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In the US, the common (though not universal) repeater offsets are ±600 kHz in the 2m band and ±5 MHz in the 70cm band. How and why did these offsets come to be standard for amateur repeaters? The minimum simplex channel spacing is much less than the repeater offset convention.

As an example, spacing between FRS channels and GMRS channels, both near the 70cm band, is 25 kHz. Is there a reason why a smaller interval is undesirable, or a larger interval is advantageous?

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Repeaters need to receive and transmit at the same time. The offset needs to be large enough to make it possible for the filters to separate the transmit and receive frequencies.

In my opinion 600 kHz is a very narrow spacing, it results in filters that are enormous, usually six cavities each 150 x 500 mm. Even so, they have several dB of loss in both bands.

5 MHz is much more reasonable, the filters can be made much smaller by helical loading of the coaxial elements inside. This lowers the Q somewhat, but with the larger spacing, it's much easier. A filter for a 5 MHz spacing might be 1/2 the length and diameter, and probably with less loss.

Reducing the offset below 600 kHz would be very difficult, the filters would be unstable over temperature changes, and the losses would be enormous.

Now the reason - I suspect - 600 kHz was chosen is because the amateur band is only 2 MHz wide in many countries. 600 allows some repeaters and some simplex traffic, space, etc. Possibly the lower parts of the band (144.0 - 144.5) were already allocated for CW, SSB and so on, so when FM came along, the repeaters needed to squeeze into the top 1.2 MHz.

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  • $\begingroup$ I've corrected my question: I was referring to the 125 kHz spacing between (simplex) FRS and GMRS channels. It appears that GMRS repeaters have a +5 MHz input offset. $\endgroup$
    – ESV
    Commented Jul 31, 2015 at 17:45
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    $\begingroup$ @esv ok. Simplex channels are 12.5 kHz apart for most FM radio. Confusingly though, Wikipedia says FRS is 12.5 kHz but lists the numbered channels at 25 kHz apart, on the odd multiples of 12.5 kHz. $\endgroup$
    – tomnexus
    Commented Jul 31, 2015 at 18:31
  • $\begingroup$ You're right. It's 25 kHz, not 125 kHz between FRS and GMRS channels. $\endgroup$
    – ESV
    Commented Jul 31, 2015 at 19:53
  • $\begingroup$ @tomnexus What about a repeater with 0 offset? What is the result? repeaterbook.com/repeaters/details.php?state_id=53&ID=7958 $\endgroup$
    – Zach B
    Commented Feb 23, 2018 at 20:49
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Was going to comment, but don't have enough rep on the ham site yet. This discussion offers a few tidbits regarding history/reasoning behind the 600kHz split and other oddities of the 2m band: https://forums.qrz.com/index.php?threads/history-of-vhf-repeater-splits.26321/

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Looking at commercial frequencies near the 2-meter band, there is no specified offset. It's all over the place. Since the use of 2-meters, 70-cm and other bands have come online and their offsets have been more orderly and match better between ham and commercial.

Ham is actually more organized than commercial in this regard. I think we have just learned more about channel spacing since the first 2-meter repeaters were built.

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