6 meter repeaters

What is the range of a 6 meter repeater?

Frequency range: 30-54 mHz. Frequency separation: 1.5 mHz or more.

Some thoughts on 10m and 6 meter repeaters

10 meter repeaters have a couple of uncommon problems...
First is a 100 KHz offset. Cavities are huge and rare. Several years ago I was asked to retune one... it was three feet in diameter and eight feet tall (and the tuning rod extending from the top was another two feet). And most duplexers need aa minimim of 4 cavities, many times more. This situation usually mandates a split site.
The second is skip. You really need to run a 10 meter repeater in tone squelch.

6 meter repeaters are similar. Cavities are smaller, but still usually 4 to 5 feet tall. Offsets can be 500 KHz, 1 Mhz, sometimes more, depends on the local coordinator and the local band plan. The repeater band is much wider, and mobile antennas rarely cover more than 500 KHz.

Local noise is a problem, and the noise floor at the repeater site can cause serious receivers desense. Rarely do you have a noise free site.

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?

GE uses the term "Noise Blanker" for the circuit in the Mastr II receiver. Motorola used the term "Extender" (and trademarked the name) for their noise blanker. The overall technique has been around since the 1950s. On low band, 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 megahertz or more apart will hear the same noise. A blanker-equipped receiver has both an FM receiver on the main channel and an AM receiver parked on a nearby quiet (hopefully) channel. The AM receiver hears only noise and the detected output is converted to narrow blanking pulses that momentarily short out the FM's IF, thus canceling those noise pulses. All of this happens at the IF frequency, long before the demodulator. At least that's the plan, and usually it works. The design of some radios blank an RF stage rather than an IF stage.

Some people say that Moto's Extender doesn'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 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 6 meter 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 wide-band 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. The plan was that the blankers heard nothing but noise, and it worked until the propagation changed and out-of area signals rolled in.

When the noise blanker (no matter who makes it) is working right, it eliminates a large majority of the noise pulses that are 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).

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 its own feedline), and a pass cavity were reused for the noise blanker receiver channel (all it had to hear was local noise).