User's Manual

UtiliNet® Endpoint User Guide 11-1-2006

Page 21 of 30

Radio Link Budget

Link Budget is a term used to describe a radio link from transmitter to receiver, and all of the factors in

between. Link budget analysis uses a simple method of allocating appropriate factors to each portion of the

link and then algebraically adding these factors. Then, the proposed link can be evaluated by how many

dB’s are left over. To analyze link budget, you must look at each component of the communication system.

Examine a sample radio link budget. Table 8.3 on page 8-9 summarizes the results.

Transmitter

For the UtiliNet product line, transmit power is +20 dBm nominally (100mW). The production minimum is

+18 dBm. It is not unusual for a radio to measure +21 to +23 dBm.

Transmit Antenna

Frequently, antenna manufacturers will not advertise the unit of measure associated with antenna gain of

their products. The commonly accepted unit of measure for antenna gain in the land-mobile industry is

dBd, which refers to dB of gain relative to a dipole. If there is a doubt, this should be confirmed with the

manufacturer. For our analysis, we need the dBi unit of measure, so a conversion factor is needed. The

conversion factor is +2.15 dB, rounded to 2 dB for this example.

Example:

A “3 dB omni-directional” type antenna is used. The antenna gain may be stated as 5 dBi. The “3 dB omni-

directional” antenna here refers to a very common, low cost antenna design consisting of a 1/4 wave

element, a phasing coil, and a 5/8 wave element, over a large ground plane. Such antennas perform well,

and are readily available from a variety of sources. It is a standard antenna for many applications, including

land mobile radios and microcellular base radios. Refer to "Use of Directional or Gain Antennas" on page

8-14 for pertinent FCC restrictions.

Ground Plane Effects

In the example above, ground plane refers to the flat, horizontal, metal chassis or enclosure to which the

antenna is secured. The ground plane works in conjunction with the antenna’s vertical element(s) to form a

complete antenna. Some types of antennas need a large ground plane to function properly, and some need

no ground plane.

When a ground plane is required, but is not large enough, effective gain reduction results. If applicable, a

reduction of 1 to 2 dB on antenna gain would be in order for ground planes smaller than about 6 to 8 inches

along the narrowest axis.

Transmitter Transmission Line Loss

This is the equivalent cable loss of any transmission line that might be used to mount an antenna remotely

from the radio assembly. Many times the purpose for remote mounting the antenna is to get it up higher and

more in the clear. This functions to reduce the Path Loss Adder term by more than the amount of this

transmission line loss term, hence a net gain is realized. The maximum tolerable cable loss is a subjective

call. It is recommended that the cable type and length be selected such that this loss term does not exceed 3

dB. A cable loss of 1.5 dB would be a reasonable goal. For this example, assume a good quality cable is

used, and the loss at 915 MHz is 1.5 dB.

Total Path Loss