Crystal frequency calculation for Air Band

JJH0906

Member
Joined
Nov 17, 2024
Messages
12
Many thanks to those who have responded to my other threads regarding crystals for the Realistic PRO-48 scanner. However, the quest continues!

I was referred to this link of the formula used to calculate the frequency crystal needed to get a monitor frequency (which I still don't quite get):


When looking at the manual for this particular scanner, the frequency bands show this:

20250129_065158.jpg

So! I guess I'm a little confused on how to calculate the airband frequencies, as they don't seem to fit in the VHF Lo or VHF Hi range.

Any thoughts?
 

mass-man

trying to retire...
Premium Subscriber
Joined
Mar 15, 2004
Messages
1,928
Location
Parker Co., TX
what are you trying to do? If you need a crystal of a specific frequency, you inform the manufacturer of the model of the radio and the frequency desired....they should do the rest.
 
  • Like
Reactions: wtp

JJH0906

Member
Joined
Nov 17, 2024
Messages
12
what are you trying to do? If you need a crystal of a specific frequency, you inform the manufacturer of the model of the radio and the frequency desired....they should do the rest.
I just want to buy some damn crystals to listen the airplanes near my house, it's that simple! :) Seems like finding the specific frequencies I'm looking for is harder than finding a pot of gold at the end of the rainbow, however!

That's how I got introduced to needing formulas and holder types and all the rest. I'm appreciating the education but also getting frustrated with the amount of effort it's taking to just get a half dozen crystals for this lil' hobby.
 

gmclam

Member
Premium Subscriber
Joined
Sep 15, 2006
Messages
6,443
Location
Fair Oaks, CA
I believe that receiver is only double conversion for VHF Hi and employs a 10.7MHz IF, similar to many radios I still run today. The crystal is likely 3rd overtone. So desired frequency minus 10.7E6 divided by 3. Let's try 110.7E6 as the desired receive (110.7 - 10.7 = 100) / 3 = 33.33MHz.
 

KevinC

The big K
Super Moderator
Joined
Jan 7, 2001
Messages
12,770
Location
I'm everywhere Focker!
I believe that receiver is only double conversion for VHF Hi and employs a 10.7MHz IF, similar to many radios I still run today. The crystal is likely 3rd overtone. So desired frequency minus 10.7E6 divided by 3. Let's try 110.7E6 as the desired receive (110.7 - 10.7 = 100) / 3 = 33.33MHz.
Which is what I told him here...


But for $50 per channel after 3 crystals he could buy a brand new BC125AT and have 500 alpha-tagged, synthesized channels.
 

mass-man

trying to retire...
Premium Subscriber
Joined
Mar 15, 2004
Messages
1,928
Location
Parker Co., TX
KevinC beat me to it! Crystals are silly expensive now. The BC125AT is an excellent air scanner! You might try looking around for one!
 

mass-man

trying to retire...
Premium Subscriber
Joined
Mar 15, 2004
Messages
1,928
Location
Parker Co., TX
Trust me I’m all for nostalgia and radios! My original Besrcst 101 still sits in the shack and happily scans away! I do remember as a kid being overjoyed that I no longer had to buy crystals!
 

ramal121

Lots and lots of watts
Premium Subscriber
Joined
Dec 5, 2008
Messages
2,242
Location
Calif Whine Country
I believe that receiver is only double conversion for VHF Hi and employs a 10.7MHz IF, similar to many radios I still run today. The crystal is likely 3rd overtone. So desired frequency minus 10.7E6 divided by 3. Let's try 110.7E6 as the desired receive (110.7 - 10.7 = 100) / 3 = 33.33MHz.
Unless air band uses high side injection. If that's the case your crystal example would be 40.67 MHz. The wiki link doesn't show the air band so you would need to confirm first before spending precious $$$ on crystals.
 

gmclam

Member
Premium Subscriber
Joined
Sep 15, 2006
Messages
6,443
Location
Fair Oaks, CA
Unless air band uses high side injection. If that's the case your crystal example would be 40.67 MHz. The wiki link doesn't show the air band so you would need to confirm first before spending precious $$$ on crystals.
Yeah agree. My thinking was to just get a "junk crystal" at a given frequency and see what you can hear. Even if it's high side, rather than low side; he'd be likely to pick up the desired frequency as an image (frequency).
 

RMason

Active Member
Premium Subscriber
Joined
Sep 18, 2003
Messages
814
Location
Colorado / Mississippi
You may want to also refer to:
 
Top