![[Picture]](../gfx/maths/x7-8.gif){kind=small}
The ERP in a given direction is the product of the power supplied to the antenna and the gain of the antenna relative to a half-wave dipole in that direction.
The product of the power supplied to the antenna and its gain, in a given direction, relative to an isotropic (ie omni-directional) antenna. By convention, ERP is used below 1GHz and EIRP above 1GHz.
In assessing the power supplied to the antenna, the loss in the feeder system must be taken into account. This is negligible in the lower-frequency bands, but significant at UHF and above.
The calculation of the field strength produced at a point distant from the transmitting antenna under practical conditions is difficult. This is because the signal is reflected and absorbed by surrounding objects, particularly those close to the transmitting antenna and especially the ground, and, of course, the ionosphere. This is not generally important in amateur radio, but from the aspect of EMC (see Chapter 9) it is often quite useful to obtain some indication of the likely field strength at quite short distances in order to assess the likelihood of breakthrough of your signal to other equipment in the immediate vicinity.
A formula derived from free-space propagation theory can be used to calculate field strength under these conditions, ie
where e is the field strength (peak) in volts per metre and d is the distance from the transmitting antenna in metres.
For example, if your transmitter has an ERP of 400W, what is the field strength at a distance of 100m? From the above expression
![[Picture]](../gfx/maths/x7-9.gif)
It is normal practice for the transmitting antenna to be used for reception. Should a separate receiving antenna be required, 12-18m of wire erected in the clear normally gives good results on all HF bands. An antenna tuning unit is often advantageous.
![[Picture]](../gfx/photos/magloop.jpg)
Photo 7.7. A Magloop (magnetic loop) antenna
