The 11 year solar cycle has brought a day of reckoning for CB radio.
The short winter days will give some respite, but skip is sometimes heard when the sun is below the horizon.
A sure sign of increased ionization supporting increased MUF up into the 27 MHz CB radio range.
Shortwave listening in the 5-10 MHz range impacts are not as dramatic for high power shortwave stations vis-à-vis SNR.
With broad frequency choices, ham radio takes advantage of a clearer diversity of frequencies.
If more people used SSB on CB, frequency sharing would be much more efficient.
Without the capture effect of FM, the typical AM mode leads to loud squealing heterodynes.
Narrowband FM on 27 MHz CB radio capture effect wouldn’t be nearly as effective.
GMRS doesn’t have quite the appeal of 27 MHz CB since the user base on 462 MHz is split between incumbent businesses who are in no mood to chat (and may even be paying for a repeater), inexpensive FRS walkie talkies, and lack of enthusiasts.
eBay sells radios from a distant city trading up from crystal controlled radios.
Even the crystal radios are often two or four channel.
We could setup a common calling frequency for chit-chat and then have a second channel for each group.
WLS 890 AM out of Chicago has joined the growing legion of stations broadcasting on the Web.
Using RealAudio format, a good dialup connection can just manage reasonable quality audio.
Usually much better audio than trying to listen with a regular radio despite their immense multi-state groundwave coverage.
The increasing electronics and metal building construction rob indoor listeners of traditional AM radio coverage, and even broadcast VHF FM radio.
This online listening format will probably only grow as internet connections and computers become faster.
Nextel makes deals with SMR operators with fully built and utilized repeater systems.
Licenses were pulled from SMRs that aren’t actually fully built and on-air.
To make a multi-channel trunked repeater from salvage parts, consider filtering and connections.
Unlike lower frequency bands where repeaters like Motorola GR300 or the Motorola R1225 repeater that are essentially refitted mobile radios for transmit and receive, 800 MHz subscriber receiver are not trivally retunable by software alone to the 806-821 MHz mobile transmit band.
The easier plan is take the 1980s scanners being dumped on eBay as people pickup TrunkTracker scanners.
For example, the PRO-2004 has specified 0.5 μV sensitivity.
Since we need a receiver distribution amplifier anyway, just leave 6 dB less padding than before–problem solved!
For filtering, that will be part of the distribution amplifier–it will filter out the strong transmit signal on the separate antenna as well as any lower frequency signals (FM/TV/UHF/VHF) and any higher frequencies (AMPS 850 MHz).
Given the location there shouldn’t be too many strong in-band signals, though overloading is possible within a km or so of the site.
If that became an issue I could put a cavity filter on that channel, again from eBay.
But for simplicity let’s stick with the receiver distribution amp 806-821 MHz filter.
The audio connection is made at the discriminator tap since DC-coupled baseband is needed to retrieve LTR data.
The volume control is all the way down to avoid bothering other workers in the radio room.
Leave the scanner speaker connected for basic diagnostics.
We use the EF Johnson 8600-series radio–get the full feature model with up/down arrows, not the single button model as you need talkaround!
Set the radios to 10 Watts transmit power so that they can handle the duty cycle, and to help reduce thermal cycling stresses.
Program each radio to a single channel, talkaround on the repeater output frequency.
Bypass the microphone jack to get the full non-pre-emphasized DC response to the modulator.
Combiner: any 800 MHz SMR combiner is fine.
Receiver distribution amplifier has three parts: 806-821 MHz combline bandpass filter, amplifier, and splitter.
These tend to come in multiples of four as it’s easier to cascade that way as per Chip of Angle Linear.
I managed to find a used one but in the future I would consider Angle Linear for even higher performance with his 0.7 dB NF amplifiers.
LTR controllers are not particular to the transmitters, but can be particular to each other.
EF Johnson and Uniden use distinct backplane signaling.
Others are switchable via jumper.
Uniden doesn’t need a terminator while EF Johnson does need a 50 ohm terminator.
Using a mag-mount antenna on a car roof as well as a trunk-lip mount, I compared the signal strength of the constant carrier on UHF (460 MHz) from a drive-thru intercom system.
The intercom leaves a constant carrier on the air, that even to a base station is only audible for maybe 2 km.
The intercoms are typically licensed in the 460-469 MHz range, 100 mW ERP.
We didn’t convert AGC measurements (voltage proportional to signal strength) from the Kenwood TK-805 to dBm yet.
Urban obstruction UHF diffraction: pull up next to a long building two stories tall, to get signal from over the building without nearly as much signal from the sides of the building.
About a kilometer from the transmitter, the building absorption would eliminate the remaining direct-path signal.
We should mostly be getting signal from over top the building.
For every distance from the building up till the end of the parking lot, the 1/4 wavelength antenna did at least as well, for the roof-mounted location.
The 1/4 wave antenna on the roof did better than the 5/8 wave trunk lip mount, because the radiation center of the 1/4 wave antenna on the roof was higher and had an evenly surrounding groundplane.
