Tuner detunes antenna
 

I recently bought an MFJ-986 tuner and have been using it will good results
on a balanced feed antenna, but I just found an odd behavior. I have a 40
and 20 meter dipole that I've used for several years, and 20 meter elements
are tuned to 14.125 which gives me a decent SWR on CW and the low end of
SSB. I thought I'd hook it to the tuner so I could use the top end of 20
meters and tune the off-resonance SWR down. I found that when I hooked the
antenna to the tuner and fed it through the Coax1 Direct, the 1:1 SWR point
shifted down about 13.950! I took the tuner out of the circuit and it again
tunes at the normal frequency. So I tried it on the Coax 2 connector and
found that that one shifted the 1:1 SWR frequency down about 14.050. I
called MFJ technical support and they, of course, had no clue. I also
fiddled with the tuning knobs, thinking that there might be some interaction
inside the box even though the coax was in the direct connect mode, but no
interaction found. The 40 meter dipole seems stable, as far as the resonance
shifting goes, but I haven't done much testing with it yet. Has anyone
seen/heard of anything like this with the MFJ-986 or any similar tuner?

Thanks and 73,
-- Dave, WA0TTN

David Cook wrote:
I recently bought an MFJ-986 tuner and have been using it will good results
on a balanced feed antenna, but I just found an odd behavior. I have a 40
and 20 meter dipole that I've used for several years, and 20 meter elements
are tuned to 14.125 which gives me a decent SWR on CW and the low end of
SSB. I thought I'd hook it to the tuner so I could use the top end of 20
meters and tune the off-resonance SWR down. I found that when I hooked the
antenna to the tuner and fed it through the Coax1 Direct, the 1:1 SWR point
shifted down about 13.950! I took the tuner out of the circuit and it again
tunes at the normal frequency. So I tried it on the Coax 2 connector and
found that that one shifted the 1:1 SWR frequency down about 14.050. I
called MFJ technical support and they, of course, had no clue. I also
fiddled with the tuning knobs, thinking that there might be some interaction
inside the box even though the coax was in the direct connect mode, but no
interaction found. The 40 meter dipole seems stable, as far as the resonance
shifting goes, but I haven't done much testing with it yet. Has anyone
seen/heard of anything like this with the MFJ-986 or any similar tuner?

I doubt that the MFJ-986 maintains a 50 ohm Z0 through the device. What
you have probably done is introduce a short section of "transmission line"
that doesn't have a Z0 of 50 ohms and thus detuned your system.

I vary the length of my transmission line as a tuning method. You apparently
varied the length of your transmission line as a detuning method. :-)
--
73, Cecil, W5DXP

The additional length of coax and the tuner changing the "RESONANCE" of
an antenna indicates that the antenna[s] did not have Zo = 50 +/- j0
ohms. Restated, there was a mismatch! The length of transmission line
from the antenna[s] to the rig tuned the apparent resonance to 14.125.
BTW resonance is NOT indicated at a VSWR of 1:1. Resonance is when the
reactance in the ANTENNA IS ZERO.

So, adding extra components in the system changed the apparent VSWR not
the antenna resonance.

BTW, I'm currently using a trap vertical as an interim antenna that has
measured resonance[s] between 26 to 37 ohms resistive from 10 to 40
meters. That's a VSWR approaching 2:1. A tuner or extra lengths of coax
will tune the ANTENNA SYSTEM to an apparent VSWR of 1:1 [but I know
better! My VSWR is still ~2:1 and my antenna resonance is still 26 to 37
ohms resistive]. MFJ 259B Antenna Analyzer used for measurements.

Deacon Dave, W1MCE

David Cook wrote:
I recently bought an MFJ-986 [SNIP]
I found that when I hooked the
antenna to the tuner and fed it through the Coax1 Direct, the 1:1 SWR point
shifted down about 13.950!

OK. There IS a mismatch in the antenna system!

I took the tuner out of the circuit and it again
tunes at the normal frequency. So I tried it on the Coax 2 connector and
found that that one shifted the 1:1 SWR frequency down about 14.050. I
called MFJ technical support and they, of course, had no clue. I also
fiddled with the tuning knobs, thinking that there might be some interaction
inside the box even though the coax was in the direct connect mode, but no
interaction found. The 40 meter dipole seems stable, as far as the resonance

{[SNIP] It's NOT ANTENNA RESONANCE!

shifting goes, but I haven't done much testing with it yet. Has anyone
seen/heard of anything like this with the MFJ-986 or any similar tuner?

Thanks and 73,
-- Dave, WA0TTN