Topband: Topband loading coil

[Tom W8JI]

>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