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Using an FM tuner as a Band II relay receiver

Posted: Sun Aug 11, 2019 3:26 pm
by Elopid

A friend of mine is looking to put another transmitter in another location and rebroadcast his existing signal out on to another frequency.

We would like to see if anyone can recommend a good inexpensive Tuner which would be good to rebroadcast the signal without suffering from RF radiation problems from the transmitter being very close to it?


Is there anything we can do to filter out the RF from the transmitter before it goes into the radio but without desensitising the capabilities of the FM tuners receiver?

Many years ago I have successfully used lower power transmitters for relays (5 to 15w) and plugged an FM tuner straight into the transmitter's audio socket but this is going to be for 150 to 300w relays,

So I am very concerned about desensitising the receiver.

All help is very much appreicated.

Re: Using an FM tuner as a Band II relay receiver

Posted: Sun Aug 11, 2019 8:04 pm
by Albert H
It's generally best to distribute to your multiple transmitters completely out of band. I used microwave out of the studio to a nearby high building, then ~520MHz from that nearby building to several sites on other high sites around the city. The output transmitters were tone-switched to allow operation from various locations, sending the "authorities" all over town and never quite getting to the site before the rig went off and another fired up....

In one Eastern European system, we had as many as eleven sites set up!

Re: Using an FM tuner as a Band II relay receiver

Posted: Sun Aug 11, 2019 11:11 pm
by thewisepranker
You may have got lucky with the filters in that particular radio. What the "big boys" probably do with the repeater sites is fit a very, very sharp notch filter (I would imagine a cavity filter) to the incoming signal centred at the TX frequency. You can homebrew notch filters to achieve about 70 dB of rejection with medium difficulty, but they need aligning. I wouldn't enjoy doing this on anything less than a spectrum analyser with a tracking generator and even that could take a while.
It's important to get the poles in the right order on the PCB.
I have built notch filters before to achieve 90 dB rejection using a network analyser but it's not much fun tuning them that far down. Beyond about 5 or 6 poles, adding poles usually worsens rejection, which really does your nut in.

Re: Using an FM tuner as a Band II relay receiver

Posted: Mon Aug 12, 2019 6:19 am
by SamTheDog
We've done this in the past without too many problems. Used a VHF link to the Band II output site, and then used that to feed a number of other TXs.

As above, you may need a decent filter on the RX if you are getting interferance or de-sense fron the TX you are feeding. But in many cases you can get away with it using careful installation methods.

We had the Band II feeding shortwave mediumwave and other Band II gear. Also for short-hops we used band II from the studio at very low power and repeated it from a local tower block. Although this is not a highly advised way of linking, it can work and get you out of (or sometimes into) trouble.....

Re: Using an FM tuner as a Band II relay receiver

Posted: Tue Aug 13, 2019 4:07 am
by Albert H
I've got a BBC "Re-broadcast Receiver" somewhere in storage. It has the tightest front-end tuning I've ever seen! It uses an eight-gang slow motion tuning capacitor with lots of screened coils and the adjacent channel rejection is stellar - these were used on sites at tens or hundreds of kilowatts to receive distant regional transmitters to re-broadcast them in the event of the main PCM link failing. I saw one of these used at Wenvoe to receive Radio 4 from Sutton Coldfield on 92.7 MHz (as I recall about 150km away), and retransmit it on 94.3MHz at 250kW!