Using one of two antenna tuners connected in series

Question

I have:

• A Kenwood TS-590SG.
• 2 antennas up, a tri-band beam and a fan dipole.
• I run both antennas to a MFJ-998 auto tuner.
• I also have a MFJ versa tuner IV.

The auto tuner works good for 80-10 meters but will not tune the dipole for 160 and will not even try on 6 meters.

I was thinking about coming out of the radio to the amp then to the auto tuner using antenna 1 as an output. Take the output of that tuner to the input of the versa tuner IV and connect both antennas to the versa tuner IV. So for 80-10 I could put the versa tuner in bypass and use the auto tuner. When I wanted to try 160 or 6 meters I would turn off the auto tuner and select the antenna on the Versa tuner. When the auto tuner is off it is supposed to bypass that tuner and be on antenna 1. I had thought about using 2 antenna switches for this but thought about the bypass feature of the versa tuner.

Does anybody see why this would not work?

Answer 1

Two tuners in series might "work" but you should consider all of the factors at play.

Since you cannot match the antenna with a single tuner, this indicates a fairly high SWR on the line. This will cause not only higher feedline losses but could lead to the development of sufficient voltage or current on the feedline to cause its failure. This is particularly true when an amp is involved as you indicated.

Then consider the tuners. With a high SWR and an amplifier, at least one of the tuners could flash over (arc) due to high voltage or it could smoke due to high current. There is also the general consideration of tuner efficiency. If the two tuners combined yielded a 2 dB loss, for example, and you run a 1 kW signal through them, they will dissipate 370 watts of heat. That is a lot of heat for a tuner to safely dissipate (and generally one tuner will bear the brunt of the matching and thus heat dissipation).

So if you experiment with this setup, I would recommend starting at QRP power levels and then slowly raise power while monitoring the operating conditions. You may find that the combined losses and risk of damage don't make it a viable solution. Then its time to put up reasonable antennas for 160 and 6 meters.

Edit:

If you are planning on putting the two tuners in series but having one in bypass, all of the above still applies. An additional consideration would be if the bypass circuit on the tuner can adequately handle the associated voltage/current that will exist with the higher SWR. When operating on 6 meters, the additional tuner, even in bypass, may alter the impedance seen by the functioning tuner.

Answer 2

In principle, this will work fine. The possible non-idealities are:

• Insertion loss from the unused tuner (the extra cabling has some loss; the connectors and bypass switches all have impedance “bumps”).
• The tuner closer to the antenna will see a high SWR and therefore high voltages on its input when the other one is in use, which is an unusual situation that it may not be designed to tolerate, as Glenn W9IQ's answer points out.

(You could even use both tuners at once to get a match that would be out of range for either one by itself, but of course just like any large mismatch it is subject to feed line loss.)

Answer 3

Out of your three tuners (including the one built-in TS590), only MFJ-998 is an L-network and the other two are T-network. If possible and equally convenient, use MFJ-998, as it causes the least insertion loss.

T-network, like one in Versa tuner, causes high insertion loss on low bands, so I would expect the inductor to be quite hot. I'd monitor that on 160. T-network is also quite lossy when the load impedance is low (and that is the case when driving a 80m dipole on 160m, except the coax may change that relationship depending on the length).