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

Topics or items to be added to this page: (if someone else would like to cover a topic please do!)
1) Photos of 6m and 10m cavities and duplexers (anybody have any to contribute?)
2) Photos of a Moto Motrac blanker, a MICOR blanker, a Maxtrac RF board with the blanker components highlighted, a GE MASTR II receiver with and without a blanker, etc.
3) Concerns and tricks in low band cavities (like WN3A's superflex capacitor trick)
4) 6m and 10m duplexer considerations - including generally the narrower the offset the higher the insertion loss.
5) Why there are multiple 6m offsets.
6) Why CTCSS is mandatory on 10m repeaters.
7) Others??? (ideas / suggestions 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 the term "Noise Blanker". The overall technique has been around since the 1950s. On 6 or 10 meters a properly functioning noise blanker can be much more effective than a preamp.

The noise blanker (no matter who makes it) takes advantage of the fact that noise is broadband, and two receivers a MHz or more apart will hear the same noise. So we have the main channel FM receiver and an AM receiver parked on a nearby quiet (hopefully) channel). Both receivers use the same IF frequency. The AM IF's is inverted and injected into the FMs IF at the same level and the noise pulses cancel. All of this happens at the IF frequency, long before the demodulator. At least that's the plan, and usually it works.

Some people say that the Moto Extenders don't work as well as GE's Noise Blanker circuit. Not having had a low band GE repeater 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 experience working on low band GE and Motorola commercial 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 noise blanker design. When he sets one up he parks the noise blanker receiver on the low end of 51 MHz and lets it run.

Motorola 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. I've heard of commercial low band system operators that have actually licensed and coordinated an extra channel and left it idle and unused just to park the blankers in every base and mobile receiver on it. This made sure the blankers heard nothing but noise.

When the noise blanker (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 noise blanker system messes up when the AM front end hears noise that the FM receiver doesn't, or when someone starts talking on the frequency that the AM receiver is tuned to (the noise blanker input).

Most low band noise blanker equipped mobiles have an antenna splitter after the antenna relay so that both receviers can share the antenna. Low band receivers are available in configurations both with and without the blankers. Those that do not have a blanker have a single receive antenna port, just as you would expect. Those that have a blanker can come with two receive antenna ports or one port that feeds an amplified splitter that allows one antenna to feed both receivers. This amplifier is not a preamp, it has just enough gain to make up for the loss in the splitter.

Note that the fact that the noise blanker has to be a certain percentage (of frequency) away from the main channel. 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 the AM receiver will not hear a thing (because the cavity pass window is so narrow), and therefore you effectively have disabled the noise blanker. I've seen a low band repeater with a conventional 6-cavity duplexer with one side feeding the repeater receiver. There was a separate noise antenna with a separate single cavity in the line the noise antenna and the noise blanker antenna input. Adjust the cavity for the least insertion loss that will get the job done as every dB of insertion loss in the noise antenna path lessens the effectiveness of the noise blanker by the same amount. Note that you can get away with a nonresonant antenna for the noise blanker - for example a high band antenna can be used for a 6m recevier. All it is listening for is local noise.

Other than increased receiver to transmitter isolation, this dual antenna requirement is the biggest argument for split site amateur machines on 6 and 10 meters. A split site configuration allows one receive antenna to be used for both the main channel and the noise blanker receivers.

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 that used two antennas on the tower and split the machine. 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 noise blanker recevier 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 0.2µV or better and I've seen a few do 0.11µV or 0.12µV for 20dB quieting (and no, it wasn't a leaky or off-calibration generator). Without a functioning noise blanker the effective sensitivity will likely be in excess of 1µV at most sites just due to the local man-made and atmospheric noise.

Contact Information:

The author can be contacted at: his-callsign // at // repeater-builder // dot // com.



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This page originally posted on 10-Sep-2008


Article text © Copyright 2008 by Mike Morris WA6ILQ
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