Question about amplification and signal strength

Discussion in 'Amateur Radio Amplifiers' started by KD0PEZ, Aug 12, 2017.

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  1. KD0PEZ

    KD0PEZ Ham Member QRZ Page

    Ok, so as I understand, to move the S meter of the receiving end, you must multiply the power times 4.

    But my question is, at what point do you start using this formula?

    Let's say for example if I have a 4 watt radio, if this holds true, then I would have to boost my power output to 16 watts for 1 S unit, correct? And if I wanted to boost my receivers end to 2 units, I would have to boost my power to 64 watts (14X4), and for 3 S units, to 256 watts (64X4)....

    Am I thinking of this correctly???

    Now let's say I have a 100 watt radio. If my thinking is correct, to move the other party's S meter an S unit, I would need to step power output to 400 watts? And another S unit, 1600 watts? (of course not legal at this rate).

    I've also "heard" that "the first 100 watts is what matters". Is that why many HF radios have a 100 watt max output?


    Thanks in advance.
  2. K9STH

    K9STH Ham Member Volunteer Moderator QRZ Page

    Theoretically, with the old RMA "S" meter standard of 1 "S" unit equal to 6 dB, you are correct. Of course, with a number of "modern" transceivers that do not adhere to this standard, it takes less power to get a higher "S" meter reading.

    The 1500-watt maximum peak power output standard is close enough to 1600-watts that you would see almost the entire theoretical "S" unit increase.

    There are all sorts of reasons for running higher power and getting a higher "S" meter reading is not much of a reason! It is the signal to noise ration of the received signal that makes the difference, not the absolute signal strength. When there is a very low noise level, a signal that doesn't even move the "S" meter can be perfect copy. Then, when the noise level is "20 dB over S-9", a lot of power, at the transmitter, may not be enough to overcome the noise level.

    Under "normal" conditions, 100-watts will make it through at least 75% of the time. But, when conditions are not good, then higher power is needed and sometimes full legal power is not even enough for reliable communications. Of course, during contests, where there are a lot of stations operating, higher power is almost a necessity to work many stations.

    Glen, K9STH
    N2EY and KU3X like this.
  3. AG6QR

    AG6QR Subscriber QRZ Page

    If you boost your power to 16 watts, you'll get one more S unit than whatever you got at 4 watts. How many S units were you getting at 4 watts? It depends on distance, propagation, and antennas, among other factors. To your neighbor across town, you might have been S-8 at 4W, and S-9 at 16W. To a guy across the country, you might have been 20 S-units below the noise level at 4W, and 19 S-units below the noise level at 16W. That guy won't notice the difference, since you were unreadable either way.

    100W is a nice medium power. It's not so much that it will strain your house wiring, and not so much that you have to take extreme precautions to avoid arcing and other bad things happening at your antenna. It doesn't require a huge heat sink on your transmitter. But it's strong enough to carry quite well over a good distance, even with a simple antenna. At least, when propagation is reasonable.

    If you run 10W or less, you'll hear many stations that won't be able to hear you. If you run 1000W or more, there will be many stations that can hear you, but who can't successfully call you back. If you run 100W, you're in the middle, more balanced so that you'll be able to reach most of the stations who can hear you, and most of the stations who hear your CQ will be able to answer you back, more-or-less.
  4. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I think the "100 watt standard" evolved for a number of reasons, starting way back with the S-line in the late 1950s which used a pair of 6146's and that's the output they could achieve.

    Heathkit did the same with their SSB/CW transmitters and transceivers. Some others pushed sweep tubes very hard and ran a bit more power.

    When rigs went to solid state in the late 70s and into the 80s, the popular transistors which could be powered by 12-14Vdc, making them usable for mobile work as well as home station use with a power supply, were mostly transistors that were rated about 100W each, and designers used two of them to afford some headroom, still yielding ~100W output power. It's about as much power as can be reasonably run with low-voltage and inexpensive bipolar transistors unless you use a lot of them.

    S-units are whatever they are. But it's true that "legal limit" (1500W output) power is only 11.7 dB more than 100W, so just about two "S" units stronger for meters that are truly 6 dB/S unit.

    However, that 11.7 dB is HUGE most of the time, unless conditions are such that signals are already strong at the 100W level. If signals are weak, to the point where you miss an occasional word due to fading, and you add 11.7 dB -- that "losing a word due to fading" is completely eliminated, and the conversation becomes rock-solid.:)
  5. AI3V

    AI3V Ham Member QRZ Page

    The first hundred watts is what matters is what stations that don't have a bigger signal tell themselves.


  6. W6RZ

    W6RZ Ham Member QRZ Page

    Agree. Here's an audio clip I made where the signal level increases by 6 dB for every iteration through the 50 seconds of source audio. This is with two SDR's hooked up back to back through an attenuator.
  7. N2EY

    N2EY Ham Member QRZ Page

    It goes back farther than that.

    The "100 watt standard" dates back to 1950 or even earlier, with many AM/CW rigs running a pair of 807s, a pair of 6146s, or a single 4D32. The Johnson Viking I and Viking 2, the Heath DX-100 and TX-1, the various Collins 32V series, and plenty of others. The S-line just continued the trend.

    When "tabletop" grounded-grid amplifiers running a pair of 572Bs, 1 or 2 3-500Zs, or a quad of 811As became popular, the "100 watt" rig standard continued, because that power level gave plenty of drive without having to run the rig at max power.

    Solid-state just continued the trend.

    73 de Jim, N2EY
    AC0OB likes this.
  8. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Good explanation, seems right.

    Although prior to the S-line, transmitters varied a whole lot in output power. I had Johnson transmitters that ran from 20-30W output up to 200W output, and they were all on the market at the same time (and mostly "before my time," so these were all "used" transmitters.)

    The Central Electronics early SSB transmitters ran 10-20W output.

    The KWS-1 ran 600W output.:p

    There didn't seem to be much of a standard, so I guessed the "dual 6146's" kind of set that, as so many transmitters used them.

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