Frequency Step and digital

[Arvetus]

Ok, I'm having some difficulty figuring out the frequency step for some frequencies I'm programming.

I have steps of 5, 6.25, 10, 12.5, 15, 20, 25, 50 khz available.

How does one figure out what frequency step applies to the frequency you're programming?

For example, I am trying to program 155.3175. I've tried 6.25 and 12.5 and neither one works. Both rounds it down to 155.3125.

I just moved to East Texas, and I'm going through trying to reprogram my Kenwood TM-V7a. I'm using the Windows program to create my list, and then will download it to the radio. I'm trying to get Nacogdoches and Angelina counties info in my radio. Or are they strictly digital now and I have no hope without a dedicated digital scanner?

 

[SkipSanders]

As it happens, that frequency is a 7.5 KHz split. ( 1/2 of 15 KHz ) They're showing up in VHF Hi band where the common channel spacing was 15 KHz.

To find out the 'official' channel, you have to look up the appropriate area in the FCC regulations to see what channels are assigned as of now. The BC396 has 7.5 KHz splits available, I don't personally know about other radios.

Of course, absolute precision isn't required. 155.315 or 155.320 are 2.5 KHz 'off', well within what works for a simple scanner receiver. Using 6.25 Split, you could get 155.31875, only 1.75 KHz off, if you don't have 7.5 KHz available.

 

[Arvetus]

I'll try using a different split and getting as close as I can. Evidently ham equipment doesn't do the 7.5khz splits yet. I don't even think I can upgrade the firmware to make mine do it.

How is the split figured based on the frequency?

 

[SkipSanders]

You're looking at problems with the 'old' splits being a wide range of stuff, depending on how the bands were originally laid out long ago.

Most VHF High band was laid out in 30 KHz steps, 'split' down to 15 KHz steps, now being split to 7.5 KHz steps. Essentially, whatever the old channel spacing was, is split in half.

The intent of all this splitting NOW is to get down to minimum possible channel spacing, to use narrowband signals and closely spaced channels to get more 'channels' out of the same amount of spectrum. In Aircraft/MilAir/UHF/700/800/900, channels were 100 KHz/50 KHz/25 KHz, split to 12.5, to be split again to 6.25 KHz. That's about the minimum spacing useful with current tech.

There are other things going on, too. APCO 25 'Phase 1', which we're seeing now, is a 12.5 KHz spacing mode. 'Phase 2', to be seen in future, is equivilent to 6.25 KHz spacing, BUT, achieves it via Time Division Multiplexing, so that one 'frequency', 12.5 KHz wide, will be carrying two voice 'channels' at the same time. Yes, that means all new scanners when Phase 2 becomes active. Joy! Or not...

 

[jastx]

How does one figure out what frequency step applies to the frequency you're programming?

The frequency is a multiple of the step size. Therefore, the remainder of the frequency in MHz divided by the step size in MHz should be zero. Since the step sizes you refer to are in KHz, they must be converted to MHz by dividing by 1000. For example:

155.3175 / 0.00625 = 24,850.8 the remainder is .8, so 6.25KHz isn't the step size.
155.3175 / 0.0125 = 12,425.4 the remainder is .4, so 12.5KHz isn't the step size.
155.3175 / 0.0075 = 20,709 the remainder is zero, so 7.5KHz is the step size.