DR3: 50's dB



In the 70's and 80's, radios' DR3 was very low.  The Drake R-4C appears to be the leader of the pack here, but we shall see in the next group that there were better radios at that time - albeit, not exactly low-cost.

DR3: 60's dB

As seen in the table on the right, most popular radios had DR3 values in the 60's well into the 1990s.  And a few into the 2000's.

A couple of radios stand out and are highlighted in green. 

• The Collins 75-S3 Wing had a DR3 of 63 dB dating back to 1961.  Unfortunately, its price put it out of reach for most hams.

• In 1970 Heathkit introduced a receiver with a DR3 of 64 dB at an affordable price. Also highlighted in green.

The Signal/One introduced some time in the 1980s was touted as a high performance radio, but its DR3 was only 68 dB.

The Yaesu FT-1000 family introduced in 1995 had a DR3  only one dB better.  It soon became a favorite radio among contesters.

The two highlighted radios  in this group (both receivers, not transceivers) used a  "tunable" IF in their first IF -  a common feature in earlier receivers. They also used a (manually) "tunable pre-selector" which improved front-end selectivity:

• The Collins used 2.955 MHz to 3.155 MHz and then was mixed to a Mechanical Filter at 455 kHz.

• The Heathkit used 8.395 MHz to 8.895 MHz and then was mixed to a Crystal Filter at 3.395 MHz.
  

The performance of most of these radios was limited by a common factor:  the frequency of their first IF (Intermediate Frequency) used upward conversion, with an IF of 45, 64, or even 70 MHz. 

This had two advantages over the tunable IF's: 

• It eliminated the need for a tunable pre-selector.

• And, it also eliminated the frequency gap (i.e., on the IF frequency) for continuous-coverage tuning of the shortwave bands.
  

The disadvantage of this technology was also twofold.  Crystal filters are subject to a law of physics called "fractional bandwidth" - a ratio of its bandwidth to center frequency.   

The result is:

• By a given number of crystals, filters at much lower frequencies can obtain a much narrower bandwidth than filters at higher frequencies.
  

• And, the lower the IF frequency, the steeper (better) the skirts of the Xtal filter.  Better filter means better DR3. A 5 MHz IF filter is better than a 9 MHz IF filter; a 455 kHz IF filter is much better than a 5 MHz IF filter.
  

In the next group of radios, we shall see how the DR3 was increased substantially by switching to downward conversion IF. 

Note: Most hams failed to understand the disadvantages of upward conversion IF and continued to buy radios with it, even after later radios began switching to downward conversion.