I wanted 24 hour (or “military”) time from a bedside alarm clock radio.
The LM8560 datasheet showed that by applying Vss to pin 28, I might achieve 24 hour time display.
First, you must ensure pin 28 of the LM8650 is NOT connected to anything else! Or you might destroy your clock radio when connecting pin 28 to Vss.
In some cases, you may find there is a spot already for a jumper wire to Vss that’s just not been put in at the factory.
After adding the wire, you’ll see that time now goes in 24 hour style…for the “2” in the tens position of hours, it will just light the top segment of what would be a numeral one instead of displaying a “2”, for those displays with only a “1” in the tens hour place.
Clear frequencies are vital for 47 CFR 15.219 unlicensed AM MW broadcasts in the 510-1705 kHz range with 100 mW input power and 3 meter antenna (inclusive of ground lead).
To maximize range one has to consider more than ground conductivity and wavelength.
For licensed broadcasters, there is a presumption (even on the heavily populated local “graveyard” channels) that there is some standoff distance to the next transmitter.
Of course, the reason local AM MW channels are useless (low SNR) beyond 10-15 miles from the transmitter at night is the extremely high congestion (too many close transmitter) on these channels.
The long range of clear channel transmitters comes largely due to the implicitly very sparsely populated channels they exist on.
Of course, that assumes all transmitters have reasonably efficient antennas and ground systems to meet the minimum efficiency requirements of 47 CFR 73.182 and 73.189.
Complex $100 transmitters can have design limitations that are constricting due to the 100 mW INPUT power regulation.
The VEC-1290K is cheap and simple enough to allow for example replacing the inductors with higher efficiency (lower loss) coils.
Add bypass caps to the power supply and check the trapezoidal (X-Y) waveform for proper modulation depth.
Turn the LM386 modulator into a low-pass filter by changing the capacitor, or add another stage with dual opamp and dead-bug wiring.
Selecting AM Part 15 license free broadcast frequency: recent encroachment by second adjacents means at night calls for audio transmit bandwidth 10-15 kHz.
Non-CQUAM car radios might have only 3 kHz of bandwidth.
Center frequency is 1640 kHz has modulation from 1620-1660 kHz at day, and about 1630-1650 kHz at night.
To get the best frequency look two channels (20 kHz) up and down from the intended frequency.
If a third adjacent is nearly (30 kHz) go an extra channel away from them for poor selectivity receivers.
At night it’s tricky to find a full bandwidth clear channel, even in the expanded band, which is the only place for license free AM that gets decent range day and night.
Some commercial stations have full 20 kHz at day but switch to a lower bandwidth 10 kHz broadcast at night on a different channel, maybe one channel up or down.
Note! Unless running less than 3-4 kHz audio bandwidth don’t use 1700 kHz center frequency since the cutoff for Part 15.219 operations is 1705 kHz–that’s absolute, not center frequency of 1705 kHz.
Another trick is for the receiver to deliberately tune off frequency, such that the carrier and one sideband are captured.
This only works for analog receivers of course, and the trend is to digital frequency receivers.
The transmitter can’t slide too far off center for the digital car radios.
Improving Part 15 AM broadcast SNR and range: at nighttime, audio compression would help make apparent SNR higher by increasing the loudness of quiet passages.
Adding an expander circuit to the receivers akin to Dolby B noise reduction would add nearly 10 dB apparent SNR.
Keep modulation clean and near 100% peak to maximize coverage.
High quality audio sources: the fidelity of even 14.4 kbps RealAudio well exceeds that of conventional AM.
Hear distant AM radio stations on an AM radio–via AM transmitter connected to computer internet streaming audio.
The usual phonograph and CD sources (remember licensing for public performance issues) work as well.
Where SNR is adequate and with receivers modified to pass 20 kHz, a Part 15 AM MW broadcast system can exceed the fidelity of FM radio.
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).
Some semipro studios have distribute music on CD-R, which gives audio fidelity improvements over plain analog cassette.
However, CD-R can fail in as little as 3 years, maybe less.
The recording contrast–the difference between binary 1 and 0 is much fainter than on a standard CD to start.
As the CD ages, particularly on cheap CD-Rs with poor sealing, the metallized layer starts to get microscopic holes that become visible holes.
Some car CD players have a hard time playing CD-Rs.
The players probably have never been cleaned, and the reduced SNR of CD-R is too much for them, and they skip on playback (audio glitches).
It’s possible to distribute music on VHS with Hi-Fi audio that most decks are capable of.
HiFi VHS audio is recorded using the video head at a video frequency.
Yes the 70 dB dynamic range uses compression, but the end result 20Hz-20kHz is nearly as good as CD, and should last a decade.
MP3 at 192 kbps nearly replicates the source CD audio fidelity for all but the most demanding listeners.
I would not call 192 kbps MP3 archival for pro studios of course, but for distributing music by amateur bands and semipro, it’s fine.
A better choice is AAC at 128 kbps, if the players support it.
Given increasing Internet connectivity, it is reasonable to assume over time that Internet purchasing and downloading of music is going to eventually dominate carrying spinning CD’s around.
64 MB of flash memory, whether SmartMedia or CompactFlash etc. can hold an hour of music at 128 kbps.
That’s like carrying a single CD.
Dropping the bitrate to 64 kbps and getting a 128 MB flash card gives you four hours of reasonable fidelity music.
In four hours of FM radio in an office setting, you’ll get worse fidelity and more repetition!
Looking at the meteoric rise in hard drive capacity vs. price, perhaps flash storage will eventually be the way to distribute music.
Not from the store itself due to cost, but by selling music downloads.
Perhaps the downloads would be cached for low-demand internet bandwidth times, and you’d sync in a dock like Palm Pilot.
Every high quality audio format targeted toward consumers except CDs has failed.
Like VHS in the early 1980s, good enough when cheap is all that matters to the masses.
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.
