6   Rotators, Aerial Corrosion water - proofing

My aerial rotator decided it was going to jam! As usual, there was an opening on 2 mtrs to the south where I haven’t had much luck. I could hear everyone else making nice contacts down into Spain etc and I couldn't. Why mention all this? Well, I hadn't’t done any maintenance on my rotator which had been up for some 15 years. Should I be surprised ? Of course not. We amateurs never seem to do any maintenance! So down it had to come and naturally the bolts holding the two halves of the rotator were corroded in. Out with WD40 and the Ronson gas jet. Once inside I found that the lower white metal bearing was binding on the claw that held it in, as the rotor had worn the soft surface down. It was a simple matter to place a greased washer onto the spindle to return it to working order. Naturally, most of the grease had to be replaced on the  gears, which had melted off over the years!  The cog on the end of the motor had worn badly too, but lifting the rotor had the fortunate effect of raising the cog to an unworn part. So it is all back and working but at the cost of most of a weekend. Chris MØECM and Dave 2EØDCM came round to give me a hand to put the pole, antenna and rotator back up again, which was kind of them. What am I getting at? Well, we should all consider helping each other get our aerials checked over and sorted out BEFORE they go wrong. Could yours fall on someone or something?

       A few years ago, Martyn, G3UKV, decided to check out the 4 mtr yagi we use on NFD. He found that there was corrosion. YOU would be corroded if you’d been out in all weathers for years! He found that there was several ohms resistance at the coaxial connections to the dipole of the yagi for a start off.

       Imagine you are getting 50 watts up to the aerial end of the 50 ohm coaxial cable. How much RF current is there? Let’s see,       P = I2 R. so P/R = I2  =  50/50 = 1,  the square root of which is 1 amp of course.  Ah, but we have 5 ohms resistance in the connections. Then what? 1amp at 5 ohms means a 5 volt loss? AND the 5 ohms is in series with the 50 ohm aerial impedance so the SWR will be wrong too. Don’t forget that the received signal strength will be less too. This could be disastrous during a contest where every contact counts, especially if you can work  the weak ones that other stations cannot hear. Point made I think.

       Whilst my aerial was down I obviously gave it a good going over. Several loose elements on the boom needed tightening up. As usual the bolts were stuck fast. So as not to damage the bolt heads, I gave them all the heat treatment and a drop of WD40 after which they all came loose  and were easily re-tightened. All the bird muck was removed as well. The acid in this eats aluminium (and plastic) for breakfast. Have you bought a new antenna? The bird muck problem will not happen if you paint it with two coats of Finnigans Hammerite beforehand.
 
      The self amalgamating tape where the coax connectors join was replaced. If rain gets into the connections or the cable you are in trouble. Talking of the coax, USING CHEAP CABLE IS NOT AN OPTION. You get what you pay for. Good coax has enough braiding so that you cannot see  the insulation surrounding the coaxial centre wire. If you can, then some of your precious RF  will get out and interference will get in! Buy coax which has foil AND braiding if you can. The more expensive stuff has lower losses too. Here you win twice, because you get more power up to the antenna and you get larger received signals down to the rig. Do unroll your new length of coax carefully. If you pull it off the roll it may kink. Then the centre wire isn’t central any more and you will have problems with the SWR. Buying second hand coax can be a bit dodgy unless it is Andrews Heliax or similar hard line, low loss cable. Even if this has been lying about in the open,  the foam dielectric won’t let the rain in, so you can buy odd lengths at a rally with confidence. Make sure there are no cuts in the plastic outer covering though. Where these cuts are could be where the solid copper outer has been dented, so check it carefully before you buy! Hard line coax is very stiff, so you’ll need to join it to the antenna with a metre or so of flexible cable such as Popes 100 (or similar) in order to get your signal nicely round the rotator!

That’s about it for this time. Vy 73. Mike G3JKX