QRO, QRP and the Decibel - dB

QRO and the Decibel - dB

Or, Why 1000W Is Enough - opinions by Jason Buchanan - N1SU

QRP and QRO can get along

QRP and QRO can get along - meaning, you can have both and have twice the fun. But having a really nice antenna makes a far bigger difference than a really nice amp. Some hams can't afford to put up a fancy antenna system so they have to get their gain via amplification, some can't afford the amp but can afford to put up a well-dressed antenna. The point is we get our kicks in different ways.

QRO has a rewarding feel when you realize you need to replace your 2000 watt wattmeter, or watching a pair of 3-500Z tubes turn color. I'll be the first to admit that I like running a 1000 watt CW signal and the full legal limit on SSB. I'd take anything Alex - AI2Q drops on the floor! But it isn't necessary to exceed 1000 watts PEP - I wish it made a difference but it doesn't. Pay close attention to The Decibel below to understand why.

The Decibel

More important than anything else that a ham should know is what the decibel is. Too few hams understand why a 5 watt signal can reach from Boston to Sacremento when 20 meters has almost folded for the evening. I'm just as guilty as most people are when it comes to running lots of power - "Bigger's better, crank it up and dim the lights!" Well, that's true to a point, but knowing where that "point" is and how to find it can sometimes be confusing. Fortunately for us, defining where the "point" is can be defined with simple terms.

The "point" that we reach where more power makes little difference is, for hams, around 1000 watts. Anything over 1000 watts is mostly pouring sand down a rat hole. It took a long time for this to soak into my brain... Why is this true? The decibel!

It is difficult for a person, especially most new hams these days, to fully appreciate the decibel. Wattmeters make it easy to see a large change in output power by watching the needle move its entire distance but this change doesn't really mean much when you think about what amplification really is. For a person to increase their output power level by 3dB, you must double your power output. For 9dB gain you must multiply your current power output by 8. For SSB operation you need approximately 10dB gain (precisely 1.67 S-units, 10x your output power) to make a clearly noticeable improvement to the person listening on the other side. So what does this mean in plain English?

For hams saying QRP is a waste of time, a 5 watt radio is only about 2 S units below someone running around 100 watts:

2 S units below a 100 operator is typically a 559 report vs 579 report. And if the band is going down for the day the dB attenuation (30dB or more) far exceeds what we are able to dump into the air.

For hams who typically run 80-140 watts output:

To make a real difference to the receiving station, 5120 watts are needed over 640! That's a kick in the pants, eh? But what about SSB operation and 10dB gain? More numbers:

It's getting worse by the minute! A typical rig at 100 watts makes a big splash at 1000 watts... but requires 10,000 watts for the same sized jump from 1,000? Yes! And for some reason, thousands more dollars are spent to get a 2500 watt amplifier instead of a 1300 watt amplifier - but why??

QRO Power!

Okay, well, there is some creedence to a 2500 watt amplifier. It's bigger than your friends' amps, you can have your own lightshow on demand by simply keying it and watching your neighborhood's lights dim, you can prove to yourself that it's really getting you around the world because your electricity bill is $200 a month more than it used to be... and it's heavier. Oh yes, there's also another 3dB going out the line but whether or not you have melted or burned things in-line or caused your neighbors' ground-fault interruption system to trip from induced current is another matter. There are those among us who will claim that the additional 3dB makes the difference between a successful or unsuccessful contact - possibly for CW but highly doubtful for any other mode.

The only thing you get out of a "Really Big"(tm) amplifier is bringing up the signal strength (aka, "loudness") of your conversation when you are not yelling into the microphone at 100% modulation. Most conscientious hams will use a little processing to compress their speech and improve intelligibility. So at an average conversation level of 60 watts PEP and using the formula above we get the figures below:

Ouch. 6,000 watts to go from S6 to S9? Yes. But from 60 to 600 you're almost at S8. Getting from S8 to S9 is one tough climb.

The best benefit that is very nice about these giant amps is you don't have to push your exciter so hard. A relaxed exciter produces cleaner output with minimal distortion products. That is really what we want - cleaner signal on the band and less strain and wear on your equipment.

I did not mention above, but no radio will show a linear S-unit gain for each 5dB of signal. Search hard enough and you will find that various brands of radios will need only 2dB for the first 3 S-units, 5dB for the next 4 and so on - others have a completely diferent pattern. You have to use your own judgement and base RST how much stronger the remote station is above the noise floor.

If you want to know if your amp is splattering, use what I call the "Wave Meter" - wave at your neighbors; if they wave back you're okay.



QRO and the Decibel - dB - 26-Oct-04
(C) 2004
jsb@digistar.com

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