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Some thoughts on 10m and 6 meter repeaters By Mike Morris WA6ILQ

Note that the contents of this page, like most here at www.repeater-builder.com, are totally dependent on donations of information. If you have a hint, or a useful trick, please consider wiring it up and sending it in.

This is a work in progress. Contributions are welcome.

An email to the repeater-builder asked:
Just what is a Noise Blanker and why do I need one on my 6 meter repeater? I've been told that my repeater will be worthless without it?

Note that the "Extender" is Moto's marketing (and trademarked) name for an IF range noise blanker. GE Marketing just used "Noise Blanker". The overall technique has been around since the 1950s.

The noise blanker (no matter who makes it) is an AM receiver (whose front end is parked on a (hopefully) quiet channel) whose IF is the same frequency as the main (FM) receiver IF. The AM IF's is inverted and injected into the FMs IF and the noise pulses cancel. At least that's the plan, and usually it works. So the noise pulses are cancelled at the IF frequency, long before they are demodulated.

Some people say that the Moto Extenders don't work as well as GE's Noise Blanker circuit. Not having had a GE to play with, I can't speak to that as I don't have first hand experience. One person's whose opinion I respect has over 15 years of working on lowband GE and Motorola gear and he says that he'll take a Mastr II over a Micor any day as a 6m repeater receiver just because of the NB design. He parks the noise blanker on the 6 meter receivers on 51mhz and lets them run.

When the NB (no matter who makes it) is working right it eliminates a large majority of the RF noise hash that is so prevalent on low band channels. A non-working noise blanker can literally make a low band system unusable. The system messes up when the AM front end hears noise that the FM doesn't, or when someone starts talking on the frequency that the AM receiver is tuned to (the NB input). I've heard of commercial low band systems that have actually licensed and coordinated an extra channel for the blanker to be set to and left it idle and unused just to make sure the blanker heard nothing but noise.

Moto recommends that the extender (the AM receiver) sampling frequency needs to be two to three Mhz away from the desired frequency and on an unassigned channel to guarantee that all that it picks up is wideband noise.

Most extender-equipped mobiles have an antenna splitter after the antenna relay so that both the primary receiver (FM) and the extender receiver (AM) hear the same thing. Low band repeaters come in single-receive antenna (without the noise blanker) configurations, and some have an separate antenna input just for the noise blanker.

This means that if you put a duplexer, or even a single pass cavity tuned to the main receive channel in front of the splitter then the AM receiver will not hear a thing (because the pass window is so narrow) so therefore you effectively have disabled the extender.

Other than increased receiver to transmitter isolation, this is the biggest argument for split site amateur machines on 6 and 10 meters. There are two relevant articles located on the Antenna Systems page at this web site, one on receiver-to-transmitter isolation, the other on horizontal vs. vertical antenna separation.

I've seen one 6m repeater where they took a single-sited machine and split it. The transmitter ended up a mile away with a low power 420 MHz cross-link with beam antennas on each end. The old transmit antenna (on it's own feedline), and a pass cavity were reused for the NB channel (all it had to hear was local noise).

BTW in most cases you DON'T need a preamp on a 6m FM receiver!!! Most already hear .2µV or so and I've seen a few do .11µV or .12µV for 20db quieting (and no, it wasn't a leaky or off-calibration generator). The effective sensitivity will likely be in excess of 1µV at most sites just due to the atmospheric noise.