Narrow band modem
The more and more bigger need for new frequencies and the saturation on the current bands force the administration to reduce the channel bandwidths for increasing their numbers.

Thus, since several years, the channel bandwidth has passed from 100kHz to 50, 25 and 12.5 kHz now. People are talking about 8.33 kHz (25/3) and the trend will continue.

For the data transmission, the necessary bandwidth depends on the desired transmission speed, of the mode and the used modulation index. Also, for transmitting faster with bandwidth more and more narrow, new modulation process had appeared such as FFSK, GMSK, DPSK, QPSK, etc…

For instance, a GMSK modulation doesn't allow to go over 9,600 bauds for a 25kHz bandwidth.

Double frequency changing
In a simple frequency changing receiver, the local oscillator (Fol) activates the mixer followed by the middle frequency amplifiers (Fi). Therefore, it is possible to receive two frequencies : (Fol+Fi) and (Fol-Fi). The choice between these two frequencies (desired frequency and "image" frequency) are realized by bandpass filters.

More the middle frequency is low, more it is difficult to realize the separation of the two received frequencies, but the amplification and the demodulation are easier and more efficient with narrow band.

More the middle frequency is high, more the separation of the received frequencies is efficient, but the amplification and the demodulation in narrow band are more complex.

The double frequency changing allows to keep the benefits of the two previous cases without having the drawbacks, and by this fact, is used commonly in high-performance receivers.

Coding
Most of the current radio modems works in frequency modulation with synthetized oscillators. In this kind of oscillators, the frequency command is realized by a vericap diode on which is overlayed the modulation. A "zero" induces a shift of -Fm, and a "one" induces a shift of +Fm. During a long sequence of "zero" and "one" level, the locking loop tends to catch the shift due to the modulation.

To avoid this undesirable phenomenon, one of the used techniques (FMO or Manchester) consists to cut these levels into a clear alternation of 0 and 1 so that the average signal value is null. In this way, what ever is the content of the modulation, the frequecy carrier doesn't vary.

Field Profile
The propoagation of radio waves in VHF/UHF is similar to the one of the light and is carried out at sight and in straight line. However, the transmission could be realized without been in this ideal case, by using particular propagation modes : Reflection, Diffraction, Broadcasting, etc.. But in these cases, tha propagation attenuations become quickly very important and it is proper to take into account during a feasibility study.

Efficient models allows to define the main parameters : transmission power, reception sensitivity, type of antenna, antenna location, etc..

Whatever is the distance to cover, it is imperative to know well the profile of the field for defining the characteristics of the material to use. 

Antennas
It exists a countless antenna types with quite different characteristics and each antenna type fits with a defined need. More, their cost is often proportional to their performances.

The main characteristics are :
- The radiation diagram : omnidirectional, bidirectional, directive antennas etc.
- The gain : the directive antennas have generally a more important gain than the omnidirectional ones
- The bandwidth : the narrow band antennas have generally a more important gain than broadband ones
- The polarization : straight (horizontal, vertical, etc.) or circular (left or right)

Their choice depends on large factors and remains a specialist job.
  
The directive antennas, beyond their interesting gain, allow to avoid perturbations of or from other users due to the fact of the narrowness of their radiation diagram, while limiting the transmission power required for a data link.

The vertical polarization in interesting in town and for buildings because it favour reflection on surrounding structures. The horizontal polarization is preferable for long range link. As for the circular polarization, it is used mainly for link with mobile stations (vehicule, plane, etc.) 

Installation
Whatever the antenna type, it's important to realize the installation carefully. The site where it will be installed will be chosen for its clearance relatively to other stations of the same network, but also for its distance from all perturbation sources or other surrounding networks. 

The antenna must be far away from all mass or structure capable to modify its characteristics and its functioning. The recommanded average distance is at least two or three wave lengths. 

It's often preferable to loose some dBs in a longer cable than to neglecte the antenna's installation itself. 

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