- Joined
- Nov 10, 2009
- Messages
- 72
Long story short.
My dad was in the NAVY back in 1954 and he flew with the Blue Angels as their crew chief. Many times he flew in cargo planes with the spare parts and tools. At times he carried a pocket AM radio with him and he loved to listen to country music out of Nashville - while he was traveling up and down the east coast.
When he came home, the Civil Defense was a big thing and they all went to CB radios - with a license, in case of a national emergency, so they could communicate with each other. Which in a way, is kind of a joke, because a 4 watt transmitter in the coal patches of Pennsylvania had a hard time traveling more then 50 miles with a 1/2 wave antenna.
Here is part of the conversation -
To continue our phone conversation from Tuesday.
The normal output from a CB radio was 4 watts.
4 watts @27 MHz is good for about 25 miles.
The distance can be increased depending upon numerous things, elevation,
antenna gain etc.
Ionosphere bounce (Skip) is very common at 27Mhz and communications even
using single element (car whip style) antenna across the US or even across
the ocean is possible. The furthest that I talked was to Sydney and
Melbourne Australia.
The beam antenna like you dad had up had a gain of about 12db. That is
like 16 times the power so a 4 watt into the antenna made it like 64 watts
into a whip antenna.
The standard modulation was AM.
AM has been around from the 20s, was easy to modulate and demodulate. It had some drawbacks.
1. It was not very efficient. If you had a 10 watt transmitter you needed
a audio power of 10 watts to modulate it to the maximum 100%. 50K signal
like KDKA am required 50,000 watts of audio.
2. Other outside electrical noise easily caused interference on the
receiver. Lightning, power line noise, car ignition, etc.
3. The receiver "threw away" 1/2 the power
In a AM signal there is the carrier and two sidebands the carrier + the
modulation and - the modulation.
Example: WCEZ in Punxsy has a carrier frequency of 1540 kHz. To make
things easy, Let's assume that they were sending a single 1KHz audio tone
for a minute. If you looked at the output of the transmitter on a spectrum
analyzer you would see the main carrier at 1540 kHz and a signal at 1539Khz
and one at 1541KHz. The two sidebands would be 1/2 the level of the main
carrier. The receiver only uses one of the sidebands and discards the
other. The main signal, the carrier, is there just to carry the
information modulation through the air.
Single Sideband was a major break through for AM type communications. In
SSB only the information is being sent, no carrier. This is great from a
sending view but the receiver needs a signal to tune to for demodulation.
So in SSB receivers there is actually a little transmitter that fakes the
receiver into thinking that the carrier is there. This transmitter is
called a BFO for Beat frequency oscillator.
The Federal Communications Commission authorized CB in 1958. At that time
they permitted Amplitude Modulation at a maximum of 100% and a final tube
input power of 5 watts DC. The reasoning was that the most efficient
design amplifier would be about 80% which would generate about 4 watts of
output power. When CB started to tale off in the early 60 and Regency
Electronics (AKA Idea Corporation) started building high quality units
Regency looked for some technical advantage to why people should buy and
pay a premium for their units. Regency determined that if they used a
combination of regular AM and SSB that they would have a slight advantage.
The transmitter had a DC input power of 5 watts meeting the FCC rules and
modulated with a 20 watt audio signal. The modulation was a Double Sideband
for several reasons. The filter to remove one of the sidebands was very
expensive at the time, and with transmitting both sidebands a regular AM
receiver would not notice any difference from a standard AM transmitter.
The power in a standard AM sideband was a maximum of 2 watts or audio
where as in the Regency the audio was equivalent to 10 watts (actually
about 8 watts when you factor in all losses) but as you can see it was
about 4 times the audio power. The model of this Regency was called a
Range Gain. Regency went further and added a BFO to the receiver and
eliminated the carrier in the transmitter in a step toward making it like a
SSB transmitter. Only this style had both sidebands being transmitted. It
was called a Double Sideband Suppressed Carrier.
Well the FCC didn't like this too much and changed the rules to limit the
"audio" power to both SSB & DSB transmitters. I don't remember the exact
specifications but it amounted to about 8 watts of maximum audio power. By
this time the Japanese were making cheap radios and the quality USA made
stuff went by the way.... Browning, TRAM, Regency, Courier, etc.