Rural UHF 5/8 wave vs 1/4 wave: going down a straight road from the intercom, with little nearby obstructions, the 5/8 wave antenna on the roof performed a little better; it filled in the “holes” of the picket-fencing signal a little bit more than the 1/4 wave antenna on the roof.
Future test thought: log AGC voltage vs. time with ADC.
A voltage-to-frequency converter would allow recording this signal on a laptop sound card, sampled at several kHz (sample rate-limited perhaps by the AGC time constant).
Drive at constant speed, with noted start/stop landmarks in lieu of GPS tagging of measurements.
Convert AGC voltage to absolute RX dBm (may be slightly frequency dependent).
We recommend 1/4 wave antennas to customers who may have overhead height limitations (garage door) and especially for those who work in forested or urban areas.
In flat rural areas, it is of course expected that a 5/8 wavelength antenna on the roof will out-perform the 1/4 wavelength antenna on the roof by a couple dB, by the design of the antenna.
Why then are high gain antennas so popular on 800/900 MHz, in locales where you see 1/4 wavelength antennas at VHF and UHF?
Because at 800 MHz the reflections from buildings and objects is stronger than at VHF and UHF.
The thought at 800 MHz is to take advantage of multipath with dual receive antennas–even on advanced SMR sites, not just cellular.
This effect is seen at 800 MHz even with yagi antennas–sometimes pointing the yagi off-boresight to the tower gives a better signal, where there is not a line of sight path (NLOS).
The EF Johnson 9755 Summit DM 800 MHz 35 Watt mobiles are designed for public safety Multinet II systems, yet suffer from reliability issues vs. their more reliable 9753 Summit DM 15 Watt mobile version.
High power 800 MHz amplifier repair:
The Q501 45 Watt MRF847 NPN transistor output (collector) pin gets burned up.
This is about a $50 part if it needs changing.
This manifests itself as a crackling noise heard on the transmit audio most often.
We have never seen a failed Q501, which is a testament to the durability.
The commercial market EF Johnson 9883 800 MHz trunked radio is rated at 30 Watts with the same transistor denoted Q651.
The 9883 adds a stabilization board with extra components not in the Summit.
The repair process for the high power 800 MHz transistor output is as follows–this should be generalizable to any high-power VHF/UHF high power transistor.
Inspect extent of damage–is transistor output pin still mostly there? Is PC burned black or just a little solder bubbling? In the case too little of the collector is left, get a new Q501 part # 576-0004-817.
Remove all solder from the collector wing, gently lifting it up in the process.
Using copper foil, rebuild trace under the collector wing. The width is impedance-critical and keep edges meeting flush to avoid a major hotspot and reburning.
Recommend using high-temperature, high-conductivity silver solder when reworking.
Note:
Don’t use a piece of wire at these frequencies and current levels. It will quickly burn up again otherwise.
In repeated cases, setting the radio to 30 Watts output may help, or send to factory for a new board.
Interconnect block breakage: J501 or J401 can break due to high levels of vehicle vibration over time.
The usual intermittent solder joint checks work here.
Microphone jack breakage:
Sometimes officers don’t realize that the Hirose microphone jack is a slide back to release, NOT a twist off.
Repairing the flex PCB after it’s torn up from being rotated is not super pleasant but unfortunately not a rare problem.
800 MHz EFJ Summit DM Programming:
There have been and will be more firmware upgrades to the radio.
The factory programmer with the high voltage switch is required to put the Summit DM into flash mode.
Don’t let the System Key get out–anyone with the System Key can program radios on the system.
OK, System Key is far better than PL/DPL/LTR but still, EFJ should be using a hardware key for Multinet II security.
Keep in mind the system policy for priority–or a mobile can talk over dispatchers!
Antenna connectors:
Use an N-N plug barrel directly to the service monitor input, to allow proper tuning with the special Johanson ceramic tuning tools for the final amplifier output at proper power levels.
Excessive power will burn up the PCB.
The N-connector on the antenna cable in the car should always be looked at for improper assembly.
The low-loss Antenna Specialists grey cable for 800 MHz is something installers may not be familiar with, so don’t overlook problems there.
In low noise and skip-free environments SSB CB Radio can exceed performance of 2 m / 144 MHz FM simplex.
A 102" whip in the center of the roof can be a quiet, amazing antenna for 10 m as well as CB.
Mobiles I can hear to 25 km range in the city.
For the base stations from a car can approach 75 km range.
For that distance you both have to be in the country or at least the base has to be in the far out suburbs and car in the country.
Preamps help at both ends with an RF-quiet vehicle.
The EF Johnson 8622 full-duplex mobile and Viking CX 8180 flip-phone full-duplex portable can stay connected in areas with poor cellular coverage.
Uniden also has a full duplex car phone-like SMR 800 MHz radio, but I don’t remember the model number, though I’ve personally programmed and used multiple of them.
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.
EF Johnson CX 8180 portable radio
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.
