|
"installing lightning protection does not just mean "Just use some #8 wire from the antenna to a ground-rod" was the advice given to someone who was desperately trying to protect his investment. Although seriously lacking the full protection required, this idea is not being shot down in flames as there are two distinct functions being performed by this earthing system. The first is any static built up on the entire antenna system, either from wind or nearby lightning storms, is drained to earth. This means that when connecting the antenna to the radio both the radio and person doing the connecting are not exposed to any serous shock with the static finding its way from antenna to earth, often through fingers! The second reason is one of safety. Should the earthing on the equipment fail then the operator is still kept safe as the equipment will be earthed through the coax outer, via the antenna, and through the earthing system on the antenna. But this is only half the story. Before the area of lightning protection is tackled, there is one fact few radio guys think about and land up installing expensive lightning protection when faced with "unexplained receiver failures". Some antennas do not offer a DC short, especially collinears. What is known to happen is the antenna becomes statically charged during dry windy days and the input of the receiver, often a capacitor, suddenly breaks down draining all the energy built up in the antenna system into the sensitive input amplifier. Such faults have often been cured by strapping a 1 to 10kΩ resistor across the antenna connector (inside the radio). In fact a world known brand of two-way radios suffered the same failure and when the suggestion was made to the R&D division the faults ceased. Back to the main issue. The reason for the lightning protection is two fold. The first is to bring the entire installation to ground, as discussed above. This is done by conveniently offering a point, being the metal case of the lightning protection, that can be taken to ground. Yes, very little more than the #8 into a ground rod, except it's done lower down the coax i.e. any energy picked up from a lightning strike along the coax is also drained to ground. The second, and most important function of the lightning protection is to control the energy picked up by the antenna. If not controlled the energy would dissipate in the sensitive circuitry of the receiver. Something the #8, or even a copper bar, would not accomplish! It will be noticed the word 'control' rather than 'drain' is used with regards the energy picked up by the antenna during a lightning strike. The reason is simple. If one were to drain all the energy, the usual day to day function of the antenna would not be available as the only way to drain all the energy would be to short the inner and outer of the coax and take these to ground. Not a very satisfactory arrangement.
With the typical circuit of a lightning protection device what will happen is when the voltage reaches the strike potential of the gas discharge tube all extraneous energy will be drained via the tube to ground. Some energy will be stored in the decoupling capacitor and during the period when the tube conducts (effectively a short) this energy is then passed on to the receiver. As the cap is small, this energy is small therefore the cap limits or controls the amount of energy the receiver needs to tolerate. However, there is one other snag with lightning protection devices, good ones that is, they exacerbate the DC short problem, not cure it. They all rely on the fact that the antenna itself is a DC short, many collinears are not. The advantage (with protection) is that the static can no longer cause damage as the tube will fire the moment it gets too much. The problem is that every time this happens, which will be a bright dry day with some wind, there is an annoying crack on the receiver. The really, really good types of protection have a resistor built in (shown in broken lines) to stop static build up.
A ½-watt carbon film resistor is used. A value of 1kΩ for up to 5 watts transmit power, rising to 10kΩ for 10 to 100 watts. Please note no wire wounds for reasons of inductance mixed with self capacitance - they resonate at the weirdest of frequencies and have a nasty habit of disturbing the SWR at points along the band. I have done plenty of experiments in the hope to prove this wrong, and have never succeeded. As soon as the wire-wound is replaced with carbon/metal film, the problems disappear. For bigger transmitters, limit the resistance to 10k-ohms but increase the power rating of the resistor to about 1% of the total transmitter power (assuming the feed is 50W). Do not place it after the tuner if there is one - impedances found here would blow the resistor. Some in the RF industry would have seen coils used as static drains. The theory is the coil is high impedance at RF and so looks open circuit, whilst still presenting a short to the DC and draining it to deck. The problem with them is they too can start reacting ('scuse the pun!) with the capacitance present on the system, especially at the lower ends of the band. Also, getting hold of the material to create the coils is such a pain. Having used resistors for most of my life with an extremely high level of success has made them my 'de facto' standard when fitting static suppression to antennas that need it. Be wary too of using diodes to protect the front-ends of receivers. Any static build-up on the system (obviously without any static drain) could cause the diodes to start conducting and become very active in mixing stronger signals. This will lower the dynamic range of the receiver and/or lead to spurious signals.
From personal experience I can confidently recommend the Polyphaser range of products www.polyphaser.com. I have used hundreds of them very successfully for years in South Africa (before moving to the UK). What is interesting is one of the leading members of Polyphaser, Dr. David Block, was one of the first people I heard talk who actually agreed with me on some of the principles of lightning protection. I used to have many heated debates regarding certain issues but when he visited South Africa the arguments stopped, and I carried on as I had been for years!
There is one final word on lightning protection. It's all very well controlling the energy during a strike. Just ensure you offer it a solid path to follow to ground. Failure to do so will merely have the energy find another path.... .... which usually happens to be through the equipment!
© 30.05.01 |
Why use Lightning
Should there not be a resistor one can be put in circuit using a T-piece with an appropriate resistor mounted in a plug which is then fitted to the T section. This arrangement is placed on the antenna side of any lightning protection.