KB1VLA
Member
I've built a doublet for which I intended to use on 20-6 meters. It is roughly 38' long (16 gauge wire) and fed in the middle with 40' of 450 ohm ladder line connected directly to an MFJ 989C antenna tuner which is rated up to 3000 watts. The antenna is about 20' in the air between 2 trees. The tuner is grounded with about 7' of 6 gauge wire to a roughly 28" steel pipe driven into the ground. The transceiver is an Icom 7300. There is roughly a foot of RG-58 coax connecting the tuner to the radio. I chose the 38' length because as I understand it, a multiband doublet should either be slightly longer or slightly shorter than a resonant dipole on its lowest intended frequency (which would be 33' for 20 meters).
I cannot get this antenna to tune up on any band. Some bands (20, 12, 10 meters) do not have any dip in the SWR no matter what the inductance or capacitance is set to on the tuner. It's also completely deaf on receive on 10 meters. On other bands (17, 15, 6 meters), I'm able to get a good match when running low power (5-10w of RTTY), but the SWR increases to well over 5:1 when I raise the TX power to over 20w (which does not make sense; impedance should be impedance whether I'm transmitting 5w or 5,000w), or wildly swings from a good match (<1.5:1) to well over 5:1 when the only part of the radio I'm touching is the PTT switch on the microphone. More alarmingly, the tuner makes a burning smell and a light smoke emits from it when I run more than 20 watts into it.
Video of SWR wildly changing during transmission (20w of RTTY): MFJ 989C doublet troubles
This is the back of the antenna tuner; I've changed the ground wire to the aforementioned 6 gauge since this picture was taken but the rest of the setup is unchanged:
I'm stumped. Everything I've read says this system should be relatively simple to get a match from 20-6 meters, but I cannot get it to work at all. Ideas?
I cannot get this antenna to tune up on any band. Some bands (20, 12, 10 meters) do not have any dip in the SWR no matter what the inductance or capacitance is set to on the tuner. It's also completely deaf on receive on 10 meters. On other bands (17, 15, 6 meters), I'm able to get a good match when running low power (5-10w of RTTY), but the SWR increases to well over 5:1 when I raise the TX power to over 20w (which does not make sense; impedance should be impedance whether I'm transmitting 5w or 5,000w), or wildly swings from a good match (<1.5:1) to well over 5:1 when the only part of the radio I'm touching is the PTT switch on the microphone. More alarmingly, the tuner makes a burning smell and a light smoke emits from it when I run more than 20 watts into it.
Video of SWR wildly changing during transmission (20w of RTTY): MFJ 989C doublet troubles
This is the back of the antenna tuner; I've changed the ground wire to the aforementioned 6 gauge since this picture was taken but the rest of the setup is unchanged:
I'm stumped. Everything I've read says this system should be relatively simple to get a match from 20-6 meters, but I cannot get it to work at all. Ideas?
