Shortwave listeners and DXers in the 1970's witnessed the rise of the solid-state high frequency (HF) communications receiver, and at the same time saw the end of an era of American dominance in vacuum tube receivers. Companies like Collins, Hallicrafters, Hammarlund and National, one by one dropped out of the amateur radio market. The void they left was quickly filled by several fast rising Japanese companies, most notably Yaesu, Trio-Kenwood and Icom, each offering compact solid state radios with good performance at reasonable prices.

Until this time most consumer-grade receivers-typically equipped with rudimentary "slide rule" tuning indicators-offered little more than "hit or miss" tuning accuracy. As a consequence many newcomers to the shortwave listening hobby were discouraged early on because of their lack of the technical skill required to properly operate the better receivers and the relative difficulty of consistently finding desired stations with more inexpensive sets.

Major advances in communications receivers during the 1970's including frequency synthesis and digital frequency readout made communications receivers easier to use, provided relatively precise frequency display and in general made the shortwave hobby more accessible to the masses-a market already primed by the CB radio boom and hungry for more radio-related toys.

Phase-Lock Loop (PLL) Synthesizers

Phase-lock loop (PLL) frequency synthesis was introduced to the HF receiver market in the mid-1960's with the National HRO-500. By 1977 it had become standard fare in professional HF receivers, with the technology beginning to 'trickle down' to the consumer level. JRC's NRD-505 (released in 1977) was among the first amateur/SWL products to offer PLL; others included the Drake R7 (1978), Kenwood R-1000 (1979) and Sony CRF-1 (1980). PLL synthesizers allowed tuning and frequency readout with accuracy and stability never before possible with conventional VFO-type receiving systems.

A block diagram of a simple PLL system is shown below:

A sample of the output from a variable Voltage-Controlled Oscillator (VCO) is input to a frequency divider and divided by a programmed value. The resulting signal is input to a phase detector and compared with a fixed reference signal to determine the phase difference. The phase detector sends a voltage back to adjust the VCO's output until the detected phase differece is zero. To change the desired output of the VCO, simply change the value of the frequency divider accordingly.

The advent of the PLL frequency synthesizer sparked a revolution in receiver design. In the early 1970's PLL tuning was a luxury only to be found in the highest of the high-end receivers. But as the cost of this technology dropped it became more and more common, first in high-end tabletop receivers, then eventually in small portable radios. PLL synthesizers are today used in all but the simplest shortwave radios today-even receivers the size of a pack of cigarettes incorporate PLL sythesized tuning.

Wadley Loop Synthesizers

A second frequency synthesis technique similar to the phase-lock loop should be mentioned. In 1970, the South African firm Barlow-Wadley introduced a frequency synthesis system known as the Wadley Loop, which was used in the company's XCR-30 portable receiver, and eventually by several other amateur and professional receivers such as Racal RA-17, Galaxy R-530, Yaesu FRG-7 (and later the FRG-7000), and the Realistic DX-300 and DX-302.

Using the Wadley Loop system was simple: one control was used to select the desired 1 MHz band, while another tuning control was used to fine tune the receiver from 0-999 kHz, usually calibrated in 1 kHz increments. In order to tune to a frequency of 11850 kHz, the operator would first set the MHz dial to "11", then dial in "850" on the main tuning dial.

The megahertz knob controlled the radio's 1st Local Oscillator by selecting the harmonic frequency of a master crystal oscillator that was appropriate to convert a given band to the radio's 1st IF. When the selected harmonic was in phase with the master oscillator's crystal, the Wadley Loop locked it there and the 1st mixer converted the band segment to the tunable IF frequency.

The kilohertz knob tuned a local oscillator that covered the receiver's 1st IF. It fed its signal to the second mixer, which did the conversion to 455 kHz for the radio's 2nd IF.

