2021-01-10, 09:49:42
Another thought:
Rob mentioned this already, but you should tune in CW mode because you need a carrier to measure. When you are in LSB/USB mode the needles on the tuner are only showing the amount of modulation which obviously varies as you speak into the mic. My MFJ tuner has a switch to measure either peak or average forward and reverse voltage on the cross needles. I leave it on the average setting so the needles only move slowly as I speak.
The correct tuning procedure is to lower output power (5 watts is enough), set to CW mode (plug a key into the radio if needed), adjust the tuner's inductance for maximum noise, then adjust the variable capacitors (they are interactive) for lowest SWR. The SWR can be read at the point where the needles cross.
I have built a simple circuit that adjusts the ALC voltage (I'll write it up for the February newsletter). When I flick a switch my Yaesu FT-897 is set to 5 watts and transmits a carrier for tuning. When I turn the circuit off the power and mode return to their previous setting. Some radios are smarter than the FT-897 and don't need that circuit.
Going back to my previous reply concerning antenna/feedline length; if each half of the antenna is cut for 80m it is going to be probably 65-70 feet long. If, as you say, you have 55-70ft of feedline it is quite likely that the overall length will be close to 130ft and that would give you very high impedance on 80m and its harmonics (40m, 20m, 15m, 10m) resulting in high SWR. Try measuring the SWR on every band and see what you get. then you will have an idea what needs adjustment.
Note: The impedance at the end of a half-wave wire is 2500-5000 ohms. Even with a 4:1 balun the impedance only comes down to, at best, 700 ohms. The tuner still has a 700/50=14:1 mismatch to deal with. We need to get the impedance down to, at most, a few hundred ohms before the balun. Let's say the impedance at the end of the feedline is 600 ohms. After the balun, that reduces to 600/4=150 ohms and the mismatch is now 150/50=3:1 which is easy for the tuner to handle.
Rob mentioned this already, but you should tune in CW mode because you need a carrier to measure. When you are in LSB/USB mode the needles on the tuner are only showing the amount of modulation which obviously varies as you speak into the mic. My MFJ tuner has a switch to measure either peak or average forward and reverse voltage on the cross needles. I leave it on the average setting so the needles only move slowly as I speak.
The correct tuning procedure is to lower output power (5 watts is enough), set to CW mode (plug a key into the radio if needed), adjust the tuner's inductance for maximum noise, then adjust the variable capacitors (they are interactive) for lowest SWR. The SWR can be read at the point where the needles cross.
I have built a simple circuit that adjusts the ALC voltage (I'll write it up for the February newsletter). When I flick a switch my Yaesu FT-897 is set to 5 watts and transmits a carrier for tuning. When I turn the circuit off the power and mode return to their previous setting. Some radios are smarter than the FT-897 and don't need that circuit.
Going back to my previous reply concerning antenna/feedline length; if each half of the antenna is cut for 80m it is going to be probably 65-70 feet long. If, as you say, you have 55-70ft of feedline it is quite likely that the overall length will be close to 130ft and that would give you very high impedance on 80m and its harmonics (40m, 20m, 15m, 10m) resulting in high SWR. Try measuring the SWR on every band and see what you get. then you will have an idea what needs adjustment.
Note: The impedance at the end of a half-wave wire is 2500-5000 ohms. Even with a 4:1 balun the impedance only comes down to, at best, 700 ohms. The tuner still has a 700/50=14:1 mismatch to deal with. We need to get the impedance down to, at most, a few hundred ohms before the balun. Let's say the impedance at the end of the feedline is 600 ohms. After the balun, that reduces to 600/4=150 ohms and the mismatch is now 150/50=3:1 which is easy for the tuner to handle.
John VA3KOT
Blog: HamRadioOutsideTheBox.ca
Blog: HamRadioOutsideTheBox.ca