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Title: DECREASING OUTPUT Post by: KB5MD on October 25, 2016, 01:25:29 PM Why do some transmitters have a decreasing output the higher the frequency? I have a 1949 model Mackay marine transmitter that is supposed to run 200 watts on 2-24 mhz, but
it will do 240watts on 160meters, 225 on 75 meters and 125watts on 40 meters. Above 7 mhz, you can forget it! All power levels listed are output watts. Title: Re: DECREASING OUTPUT Post by: W7TFO on October 25, 2016, 03:15:32 PM Tube inefficiency & dielectric losses as frequency goes up.
73DG Title: Re: DECREASING OUTPUT Post by: K1JJ on October 25, 2016, 06:22:18 PM Sounds like the rig may have a problem, since the commercial specs say 200 watts up to 24 MHz but you are seeing only 125 W on 7 MHz. Alignment maybe? Swap tubes?
In General: Skin effect (IR losses) will rear its head on ALL RF surfaces as frequency goes higher. This includes RF connections, switches, tuning caps, coils, coupling caps, relays, bypass caps, etc., - anywhere RF flows. So coil stock that worked well on 160M may actually be very lossy on 10M unless the cross sectional wire diameter is increased substantially. L/C Q will also have a big effect on various bands. Excessively high Q will eat up power needlessly. Keep the linear amplifier final Q around 12-15 for a good compromise. T Title: Re: DECREASING OUTPUT Post by: KA2DZT on October 26, 2016, 09:41:40 PM Excessively high Q will eat up power??
Title: Re: DECREASING OUTPUT Post by: K1JJ on October 27, 2016, 12:47:46 PM Excessively high Q will eat up power?? Sure, in the form of heat from resonant circuit circulating I/R losses in an RF power amplifier tank. As an extreme example, tune up a standard final amplifier with a Q of 12 at 1 KW and the tank coil runs cool. Change the Q to 100 (less L and more C) and that same tank coil will have a meltdown due to high circulating RF currents within the LC resonant circuit. IE, higher Q in a parallel resonant tank circuit results in higher circulating current. It can be countered by using physically larger sized tank components. T Title: Re: DECREASING OUTPUT Post by: KA2DZT on October 27, 2016, 02:16:18 PM Excessively high Q will eat up power?? Sure, in the form of heat from resonant circuit circulating I/R losses in an RF power amplifier tank. As an extreme example, tune up a standard final amplifier with a Q of 12 at 1 KW and the tank coil runs cool. Change the Q to 100 (less L and more C) and that same tank coil will have a meltdown due to high circulating RF currents within the LC resonant circuit. IE, higher Q in a parallel resonant tank circuit results in higher circulating current. It can be countered by using physically larger sized tank components. T The higher Q will create higher peak voltages which should create higher output power or am I wrong. You're right the I/R losses can be lessen by a larger tank coil. Title: Re: DECREASING OUTPUT Post by: K1JJ on October 27, 2016, 02:54:05 PM In theory you are correct. By right, if the tank coil and surrounding associated RF components were of infinite size and had zero loss, (or were "super conductors") then the Q increase would have little effect in a power amplifier. Higher Q will even create the benefit of a slightly better linearity. However, in the real world, we usually work with smaller tank components that become lossy and get to a point of diminishing returns when the Q goes much above 25 or so, especially on the higher bands with skin effect. AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
It's all about real conductor size, skin effect and resultant I/R losses generating wasted heat. T |