Recently in Commercial Radio Category

Moseley TRL-1 and CIP-2 Manuals

Highly unlikely you'll need these unless you're a broadcast engineer servicing some old equipment.

If you're interested in two-way or microwave, you might take note of the 200KHz IF filters the device has in it. One can cram a lot of data through that interface. (GRAPES modem anyone?)

TRL-1
CIP-2

These are meant to be printed with "auto-rotate" turned on, except the backsides of the 11"x17" pages were not scanned. Everything has been rotated so that one may view the PDF without printing or turning one's head sideways every other page.

Tower Work

Do you have an abject fear of heights? Do you quake looking through a glass or metal floor above twenty feet? If so, you probably shouldn't be doing tower work. But if you are, here are a few other things you should be aware of:

If you need a rope to haul anything other than your tools, you need a ground crew.

Just because an antenna and/or bracket moves smoothly on the face of the tower doesn't mean you can lift it, or take it off the tower by yourself.

Never estimate the amount of time it will take to finish a task working alone. It will take longer, and eventually you'll find some point where you will need more power from the ground than you can exert on your own.

I'm going to update this as I go, so look for more lines to be added over time...

Understanding I and Q modulation

This article is borne out of an attempt to explain modern modulation methods to a friend who has only heard of AM and FM.

http://education.tm.agilent.com/index.cgi?CONTENT_ID=4

This article explains and illustrates how a difference between the "I"  and "Q" produces modulation. I can be thought of as In phase. Q is short for Quadrature phase, which is the phase difference when measured from a fixed distance of -90 degrees from the I signal.

Is there is fixed shift in frequency, the resulting modulation is a circle. When viewed along the time axis, the circle becomes a spiral. If the spiral moves to the left (left-hand circular polarization), the frequency is higher than the carrier frequency (I leads Q). The converse is also true; the spiral turns to the right if the frequency is lower than the carrier frequency(Q leads I). Either of these signals, viewed on an oscilloscope, appears as a circle turning one direction or the other depending on the leading component.

If you have the ability to modulate the I and Q phases directly, any known modulation may be synthesized using a mathematical expression. AM is perhaps the most simplistic to represent since resulting occupied frequency is symmetrical. Holding the I phase steady in amplitude and phase while varying the frequency of the Q phase results in frequency modulation. It is almost as if varying the I phase amplitude modulates the signal and the Q phase frequency modulates the signal and the two together make any modulation, but this not accurate since QPSK is a phase modulation. It would be more appropriate to say that phase modulating both axes produces QPSK, and through that ability QAM, FM, and AM may be created. For this example I do not distinguish between AFSK, BPSK or FM, nor MSK or GMSK since they are all frequency-modulated modulations despite individual complexities.

One point of note that I have not seen in the books yet is whether or not the QPSK or PSK signals are subject to pre- and de-emphasis of the signal as a result of the modulation. In two-way radio, phase-modulation came about as a cheap way to frequency modulate transmitters, however it has a built-in pre-emphasis of the transmit signal, amplifying higher frequencies more than lower frequencies as a result of the modulation process. In the data world, this causes the resulting signal to occupy more frequency bandwidth than the symbol rates would otherwise imply.  

Here is another good explanation.

Single-Frequency Networks

I'll expound on this later after I've thought about it more.

I just ran across Single frequency network on Wikipedia, and was struck by one paragraph:

"An SFN may also increase the coverage area and decrease the outage probability in comparison to an MFN, since the total received signal strength may increase to positions midway between the transmitters."

The FCC provides a "protected contour" of estimated signal level to a broadcaster according to the license of that broadcaster and other agreed upon terms and conditions. The broadcaster is responsible for engineering, construction, and maintenance of such facilities to permit distribution of an electromagnetic field over that contour without impacting the operations or protected contour of another broadcaster. The strength of the electromagnetic field determines received signal quality and essentially limits the possible audience of that broadcaster.

Within the above in mind, it may be possible that by switching to OFDM or "Digital TV," future television broadcasting opportunities may be presented through the use of translators.

At present, translators are used in television broadcasting, but are licensed to different television channels and are assigned callsigns which may bear no resemblance to the registered marks or brand of the owner, operator, or beneficiary of the translator. I say beneficiary as the translator may not be owned by the television or FM radio station, and may or may not be claimed by the broadcast station as an additional market presence.  In the FM radio band, the licensure of every available combination of frequency and power has turned most formerly small markets into what metropolitan radio markets resembled a decade ago: A wall of radio stations from band-edge to band-edge, with seemingly no space between stations. The translators further compound the problem by using another radio channel at a lower licensed power than the FCC would otherwise permit a broadcaster or would-be broadcaster to implement -- anywhere from one watt to four hundred watts. 

However, back to the idea of the single frequency network, it is possible to implement a signal frequency network or simulcast network using multiple transmitters in multiple locations to completely saturate the protected contour of that broadcast station. The chief difficulty in implementation is that each transmitter should have an extremely stable source of frequency synchronization, as interaction effects between the two radio waves will cause signal nulls and other phenomena which will reduce the coverage of the transmitters in spaces where both signals may be present.

This entry is at present incomplete.

