[Milsurplus] big crystals

[Bob Camp]

Hi

As you go lower in frequency a crystal has to get thicker. There is  
also a minimum width or diameter that will do well for a given  
thickness. The diameter / thickness  stuff is mostly noticeable below  
3 MHz with WWII era crystal holders and resonator designs.

One simple way to look at it: If a 3 MHz crystal needs to be 1/2 inch  
across, a 300 KHz crystal would need to be 5" across.

Obviously nobody is going to make a lot of 5" crystals, either for  
fixed or portable (!) use. As you go lower in frequency you go to a  
cut that does not require as large a diameter / thickness ratio.  
Another  trick is to swap things around so you only need one large  
dimension and the rest can be small. You also can trade performance  
for size if you have to. All of that helps some, but you still need a  
larger width at the lower frequencies. .

If you are building a transmitter that is going to work below the AM  
band, you are going to need big crystals. If you are using pressure  
holder technology, they are going to be rectangular blanks. That  
gives you great big holders. A lot of designs start at the low  
frequency end of the range and then work up. The same crystal holder  
is retained over the entire range. It's far easier to stock one set  
of parts in the factory than to fiddle with several variations.

Of course if you get low enough, you just use a bare quartz bar in a  
pine box. Take a look at an old GR 100 Kc standard if you ever get a  
chance ...

Bob
KB8TQ