Repeater Offset

Discussion in 'General Technical Questions and Answers' started by KC5WVL, Nov 28, 2003.

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  1. KC5WVL

    KC5WVL Ham Member QRZ Page

    Can someone tell me what the + or - offset means with repeater frequencies? How will that effect how I program it into my radio?

    Thanks!

    cc
     
  2. wb6bcn

    wb6bcn Ham Member QRZ Page

    Very simply put:
    2 meters has a standard offset of 600KHz,  and 70cm uses 5.0MHz as a standard. On 70cm any receive frequency above 445MHz you will transmit 5.0MHz lower. If the receive is 445.775 your transmit would be 440.775,  or  -5MHZ offset.  If the receive is 444.775 your transmit would be 449.775,  or  +5MHZ offset.
    No offset means you transmit and receive on the same frequency,  also called simplex.  On 2 meters it isn't that simple.  144 to 148MHz the offset changes each MHz.  You can download a two meter bandplan from ARRL.org and other sites to see just how the band is partitioned.

    I also should mention that some repeaters use non standard splits, or in the case of 2 meters they won't be 60KHz.
    There is another item to program in callec CTCSS also known as PL. This a tone below 300Hz that is required to access most repeaters.
     
  3. KC0QNB

    KC0QNB Ham Member QRZ Page

    Even more simply put, you have a 2 meter rig, your local repeater is (as posted on a repeater list website) is 146.850 this is the freq you receive on, but in a standard system you transmit on 146.250.
    Now if the repeater you want to use is over 147.000, and your local repeater is on 147.315 you transmit on 147.915.
    By gentlemans agreement the freqs below 147.000 on a 2 meter system use a - offset, and those above 147.000 use a + offset.
    Will this make a difference in setting up the programming your rig? yes.
    Most modern radios already have the standard 600kHz offset already programmed in, any other offset you will have to manually set.
    73 Ryan
     
  4. N8CPA

    N8CPA Ham Member QRZ Page

    Watch how your display changes as you key your mic. Thats' the frequency you're actually tranmsitting on. As an experiment, while listening to a conversation on a repeater, press the REV if your rig has one. You will be listening to the repeater input frequency. If you are able to hear the station transmitting while in reverse, generally speaking, you are in simplex range.

    That's also a handy feature for DF-ing illegals. Use two rigs. One set for normal, one in Reverse.

    BTW, Welcome to the ham bands! [​IMG]
     
  5. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    Although the "standard" repeater offset is + 5.000 MHz on the 70 cm band, and is -600 KHz on the 2 meter band below 147 MHz and +600 KHz above 147 MHz there are definitely repeaters that use just the opposite offset. In many areas when repeaters are on the terciary split on 2 meters (either 15 KHz or 10 KHz away from the next repeater) the opposite offset is commonly used. For example, if the repeater output is on 146.355 MHz (the "normal" repeater input for 146.940 MHz is 146.340 MHz but terciary splits are often inverted) the input frequency would be on 146.955 MHz. If the repeater output were on 147.775 MHz (the "normal" output of the next repeater channel might be 147.160 MHz with the input at 147.760 MHz) then the input would be 147.175 MHz.

    Also, in some areas, the 70 cm band is inverted and the input frequency is 5.000 MHz below the output. These are not that common, but they do exist. The reason that the +5.000 MHz "standard" existed was because that is the "standard" for the commercial 450 MHz to 470 MHz range. The commercial standard for "T" band (470 MHz to 512 MHz) is +3.000 MHz. In those areas where the -5.000 MHz input was used on the 70 cm band this was done because on the older commercial equipment the receivers sometimes could not be tuned down to frequencies below 445 MHz without considerable modification. But, they would tune in the 449 MHz range without any problems. Since the transmitters would make it down to frequencies below 445 MHz with little, or no modification, the -5.000 MHz input was sometimes used.

    As for the repeater output frequency of 147.000 MHz the input frequencies are about evenly split between +600 KHz and -600KHz. That is, the input may be 146.400 MHz or 147.600 MHz. In fact, from my location on the north side of Dallas, Texas, I can "hit" repeaters with both offsets!

    The original reason for the "+" offset on frequencies above 147 MHz was that when the old Motorola, General Electric, RCA, and other commercial manufacturers equipment was the "norm" on amateur FM, the transmitters could usually be tuned over a 2 MHz wide range but the receivers had a lot of problems even making 1 MHz without too much loss in sensitivity. Thus, the trend was to keep the repeater output frequencies concentrated as near to 147 MHz as possible while allowing the input frequencies to be of the greater frequency range. Thus, the mobile (and base station) transmitters would have to make a larger frequency excursion while keeping the receivers much closer to the center tune-up frequency. Since the repeater only had to worry about a single frequency there were no problems with them.

    With the advent of equipment designed especially for the amateur bands and with the newer commercial equipment that could make larger frequency excursions in the receivers, the need for keeping the receive frequencies so close together no longer was needed.

    Glen, K9STH
     
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