- Stu WØSTU
Repeater Offsets in 2-meter band (T2A01)
Question pool item T2A01 in the 2018-2022 Technician pool asks: Which of the following is a common repeater frequency offset in the 2 meter band?
A. Plus or minus 5 MHz
B. Plus or minus 600 kHz
C. Plus or minus 500 kHz
D. Plus or minus 1 MHz
Let’s start by defining frequency offset. Remember, a repeater receives your signal on one frequency and retransmits it on a different frequency. This is necessary for simultaneous retransmission – a repeater cannot simultaneous retransmit on the same frequency on which it is also receiving, as this would create a vicious feedback situation. The difference between the repeater’s receive frequency and its transmit frequency is called the offset. So, the offset is a frequency value in units of hertz, kilohertz, or megahertz.
When determining this difference between frequencies, the order matters to ensure proper '+' or '-' offset results. Let’s establish the following convention, in agreement with terminology used in the ARRL Repeater Directory. Let’s call the frequency that the repeater transmits the output frequency, also known as the listen frequency or downlink from the operator's perspective. Let’s call the frequency the repeater receives the input frequency, also referred to as the talk frequency or uplink from the operator's perspective.
To properly compute the offset you must subtract the output frequency from the input frequency, or:
Input – Output = Offset
Alternatively: Talk - Listen = Offset
So, a frequency offset may be a positive or a negative value. If the repeater’s input frequency (talk) is higher than output frequency (listen), the offset is a positive value. If the input frequency is lower than the output frequency, the offset is a negative value.
Most repeaters will operate with a standard offset frequency difference, either positive or negative in value. But be aware that sometimes the standard values are not adhered to, so don’t assume the standard in all repeater cases. Check a repeater directory or the repeater's online information to be sure, along with the specific frequency pair and CTCSS tone requirements for the machine.
The standard offsets are determined by the Amateur Radio Service band on which the repeater is operating. The following are the standard offsets for bands:
70 cm Band +/- 5.0 MHz
1.25m Band +/- 1.6 MHz
2m Band +/- 600 kHz (+/- 0.6 MHz)
10m Band – 100 kHz (-0.1 MHz)
On the 2m band suppose a repeater has an input frequency of 146.250 MHz and an output frequency of 146.850 MHz. The offset is 146.250 – 146.850 = -0.6 MHz, or -600 kHz. This is a negative standard offset.
On the 70cm band, suppose a repeater has an output frequency of 442.600 MHz and uses a standard positive offset. The input frequency is 447.600 MHz. [447.600 – 442.600 = 5.0 MHz]
The answer to Technician question T2A01, “Which of the following is a common repeater frequency offset in the 2 meter band?” is B: plus or minus 600 kHz.
-- Stu WØSTU