ELT Practice Frequency Change – National

CASARA National has been advised by DND to change the ELT practice homing frequency to 121.775. There are already some provinces using this frequency, however it has recently been approved by the Government of Canada Technology Branch (replacement for Industry Canada) for national use.

In BC we currently use 123.1 for our training ELT’s however, the reality is that this frequency is an operational SAR frequency for Snake Operations with 442 Sqn. There are two operational frequencies; Snake 1 ( 123.1 MHz ) and Snake 2 ( 135.9 MHz ). 123.1 was decided upon sometime in the 80’s as it is closer to 121.5 which left 135.9 (Snake 2 ) for tactical comms for PEP Air/CASARA aircraft normally for training tasks.

Snake 2 frequency of 135.9 is way at the top end of the aviation spectrum, and the efficiency of transmitters and antennas are least efficient at the end of the performance scale.

Aircraft antennas are normally tuned to center band around 121.5 as this is the emergency frequency, and most quarter wave antennas have a lower “Q” (wider bandwidth) but even still, by the time you get up to 135.9, you are usually outside the best performance range of the antennas, ant there is a lot of return loss… especially on the transmit side.

Moving the ELT training frequency to 121.775 came with a bit of push back stating that it’s “too close” to 121.5.
Actually it is 275 kHz away from 121.5 and an airport emergency receiver on 121.5 has a receiving bandwidth of maximum of 6 KHz. ( 3 KHz on either side of 121.5 ) which shouldn’t be an issue at all.

Just for a test, I sent a modulated ELT signal on 121.775 into a 100 watt amplifier into a gain base antenna, less than half a kilometre from the Pitt Meadows Tower, and they opened the squelch on thier emergency receiver, and heard nothing. We did some additional intermod tests to see if there were any harmonics, and nothing came through at all.

This proves that with clean setups, technically there shouldn’t be any issues, however, the unknown is the integrity of the vintage and somewhat geriatric ELT transmitters we will be updating may create problems when close to aircraft or other towers, however, the low power outputs should resolve any problems if the battery is charged and the antenna is good.
ALWAYS check with the tower that you are not interfering with anything if you are close to one, and always let JRCC know that you have one operational.

By freeing up 123.1 for use in training and operations, I believe we’ll see a much better communication performance as we’ll be operating well within the design specifications of our air radio equipment. 135.9 can then be used a the “backup” frequency, but it has always had it’s limitations in basic spectrum design.

We will be calling for all Pointer ELT’s and the Little Helper receivers to be returned to PEP Air (Zone Commanders will be advised of where to send them) for upgrading to the new frequency so we can have them all converted before the good spring weather arrives.

Fred Carey
Deputy Director (BC)

Share this post: on Twitter on Facebook