“Mains” stability during transmit

Here in the US, standard voltage of electrical outlets is 120 VAC via a tapped 240 VAC service available at the breaker panel. Everything in my shack operates from the normal 120V outlets, including my AL-811/AL-80BQ.

I thought it might be interesting to measure the voltage drop caused by transmitting while QRO. The extent of that drop surprised me but what really caught me off guard was the voltage drop caused by transmitting at near-QRP levels.

Here are the measured (Fluke 87) voltages at different RF output levels from the K3/AL-811:

RF Out   Outlet Voltage
0 watts   120.8
25 watts   117.7
50 watts   117.1
100 watts   116.2
500 watts   112.1

I’m not sure if this is typical or the result of my shack being located at the opposite end of the house from the electrical service entrance with the subsequent copper loss.

I’m also not sure what the current draw is on the AL-811 when producing 500 watts out, but with an 8-1/2 volt drop at every Morse character, I’m starting to question the commonly advertised notion of such amps being operated at 120V.

Yes, they can be, but…


On another note, I have recently replaced some of the incandescent light bulbs here with those new low-wattage flourescents and notice that they do not flicker in tempo with my CW keying like the incandescent ones did. No additional noise was noticed with these bulbs as some have reported.



  3 comments for ““Mains” stability during transmit

  1. May 14, 2011 at 2:51 pm

    Hi John, this has to be copper wire loss. You need a thicker cable for your QRO equipment ;-) We have 230VAC in the Netherlands. Never measured the voltage drop actually, but I guess it always drops a little. 73, Bas

  2. Oli, DL1OLI
    May 16, 2011 at 2:53 am

    Here in Germany we have 230V. But here only a voltage drop of 3 % is allowed in house installations. You have about 7 % voltage drop. That is a lot. How many amps draws your AL-811 on 120 V? About 8 A ? I would suggest using a thicker wire, or better switching the amp to 230V. or both …

    The incandescent lights have a small switching power supply with condensators. The voltage for the incandescent tube is regulated. Therefore you will not see a flickering.

    73 Oli

  3. May 16, 2011 at 7:42 am

    I think you’re both right. I did another experiment with a 1000-watt hair dryer. It causes a significant volrtage drop here in the shack but much less of one near the electrical service entrance to the house – in fact, almost no voltage drop there.

    So rather than run thicker cable to the shack to maintain 120V, I’ll probably have an electrician run a 240V service here and will then have the option of upgrading to a more QRO amp if I ever want to.

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