A Galaxy Saturn Turbo (a.k.a. RCI-2990) pairs well with an Astatic D-104 microphone.
The heatsink cools power supply pass transistors and the RF amplifier.
The power supply is a known weak point in this radio, so don’t want to overstress the pass transistors with excessive transmit power.
To improve cooling, an idea is to run the fans at 9 volts when not transmitting, and use a MOSFET to short the resistors to provide full 13.5 volts when transmitting.
A 555 timer provides the delay after transmit, to allow for cooling 15 seconds additional after the transmission ends.
These tips apply in general for AM / SSB transmitters.
Don’t crank up the ALC control for SSB.
Pushing the SSB transmitter into the non-linear range splatters the band for tens of kHz with decrease in communications usefulness.
Set SSB transmit power with two-tone test input to the rated 100 watts, which is comfortably linear and clean (and doesn’t overstress the power supply).
The AM Limiter and carrier should be set to allow clean modulation with an oscilloscope.
Maximum transmit carrier 25 Watts, minimum 1/2 Watt.
Monitoring transmit quality: on AM, use a germanium diode with a pickup wire in the air into a microphone input of a Radio Shack SW/PA radio.
This gives very clean broadband monitoring with headphones.
For SSB, one needs an oscilloscope with sufficient analog bandwidth for 30 MHz.
You can get a sense if something is amiss by monitoring the adjacent channel with an AM CB, where the receiving CB has no antenna.
If one hears spits and splatter, the transmitter SSB ALC is clearly set too high.
For FM, don’t touch the transmitter deviation control unless a spectrum analyzer or FM deviation meter is available.
The spectrum analyzer is used with
Bessel function table
(or an FM deviation meter) to precisely set the maximum transmit FM deviation.
For FM CB radio and 10m FM simplex, the maximum FM deviation is 2.0 kHz.
This was determined by regulators considering the standard 10 kHz channel spacing.
In contrast, in some parts of the world, 10m (29 MHz) FM repeaters may use 5 kHz maximum deviation despite the 10 kHz channel spacing.
This practice may have evolved from the earlier days of 10m FM repeaters repurposed from the commercial low-VHF FM business band, where 5 kHz deviation and 25+ kHz channel spacing was common.
We recommend setting the FM deviation to 2.0 kHz for widest compatibility with FM simplex operation.
When the FM deviation is set too high, the FM signal splatters into adjacent channels and causes unintended muting of squelch on receivers “talkoff” set for 2.0 kHz deviation because the wider deviation signal is interpreted as noise by the squelch circuit.
Throughout the mid-late 80’s, the FCC worked to help out the AM band, which has been in decline since the late 70’s.
One of the key actions was FCC Docket 87-267, which along with other moves simplified AM stations from ten classes down to four classes A-D.
Old Class I-A are the stations heard from half a continent away, 50 kW day and night with protected skywave coverage.
The new Class A subsumes the clear channel and clear channel-like licenses and allows daytime Class D stations to tuck in at the edges of skywave coverage with as little as a few watts of power at night.
No new Class D stations are authorized as of FCC Docket 87-131, but their ability to operate at low power at night is a big improvement over forced dusk to dawn shutdowns heard from “daytimer” AM stations only a few years ago.
Class B means in effect > 250 watts, and Class D can be 50 kW during the daytime, but less than 250 watts at night.
Real-world crystal radio performance: high power stations were too far away.
At night the local and regional AM channels are difficult to hear more than about 15 miles from the transmitter.
The only clear reception one can count on throughout is on the clear channel 50 kW stations.
Thus, the signal (field strength) is too weak to make much in a crystal radio set, when a superheterodyne receiver can hardly pull them out.
The answer: better coil (higher Q), better antenna (capture more of the available field strength).
The bigger antenna is not so practical indoors with noisy fluorescent lights overhead.
During WWII, improvised detectors ranged from cuprous oxide to coke or razor blade.
Modern radios might use a 1N34 germanium diode, which have a low threshold voltage and are often used as detectors in cheap (and moderate) superhet AM radios.
A CB radio base station antenna can be made with a piece of 50 ohm coax cable (such as RG 58) and 20-30 feet of wire of any gauge.
CB radio antenna dipole antenna: from ARRL Antenna Handbook:
468/27 ~ 17.3 feet
Construct CB Radio dipole antenna:
cut an 18.5 foot piece of wire (deliberately long to account for effects of environment)
cut again in the middle yielding two 9.25 foot wires
soldered each piece to the center and shield respectively of RG-58 coax
hung dipole up in the attic using a nail at each end into the wood.
Tune CB Radio dipole VSWR: instead of cutting the dipole wire for best VSWR, fold it over at each end until the VSWR was minimum at channel 20.
SWR under 2 is possible.
Physical space constraints are overcome by putting the dipole diagonally.
I waited for a semi-truck to drive by, and asked on channel 19 if they could hear me using the company name on their door to help ensure a response.
The first truck did have a CB on 19 and replied!
The old Johnson Messenger 123A AGC apparently was set to a soft background noise, or perhaps due to aging had reduced RF sensitivity.
The net effect was a gentle swoosh of static to blasting loud voice 10 times louder!
I only got about 2 miles range, but he was mostly off the end of the dipole.
So despite being about 25 feet above ground level, I only got 2 miles range.
I think I lost him before he lost me–only moderate receiver sensitivity perhaps.
The S-meter barely deflects with no one talking, so I don’t think it’s man-made interference as the range-limiting problem.
After a few more tries with similar results, I decided the issue was I needed a vertical antenna, to avoid:
possibly severe pattern losses off the dipole ends
cross-polarization losses (also possibly severe)
coupling with household wiring beneath dipole (raising peak elevation angle of radiation pattern)
I could not simply find 18 feet of vertical space.
I was not going to drill a hole in the ceiling or roof.
I cracked open the antenna book for the next iteration: vertical monopole.
The coax center wire raises vertically.
I found bending a few inches of the top perpendicular like a capacity hat gave better VSWR.
I soldered two additional wires to the shield and spread them out approximately evenly with 90 degree azimuth separation.
The VSWR was now 1.4:1, excellent!
Even better, base to mobile range is about 10 miles.
Base-to-base, I can hear about 30 miles and I say that because of low ambient noise level, I can hear base stations in nearby cities that can’t hear me due to their local noise level.