Daytime Wave Pockets of Medium Wave Mast Antennas
By F. M. Kabbary

n the summer of 1999, we were doing field measurements of one of our working CFA sites, the Tanta 1161 CFA site and it was running 25 kW to an old 30 kW Tesla transmitter. We were making A/B comparisons with a TCI tower connected to a 100 kW programmable Harris DX100 transmitter at 1188 kHz running continuously during the tests. Of course we could not run two antennas at the same frequency of 1161 kHz. Therefore, we chose the closest frequency to the 1161 kHz which was 1188 kHz rather than 1152 kHz in order to get more gain on the ground with less sky wave when using the 125-meter tower antenna on 1188 kHz. (The 1188 kHz frequency happens to be same frequency as the San Remo CFA.)

THE TESTS
Tests were started at 11:00 AM and ended at 2:00 PM. The measurements were conducted in the zone south of Cairo and Giza along the main Egyptian road and Fayoum road with both roads heading south across the western desert. The distances used for measurements on the Fayoum road were at the following road signs of distance to Fayoum: 83 km, 73 km, 63 km, and 53 km. When the field measurement at the 73 km sign was made, I realized that the Tanta CFA measured 64dB µv/m in the desert, which was at a point of 125 km from the Tanta site. The 100kW tower antenna measured 37dB µv/m at 12:00 noontime and then dropped 22 dB µv/m at 2:00 PM (changing slowly from 22 to 33 dB up and down). This was purely a coincidental discovery about the behavior of the medium mast antenna during the daytime.

THE DISCOVERY
The sharp drop of the signal from 53dB to 22dB within only 200 meters is a very surprising 31 dB sharp drop of the signal! This is really what I would characterize as a "wave pocket" of sharp fading. The CFA field readings all over the 160 km distance from the antenna were all normal without any indication whatsoever of these same wave pockets or fading, Thus, in my opinion, the CFA helped to confirm that the mast antenna wave pockets are due to sky wave reflections from the D layer or another ionospheric layer rather than the normal D, E, F1, F2, layers. We may call it the C layer, if it is lower than the D layer as it may be diffracted when incident at some angles on the D layer, providing that underground metal deposits are not the cause of the fading. In this case, the comparison confirms that the CFA has no sky wave, i.e., no fading at any time, and perhaps very little sky wave out of the target area for the antenna.

Further, the CFA comparison confirms that the medium wave antenna's behavior exhibited these areas of sudden, sharp fading and signal loss exists due to characteristics of the mast-type antenna. An indication of these wave pockets is demonstrated by the sudden abnormal decrease in signal strength and indicates that one is approaching the wave pocket.

MORE TESTS MADE
After making several trips back and forth to the wave pocket location, I have found that the location of the wave pocket remains fixed and has been constant in that way now in this third year since discovery. Three km immediately before the wave pocket location, the signal is 59 dB; two km after the pocket is passed the signal reaches 51 dB. I have been specifically checking this area for two years and the condition has not changed.

I have conferred with my friends about this phenomenon, many of them experts and some of them did not agree with my findings. I too was in agreement with them until I made this discovery that there is no daytime fading of medium waves because the D layer absorbs them. Even though some of the signal could conceivably pass through the D layer will absorb the reflected wave from the E layer and therefore not reflected. This is so only during the daytime. Due to the construction of the D layer ions, the signal should be absorbed completely and at night the D layer vanishes completely leaving only the E and the F layers. The D layer is 60-90 km high. Repeating these tests for the seasons in 2000 and 2001 the results were the same at the 73km sign, however, the wave pocket was determined to be more severe in winter than in summer.

SEARCH FOR MORE WAVE POCKETS
Out of curiosity, we searched for other wave pockets and one was found exactly at the 63 km road sign. It was 25 dBµv/m while the next maximum wave pocket effect was at another 5 km measurement. Then, another minimum point was found at the 53 km road sign and then at the 43 km road sign as shown in Fig. 1.

Fig. 1

At the 83-km road sign, 115 km from Cairo, it was less severe. Continuing the next day, back to the station at Tanta, wave pockets were found at almost every 10 km as before, but when getting closer to the station, it was noticed that the distance between each wave pocket was decreasing when moving toward the station and were down to 5 km apart closer to the station. Also, the wave pockets were becoming softer and weaker.

We can understand that the far distant weaker ground wave signals would have the worst wave pockets because the sky wave signal is relatively stronger than the ground wave signal while close to the mast antenna. The ground wave is much stronger than the sky wave so softer wave pockets can be found at the closer distances. This is shown in the same Fig. 1 too.

Therefore the maximum signal is the superposition sum of the two signals, ground and sky waves, while the minimum readings are the difference between both the sky and ground waves. This means the true field values in the near and far distances are the average values of the peak levels and the wave pocket levels as shown in Fig. 2. So the actual field strength values of the mast antennas will be much less than that given by the ITU, FCC, EBU or ABU.

Fig. 2

RETHINK THE RULES?
In light of the above condition scenario, all radio regulations should now be reconsidered and reanalyzed now after the discovery of this new wave pockets phenomenon!

OTHER ANTENNAS TRIED
Similar measurements were conducted during the daytime for other mast antennas of various, but different heights and at different frequencies. Some of these mast antenna heights were less than standard as follows:

Fig. 3: 150-meter mast antenna, East of Cairo, 1071 KHz 100kW

Fig. 4: 60-meter mast antenna, at 1197 KHz, 25 kW

Fig. 5: 40-meter mast antenna, 936 KHz, 25 kW east of Cairo

Fig. 6: 140-meter mast antenna, Tanta, 100 kW 711 KHz

The same tests were performed on various other random days for the other masts during the daytime and wave pocket results were found for those tests as well.

ANTENNA HEIGHT IS A FACTOR
For shorter antennas, the fading, i.e., the first wave pocket occurred at shorter distances of only 2.5 km from the antenna, then increasing to greater distances. For the less medium wave bands, for instance at 711 KHz, the wave pockets became softer. This is interpreted to mean that the lowest (bottom) of the ionosphere layer absorbs some of the lower frequency sky wave and reflects a lesser amount of the of the sky wave back to the ground at the lower frequency of 711 KHz. See Fig. 6.

The average value of the readings in the various diagrams represents the actual new values of the antenna field strength readings. This will be less in value than the standardized field strength readings that are previously known. (see Fig. 2) As a result of this finding, new regulations should be considered for the taking of daytime measurements, after further confirmation and validation of the daytime fading of medium wave signals, which is an entirely new concept.

This investigation indicates that the D layer reflects the signals of the medium wavelengths, but why this happens every 10 km at long distances, and less at closer distances to the mast antenna is not yet known. This indicates that the bottom of the D layer is rippled or corrugated and that each ripple occurs every 10 km, or that the reflected wave occurs at a specific incident angle on the bottom of the D layer during the daytime.

MORE TESTING AT MORE PLACES
Clearly this phenomenon needs more attention and much further investigation from other locations, with similar as well as different environmental conditions to see if this is a consistent behavior that should be considered while conducting field strength measurements of this nature. Only a sufficient number of other observations in different parts of the world under a variety of environmental conditions will confirm whether this phenomenon is attributed to ionospheric conditions or other environmental conditions, or some combination thereof. -30-

Fathi M. Kabbary, PHD, MIEE, C. Eng, MIEEE
Consultant
Engineering Section
Egyptian Radio TV Union

~ antenneX ~ March 2002 Online Issue #59 ~

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