The Wide Boys of 2m July 7th, 2013
I am writing this whilst the annual UK VHF NFD contest is in full flow. In fact I got up at around 6am this Sunday morning and had a listen on 2m, and some of the contestants were making full use of their voice recorders. I obviously cause panic at one station who had clearly had their voice recorder playing for some while. When I called them shortly after 6am there was suddenly silence for around a minute and then a voice appeared explaining that they had picked up the wrong microphone. However that was not the purpose of this blog.
I noticed yesterday and today that, at least in my corner of the South East, there were two signal from fixed stations that had SSB signals far wider than they should have been. The picture above I hope gives an indication of the problem. The stations were around 15km away and I made sure that I turned my beam away from them so that there was no chance that my Elecraft internal transverter was being overloaded. I had been trying to work a GI station but it proved impossible whilst these two stations were transmitting nearby and then the GI station faded out. I was measuring SSB at least 30kHz wide on my P3 panoramic display from both signals and it set me wondering how operators can take simple precautions to avoid this.
I am pretty sure that the stations were using “linears” and so I am going to make the assumption that the driving source was clean. Most commercial transceivers are very docile and well designed and it is very difficult to over drive them in the sense that they will cause RF distortion. However, the commonly used “brick” amplifier has less protection and there is often no metering to help. So the temptation is to drive them until you get maximum power out, but that does not mean that it is clean power when you are hitting the peaks. So is there a simple way that at least gives an operator a fair chance of putting out a clean single without investing in a spectrum analyzer. Well here is one idea.
Let’s assume the driving source is clean. Drive that source to full power with a steady carrier and note the indicated power on an external VSWR meter. It helps if that meter has a “PEP” function as this will make the next step a little easier to get a reasonable reading. It really does not matter how accurate the PEP reading is as we are not going to deal with absolutes here.
So having taking a note of the maximum power from the driving source using a steady carrier, now set the driving source up for SSB. Now transmit SSB in the normal way and make a careful note of of the average indicated power. You now have two measurements. Let’s say that maximum power is 100W and the average SSB indicated is 50W. Yes that is typical. Don’t forget the “PEP” readings are rarely PEP and simply introduce a “HOLD” function. But as I said we are not really concerned with absolute measurements, what we are going to do is to calculate a ratio. In this case the calculation is easy. The ratio of SSB indicated power is 50% of the amplifier’s maximum power. So now let us switch on our brick amplifier. Again send a carrier and measure the maximum power output. Here we will assume it is 300W. For safety let us build in a 10% reduction and say the maximum clean power is actually 270W. Now switch to SSB and adjust the drive level for 135W indicated on the VSwR power meter. That should be about right and unless the amplifier has a fault on it, the signal should be clean. Resist the urge to increase the drive until the meter won’t go any higher, that is when you become anti-social on the bands. What is more, you actually don’t achieve any advantage by pushing hard. The place for compression is in the driver and not the linear.