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Narrow Band FM bandwidth and channel spacing
What is the bandwidth of standard narrow band FM as we use it on 2 meters?
Is there a rule of thumb based on deviation? (I don't want to get into the heavy math, I just know it's more complex than AM or SSB.) As an old timer, I remember 2 meter amateur FM started out with 30 KHz channel spacing. Then, it went to 15 KHz. Then some deviants went to 12.5 KHz. New rigs have 5 KHz tuning. Is there an official channel spacing? -- Bob D. ND9B |
#2
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Narrow Band FM bandwidth and channel spacing
There is a diffrence between FM bandwidth and channel spacing, though they can
be related. The original two meter FM gear used in the 60's were converted surplus land mobile radio service equipment. Due to the FCC imposed reduced channel spacing of 30 kHz in the VHF LMR band, the two meter band inherited the same 30 kHz spacing. The actual IF bandwidth of the receivers was 15 to 20 kHz, since the combination of crytal frequency drift over temperature, the poor IF filter quality available then and the signal bandwidth of a 5 kHz deviation transmitter would stay within that passband. Later improvements in IF filters (ceramic compound filter blocks) and crystal oscillator temp stability allowed using 14 to 16 kHz bandwidth and that became the norm for 5 kHz deviation in the two meter band too. At UHF, the situation was similar, converted LMR UHF gear was adequate for 25 kHz channel spacing since that was what they used. Same 5 kHz deviation and typically 20 kHz IF bandwidth since temp stability was less than VHF. Again, improved fitlers and xtal osc design allowed narrowing to the same 14 - 16 kHz IF bandwidth. Now when you say narrow bandwidth, I wonder if you mean sliver bandwidth FM? That was the latest change to hit LMR. VHF channel spacing went to 15 kHz (or 12.5 Khz at the top end) in the VHF LMR band and deviation dropped to 2.5 kHz, plus 12.5 kHz spacing at UHF and 2.5 Khz deviation too. When I worked for Midland years ago, I came up with a dual mode modification for their LMR gear that used a compromise IF bandwidth of 9 kHz (four pole xtal filter in the first IF) and a set of audio switches to go between 5 kHz and 2.5 kHz deviation, programable by channel. On Thu, 31 May 2007 01:52:03 -0400, "Bob D." wrote: What is the bandwidth of standard narrow band FM as we use it on 2 meters? Is there a rule of thumb based on deviation? (I don't want to get into the heavy math, I just know it's more complex than AM or SSB.) As an old timer, I remember 2 meter amateur FM started out with 30 KHz channel spacing. Then, it went to 15 KHz. Then some deviants went to 12.5 KHz. New rigs have 5 KHz tuning. Is there an official channel spacing? |
#3
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Narrow Band FM bandwidth and channel spacing
On Thu, 31 May 2007 01:52:03 -0400, "Bob D."
wrote: What is the bandwidth of standard narrow band FM as we use it on 2 meters? Is there a rule of thumb based on deviation? (I don't want to get into the heavy math, I just know it's more complex than AM or SSB.) As an old timer, I remember 2 meter amateur FM started out with 30 KHz channel spacing. Then, it went to 15 KHz. Then some deviants went to 12.5 KHz. New rigs have 5 KHz tuning. Is there an official channel spacing? There is no specific requirement in the rules other than we must use good engineering practice. 97.101 (a). For FM, 6 kHz ( 3 kHz in each sideband) is needed. I.E. Don't use more than you need, and if you do, other hams will quickly let you know about it. Dick - W6CCD -- Posted via a free Usenet account from http://www.teranews.com |
#4
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Narrow Band FM bandwidth and channel spacing
In article ,
"Bob D." wrote: What is the bandwidth of standard narrow band FM as we use it on 2 meters? Is there a rule of thumb based on deviation? (I don't want to get into the heavy math, I just know it's more complex than AM or SSB.) Bob- Although 5.0 KHz peak deviation has been a defacto Ham standard for around 30 years, equipment manufacturers frequently also have a commercial product line. They will often adjust equipment the same for both lines. Recent Ham equipment is being shipped with peak deviation ranging from about 3.5 KHz to 5.0 KHz. That represents a 3 dB range of adjustment, which is just noticeable to the ear. A more appropriate question might be what should the average deviation be? Many people like to hold the microphone several inches away from their mouth. They may not like the sound of wind-blast resulting from holding the microphone next to their mouth. Unfortunately the radio may be set up for close talking, and might easily have 10 dB lower audio as a result of holding the microphone away from the mouth. Ham Radio is not Broadcast Radio. To keep background noises low and voice level high, I would vote for close-talking and an average level of around 3 KHz deviation. Fred K4DII |
#5
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Narrow Band FM bandwidth and channel spacing
I note that my Yaesu VX-100 allows me to change the deviation, if I
wish. A "wide" and a "narrow" setting. /paul W3FIS |
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