Rob's web

Home - Techniek - Electronica - Radiotechniek - Radio amateur bladen - QST - Power increases and their effects

DB rears its ugly head.

If you are considering an increase in power in the near future, this commonsense article on the merits of power increases is right down your alley. Or if you have already booted the input up above what it used to be and have been slightly disappointed in the results, here's why.

Now that most of the amateur bands have been returned to us and a lot of fellows have that old (or new) rig tuned up and on the air, we can sit back and take it a little easier and make plans for the future. Perhaps a lot of you fellows are already burning the midnight oil planning a new rig or a new final stage that will handle two or three times the power you are now running. Or perhaps you are scheming and figuring as to how you can squeeze just a little more power out of that already-overloaded final tube.

How many of you fellows have dreamed of a rig with maybe double, triple or even four times the power of your present rig and thought, "Boy, would I burn holes in the ether and would I go places with that much power!" Later on perhaps you acquired the increase in power, after a considerable outlay of money, and then were disappointed when you didn't shove the S-meter up several points on the other fellow's receiver. What was wrong? Or perhaps you have been in contact with some fellow who asked you to stand by while he raised his input 5 per cent. Why weren't you able to tell the difference in the strength of his signals?

Before you take that roll of lettuce (money to you) which the XYL was probably figuring on using for a new outfit and spend it on some higher-powered equipment, let's take a look at this business of power increases and what one can expect from them.

The decibel

To do this we are going to have to take a look at that somewhat unpopular fellow the "decibel." Now hold on! - don't throw down your copy of QST and go out and blow that cabbage just yet. I'm not going into a long-winded explanation and discussion of the decibel - I'm not well enough acquainted with the fellow for that - but I do hope to give a simple idea of what the decibel is with respect to this business of power increases, so that it will mean a little more to a lot of you fellows than just the top row of figures on the receiver S-meter.

What is the decibel, or "dB" as it is commonly called? Basically it can be said to be a measure of the minimum variation in the intensity of sound that the average human ear can detect. The response of the human ear is not linear but it is logarithmic. If we have an amplifier delivering 5 watts of energy to a loudspeaker and then increase that energy to 10 watts, the sound won't be twice as loud. The increase in level will amount to about 3 dB and should be around 50 per cent louder to the human ear. To make the sound appear twice as loud to the ear, we have to increase the level about 6 dB, which represents an increase in power of 4 times. If you wish to get an idea of how different variations in db. level sound to the ear, make a visit to your local broadcast station and have the operator give you a demonstration on the monitor amplifier. This is a simple matter, as practically all audio controls used in broadcasting are calibrated in dB loss.

Fig 1
Fig. 1. A plot of power ratio vs. dB.

Now how can we figure how many db. a certain power increase will give? If one wants to be fancy he can get it from a table of logarithms and the knowledge that

eq 1

P1 = original power
P2 = power after increase

But this involves a knowledge of logarithms and all we want to know is how much good our contemplated power increase will do. So the lazy man's approach is the best in this case, and Fig. 1 shows dB plotted against power ratio. It is an easy matter to read off the necessary dB once the power ratio is calculated.

Suppose you plan to increase your power threefold. Then, from Fig. 1, the increase equals 4.8 dB. Simple enough, isn't it? Now that we have found that our power increase of threefold corresponds to a gain of 4.8 dB, let's see what this means on the S-meter of our communications receiver. The one here is an older-model RME-69. The meter on this receiver is calibrated in R units, instead of S units as in the later models. However, the meaning is the same. The meter is also calibrated in dB, zero dB corresponding to R1 and each increase of 6 dB representing an increase of one R unit. Thus our power increase of threefold represents 4.8/6.0 or 0.8 of an R unit. This is what you can expect the other fellow to report to you on your signals, assuming identical receiving conditions for the two powers. It doesn't sound very impressive, does it? However, the signal will sound about 80 per cent louder which in many cases may mean quite a difference between copying the signal and not copying it.

Now that we have a true picture of what an increase in power means to you, stop and think: Is it worth the extra cash to shove your signal report up less than one S- or R-point? This is not an argument against high power, because to the fellow who has the money to put into high-powered equipment I say, "Go to it," provided you know how to handle high power and use it with discretion. Of course it is up to you to decide whether that double or triple power increase is worth it, but don't be disappointed when the fellow on the other end reports you only a fraction of an S-point louder than you were with the lower power. Also, before you screw up the plate voltage another notch on the final tube (which probably puts the plate on the verge of dripping), stop and figure out what those few extra watts are going to mean to the fellow on the other end. Remember it takes an increase of approximately 26 per cent in power to raise that signal 1 db., which difference is barely perceptible to the human ear.

During the war all broadcast stations were required to reduce their power output 1 dB. This represented a power reduction of slightly over 20 per cent. A 5 kW broadcast station actually put a little under 4 kW into the antenna during the war. It was conceded that the average listener could not tell the difference between the two powers. This rule has of course since been rescinded, and all broadcast stations are now running their full licensed power.

It seems to me that a lot of us might get more for our money if we change our plans about that increase in power until we can really afford an increase in power that will mean something. Instead of spending that moola now for some higher-powered equipment (and being relegated to the dog house by the XYL), it might be better if we were to concentrate on the old rig for the present and try to get it to operate a little more efficiently and smoothly. Perhaps a few bucks spent for an antenna ammeter, a field-strength meter, a better transmission line or a new beam antenna, might give us just as much increase in results as would the contemplated power increase, and at less expense.

Remember if you can get an increase of ½ of an S-point, or 3 dB, at the receiving end with the same power input to the rig, you have accomplished the same goal as doubling your power into the antenna, and if a new beam antenna should give you an increase of 6 dB at the receiving end, brother, pat yourself on the back. You have really done something. That's just the same as increasing your power 4 times. Also Ye Olde Lighte Bill will be smaller and you can pass most of the folding stuff over to the XYL for that new outfit (she would probably have snagged it away from you anyway before you got down to the radio store).

Personally I think I'll give that ham in the next county a buzz and see if I can figure some way of raising the S-meter on his receiver a fraction of a point farther with my 807 and the 35 watts input.

Laurence Smith, WTFOM.