D-layer ionoscatter on 50MHz
by Jan Hubach, OH1ZAA

Scatter is a well known word in the radio propagation "theatre". Most of use have experienced some form of scatter path in real life. Operators enjoying the use of large antenna systems deal with it on a daily basis, almost every time they are operating. Hardly ever is it possible to hear ones own scattered signal, as turn around propagation time in terrestrial communications is fairly short. However, when sometimes using break-in keying one can hear the echoes of self-transmitted signals coming back from distant reflections or scatter. On the HF bands this is most prevalent when operating at times when your station is in the grey-line zone and signals may circle the earth a number of times with surprisingly low attenuation. Delays may grow to a substantial fraction of one second, which is after all a considerable delay, when exercising fast CW-transmission.

During the numerous F2 openings of the last couple of years on 50 MHz we have often observed the hollow sounding back-scattered signals of other stations in our region. This effect is similar to the things we hear when listening with high gain antennas on the lower frequency bands when the propagation is open and signals are covering large geographical areas. This time we will briefly touch a scatter effect that is present even when normal skip conditions do not exist: D-layer ionoscatter, an always existing phenomenon.

Very little has been written about this scatter effect probably due to the fact that the ionoscatter mode is not accessible to low ERP stations. However, with ever expanding sizes of stacked arrays and a more liberal licensing policy in some countries leading to higher legal power outputs, the D-layer scatter propagation is showing up more frequently. Soon it will be possible to enjoy it regularly at least in reception mode if mid-size antennas are employed. For reference a good friend of mine: Don, W0PO, used this mode in his 50 MHz pioneering days. He lived in Colorado, just outside Boulder. With a 8877 amplifier and a 6-element Yagi he could work the Californian stations of similar calibre, any time when beaming from the Colorado plains over the Rocky mountains. The following information supposedly adds to the confusion about the numerous propagation modes in this interesting VHF-spectrum range, but also to a better understanding of the many faces of the 50 MHz band.

Lately many questions have been thrown up regarding gaps in explanations of certain phenomena. Depending on the reader this may rise more questions or fill in some minor gaps in the "labyrinths" of the lower ionosphere. The D-layer is situated at approximately 60 Km height, and for proper scatter coupling the antennas of the stations involved have to see a substantial "common volume" of the D-layer to be able to communicate. In this respect the process is very similar to troposcatter at higher VHF frequencies. In troposcatter the signal is scattered by natural irregularities of the refraction index in the lower atmosphere, i.e.: variations of humidity (clouds), whereas with ionoscatter it is due to irregularities in the electron content of the D-layer. Simply due to the height difference of the "responsible" layer, troposcatter is useful up to distances of 700 Km, whereas D-layer is at best in the 600/ 1300 Km range.

The general rule for a high power station is that he will cover any 1300 Km path with first line of sight, ground wave, troposcatter, and finally ionoscatter propagation. This parade will mostly be topped by sparking "displays" of meteor bursts, some of them with spectacular Doppler.

Anyone having monitored the vast number of 50Kw and stronger TV-station carriers around 48.25 and 49.75 MHz will know what I mean. A very similar effect occurs when the E-layer is in a state of ionization so that the MUF is a few megahertz short of full reflection for the distance in question. At that moment E-layer scatter occurs, and again due to its natural height of 90-110 Km the "easiest" communication range of 1800 to 2200 Km is observed.

For multi hop circuits all points except one have to support virtually full reflection as power transferred forward through a scattering region will not be sufficient to "feed" a second scattering region. Therefore we can assume that multi hop E-layer propagation can take place. However, at short wave frequencies the D-layer will never reflect signals as such. Hence, it is safe to assume D-layer communications strictly as a single circuit case, unless very fancy geomagnetic conditions are prevailing. There is very much to be said about the characteristics of ionoscatter. and one need not be surprised that there is a diurnal variation of scatter level as well as wave polarization dependence. Irrespective of the variations, it is for sure that D-layer scatter is always there, and this is the reason why the 40-80 MHz range has been so popular among certain organizations wanting to be absolutely sure that some form of channel for distance communication is always available, no matter what natural or human-induced conditions are prevalent. This also partially explains, why it has been very difficult in some countries to negotiate the availability of the 50 MHz band for experimental use in the form as we experience it these days. In many countries the licenses do not impose an ERP limit. Thus ionoscatter experiments encourage the construction of large antenna arrays. Apart from the fact that the bigger antenna systems make all the difference in the world in accessing previously undiscovered (combinations of) scattering modes and related impressive results in long-haul DXing, the concentration of energy in a narrow azimuthal sector also effectively reduces interference in many other directions. Finally we can look at the map of Europe and estimate the useful potential of ionoscatter communications. Given the licencing policy of Denmark with the considerable power level allowed, we find OZ stations in a fairly central position. With the 1300 Km ionoscatter range they can cover most of Europe with this mode. Stations running 400 Watts and a stack of two Yagis spaced 0.6 wavelengths, can be copied almost 24 hours a day with similar arrays for signal reception. With smaller antennas signals can be copied during the better peaks of ionoscatter on a diurnal basis. The intention of this article is not to go into extreme detail, but to leave the secrets of ionoscatter for experimental work and discovery in real life. Other interesting paths are the OH-PA/DL, 1-PA/G, CT-G/EI paths and all other combinations in the 600-1300 Km range.

To facilitate the experiments we propose a 50.270-50.290 MHz ionoscatter range in all future band plans. Due to the proliferation of computers, modems and other digital equipment generating interfering carriers at unpredictable frequencies in the VHF range, we do not like the idea of a single calling frequency. A multi frequency calling sequence is proposed, starting at the 50.280 MHz central activity frequency, followed by calls on 50.282,50.284 MHz until the call is answered. Spare frequencies of 50.276, and 50.278 MHz may be used and of course any other prearranged frequency in the 50.270-50.290 MHz range during schedules is acceptable. The special experimenting range is proposed to facilitate acquaintance of the mode, and to avoid local interference to DX-traffic around 50.110 MHz and to the poor dynamic range of other receivers during experiments of long duration.

In practice, however, a ionoscatter QSO can be conducted anywhere in the non-DX part of the band, and it is in no way different from a regular QSO supported by any other propagation mode. Highly directional antennas will effectively reduce average overall interference.

Wishing you all the personal discovery of a new and exciting propagation feature.

73 Jan OH1ZAA / 0H2MZA.

UKSMG Six News issue 30, June 1991