You may already be familiar with Wireless Waffle's FM DX Logbook which not only provides the log of FM transmissions received but also analyses them to try and make a comparison between the effects of tropospheric and sporadic-E propagation.

In addition to this, a record of medium-wave (often referred to as AM) radio stations has been kept. And it wouldn't be Wireless Waffle without conducting some kind of analysis of their associated propagation. The log is fairly straightforward and considers only whether the station was receivable during the day (ground wave), or whether night-time (sky wave) effects were necessary to hear the station. The transmitter power of the station has then been considered, as well as the frequency to try and see whether and how these may effect propagation.

There are a number of inherent inaccuracies associated with such an analysis, specifically:

• No account is taken of whether the received signal was 'loud and clear' or 'at the back of the box'. That being said anything that is strong and local-sounding has not been included on the grounds of it not qualifying as 'DX'.
• The transmitter power indicated may not reflect the radiated power in the direction of the receiver due the use of directional antennas.
• No account is taken of the gain or effectiveness of the receive antenna. Though most were received using a simple portable receiver, some had the luxury of a long wire antenna.
• No account is taken of the extent to which the path between the transmitter and receiver are over land or sea.
• No account has been taken of seasonal variations (which can account for 5dB of path loss difference¹).
• The impact of co-channel interference which makes the reception of even potentially strong stations more difficult is not factored in.

Notwithstanding these inaccuracies, the more stations that are received, the more the results should begin to show some common tendencies.

The following equation is a very basic form of that which should dictate reception:

Received Field Strength ∝ P_t^½ ⋅ f ^-½ ⋅ d

where:

• P_t is the power of the transmitter;
• d is the distance between transmitter and receiver; and
• f is the frequency of the transmitter.

¹ According to ITU-R Recommendation P.368-10.

The logs are presented in the table below.

Frequency versus Distance

With free space propagation attenuation increases with frequency, and though ground-wave and sky-wave signals use different propagation methods there should be a similar effect. The results show the opposite though, with higher frequencies producing longer distance reception than lower frequencies. This may be due to the receive antenna - as the wavelength becomes longer (i.e. the frequency gets lower) a fixed length receive antenna will be less effective.

Note that the effect of different power transmitters has been taken into account by dividing the distance by the transmitter power to the power of 0.5.

Distance versus Transmitter Power

In theory, the higher the power of the transmitting station, the further the signal will be able to propagate. The relationship between power and distance is best demonstrated if the range of powers and distances are presented on a logarithmic chart. The chart below shows the relationship between distance of the station (in km) and the transmitter power (in Watts).

The chart below shows the same calculation, but with the a frequency factor (with frequency to the power of -0.5) taken into account. If the frequency factor is making a difference, this should cluster the various stations closer to the line of best fit.