The following articles have been "Scanned" from our archives of "Pre-Web site " newsletters.

 IONOSPHERIC, TROPOSPHERIC PROPAGATION & OTHER PHENOMENA 

Radio waves at VHF/UHF (i.e. 50 to 950 MHz) are subject to the same laws of propagation as other waves but because they are higher in frequency than the HF bands the results achieved by their use differ enormously from those on the lower frequencies.  On frequencies below 50 MHz the dominating feature is ionospheric reflection and refraction, this limit depends on the time of day, the season, direction and sunspot activity but is usually between 15 and 50 MHz. The upper limit is reached only during periods of high sunspot activity, occasionally but very rarely will it exceed 50 MHz to propagate around the curvature of the earth by F2 ionisation, but on frequencies above this, tropospheric propagation becomes of major importance, particularly when chasing D.X.   Most of the effects that interest us take place more or less between 2 & 12 km in altitude but the lower layers of this height are more affected by the weather, so there is a close link between the weather conditions and propagation on these frequencies. 

There are times when signals can be heard from distances well beyond what you normally receive and this is because of differences in the refractive index at different locations along the signal path, this produces increased levels of refraction.  If this index rises above a certain amount then the signals are refracted at a greater rate than that of the curvature of the earth and so are returned to the ground and reflected upwards again similar to the HF spectrum, so this is one form of tropospheric ducting.   At other times signals may become trapped in an elevated duct and can travel for several hundreds of miles without being heard at ground level along the normal transmission path similar to the skip zone on the HF bands under normal ionospheric propagation.  The 4 and 2 metre bands are very good for this and some spectacular distances over 1500 miles can be covered.  Similarly 6 metres will show this phenomena at times with somewhat greater range.

One of the features of "Tropo" is that of fading, quite often signals being copied from some considerable distance will be very strong one minute and then disappear into the noise the next and then return. This is because conditions which affect propagation are consistently changing as winds displace the air and this changes the path supporting the signal to also change, giving rise to the signal being received coming in over more than one path at a time and as these path lengths vary in distance the signals being received pass in and out of phase with one another.  There will be times when the paths combine and increase the signal and at other times will interfere and cancel each other out, so reducing the signal strength and so on.

'Sporadic E'

In summer months during daylight hours, very intense ionisation known as 'Sporadic E' sometimes occurs within the E layer, its effect is capable of reflecting signals up to145 MHz giving enhanced communication in the region of 500-1000 miles but signals can be very erratic and unpredictable and so cannot provide reliable contacts.  When using yagi beams best results do not necessarily mean pointing the aerial in the direction of the distant station on the great circle map. Under normal conditions the E layer upper limit is very much less than the F layer.

LIGHTNING. *APRIL SHOWERS & THUNDER STORMS* prompted the following -

When Benjamin Franklin tried out his now famous experiment, flying a kite with a key attached to it while lightning danced all around him, he didn't appreciate the dangers.  In fact he was extremely lucky to have survived, others who tried the same thing were indeed killed.

But Franklin followed up his experiment by designing the Lightning Rod, which was a device that provided a discharge path for the electric charge built up on the clouds above, thereby providing a path to earth should lightning strike it, rather like our Aerials can attract a discharge path.

Since the development of the lightning rod (circa 1752) lightning damage has been drastically reduced. At that time the damage was generally structural.  These days, especially to those of us who use modem electronic equipment, storm damage is something quite different.

If Benjamin were alive today and could see some of the effects of lightning on today's gear he might wonder whether his invention was proving to be of any use whatsoever.

There is a saying that lightning never strikes the same place twice but don't you believe it. Really it's rather frightening. During last weeks storms I was waiting for the flash (as my aerial spark gap was clicking) and the immediate ear splitting thunder crack that would ensure the instant demise of some electronic device in the vicinity. Sure enough there was a flash and a colossal, ground-churning, window-shattering explosion that spread from Earl Shilton to Burbage and beyond. The fact about lightning, is that it doesn't have to strike directly to cause disaster.  The pulse energy can easily upset and/or damage sensitive transistor equipment and this repeatedly occurs.

So make sure that your aerials are always earthed when not m use and have some form of spark gap in use at all times. A Car spark plug is ideal with a small gap 2 -3 mm being sufficient for HF aerials. * G3SVT.

OPEN OR SHORT CIRCUIT CHECKS ?

According to theory only two basic faults can occur in an electrical circuit ! either an open or a short circuit. Complication is added by the faults degree. E.g. a resistor can go high in value ( a partial o/c) or a capacitor can leak (a partial s./c). thus the faults that blight us when our equipment goes on the blink are only variants on a very old theme.

The short or open circuit hold true for most failures but it doesn't take into account such things as phase errors or incorrect alignment of circuits. For the vast majority of faults however the theory holds true. *G3SVT.

TELEVISION & VIDEO INTERFERENCE

When Graham MOBVR commenced operating on HF his XYL Margaret had great difficulty in watching television or the video. The aerial installer had fed all the coax aerial leads in close proximity to each other. The only option was to fit a braid breaker/highpass filter at point of entry at the video. This simple operation cured the problems.

An explanation follows :-

The coax braiding acts as a long wire aerial to any RF energy induced in to it and the signals picked up flow straight through to the chassis of VCR & TV an ultimately to earth with the result that RF voltages give rise to interference in various circuits.  Amateur transmissions on 3.5 and 7 MHz affecting the colour decoder circuits in the TV while 3.5 MHz falls within the pass band of the luminance signal FM sidebands, so any unwanted signals at this freq. will get into the demodulator during playback and show up on the picture. In this situation earthing the VCR's chassis / bottom metal screen will only make matters worse since this will improve the performance of the long wire aerial formed by the braiding, the fitting of a braid breaker including a high pass filter eliminates this problem, as recommended by the R.A. for the purpose. * G3SVT.

FREQUENCY STABILITY- CRYSTAL- QUARTZ !
WHAT IS QUARTZ ?

Whenever we have a requirement for a simple stable source for a fixed frequency we invariably make use of a 'crystal slice' of the stuff to determine the frequency required. So what is quartz ? We all make use of it but very rarely consider what or where it comes from, so here goes.

In its basic form we see and use quartz every day without knowing that its quartz.  It's one of the most widely
distrlbuted and useful mineral known to man.  It forms a large percentage of the rock we call granite, white sand Is almost pure quartz. It is composed of silicon and oxygen, it's harder than steel and clearer than glass!

When quartz is pure. it's devoid of colour or it's white, but when mixed with impurities it may be in various shades of
red, yellow, green, blue, lavender, black or brown.

Even many of our semi-precious stones are only quartz coloured by small quantities of other substances- Amethyst or onyx are a couple of examples.

For radio work pure silicon quartz is cut into very thin slices and processed.  It is occasionally found In large, clear six-sided crystals with pyramid ends called •Rock Crystal*

When pure rock crystal is melted at 1,710 deg C. we obtain fused quartz.  It's co-efficient of expansion / contraction is far less than other known materials and is more transparent than glass.  Special quartz lamps are used to provide artificial sunlight for medical use of course. ~ Carstein.

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