 years. I still don't have time, but I have started thinking about it now, and I don't seem to know how to stop! But it may take a while...

Best wishes,

Lew.

[VK2ZIW]

Can I ask why one would use the 1980 vintage 828 series in 2017?

These are not synthesised either receiver or transmitter.
So, expensive crystals will have to be bought. Far outweighing the cost of
two much better mobiles such as the PRM80s.
Also, the Rx IF bandwidth and oscillator temperature drift will be a problem.

There were plenty of suitable mobiles at the CCARC Wyong field day swap-meet on 26/2/17.

Alan VK2ZIW