ADVANCED LESSON 60logomidi2.gif

             
LEARNING  OBJECTIVES and NOTES
EMC - Field Strength
7c.1 Recall that reducing field strength to the minimum required for effective communication is good radio housekeeping. Recall and apply the formula for the field strength surrounding an antenna given the ERP and distance from it.

The lower the power transmitted, the less the risk of causing interference. It is therefore advisable to reduce the transmitted power to a point where reliable communication is still possible.

Formula for calculating the field strength surrounding an antenna

Form_E=7.gif

E=the field strength (peak) in volts per metre
d= d is the distance from the transmitting antenna in metres
erp=effective radiated power from the antenna

This formula is intended to be used to calculate field strength over sort distances for EMC purposes.
Over longer distances the formula is less useful because of absorption by the ground, ionosphere etc.




Example 1
calc_Field_Strength_E.gif
EMC - Feeders and Antennas

7d.1 Recall that balanced antenna systems tend to cause fewer EMC problems than unbalanced antennas.
Recall that the feeder (balanced or unbalanced) should leave the antenna at right-angles to minimise coupling.

There are two main types of antenna feeder:
  • Coax
  • Balanced feeder
Coax is unbalanced and although convenient to use is more likely to cause EMC problems because of common mode currents on the outside of the coax braid. This can be overcome by the use of a 1:1 balun (balanced to unbalanced) where the antenna joins the coax.

Balanced feeder, for example 300 ohm ribbon feeder, 75 ohm ribbon feeder and open wire 600 ohm feeder, has an equal and opposite current in each wire which cancel each other out leaving nothing to be radiated from the feeder. This makes balanced feeder less prone to EMC problems. Of course if used with a dipole the impedance at the end of the balanced feeder is unlikely to be 50 ohm and so an ATU is required to match what ever impedance is present.

An advantage of the use of balanced feeder is that a dipole with the aid of an ATU can be used to tune the antenna on several amateur bands. This antenna is commonly called a doublet.

What ever type of feeder is used it should leave the antenna at right angles. If it is nearer one half of a dipole than the other, it could cause a change in the feeder current, leading to inbalance in the radiation pattern and EMC problems.