# Ionospheric Narrowband and Wideband HF Soundings for Communications Purposes: A Review
## 3 State of the Art of Sky Wave Sounding
### Recommendation ITU-R P.845-3
#### 3.1.1 Measurement of Sky-Wave Signal Intensities above 1.6 MHz  
ITU, as a standardization organization, recommends that the measurements of sky-wave signal intensities should be undertaken in a carefully controlled manner to help improve the accuracy of methods for estimating field-strength and transmission loss. Ideally, measurements should be carried out systematically over as wide a range of conditions as possible at a series of frequencies over paths of different lengths in all regions of the world. Measurements should be done at each hour of the day in the separate seasons and for a whole solar period.  
ITU recommends that, for the sake of interpretation and being statistically meaningful, measurements should cover a minimum period of one year at a given fixed frequency. ITU also exposes the advantages of recording signals simultaneously over a path at a series of different frequencies, both to aid the understanding of propagation effects and to permit quantitative data to be obtained by night when maximum usable frequencies are low, as well as by day when there is much absorption at the lower frequencies and signals might be masked by background noise.  
Transmitter  
The transmitter should preferably operate 24 h per day and should transmit an unambiguously identifiable signal to prevent mistaking the signal of interest for co-channel signals, adjacent channel signals or interfering noise. The transmitted signal should be stable both in frequency and radiated power, which should be higher than 1 kW for short paths and higher than 10 kW for medium and long paths. If the transmitter uses modulated signals the type of modulation should be constant and the mean percentage modulation should not vary. ITU suggests to better use narrow-band transmissions (i.e., 1 kHz or less) rather than wide-band transmissions to reduce the probability of being interfered.  
According to ITU recommendations non-directive antennas have advantages over directive antennas because the relative strength of the received signals coming from different paths are mainly determined by propagation effects and not for the radiation pattern. Another benefit of omnidirectional antennas is the fact that in absence of knowledge of wave launch directions, valid deductions can only be made in case of a single transmitter-antenna gain. Vertical half-wave dipoles with all-round azimuthal coverage are preferred for monitoring and sounding purposes since radiation patterns may be fairly estimated, even at low elevation angles.  
Receiver  
Prediction methods of signal strength do not take into account local characteristics at the receiver site and consequently the receiver antenna should be clear of obstacles (e.g., foliage and buildings) and adjacent antennas should be separated more than ten times the antenna length. Due to the high level of noise and interference at the HF band, ITU recommends not to use high gain receiving antennas. Therefore, appropriate to employ should be a short vertical antenna or a grounded vertical monopole shorter than a quarter wavelength or a small loop antenna. In case of using a loop antenna it should be aligned in a vertical plane containing the great-circle path to the transmitter and in case of long-distance paths, where off-great circle propagation contribution is important, vertical monopoles are recommended due to its omnidirectional azimuthal radiation diagram. Receiver setup should also take into account other issues such as the ground slope, the radiation pattern elevation angle and the ground conductivity.  
Appropriate impedance matching should be provided between antenna and transformer or wideband pre-amplifier. In order to maximize signal-to-noise relation the receiver bandwidth should be consistent to the bandwidth of the transmitted signals.