160m dipole on 40m

[Steve W8TOW]

To start with, I've got a great working 80/75m dipole fed with
600 ohm feeders to a J KW mbox...AND it works FB on 20m....

In years past, I've used a 40m dipole on 15m with little issues...

Why does my 160m full size dipole fed with 600 ohm feeders
fail to load on 40m?
I have a HB link coupled tuner similar to the KW Mbox design,
(a edge wound coil about 4" diameter and 12" long with about 500 pF of "C")
and regardless where I "tap" the coil, I cannot achieve resonance...
My 80m dipole actually resonates easier on 40m...

[W2VW]

The combination of doublet length and feeder length presents a load to your coupler which is out of it's tuning range. Not unusual.

A different coupler configuration such as series tuning might be one way to make it play.

Another way would be to take a matched pair of doorknob capacitors and put them in series with each feeder where they connect to the coupler. This would take experimentation. I suggest 75-100 pf each for the first guess.

If you try this make sure there is no heat from the caps after a long test transmission. If there is, too much power is being used up by components with higher than necessary circulating current in the name of convenience.

Another cheap trick is to place a reactance across the feedline. Roller inductors are en easy way to try this. Sometimes the small inductance values that work can seem unusual.

[Steve W8TOW]

Thanks for the suggestion...will give that a shot!
73
8tow

Oh, the current config of the ATU is a Parallel TU...as you might assume...