Mobile Operating and More
Harry Leeming G3LLL is back with more wise words of advice and caution.
Unless you drive a mammoth heavy goods vehicle, you will almost certainly have to operate your mobile rig from a 12 volt supply, or will you?
The actual voltage from a car battery while the car engine is running, is normally in the region of 13.5 to just under 15 volts. When the car has been parked with the engine stopped for a while, even with a good battery, this can be expected to fall to around 12 volts. If the battery is a few years old, it may fall even lower long before it becomes discharged, so mobile equipment should be designed to cope with these battery voltage variations, but this is not the only problem.
Pete asked in the shop for ‘a bit of wire’ to make up a mobile lead for his HF rig and was somewhat shocked when I tried to sell him a length of black and red heavy duty cable. “I don’t have to pay for this thick stuff” he said, “The rig takes less than 20 amps on the odd peak, and ordinary mains lead should stand that”. He had missed an important point.
Let us say that a heavy-duty electric water heater takes 20 amps and the electricity supply entering your house is 230 volts. If the total resistance of your house wiring from the meter to your heater is 0.1Ω, by Ohm’s Law you will lose 2.0 volts in the wiring, but out of 230 volts, you are unlikely to notice this. If, however, the resistance of a mobile lead, plus internal wiring from your car battery to your 100 watt SSB mobile rig, measures even half of that, it will drop over a volt on voice peaks. When your car is parked and the battery is only giving out around 12 volts to start with, this may then drop the voltage to below that at which the voltage stabilisers in your rig can hold it steady. The result will be distortion and FM on your SSB transmissions, and some very bad reports. You could, of course, start the engine to increase the supply voltage by a couple of volts or so, but you would not want to operate for too long with the engine running when you are stationary. The real answer is to use thick, fused, short heavy duty leads that go directly to the battery. If you are in doubt about your lead try checking the voltage that is arriving at the socket on your rig, as you speak into the microphone.
Power Distribution
Have you got a really big PSU Harry, that I can install in my workshop and distribute the 12 volt supply to all my benches? I told the customer that this would not be such a good idea, due to the voltage drop on any longish interconnecting cables, unless his current requirements were very low.
The electricity suppliers are well aware of this problem. In the early 1900s they did not have power stations belching out smoke on the outskirts of towns, they had them polluting the atmosphere right in the town centre. Why? They generated a 220 volts of DC supply, and if you were far from the generator, the voltage drop caused by the resistance of the mains supply cables would be considerable, so much so that villages and farms a mile or two from the centre could not be supplied with electricity. Just think of it; to bring a few hundred megawatts of electricity at 220 volts to a large modern town, would require a current of millions of amps. Even if you could construct a thick enough cable, the voltage loss would be colossal.
Now with AC supplies, which can be stepped up and down in voltage by transformers, we have a have a National High Voltage AC Grid that distributes electricity nationwide at 400,000 volts, so even megawatts of electricity requires only tens of amps. From this supplies are tapped off and stepped down to a few thousand volts to supply towns, and further transformed down to 230 volts for local distribution to homes, so that at all times the voltage loss is only a small fraction of the whole.
Static‘Mobile’HF Operation Can Be Fun
Years ago, operating from a home in a very built up area was difficult. I was trying to develop and evaluate various kinds of speech processors, but I suffered from local interference and the constant worry of a knock on the door regarding TVI and BCI. I fought back by using my rig in my car, and until then had never realised just how successful operation from a car could be.
The mobile antenna I used was a multiband G-Whip, which was ‘top loaded’. The advantage of this arrangement was that an ‘Extension Rod’ was available. This increased the height to about 6ft, and while only suitable for static operation, it really did boost the performance and bandwidth. It’s a pity that it does not seem to be made any more. As I found, however, the success
of this type of operation was, as the estate agents say, “Location, location and location”.
