Taking a receiving station at TM194450 which has a reasonable line of sight to Sudbury

from Wolfbane BBC2 analogue from Sudbury has the following characteristics

TX e.r.p 250kW on channel 44 18 miles away (corroborated by http://www.bbc.co.uk/reception/tv_transmitters/tv_sudbury.shtml )

Convert these to standard units

TX power

**p= 250000 W**

Distance

multiply by 1.60943 to get km, multiply by 1000 to get metres

d=18*1.60943*1000

thus

**d=28970 m**

from Ref(1) formula(1)

where e is in V/m

thus

e = (sqrt(30*250000))/28970

**e=0.095 V/m**

Convert to dBuV/m (=20log_{10}(e/1uV))

e=20log_{10}(0.095/1e-6)

**e=100 dbuV/m**

Conversion of this into a signal strength

From Ref 2 the open-circuit voltage across the terminals of a half-wave dipole aligned for maximum signal in a field strength of e V/m

e * lambda

V = -------------

pi

where lambda is the wavelength and pi = 3.14159... The terminated voltage at the receiver is ½ this, as half is lost between the source impedance and the matched termination.

wavelength is related to frequency where

speed of light (3x10^{8}m/s) = frequency * wavelength

Deriving the frequency from the channel number. We know the channels are 8MHz wide, and channel 21 corresponds to about 470MHz. Channel 44 is therefore (44-21)*8 up on this

f = 8*(channel-21)+470 MHz

= 8*(44-21)+470 = 654MHz

lambda = 3*10^{8}/654*10^{6} = 0.46 m

received signal voltage for a dipole

V = 0.5 * 0.095 * 0.46 / 3.14159 = 0.0069 V

Convert this to dBuV

V = 20log_{10}(0.0069/10^{-6})

**V = 77dBuV**

This is now increased by the antenna gain. I am using a standard cheap and nasty Maxview 14 element aerial cited by Maplin at 7.3dB gain

so I expect a signal level of 77 + 7.3 = 84dBuV

**Final received signal level = 84 dBuV**

(the form above also allows for masthead amps and CT100 downlead, not accounted for here)

I measure 71-74dBuV for BBC2, a difference of 10-13dB. This seems reasonable to me, given the non-perfect roof mounted antenna

I don't know is the honest answer to that. The calculation up to field strength has been checked twice and independently calculated to arrive at the same result. CCIR rec 417-3 (now ITU BT417-5 but I'm not going to pay them to look at the update) quotes a minimum field strength of 65dBuV/m in band IV and 70dBuV/m in band V, in which case the test receiver would technically be outside the service area of Sudbury if the field strength were 62dBuV/m. All I can say is that the value I arrive at agrees reasonably with experiment in the test case.

Ref 1 ITU-R PN525-2 available here

Ref 2 RSGB Radio Communication Handbook p11.3

M E g A L i T h i A

Text and photographs © RM 2003 unless otherwise credited