Written by Wayne Harrah    Friday, 23 January 2009 18:00    PDF Print E-mail
Skywarn Repeater Basics

Skywarn Storm Spotter N2KNLThere are 4 things that make a repeater work.

It listens to the world on frequency "A". Then, it talks (simultaneously) to the world on frequency "B". Each radio the USER is using knows when its time to listen, that it has to be on the "LISTEN" freq. That radio also knows that when its time to TALK, it automatically changes to the correct "TRANSMIT" freq when you push the PTT.

OK, lets assume you want to be able to TALK on the repeater and give reports as well as receive them. When you program a RECEIVE FREQ into a radio, it ASSUMES (unless you set it differently) that a standard OFFSET will be used. The easy thing to remember is that a "FREQUENCY" shown in a listing (for the purposes of the listings anyway) is the frequency that you will RECEIVE on, such as 146.940 Mhz... That is the freq you would see on your radio display when you tuned to that repeater... With me so far? "OFFSET" (sometimes refered to as a + or - offset), is the difference between the RECEIVE FREQ and the TRANSMIT FREQ. Remember, we are talking about a "REPEATER". It simultaneously retransmits what it HEARS on frequency "A" (such as 146.940) out onto another predetermined frequency "B". The difference is called "OFFSET". Usually its given (in a standard repeater pair) in terms of -600khz or +600khz of the receive freq.

REPEATERS are only allowed to be on certain frequency ranges in each Amateur band. If you aren't on a repeater, youre talking SIMPLEX, where you transmit and then receive on the same frequency, by taking turns.

The radio knows what these repeater "sub-bands" are, and depending upon what freq you're using to LISTEN will determine whether the standard OFFSET (where the corresponding transmit freq is for that receive frequency) will be [above] the RECEIVE FREQ or [below] the RECEIVE FREQ. Transmit freqs ABOVE the receive freq are called "POSITIVE OFFSET", and transmit freqs BELOW the receive freq are called "NEGATIVE OFFSET'. With me? Repeater Tower

Repeater RECEIVE freqs from 145.000 to 146.995 usually (in a standard pair) specify a NEGATIVE OFFSET of -600khz.
Repeater RECEIVE freqs from 147.000 to 147.395 usually (in a standard pair) specify a POSITIVE OFFSET of +600khz.

So, the Shellsburg Iowa repeater 145.230 (the receive freq, remember) is -600khz, or 144.630Mhz. Another example could be the Waterloo Iowa 146.940 repeater, which also has a negative offset of -600khz, or 146.340 for its transmit freq. Now, the Coralville Iowa repeater on 147.150 has a POSITIVE OFFSET of +600khz, which makes the transmit freq (from your radio) of 147.750 Mhz. Make sense?

The transmit frequency is HIGHER THAN the RECEIVE frequency... Thus, they call it a POSITIVE OFFSET. The BEAUTY IS, your radio will ALREADY KNOW THESE STANDARD FREQ PAIRING RULES, so in 99.9% of all cases, all you have to know is what the RECEIVE FREQ is you want to listen to and the offset it automatically stored. When you program each channel, the radio will assume the "STANDARD PAIRING RULES" of OFFSET are correct, and will store those unless you manually change them. You almost never have to do that. There are cases where the offset is not STANDARD, but those have to be programmed specially, and they usually only happen in the largest of cities, like Chicago or New York City. So, If you want to LISTEN to traffic on the Waterloo Iowa 146.940 repeater, you would program into your radio the freq 146.940. That's all you'd need to do IF YOU DIDN'T WANT TO BE ABLE TO TALK, TOO.

Now. Think of a repeater as a shared WAREHOUSE. In order to gain access, and use the warehouse, you might have to have a "key", just like in a lock to the door of a garage or house. A subaudible tone "in the old days called a "PL" tone, standing for "Private Line" tone, is superimposed along with your voice when you key the PTT. The repeater software is set up to ONLY activate when it hears a signal on the correct INPUT frequency (the frequency you are transmitting towards the repeater on) AND (this is important) if the PL tone is ALSO present. It can hear it, but people usually can't. That's why they call it 'subaudible'. So, if the 146.950 repeater has a subaudible tone that has to be present to use it, you have to make sure that when you program your repeaters RECEIVE and OFFSET (although offset is almost always the usual + or - 600 khz), you also have to select the correct subaudible tone frequency that was set up for that repeater. Two repeaters on teh same frequency can sometimes interfere with each other during "skip" conditions... So, installing a PL tone on a repeater cuts down on interference, because repeaters on teh same frequency never have the same PL tone too. I hope this makes sense a little bit better.

RECEIVE FREQ: the freq usually specified in a repeater directory for a given repeater.
OFFSET:the difference between the TRANSMIT FREQ and the RECEIVE FREQ (specified as positive or negative, (usually 600 khz + or -)
TRANSMIT FREQ: RECEIVE FREQ plus (or minus) the OFFSET gets you the TRANSMIT frequency.
PL tone: the subaudible tone used as a "key" to tell the repeater a local (or authorized) user is trying to get access to the repeater.
Good luck! your radio manual should explain this stuff to you, or any HAM RADIO manual on how repeaters work can also help clear it up.


A special thank you to Wayne Harrah for writing this article.

Last Updated ( Saturday, 24 January 2009 13:08 )

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