The 2-Meter to 20-Meter Magic Number
Historically, amateur radio operators wanted a way to receive and transmit on the 2-meter band (144–146 MHz) using their highly sensitive, existing 20-meter HF receivers (14 MHz) as a tunable Intermediate Frequency (IF).
To mix a 144 MHz signal down to a 14 MHz IF, you need a highly stable 130 MHz local oscillator (LO):
Creating a stable, fundamental-frequency quartz crystal at 130 MHz was physically impossible for decades because the quartz wafer would have to be sliced microscopically thin and would easily shatter.
Instead, designers utilized a robust, lower-frequency third-overtone crystal operating at 43.333 MHz. When you multiply 43.333 MHz by three in a simple tripler stage, you get exactly the 130 MHz LO signal needed:
Why They Flooded the Market
Because the 2m-to-20m conversion was the gold standard for VHF operation in the 1960s, 70s, and 80s, these crystals were mass-produced. They were the heart of legendary gear like the Drake SC-2 receiver converter and dozens of homebrew transverter designs featured in the ARRL Handbook and 73 Magazine.
If a ham wanted to monitor the popular 146.94 MHz repeater frequency of the era, they would use a 2-meter converter with this exact crystal, allowing them to tune their HF dial to precisely 16.94 MHz.
Other Multiplier Matches
Additionally, 43.333 MHz has convenient harmonics for other bands. For instance, multiplying it by 10 yields 433.33 MHz, which sits perfectly inside the 70-centimeter amateur band and the widely used 433 MHz ISM band (common for low-power key fobs, weather stations, and remote controls).
Whenever you see a strangely specific, non-integer crystal frequency like 43.333 MHz, 38.667 MHz (used for 2m to 10m conversions), or the famous 3.579545 MHz color burst crystal, there is almost always a legacy of mass-production and clever math behind it!
I would be interested to hear if any Soldersmokers
ReplyDeletehave successfully built something like this that
they are presently using. --Walter KA4KXX
Good point Walter. This might not be what you had in mind, but one thing that I thought of was our beloved 3.579 MHz Color Burst Crystal. We use them in Michigan Mighty Mites, but the origin of these rocks is also quite interesting. More on this, perhaps tomorrow. For now, perhaps hams should check their crystal junk boxes, looking for 43.333 MHz, 38.667 MHz, and, of course, 3.579 MHz 73 Bill N2CQR
DeleteMy advice to Michigan Mighty-Mite
ReplyDeletehomebrewers would be to buy a 40M band
crystal which are readily available on
several frequencies for only about $3 each
from Nightfire Electronics, QRP Me, and
several other sources. Three big reasons for this
is because there is many times as much CW activity
on 40M as 80M, the 40M band is open for a greater
portion of the day, and the antenna is much easier
to construct. In fact, I have never operated CW
on 80M, and my 75M antenna would not even
tune that far down if I wanted to.
Even better, buy a crystal on Mouser for 7.123 MHz
and participate in the Sunrise Net every day of the year!
--Walter KA4KXX
Thanks for the tips on Nightfire and QRP Me, Walter. List any other sources you can, because several have gone out of business over the years. It's great to see these guys are still active.
ReplyDelete73! WN2A
Another source for several types including FT-243 and HC-49 is AF4K Crystals, although their website is a bit difficult to navigate. --Walter KA4KXX
ReplyDeleteAnd another source of cheap QRP CW crystals is Kanga Electronics. I bought 7030, 10116 and 14060 rocks in the past year.
ReplyDeletePaul VK3HN
I want to hear from Bill, not Gemini.
ReplyDeleteBut I get a lot of info from Gemini, in a way that is similar to how I got info from the Google search engine. I'll only post the Gemini stuff if it sounds mostly correct, just as I did with the results of search engine results. 73 Bill N2CQR
Delete