I have an inverted L antenna resonant on 80m, I decided to add a loading
coil to the base to bring it on frequency for TopBand. I wound a 2.5mm wire
coil 16 turns on a bit of square Paxoline board by drilling holes into the
board each side and feeding the coil through the holes, a bit of a struggle
but it worked, not very pretty, it is 146mm diameter and 120mm long 30.5uH.
When I first tried it the SWR after small trimming was 1.2:1 band centre
and 2.0:1 band edges. Now it looked a but ugly, so I decided to neat it up
a bit by adding two fibreglass webs so the coil was now circular and held
by a cross section instead of just the one plane, now when I retried the
SWR it is still resonant on 1.800 but the SWR is now 2.0:1 in the middle of
the band and 4.0:1 at the edges the turns are the same, the inductance is
the same but I have lost my fine SWR curve.
It sounds like when you neatened up the coil the Q increased, although
perhaps something else changed.
If your earth losses are very low, the inductor Q can have a noticeable
effect. The better the Q, the further from your goals of low SWR and wide
bandwidth.
If you get the SWR lower in band center, the ends may come down. You could
put a shunt coil to ground across the feedpoint to do that.
73 Tom
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