Extremely powerful transmitters may be rendered useless if the type of antenna chosen, height of antenna from ground, or path loss considerations are not taken into consideration. Due to the many variables within the early design of a system, the best approach is to perform a link analysis determining the overall margin of the link, and from this, decide where the cost effective tradeoffs may be made.

Atten (decibels) = 36.6 + 20 * log F(MHz) + 20 Log D(miles)

Printable version of above chart.

1. Use the nomogram to determine the attenuation in decibels or use the equation:
Atten (decibels) = 36.6 + 20 * F(MHz) + 20 Log D(miles)
Example: A 30 mile radio line of sight distance at 174 MHz will equate to
Atten = 36.6 + 20 * log(174) + 20 Log(30) = 110.95 dB

2. Next, verify that the distance in miles between the transmit and the receive antennas is acceptable, given the installed height of the antenna on a tower at each end. This will confirm that the curvature of the earth will not interfere with the radio wave propagation. Each sight under consideration needs to be investigated for obstructions between the transmitting antenna and the receive antenna. For example, mountains, major buildings, heavy wooded forests, etc. all may have a substantial impact to the quality of the link especially when rain, ice, and snow are covering the above obstructions. Use the following nomogram to verify antenna height or distance in miles between stations.

3. Determine the level of reliability desired within a communication link in terms of percent reliability and read the corresponding value of fading/multipath losses:

Percent Reliability (%)	Fading Margin (dB)
90	10
99	20
99.9	30
99.99	40

4. Calculate the path margin using the following equation:
Link Margin = [Transmitter Power] + [Transmit Antenna Gain] + [Receive Antenna Gain] - [Cable Loss at Transmitter] - [Cable Loss at Receiver] - [Attenuation Loss] - [Fading Margin] - [Receiver Sensitivity]

Example: Using the following typical equipment specifications coupled with the desired reliability of 99.9% over a 10 mile path:

Radio Transmit Power (174MHz)	4 Watts (+36 dBm)
Radio with Receiver Sensitivity	-105 dBm
Transmit Antenna Gain	+7 dB
Receive Antenna Gain	+3 dB
Tx Cable Loss (RG-8)	.5 dB
Rx Cable Loss (RG-8)	1.2 dB
Attenuation over 10 Miles (above)	110.95 dB
Desired Data Reliability (Fading Margin = 30 dB)	99.9%
Link Margin = 36 + 7 + 3 - .5 - 1.2 - 110.95 - 30 - (-105)
Link Margin = 8.35 dB

Final Comments
1. Always use the lowest loss cable possible (or within budget) for antenna cables. Typically, remote sites have short cable lengths but base stations most often have over 100 feet of cable between the transceiver and antenna. In this common application, standard RG-8 cable is suitable for the remote antenna cables but the base station may benefit from use of a low loss rigid antenna cable for the long distance between the antenna and the transmitter. Quite often, 3 dB of loss may be recovered through the use of low loss cable instead of increasing the transmitter output power by a factor of two.

2. The above link analysis is calculated on the basis of a clear Line of Sight path meeting the above nomogram requirements. If dense foliage, trees, buildings, or other obstructions are between the transmitter and the receiver, then much more margin will be required to compensate for the poor weather conditions.