Solution- A homemade 1 megahertz Crystal Calibrator for quick checks on 20 MHz  ^^
+ The Availability of WWII Surplus Electronic Parts for the Radio Enthusiast  ($)
^^ A crystal calibrator puts out a marker signal at a precise frequency which a radio receiver is tuned to, in order to help derive a  
correct frequency of interest, that will be monitored.  Older radios used a variable capacitor for tuning before the advent of the
precise digital tuning that we enjoy today.  Most old radio sets had a tuning capacitor which was linked to the tuning knob by a cord
and pulley arrangement.  The tuning indicator was attached to the cord and when the tuning knob was turned
the indicator slid left or
right under the glass that had the frequency markings.  Mechanically this system was loose and introduced error into the indicator
reading.  Example- The dial indicator might be set to read 20Mhz-(note- Mhz= Megahertz) but electrically the tuning capacitor might be
operating at 20.132Mhz.  This is why radiomen had to use a crystal calibrator.  You can think of the crystal calibrator as a low power
transmitter which puts out a marker signal at a
precise frequency, which is determined by the thickness of a wafer of quartz crystal.-
(see- Piezoelectric Effect)   For example, the crystal in my calibrator is ground for 1Mhz and it puts out weak marker signals at each
multiple, or harmonic, of 1Mhz= 2,3,4,5 and on up to 20 Mhz and beyond.  To listen for space signals, I would set the receiver dial on
20Mhz or 15Mhz and then I turned on this calibrator and tuned for the nearest audible marker.  This was so easy,
but, calibrating down
to a smaller frequency resolution required another calibrator and the tedious process involved, tuning slowly, and counting each
audible marker to arrive at the desired frequency.  This is the complicated process used to calibrate a receiver.  The dial error offsets
differed from radio to radio and it was best to be familiar with the tuning peculiarities of each radio in order to calibrate it properly.  
Thank God for modern digital tuning.

It should be noted that the cheaper tube radios also suffered from slow frequency drift as the tubes heated the radio circuits up to a
stable operating  point.  Reaching thermal stabilization might take a few hours, all the while, requiring the operator to repeatedly touch
up the tuning to the desired frequency.  This drift effect was especially noticeable from a cold start, as the heat increased, the
aluminum tuning capacitors, Bandspread & Main,  would slowly get a tiny bit bigger in size, causing the tuned circuit of the Local
Oscillator to shift frequency.  Further, the operator had to be mindful of avoiding abrupt events, like dropping a book on the operating
table which could cause the temperamental tuning capacitors and coils to jump off frequency.  In the old days, monitoring a precise
frequency was truly a challenge.  The hobby grade receivers of that era were better suited for casual tuning on the shortwave bands.  
However, in a dimly lit room the old tube radios had a soft glow and were warm to the touch and they had real control knobs that you
could grasp hold of.  Todays computer screen radios have collections of pixels, that look like knobs, knobs that turn when you click
on them with a mouse.  Admittedly, digital radios are much better, but in a strange way, I  miss the old tube radios.
(&) I didn't have the above calibrator until the early 1970s.  This simple circuit was key to jump starting my radio tracking of Soviet
space operations.
 So back in the early 60s, when the Russians launched a manned mission,  I would man my receiver, most of the
time, just tuning blind for AM voice signals, by constantly rotating the Bandspread Tuning plus and minus the 20 megacycle dial
marker.   When the skip was right, I would use the powerful TIME and FREQUENCY radio station
WWV to set my receiver on 20
megacycles.  When audible, it was so easy to tune WWV on 20Mhz, right in the center of the Soviet satellite band, and then set the
Bandspread (fine) tuning, plus and minus, slightly.  At the beginning of the Space Age, the designers of Sputnik-1 wanted peoples of
the world to tune in the baby moon directly from their homes, mainly for propaganda reasons and to help validate their satellite
accomplishment.  Shortwave radio then was like CNN now.  However, the Sputnik builders were also mindful that
most hobby radios
of that era had inaccurate tuning, (as described above).  For this reason, Sputnik transmitted on 20.005Mhz, right next to the powerful
WWV station, which made tuning for Sputnik easy for the average listener, how clever of Sergi Korolev and team.
($)  A military surplus 1Mhz precision grade crystal is used in my homemade calibrator.  After World War Two there were tons of
military surplus electronic parts, all high grade, built to military specifications and available to radio amateurs at cheap prices.  Below
is the Radio Shack of Aluminum Overcast, a restored B-17 bomber.  The BC-348 receiver (1st photo) and the ARC Command Sets (2nd
photo) were made by the thousands and flooded the surplus market after the war.  These rugged radios could be purchased at a
reasonable  price and many HAM radio enthusiasts, myself included, started out  with this type of gear.

Note - I made the 4 photos below on Oct 15, 2006 when I toured inside Aluminum Overcast at North Little Rock airport.
During slack time the B-17 Radioman listened out on the BC-348 (above).  Searching, he gathered intelligence by tuning in enemy
radio signals,  jotting down enemy code messages and noting the transmission frequency used.  Flying against  the mighty Luftwaffe
in WWII with no place to retreat, imagine, trying to tap out a
Morse code message while being fired on.

I also collect WWII radio gear and have all the sets in these photos.
World War Two fliers decorated their planes with all types of Nose Art but variations
using the female form was most popular.

($) Aircraft production during WWII numbered in the tens of thousands as the industrial strength of America was an
important factor in defeating Germany and Japan.  The radios above were made by the hundreds of thousands.