The EF Johnson 8625 full-duplex mobile and Viking CX 8180 flip-phone full-duplex portable can stay connected in areas with poor cellular coverage.
It’s possible to be as much as 50 km from the repeater tower with a good antenna system on the car and the high-power mobile radio, making full-duplex phone calls.
A main difference is that like legacy IMTS systems, the repeater tower is selected manually by the user.
An advantage over cell phones is that traditional group calling (and individual calling) of two-way trunked radio is also included.
Since Nextel’s Jan. 1997 public relaunch as a cellular competitor outside of its Fleet Call dispatch legacy, this type of service is nationwide.
Note that Nextel dispatch group/individual calls are restricted to local geographic areas.
This is necessary to reduce burdens on Nextel’s backbone network most likely.
Nextel telephone calls essentially work like any other TDMA cellular phone.
The EF Johnson full-duplex Viking CX 8180 handset actually looks like a flip phone with 1 watt transmit power vs. full power 3 watt handheld two-way radio on 800 MHz.
It’s like a thick flip phone, but with the super power of dispatch group/individual radio chat.
Before this, the most svelte two-radio was the Motorola Visar launched in 1993, as a very small handheld two-way trunked and conventional radio.
The manufacturing difficulty is not too different from the Motorola Visar, which is even thinner, such that the battery is twice as thick as the radio with the typically used battery.
Putting a duplexer in handheld phones, including the Viking CX 8180 is facilitated by the wide frequency separation.
For SMR, the mobile transmit band is from 806-821 MHz, and the base transmit band is from 851-866 MHz, a 45 MHz separation between transmit and receive.
AMPS analog cellular in the US and Canada also uses 45 MHz separation in the 850 MHz band.
The 8625 has 12 Watts of transmit power, unlike any of the 3 Watt bag phones.
Three watts are lost in the duplexer, which prohibits talkaround in this and other full-duplex models.
Instead of say 10-15 mile range of a bag phone, 50+ mile range is possible from the tower with SMR interconnect.
800 MHz full duplex interconnect:
incoming caller calls common number (without a DID controller), then enters repeater-group number (5 digit)
outbound: user select closest repeater, enter number and press send. half-duplex get dial tone or busy.
For reference, first generation EF Johnson Challenger radios are from the mid-1980s.
Second (1.5 gen) generation Challenger radios are from the late 1980s.
In general, brands like Regency and EF Johnson make radios that are cheaper initially than Motorola Maxtracs that target the same low-mid two-way radio market.
The key tradeoff is higher total cost of ownership in maintenance and reprogramming, as the Challenger may require opening the radio to mechanically tune for new, distinct frequencies.
These fixes require a 5/16" nutdriver, Phillips screwdriver and a scouring pad.
Here are some issues EF Johnson synthesized radios have, tailored to the EF Johnson Challenger.
Microphonics are a squealing sound on transmit and especially receive, with severe cases causing loss of lock and broadband splatter on transmit.
It is related to oxidation of grounds after screws loosen from vibration.
Some of the later synthesizers are packed with non-conductive foam to help mitigate this issue, with absorber (conductive) foam on the shield underside (not touching circuitry).
For the Challengers, the following items are key suspects:
shield over the final/pre-final amplifier is a key suspect
shield over the mid-bottomside of the main PCB is also often the culprit
shield over the component side of the main PCB is a secondary culprit
in the worst cases or proactively, the main board itself can be cleaned to ground, taking care not to damage the RF cavity filters.
Scrape lightly on the chassis to remove oxidation, desolder and re-tin the ground, and apply an anti-oxidant suitable for electronics.
The suitable anti-oxidant is Tuner Lube type products as are commonly available from GC Electronics and other sources.
Tuner Lube also fixes gaming TVs using a mechanical tuner permanently set to channel 3 that build up oxidation.
The TV tuner self-cleaning mechanism that doesn’t get used when they’re always on one channel.
Heavy vehicles vibration can loosen the screws holding the power amplifier bolted onto the EF Johnson Challenger radios.
This manifests as a scratching/hissing noise due to high resistance ground contact.
Sometimes the heat sink is loose.
Tap the heatsink with a non-conductive object while transmitting to verify.
For repeat trouble cases, use anti-seize compound on the screws, which makes them tack in better while still being removable.
The EF Johnson radios have a DC powered microphone amplifier, that under extreme dirty conditions can become shorted in the microphone jack.
Usually a brush and electronics cleaner will cure this, unless the filth has gotten between the jack and board.
A jack replacement may be warranted if the contacts are oxidized heavily.
The Audiovox FMC-1C is an FM converter (listen to FM radio on an AM-only car radio) from the late 1970s.
The user-tunable FM broadcast 88-108 MHz front end downconverts to 10.7 MHz IF, then into a TA7130P IF amplifier / detector IC, that is then modulated to 1.400 MHz AM signal.
In effect, the FM broadcast receiver connects to an micro AM transmitter.
The AM-only car radio is tuned to 1400 kHz and then tune the FM converter to the desired FM frequency.
Even if the AM radio is AM stereo, the FM signal will be mono.
The music fidelity is primarily limited by the IF bandwidth of the AM car radio, perhaps 3-6 kHz or so.
As digital cellular continues its march across America, the network fragmentation leaves those needing mobile terminals such as the transport (trucking) industry stuck in difficult situations as the terminals are locked to the particular network used by the provider.
With competing standards such as CDPD (AMPS) and the wider coverage offered by Mobitex and ARDIS, not to mention IS-136 and IS-95 networks, regional transport providers find substantial gaps in their dispatch networks, leading to the trusty payphone or bag phone being the last-mile link.
On the fixed routes where companies have multiple round trips daily, drivers will look out for other trucks from the same company and tell them dispatch wants to talk to them.
When drivers are issued bag phones that go from truck to truck, the lack of external antenna means calls can be missed while sitting at a loading dock, behind/inside buildings, or in wooded/hilly areas.
Even worse are handheld phones with the antenna retracted.
As an alternative, some companies use high power CB radios to give them several miles range outside the legal 40 CB channels.
Companies are reluctant to outlay for biz-band gear with FCC PR 92-235 narrowbanding hanging over them.
The push to 6.25 kHz equivalent efficiency is going to take more than FM as the advantage of FM over AM disappears with such low modulation index.
It means that a lot of 25 kHz gear is going to come on the used market (including AuctionWeb/eBay), so either companies are waiting for that, or more likely the rapid expansion of cellular.
People really value one to one communications, despite the many situations where one to many and many to many is advantageous.
In the end, I think one to one comms will win out, and there will be a massive hunger (more than exists already) for frequency spectrum to host all these communications.
As handheld digital terminals become more important, this will reduce the frequency crush temporarily, until everyone wants one.
Paging delay is becoming an issue in some areas as pagers become cheaper and more popular.
Like the rural telephone entrepreneur of a century ago, acquiring rights to string wire throughout town and countryside, eventually there are too many wires, which required investing in multiplexing technology, driving consolidation of rural telephone companies due to economies of scale.
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 ones 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.