[TowerTalk] Inverted L Tuning Question

[Jim Brown]

On Thu, 4 Dec 2008 18:06:16 EST, RLVZ at aol.com wrote:

>It was my understanding that the coax braid should be connected to both the 
>radials and the base of the towers gnd system, but as you can see the the 
>minimum SWR and resonant freq. go crazy when I do. 

The coax shield should be going to the radial system. Earth is NOT a good 
return for antenna currents/fields. You don't want/need an EARTH connection. 
You DO need radials. 

The antenna is electrically longer than a quarter wave, thanks to the 
capacitance of the top section to earth. That means it looks like R + jXL. 
You can tune out the XL term by simply adding capacitance in series. I'd 
start with values in the 200-300 pF range. These values can be obtained by a 
suitable length of coax. RG8 or RG11 is plenty good enough for legal power. 
You'll need to keep it above the earth so that there's minimal shunt 
capacitance. 

Two radials is not very many. I'd try to add a few more. A total of 8 is a 
reasonable stopping place for a quickie job, and will probably add a dB or 
two to your signal. 

73,

Jim K9YC