Help with math....

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westcoaster
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Help with math....

Post by westcoaster » Sat Apr 01, 2006 9:11 pm

This will be elementry for some but here goes...

I know channels can increment in 6.25/12.5/25 Khz steps

How is the math done to figure out what are the valid frequencies in each of the steps?

I know 154.1 and 158.94 are valid frequencies but I can't get the math to line up on either of them....




Thanks,

k2hz
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Location: Rochester, NY

Post by k2hz » Sat Apr 01, 2006 9:53 pm

The math does not work for those frequencies because the 6.25/12.5/25KHz steps do not apply to FCC Part 90 VHF channels.

The old FCC VHF Part 90 channels are 15Khz steps and the synthesizer works on 5 Khz steps so those frequencies are all divisible by 5.

The new FCC narrow Band VHF part 90 channels are 7.5 KHz steps so most wideband radios with a 5 KHz step synthesizer will not work. The new narrow band radios use a 2.5KHz step to cover both old and new frequencies.

The 6.25/12.5/25 steps do apply almost everywhere in the UHF bands and on VHF in the US Government bands, Marine Radio and most overseas use.

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westcoaster
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Post by westcoaster » Sat Apr 01, 2006 11:18 pm

:oops: If I had looked at my radio before posting I would have saw the 5Khz steps....

This also explains why some overseas radio's are not licensed for use in the USA and Canada.

I am a little concerned though, The radio I have (kenwood tk-760) while narrow band capable, does not do the 2.5 Khz steps it appears to only do the 5 or the 6.25.

Beyond actually getting the frequency number to fit in the radio what other role does the channel step play? can a frequency that is programmed with a 6.25Khz step communicate with one that is programmed with a 5Khz step? "harmful interference"??

Any links where I can do a little reading on this subject would be greatly appreciated...

Thanks,

k2hz
Posts: 150
Joined: Sun Jan 30, 2005 1:02 pm
Location: Rochester, NY

Post by k2hz » Sun Apr 02, 2006 12:21 am

While some older VHF radios are capable of "narrow band" (11 kHz bandwidth) they are not capable of operating on the new FCC narrow band frequency plan.

Most older radios VHF are not. If the radio was not designed in last few years, since the FCC adopted the 7.5kHz channels, it generally will not work. This is only an issue with VHF radios. UHF "narrow band" radios generally can do the associated 12.5kHz frequency steps.

FCC frequency tolerance at VHF is 750Hz. Anything more than about 1 kHz off frequency will have degraded reception in addition to the adjacent channel interference issue. Operating 2.5kHz off frequency, in addition to the FCC rule violation, will just make a total mess of your communications and adjacent channel users.

The radio manufacturers specifications really muddy up the issue of frequency steps and bandwidth. To operate on the new FCC VHF channels the radio must be capable of operating with an "11k" emission designator and a synthesizer step of 2.5kHz. Most manufacturers "channel spacing" specs are meaningless on VHF. They quote 25/12.5 specifications because they look better than performance on the actual 15/7.5 channel spacing.

This has been discussed many times on this board. Check the Kenwood Commercial forum. For example:

viewtopic.php?t=2024&highlight=

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westcoaster
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Post by westcoaster » Sun Apr 02, 2006 10:52 am

Hmmm.... All I can do at this point is hope Canada hasn't adopted FCC part 90 rules. My radio can't program 158.9175 either, the closest I can get is 158.91875 (.00125 off)

Thanks for the help,

k2hz
Posts: 150
Joined: Sun Jan 30, 2005 1:02 pm
Location: Rochester, NY

Post by k2hz » Sun Apr 02, 2006 11:59 am

I hope Canada did not make the mistake the FCC did with going to 7.5kHz channel spacing. In addition to the fact that it was not compatible with any existing radios, there are other problems.

If you "do the math" on the fact that authorized bandwidth is 11kHz but channel are spaced every 7.5 kHz you will see that there is a major overlap of adjacent channel bandwidth. So, in reality the "new" frequencies are virtually useless if there is any nearby adjacent channel activity.

I checked the Industry Canada frequency coordination website for 158.9175 and I see no assignments anywhere in Canada. There are assignments on 158.910 and 158.925 but nothing in between.

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