I got this email newsletter from the lovely people at Radiozing and thought it worth sharing as it answers a lot of questions people have with linears and power etc.
If you are buying any kit they are always worth checking out at http://radiozing-uk.com/ as they have some great prices and know the stuff they sell.
Radio or Amplifier not reading enough power on your PEP power meter ?
You need to understand how to measure Peak Power !
A peak-reading meter is one which measures the peak value of a waveform, rather than its mean value or RMS value (like an average)
There are power meters sold that have switches labeled PEP or Peak, but MOST do not provide accurate PEP or Peak readings. A true peak reading circuit can read the peak value and hold that value long enough to drive the meter even when the input signal is dropping. Most meters use a capacitor as a simple “hold” on the charge, but the peaks still only charge the capacitor to an average value – a value higher than when switched in RMS /Average mode but this is not a true peak either. Also, the best readings are done on a dummy load as your antenna’s reactance can cause different readings than a good properly matched dummy load.
All of the meters sold by Dosy, Diamond, Avair, Astatic and many others that say Peak or PEP on them will only read an enhanced “average” power. Generally, they display about 60-75% of actual true peak power. If you are set up for low dead key and high peak power these meters will be even further off and read only a fraction of your true peaks, more like 30-40% of true peak power. Even with this said, these meters are still of decent quality, relatively inexpensive and for most people a good choice for personal use.
A Bird 43P meter (with a real peak reading circuit installed) can hold the value and drive the meter to show true peak readings as verified on an oscilloscope. The peak RF voltage is input to an op amp circuit that will hold that charge for as long as it needs to. The Bird meter is very expensive and not practical for most users. These meters are usually used by technicians and by users who operate at increased power levels (above 2KW) that can only be accurately measured by these type of meters.
The oscilloscope is the most accurate PEP measuring tool, as it will display the crest (peaks) of the modulation envelope independently of the rise-time or audio-frequency response. An o-scope is also very expensive and not practical for most users. These are usually used by technicians. The problem is they are large, complicated, have many controls and tend to scare off the average user.
Do you have to own a Bird 43P meter or an O-scope?
NO, a regular meter is perfectly fine for a consumer to monitor his equipment. You do have to realize the limitations to average commercially available meters and know that your true peak power is more than you are seeing.
A version 6 CRT6900N / AT5555 can be tuned for a peak power output of 40 watts SSB on the Bird 43P. The same radio as measured on a Dosy / Diamond meter would show 26-28 W. You must understand that the 28W you see is actually only 70% of the true value.
Lets say you are trying to set up your linear amplifier for good drive and clean audio. To do this, as you know on AM and SSB that you want a 4:1 ratio power. So you want your peak power to be 4 times your dead key. First you read your peak output using a Dosy meter and you might read 150W. So by knowing the limitations of the meter you decide that your true peak power is really about 200W and set your dead key to about 50W.
Say your technician sets up your radio and amp and tells you it is dead keying 100 watts and peaking 400 watts (perfect and within the legal limit for full class UK license holders). When you get home and check out your meter you see 96W with voice peaks to 280W. Did he do it right ? Did you set everything up right ? Is something wrong with the amplifier? What should you do? NOTHING, because now you know the limitations with meters and how to read true peak power. So you smile and say yep, that’s just about right.
The other thing to keep in mind is RM and Zetagi (and others!) don’t make clear their claims of power output and these amps don’t achieve much more than 60% to 70%% efficiency. So a 100 watt amplifier, expect to see 60 to 70 watts out on FM/CW.
Also keep in mind that amplifiers are very voltage reliant – so long power cables you will get voltage drop and lower output power from your amp. Always use shortest power cables – and if running with a power supply, increase the voltage to compensate.
“My preamp on FM doesn’t work and is faulty – it just increases the S-meter on all of the signal”
Yes, – that’s correct and it is not faulty. Pre-amps are only effective when you have a low SSB signal with low noise floor level – if you use it on FM it will seem to just increase the S-meter reading and not improve the actual signal to noise ratio level – in other words Preamps on FM don’t help and are only effective on low SSB signals.
“My amp is faulty as I have gone from 100W to 300W and there is not much difference on the received signal”
Each increase of one “S point” (e.g. from a S5 to a S6) on the receive meter is equal to 6 DB difference. When the manufacture calibrates todays HF rigs to the international standard, therefore, to go from a reading of S6 to S7 you will need to increase your output power by 6 DB. Using this scale, it means that every time you double your output power you are increasing your signal by only 3 DB. If you again double your output power you will get another 3 DB increase in signal strength. Together both these increases total 6 DB from your original power level.
Here is a real-life example: you are transmitting at the 100 watts output power level. The station on the receive end of your signal is seeing an S5 on his S meter. In order for him to see an S6 on his meter, you would have to increase your power output level by 6 DB or go from 100 watts to 400 watts of output. In order to go from an S6 to an S7 on the receive end you would then have to go to 1600 watts of output
Overdriving power input!
“Its safe to put more than the stated amount of power input into an amp because it is not turned on (although its still in line)!”
Well in most cases – yes – as the amplifiers have a change over relay that in power off mode routes the input power bypassing the components, but in some amplifers – especially the lower cost versions when powered off the relays aren’t switched so it is always better practice to remove the amp if not in use. But how much power are these relays designed to take ?
Too much power input destroys transistors and can be very costly to replace !
“But my radio only produces 10W so it cant overdrive it. The amp has to be faulty”
Both older and todays 10 metre multimodes and older and todays HF radios are known to “spike” power – ie overshoot the power that you have set. Ie, you set the radio at 20 watts and the amplifier blows. Search on google for TS590 power spike.
High SWR can cause transistors to fail
Aswell as damaging or destroying transistors, higher input power draws more current and puts other components under extra stress. We have seen the fuse holders melt because of this.
In an ideal world, buy an amplifier that can produce more output power than you require. Don’t run it with maximum input, and don’t run it on the highest level setting. What would happen to the Ferrari engine if you constantly drove it at maximum speed ?