TDM, The only way to go

I have read to so much about various VoIP systems being pressed into radio use, but many of them ignore time as a requirement for audio. The chief reason why time is so important is that in a voted or simulcast system, you must have constant time figures across the link, or the noise comparators will be comparing dissimilar signals, resulting in unpredictable voting behavior. It may be simple enough to build a conventional FM repeater to pass a signal between two points, however the repeater has the notable disadvantage of "coloring" the audio passed through it. The audio that is recieved is subjected to the reciever's de-emphasis network as well as the transmitter's pre-emphasis (which is not a network, but a side-effect of phase- or frequency-modulation). This is documented and explained in depth here: http://www.repeater-builder.com/tech-info/flataudio.html However, due to this constant cycle of audio processing and deprocessing, the passed audio is "colored" after crossing each stage. This requires equalization of the path, to restore the audio back to something resembling a "flat" path, which does not express such characteristics. The simple way to avoid all of this complexity is to digitize the audio as soon as possible, and send it over the air as a digital signal. This is what Ma Bell started doing in the 1970s, and continues to do to this day.

Furthermore, if E&M signalling is used in the link, with E&M four-wire line cards, there is a seperate transmit, reciever and signalling pair presented to the end user. E&M signalling is bi-directional, so there is a local switch input, and a remote relay output. These two signals are very simple to connect to a reciever and transmitter, allowing carrier squelch detect to be sent and push-to-talk (PTT) signals to be sent!

There is Part 15 Microwave equipment manufactured which support a full-duplex T1 (1.544Mbit/s) or E1 (2.048Mbit/s) radio link. The equipment was manufactured by Western Multiplex, and put in use all over the country. One well known radio station located in Athens, AL used a T1 radio link to get a Huntsville-local phone number, while having a physical station location that is two LATAs away. Even if the station had subscribed to the "Area Calling" service from the telephone company to defray the cost of what would have otherwise been long-distance telephone calls to Huntsville, the stations's listeners would have had to dial a phone number that was not in the local Huntsville calling area to reach the station. Using the microwave TDM link prevented this, and subjected the station to telephony outages when the link failed during the projected downtime for the wireless link. The station management felt at that time (and still to this day) that the outages were an acceptable trade-off.

 
In the modern telecommunications arena, the sort of circuit is frequently implemented using wireless OC-3 hardware carrying DS1 trunks, as the ATM circuit may be seamlessly and automatically rerouted if a physical link goes down. One local public service provider has implemented such equipment in the fashion of a large ring around a tri-county area, and is using it to provide radio backhaul to a centralized switching location as well as supporting remote bases in that tri-county area. Since the system is full-duplex, the audio delays are static in length, and audio circuit response is flat. Predictably, the system is a smash hit. To date, the only complaints I've heard from the operators are that the combiner networks used to combine ten transmitters into a single antenna have too much loss. Unfortunately, big cavity resonators have quite a bit of loss outside of thier resonant frequency.

In amateur radio, the attempts to implement a similar solution have entirely too many cut corners, brought about largely by the availablity of affordable equipment, and a lack of solutions to fill a smaller need. For instance, in planning to accomplish a goal of a radio link between Birmingham, AL and Huntsville, AL, it is necessary to make at least one hop in the middle of the path, or two. Ma Bell chose to make five, but she was picking up other communities along the way. I have identified a site that would be perfect as a midpoint, however, I do not need twenty-four channels of voice, nor do I think that I could "resell" any of those channels to other amateur radio operators, or the owner of the tower on which the equipment would have to sit. Furthermore, the need to have two different frequencies complicates matters, as each T1 radio would need to have a solid dish, and hopefully nothing in near sight of the dish that would allow the signal to be bounced back into the other dish. The typical ham implementation uses 802.11b, 802.11g, or 802.11a for a wireless backhaul, each of which is half-duplex! This results in unpredictable delays, not to mention much wasted "air bandwidth" waiting for the reciever to lock once the transmitter is on the air. And since all transciever control is handled in the protocol and driver levels, it's virtually impossible for the end-user to tune the transmit-to-recieve times the way that a packet radio (AX.25) user may by altering PACLEN and TXDelay. 

A perfect solution for this problem would be a radio link that had an ISDN or double ISDN width over the air. This would permit call-status information to be sent, as well as two full-duplex audio paths. If one so desired, it would be possible to use the entire bandwidth to support a data transmission at 128 to 144 kbit/s. This would obviously require about 256KHz worth of radio bandwidth, not including guardbands for an MSK implementation. While QPSK and QAM may be used to lessen the radio spectrum requirements, delay is introduced as well as a requirement to use linear amplifiers. Using a linear amplifier increases cost, reduces sources of the amplifier (if pulled from other surplus or used equipment) and increases operational costs, as the amplifier efficency drops from eighty percent to thirty to fifty percent. GMSK is a possible modulation solution which may be implemented in such a fashion as to allow class-C amplifiers to be used.

GMSK achieves approximately 0.7 bits per Hz of occupied bandwidth, which for a 144kbit/s data channel (2B + 1D, ISDN standard) results in about 206KHz. Push that out to about 225KHz to allow a little guard banding, then realize you've got to have the same coming back in on another channel. Provided that the baseband doesn't experience a large group delays, and you can get filters wide enough, it should be possible to retrofit a two-way radio to handle the RF duties. From a signalling perspective, ISDN is clunky at best. There's no less than 500 different parameters, and you've got to order the standard from Bellcore or ITU in order to choose which ones to ignore and which ones to use. It would be a far better idea to pick two channels out of the twenty-four in a DS1 signal and send those over the link, rather than attempt to implement ISDN. Personally, I doubt that an implementer would suceed in finding E&M modules for an ISDN station adapter. It would be simple to use a DS1 channel bank or CSU/DSU with an E&M module or an Add/Drop CSU/DSU. The difficulty here is in implementing a solution cheaply and efficently, given the bar of providing a DS1 interface (and the 1.5Mbit/s signalling) or finding DS1 channel banks for cheap or free. 

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