3rd decimal on radio freq, why?

exncsurfer

exncsurfer

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exncsurfer
I was playing with 6pc's kneeboard generator just now and a question sprang to mind.

Can someone tell me why some frequencies have a 3rd decimal place value, when the radio tuner only has 2. .975 vs. .97

Thanks

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That "136.12" is actually tuned to 136.125. The '5' is superfluous on the display because with our system of 25kc spacing it can't be anything else if the second decimal place is a '2' or a '7'. Likewise, anything that ends in '.x7' is actually .x75.

On that radio, the manufacturer just dropped the third decimal place to save room on the display.
 
And on some older radios that were built in the "inbetween" time between the channel spacing standards got tighter, some manufacturers made you pull out the knob or do other gyrations to get to the "splinter" channels.
 
denverpilot said:
And on some older radios that were built in the "inbetween" time between the channel spacing standards got tighter, some manufacturers made you pull out the knob or do other gyrations to get to the "splinter" channels.
Click to expand...
Yes, like on this old boat-anchor Cessna-brand nav-com from the mid-1970s. It was confusing, but not as confusing as the instruction in the manual. o_O

RT-328T.jpg

I didn't have to worry about that when I got my IR, though ... the ol' vacuum-tube Mark 12 only had 90 channels, from 118.1 to 126.9 with one decimal place. :cool:
 
denverpilot said:
And on some older radios that were built in the "inbetween" time between the channel spacing standards got tighter, some manufacturers made you pull out the knob or do other gyrations to get to the "splinter" channels.
Click to expand...
Change "made" to "make". I've used plenty of those radios...recently.
 
Pilawt said:
Yes, like on this old boat-anchor Cessna-brand nav-com from the mid-1970s. It was confusing, but not as confusing as the instruction in the manual. o_O

View attachment 48323

I didn't have to worry about that when I got my IR, though ... the ol' vacuum-tube Mark 12 only had 90 channels, from 118.1 to 126.9 with one decimal place. :cool:
Click to expand...


This is what I am using to fly IFR. Seriously. At least I have two of 'em.
 
And when you fly in Europe and they give you the 3rd decimal number it will drive you up the wall if you're used to just putting in the frequency like I do. I key xx.x and enter. Example "N12345 contact tower on 120.9" I push 2-0-9 then enter. The radio won't let me do the ten thousandths without the number 1 in front to signify all six places.
 
This is sort of related to the OP's question.

I recently flew to Montgomery Field (KMYF) in southern California. After clearing the runway, tower told me to contact ground on 118.22.

I was in a Cessna 172 that has a 25 kHz version of the Bendix KX 155A Nav/Com, like this:
Radio1.jpg

This is a “25 kHz” radio that moves in 25 kHz increments, aka .025 MHz increments.

Dialing in ground on "118.22" should have been easy because the radio should move through this sequence of frequencies (in MHz):

Radio2.jpg

However, no matter what I did, I couldn’t get the radio to display 118.22. I was stumped and getting a little frustrated.

I finally figured it out after a minute or so and never looked into more closely until I saw this thread.

As it turns out, my problem related to the “PULL 25K” legend right below the big and little frequency knobs. Somehow I or the previous pilot had pushed it in. According to the manual, “it will change in increments of 50 kHz when the knob is pushed in and 25 kHz when the knob is pulled out.”

If the radio is limited to increments of 50 kHz (aka .050 Mhz), there’s no way to dial in 118.22, because it skips over the .025 and .075 frequencies. Apparently, the goal is to let you move more quickly through whole and half frequencies:

Radio3.jpg


Oops.
 
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