A quarter-wave loaded whip is only half the antenna system, the rest is the earth return. When a vertical antenna is used at home you normally drive a spike into the ground, add a few radials, and hope for good results. However perhaps due to poor soil conductivity, absorption by local buildings, and local QRM these can be disappointing.
When operating from a car, however, you have a wide choice of possible operating locations. I was very lucky and found that about a mile from my Blackburn home, there was a track on the edge of a playing field, well away from any electrical equipment.
Besides being fairly well elevated it was below the peak of the hill, and so was damp and had deep soil, hence the ground conductivity was good. The car’s capacity to ground acted like a capacitor in series with the earth return of my antenna, and the results seemed too good to be true. Perhaps it was coincidence, or the /M callsign standing out, but for the first time ever a ZL station came back to a 15m CQ call.
Such operation, however, has its hazards. Once the police came round, as I had been reported as a ‘Suspected Spy’, and once the car battery went flat and I could not start the engine. Fortunately, an obliging courting couple gave me a push. If you try static mobile yourself, avoid using an automatic car, and always park facing downhill unless you carry, and preferably operate from, a spare fully-charged battery.
Coil Cores
I had an e-mail from Patrick, who was trying to align an old rig but had found that many of the cores in the coils were jammed solid with what appeared to be hardened wax.
Many rigs have ferrite cores with hexagonal holes in them locked in place by wax. The holes in the cores may seem to be the correct size for an Allen key, but don’t be tempted because while a metal tool may fit, it will throw the circuits wildly off tune and may well damage the cores. They should be adjusted with a plastic trimming tool, Fig. 1, but if they are stuck, don’t try to force them but apply a little heat. The best way I have found to do this is to wrap some 18 or 20SWG wire round a soldering iron, leaving about an inch poking out as per Fig. 2 and then insert this into the hole in the core to warm them from the inside (don’t get them too hot or you may melt the coil former). This will usually do the trick, and enable you to peak them up with a trimming tool. An alternative, which I have heard about but not tried, is to use a few wax softening ear drops, but whether this will do any harm I am not sure.
Undocumented Modifications
Many of my customers had left instructions with their families, on the line of, “If anything happens to me, and you want to get rid of this lot, see Harry”.
I was flattered that they trusted me, and it brought in valuable business, but also a few problems. Sometimes I would go to a house, glance at a pile of equipment, hunt down the accessories and the instruction books, and say something like, “Well most of it seems in good condition, and so I should be able to get a good price for it, let me take it with me, and I will check it over, and phone you”. Usually I would then be able to check what repairs were needed, and phone them with an offer that covered my costs and a reasonable profit margin.
When I sold equipment on behalf of a customer I did not usually give a long-term guarantee, but sold it on the basis of seven days approval. I always wanted, however, to be in a position to carry out repairs if there was ever trouble in the future, and this was a problem if there were undocumented modifications. Sometimes I would find bits of Veroboard hung in the wiring with no details added to the manual, and occasionally when I did not know what the bits were for, I would have to contact the seller and ask them to come in and have a look at the equipment. I would then point out the problem and quote them for restoring the rig to its original state. Sometimes they just took it back.
“There is nothing so certain about life as its uncertainty” Do your family a favour then, and jot down the details of any mods in the back of the manual, along with any instruction leaflets, now!
Bert said it was his Biggest Mistake?
Bert’s wife was getting fed up with him “Making the house seem like a junk shop”, and one day things came to a head.
Bert had spotted a pile of WW2 radio equipment in a local auction, and as it looked quite clean he wondered how little it would sell for. No one else seemed interested, so he did not start bidding until the auctioneer came down to £10 and then, much to his surprise, the hammer came down and it was all his.
He arrived home with the car boot and the back and passenger seats full, to be greeted by his wife. She looked in the car with horror and exclaimed, “If that lot comes in the house, I’m leaving”.
That was when Bert said that he made his mistake. He got rid of the radio equipment and tidied up the spare room.