B&B Electronics Modem Wireless Proprietary RF, Antenna and Cable Selection, More is Not Better…

Communication

Wireless Serial USB USB

Wireless Proprietary RF

R.F. Antenna and Cable Selection

When designing an antenna system there are several items to	level arrives at the receiver. This will allow some “link margin”
consider. Factors that affect the performance of the antennas	in situations where conditions change and the environment
are highlighted below. An RF communications system should	degrades the signal.
be designed such that more than the absolute minimum signal
Radio Output Power. RF power is rated in watts or milli-watts.	Antenna Type. If at all possible your system’santennasshould be
Typical B&B Electronics Zlinx products are rated in 1mw,	able to “see” each other.This is not alwayspossibleso it’scritical
10mw, 100mw, 500mw and 1watt, outputs. As RF power	to select the proper antennacomponents to keep signalloss or
increases so will the current demand on the power supplying	decrease receive sensitivity.If obstacles are present,you may
your system.	consider a higher gain antenna.
Radio Receive Sensitivity.Receive sensitivity is basically the ability	Cable Type. Selecting the incorrect cable could cause
to detect and decode a specific radio frequency.Typicallythis is	significant signal loss. A rule of thumb is for every 3db of loss,
measured in dBm - the lower the better.Typical Zlinx products	your system will lose one half the output power emitted from
are approximately -114dBm.	the radio. Visit our antenna selection tech paper a helpful
	selection guide and more information:
Transmit/Receive Distance. Keep in mind that if you have lots of	www.bb-elec.com/techpapers.
transmit power but poor receive sensitivity,bidirectional	Obstacles. Obstacles are always a problem. General rule: Try
communicationmay be a problem. The type of cable and antenna
used will affect the receive sensitivityand RF output power.	to avoid them. When placing an antenna, keep in mind that
Cable Impedance. It’s important to select a cable that matches	trees grow, new buildings are constructed. Plan for future
	obstructions.
the radio’s impedance. A mismatch will cause the radio link to
become inefficient or may cause damage to the radio.

More is Not Better…

Keeping the above in mind, one of the most important components in an antenna system is the transmission cable. The wrong type, wrong impedance, wrong length, etc., and your system could go south quick. When selecting RF coax type cable keep cable runs as short as possible. Long cable runs will cause additional signal loss.

Peer to Peer vs. Peer to Multi-Peer…

When selecting an antenna also keep in mind the type of system architecture. Simple systems, such as peer-to-peer, can be easily designed using Yagi antennas on both ends. A multi- peer system, such as a Modbus master to several slave devices, needs a little more thought. When designing a multi-peer system the master needs to communicate to all the respective slave devices. This is typically accomplished by designing an

omni-directional antenna on the master and Yagi antennas pointing at the master antenna.

Antenna Selection…

The same rule applies for the gain of an antenna: for every 3db of gain, you will double the effective radiated power out of the respective antenna. A gain of 8.1db using a 100wm radio is approximately 270mw of output power. (Keep in mind we have not included any cable or connector losses.)

70

Cable Specifications
Cable Type	Loss at 900 MHz - per 100 feet	Loss at 2.4 GHz - per 100 feet	Diameter
LMR-195	11.5 dB	19.0 dB	0.195 in.
LMR-400	3.9 dB	6.8 dB	0.405 in.

Antenna Gain Rule-of-Thumb
Frequency and Wattage		300 feet	1000 feet	1 mile	5 miles	10 miles
900	MHz, 100 mW	2.1 dB	2.1 dB	>6 dB	>10 dB	n/a
900	MHz, 1 W	2.1 dB	2.1 dB	>3 dB	>6 dB	>10 dB
2.4	GHz, 50 mW	2.1 dB	>6 dB	>10 dB	n/a	n/a
2.4	GHz, Zigbee	2.1 dB	>6 dB	>10 dB	n/a	n/a

Typical radiation patterns using Yagi and Dipole type antennas.

www.bb-elec.com 1.815.433.5100