Topband: 160M/80M Vertical - Using a capacity hat below a trap

[Tom W8JI]

> I’m finalizing design for my 160M/80M shortened wire tree-mounted
> vertical.  An Unadilla 80M trap will be
> placed approx. 65’ feet above the feedpoint; above the trap is another 20 
> feet
> with a small cap hat.  An autotuner
> handles the Z matching for the anticipated 500 watts on both bands.
>
> QUESTION: Can one install a capacitive hat directly below
> the 80M trap to help shorten the length of the 80M section?  For example, 
> shortening the 80M radiator to
> 60’ (from 65’) and making up ‘the difference’ with a capacity hat?  Any 
> downside?


The only downside of adding capacitance below the trap is the chance of 
increasing capacitance across the trap and changing trap tuning. If you just 
keep the loading hat or wire away from the section above the trap, you 
minimize that effect.

You really want the trap slightly up out of the 80 meter band to minimize 
trap losses.

Almost anything can appear to work OK, but technically a trap of given 
components handles the least power and has the highest loss when it is at or 
very near resonance. If you did the trap resonance for 3.65 MHz, loss would 
peak and power capability would be minimum right around that frequency. The 
trap would have reduced loss and higher power rating as frequency moves away 
from 3.65.

The loading wire or hat below the trap actually can decrease loss and 
increase power rating of the system, so it isn't a bad idea if you also 
minimize coupling to the area above the trap.

73 Tom