My dad was in the NAVY back in 1954 and he flew with the Blue Angels as their crew chief. Many times he flew in cargo planes with the spare parts and tools. At times he carried a pocket AM radio with him and he loved to listen to country music out of Nashville - while he was traveling up and down the east coast.
When he came home, the Civil Defense was a big thing and they all went to CB radios - with a license, in case of a national emergency, so they could communicate with each other. Which in a way, is kind of a joke, because a 4 watt transmitter in the coal patches of Pennsylvania had a hard time traveling more then 50 miles with a 1/2 wave antenna.
Here is part of the conversation -
To continue our phone conversation from Tuesday.
The normal output from a CB radio was 4 watts.
4 watts @27 MHz is good for about 25 miles.
The distance can be increased depending upon numerous things, elevation,
antenna gain etc.
Ionosphere bounce (Skip) is very common at 27Mhz and communications even
using single element (car whip style) antenna across the US or even across
the ocean is possible. The furthest that I talked was to Sydney and
Melbourne Australia.
The beam antenna like you dad had up had a gain of about 12db. That is
like 16 times the power so a 4 watt into the antenna made it like 64 watts
into a whip antenna.
The standard modulation was AM.
AM has been around from the 20s, was easy to modulate and demodulate. It had some drawbacks.
1. It was not very efficient. If you had a 10 watt transmitter you needed
a audio power of 10 watts to modulate it to the maximum 100%. 50K signal
like KDKA am required 50,000 watts of audio.
2. Other outside electrical noise easily caused interference on the
receiver. Lightning, power line noise, car ignition, etc.
3. The receiver "threw away" 1/2 the power
In a AM signal there is the carrier and two sidebands the carrier + the
modulation and - the modulation.
Example: WCEZ in Punxsy has a carrier frequency of 1540 kHz. To make
things easy, Let's assume that they were sending a single 1KHz audio tone
for a minute. If you looked at the output of the transmitter on a spectrum
analyzer you would see the main carrier at 1540 kHz and a signal at 1539Khz
and one at 1541KHz. The two sidebands would be 1/2 the level of the main
carrier. The receiver only uses one of the sidebands and discards the
other. The main signal, the carrier, is there just to carry the
information modulation through the air.
Single Sideband was a major break through for AM type communications. In
SSB only the information is being sent, no carrier. This is great from a
sending view but the receiver needs a signal to tune to for demodulation.
So in SSB receivers there is actually a little transmitter that fakes the
receiver into thinking that the carrier is there. This transmitter is
called a BFO for Beat frequency oscillator.
The Federal Communications Commission authorized CB in 1958. At that time
they permitted Amplitude Modulation at a maximum of 100% and a final tube
input power of 5 watts DC. The reasoning was that the most efficient
design amplifier would be about 80% which would generate about 4 watts of
output power. When CB started to tale off in the early 60 and Regency
Electronics (AKA Idea Corporation) started building high quality units
Regency looked for some technical advantage to why people should buy and
pay a premium for their units. Regency determined that if they used a
combination of regular AM and SSB that they would have a slight advantage.
The transmitter had a DC input power of 5 watts meeting the FCC rules and
modulated with a 20 watt audio signal. The modulation was a Double Sideband
for several reasons. The filter to remove one of the sidebands was very
expensive at the time, and with transmitting both sidebands a regular AM
receiver would not notice any difference from a standard AM transmitter.
The power in a standard AM sideband was a maximum of 2 watts or audio
where as in the Regency the audio was equivalent to 10 watts (actually
about 8 watts when you factor in all losses) but as you can see it was
about 4 times the audio power. The model of this Regency was called a
Range Gain. Regency went further and added a BFO to the receiver and
eliminated the carrier in the transmitter in a step toward making it like a
SSB transmitter. Only this style had both sidebands being transmitted. It
was called a Double Sideband Suppressed Carrier.
Well the FCC didn't like this too much and changed the rules to limit the
"audio" power to both SSB & DSB transmitters. I don't remember the exact
specifications but it amounted to about 8 watts of maximum audio power. By
this time the Japanese were making cheap radios and the quality USA made
stuff went by the way.... Browning, TRAM, Regency, Courier, etc.