(Source: Shortwave Receivers Forum, Kulpsville 1993 by Clark, Johnson and Byington)

Digital Readout

At the same time frequency synthesis came into the mainstream, digital frequency readouts became more and more commonplace. could be found on several receivers, such as the Panasonic RF-2800 and RF-4800 and the Realistic DX-300, and although they were not always as precise as many DXers hoped for, they were clearly a hint of things to come.

SWL Receiver Market in 1980

By 1980, an SWL shopping for a new communications receiver had a variety of models from several manufacturers to choose from. Here's a sample:

• Yaesu's FRG-7 was a solid, affordable tabletop performer very popular among hobbyists. Combined with a custom GAR-7 digital readout from Gilfer Shortwave and some high-quality ceramic filters, the "Frog-7" was also suitable for some serious DXing. Eventually it was sold under private label through Sears-Roebuck stores. Later, Yaesu introduced the FRG-7000, which used the same basic Wadley Loop platform as the FRG-7 but added several useful features such as a digital readout.
  
  
• The Drake SPR-4 was a crystal-controlled receiver who's design was a holdover from the late 1960's and early 1970's. The Japan-built SSR-1 offered full-coverage and direct analog frequency readout, but was a marginal performer compared with earlier Drake models. When their R-7 was introduced in 1979, it quickly attained legendary status among DXers and became the standard of comparison for the early 1980's period.
  
  
• Kenwood's R-300 was an interesting model, with twin drum dials instead of the standars slide-rule-plus-bandspread readout of similar receivers. But Kenwood's first successful communications receiver was the R-1000, a compact, well performing radio with PLL synthesis and digital frequency readout.
  
  
• Realistic's DX-160, like its predecessor the DX-150, was quite popular and widely available through hudreds of Radio Shack outlets nationwide. While performance was a far cry from being acceptable for serious DXing, the DX-160 was probably responsible for getting more people started in the hobby than any other radio of its time. It was later replaced by the DX-300 (and later DX-302), similar to the Yaesu FRG-7/7000 using the Wadley Loop system.

The above receivers were among the most popular table-top models of that period. In addition, many table-top and portable shortwave receivers were made by companies better known for consumer electronics than radio communications equipment. These receivers helped bring shortwave listening to the masses.

• Sony's ICF-5900W, a direct analog-readout portable, eventually gave way to a revolution: the ICF-2001, the market's first affordable, microprocessor-controlled portable shortwave radio. Other Sony offerings around the same time included the ICF-6700W and ICF-6800W, both large "portables" that provided nearly hi-fi audio quality and outstanding SWL performance.
  
  
• Panasonic's "Command Series" of shortwave receivers RF-2200, RF-2800, RF-4800, and RF-4900.
  
  
• Grundig, though not readily available in the USA, had many fine portable shortwave radios.

The following table shows the most popular receivers of the late 1970's. The overall ratings for SWL and DX listening purposes are taken from the 1980 World Radio TV Handbook receiver review section (edited by Larry Magne).

Model	Price (US$)	Overall Rating (SWL)	Overall Rating (DX)
Yaesu FRG-7	$370.00	F	F
Realistic DX-300	$379.95	PF	P
Sony ICF-6700W	$439.95	F	F
Panasonic RF-4900	$499.95	F	F
Trio-Kenwood R-1000	$499.95	G	FG
Sony ICF-6800W	$650.00	FG	G
Yaesu FRG-7000	$655.00	F	F
R.L. Drake R-7	$1,295.00	GE	E
McKay Dymek DR-33C	$1,500.00	F	GF
Japan Radio Co. NRD-505	$2,275.00	E	GE

Sources and Recommended Reading:

Shortwave Receivers: Past and Present by Fred Osterman (1997 Universal Radio Research)

Radio Receiver: Chance or Choice by Rainer Lichte (1985 Gilfer Associates)

World Radio TV Handbook 1980 - Receiver reviews by Lawrence Magne (1980 Billboard)

The Buyer's Guide To Amateur Radio by Angus McKenzie (1986 Radio Society of Great Britain)