Info

Noise-Operated Squelch:

Bob Eldridere, 2-1 f ay Mobile Squelch Problems, Radio & Television News, February, 1959, page 72,

Audio-Cutoff Circuit:

Amrricnn Radio Relay League. The Radio Amateur's Handbookt

Lantfford-Smitlu op citf page [ 127.

F. E. Tor man. Electronic and Radio Engineeringt fourth edition.

HPrPUHÉR I960

In 1957 two radio amateurs, John Chambers (W6NLZ) and Ralph Thomas (KH6UK), astounded the scientific world by doing the impossible. They communicated with each other over the 2500 mile path between Southern California and Hawaii on 144 mc, a line-of-sight band! Only a short decade ago, radio textbooks described this frequency as having very limited range, and actually gave a formula for computing distance, The answer was usually 25 miles, or so!

Cascode Amplifier

You can be assured John and Ralph did not accomplish this tremendous feat using superregen receiver or even superhets with a simple 6BQ7 cascode rf amplifier. The received signals were a small fraction of a millionth-volt (microvolt). At this level the rf amplifier stage, the antenna, and even the cosmos, gang up on the signal and try to push its head under a sea of noise. This noise is the hiss you hear on a television receiver between channels, and is the mating call of electrons in motion.

You can't do much about cosmic noise, or the antenna for that matter, but you can construct an rf amplifier which will contribute as little noise to the signal as possible. This converter incorporates such an rf amplifier. Its impressive performance is indicated by the signals received when it was tuned to the satellite frequency of 108 mc> Excellent recordings were made of the 10 milliwatt Vanguard" transmissions when it was at the zenith of its orbit, some 23,000 miles away! Although no spectacular dx has been received on two meters, due to a poor location, the receiver noise increases considerably when the ¡0 element beam is turned toward the sun1'

Theory of Operation

The purpose of this type of converter is to translate signals from the transmitted frequency down to a more convenient range- Most amateurs possess a communications receiver covering 1.5 to 30.0 mc, but few have the time or inclination to construct a receiver for 2 meters. By building this low-noise converter, they can have a first class receiver with only a few hours work.

n PTCLJRZD IOAA

Fig. 1 is the schematic diagram of the low-noise converter. The antenna^ connected to Jl, is coupled to the rf amplifier grid coil (LI) :hrough a 7-45 mrrifd trimmer* This capacitor 3rings about an impedance match between the transmission line and tuned circuit and avoids experimenting with various tap points on LI, The rf amplifier circuit is an offshoot of the common 6BQ7 cascode circuit found in most television tuners. However, this circuit, which was designed by W2AZL3, has many innovations which combine to provide an extremely low noise figure (a figure of merit which determines how weak a signal can be detected), A 6BQ7 in a television tuner might have a noise figure of 7 or 8 dbf when set to channel 13, since the response must be at least G mc wide- If the same? tube and circuit was peaked on a small portion of the 2 meter band, the noise figure might drop to 5.5 fib- Replacing the 6BQ7 with a lower noise tube (such as the 6AJ4 or GAM4) would knock off another

Lester Earnshaw ZUAAX

Box 51

Wdrtworth. Ne-* Zea ^rrj

Was this article helpful?

0 0

Post a comment