Johnson's Techworld

 Tropospheric Scatter Propagation is known in short as Troposcatter Propagation. Troposphere is the lowest layer of the atmosphere. Though Troposcatter is typical at microwave or super high frequency or SHF band , it can also occur at VHF and UHF bands. The height of the troposphere is about 20 km near the equator, which comes to about 9 km in polar regions in summer. Height of the troposphere is more in summer than in winter. Troposphere can refract higher end of the radio spectrum. HF is reflected typically by the ionosphere, much above the troposphere. Tropospheric scatter was used mainly by the military prior to the era of satellites. When the transmitting and receiving stations point their antennas to a common scatter volume in the troposphere as shown in the illustration, troposphere refracts the signal towards the receiving station. Troposcatter may work from frequencies as low as 144 MHz to 10 GHz and useful communication can occur from 100 km to 700 km. Typically the transmitt

 Transequatorial VHF propagation was first noted by radio amateurs in August 1947 when Mexican stations could work Argentine stations regularly across the equator in the late afternoon and early evening on 50 MHz. This occurred for few more years during the maximum of Solar Cycle 18. Transequatorial Propagation came down during the period of sunspot minimum and reappeared in 1955 and continued through the maximum years of Solar Cycle 19. We can expect similar conditions as we are heading for the maximum of Solar Cycle 25 in 2025. Two types of transequatorial propagation has been described. One occurring during late afternoon and early evening with maximum distance of about 6000 km in low VHF band of 6 m. Second one was between 7 pm to 11 pm local time with contacts on 2 m and sometimes on 70 cm band. Mechanism by which transequatorial propagation occurs is the presence of equatorial anomaly, which is a high concentration of electrons on either side of the equator in the region of 10 to

 Semiconductor rectifier diodes are PN junctions with P type semiconductor for anode and N type semiconductor for cathode. They are used to convert alternating current to direct current and is widely used in various electronic devices. Simplest form is a half wave rectifier which uses only a single diode and the output has only half of the alternating current as output. It is a pulsating DC at the frequency of the input AC waveform. Full wave rectifier uses a couple of diodes fed out of phase using a centre tapped transformer so that both halves of the AC waveform appears in the output as a pulsating DC with twice the frequency of the supply voltage. Bridge rectifier uses four diodes and a transformer without a central tap. The output waveform is just like that of the full wave rectifier with two diodes. As both these are DC with ripples, smoothening out of the waveform can be done by using a high value capacitor across the output. Further smoothening out can be done using an inductor

 When I started off in amateur radio in 1985, Solar Cycle 21 was nearing its end with sunspot activity going down. Solar Cycle 22 started in September 1986. So I had the opportunity to witness propagation on 40m band going down and that on 80m becoming better. In those days, I had no option for higher bands as my Philips Prestige home radio which I used for RX did not cover them. I was unaware of regional activity on 160m and I do not remember whether my radio had covered that band. I still remember the EC Net also known as East Coast Net on 80m at 10 pm, to which I used to check in, mostly on CW as my homebrew radio, 3 x 807 vacuum tube radio, was not that great on AM for 80m. Now we are in Solar Cycle 25, which started in December 2019 and we are headed for the maximum sunspot activity soon. When we pass that maximum, we will see higher bands going down and lower bands picking up. In the beginning of 2024, my favourite DX band was 10m, due to high sunspot activity. As we go beyond th

 Phased array antennas are those in which the radiation pattern of the antenna can be changed without physically moving the antenna. For the ham radio operator, it would be quite interesting if you can steer the beam without the need for an antenna rotator! The same principle is used in ultrasound imaging to steer the ultrasound beam. If several antenna elements are fed from the same transmitter with a phase shift, the direction of the beam can be changed by changing the phase shift. This is possible when a computer controls the phase shift to each element. It is widely used in the field of radar imaging. In the illustration, computer controlled phase shifter changes the phase of the signal fed to each element. The progressive delay in signals would in effect change the direction of the superimposed signal wavefront to an angle depending on the phase shift given. By altering the phase shift, the angle theta at which the beam is radiated, can be changed without physically rotating the a

  VU2SV Memorial Net on 7090 kHz with VU2NSL and VU3LLL as net controls. Plenty of signals on 40m waterfall display. Excellent band condition here today morning.