© WB5CXC 2011
Adjustable J - Poles
I have seen on the Internet where adjustment screws have been put on Copper Cactus J-Poles.  I was wondering if this was worth the work to add them.  I calculated the dimensions for a Copper Cactus, I then did an EZNEC model of the Copper Cactus.  The SWR was not too good.  I then then added an adjustment of ~ 1/4” & 1/2” to the model an recalculate the SWR.  I determined that adding the adjustment screws is well work the effort.  
I used the above for the model at 20 foot height.  I was investigating if an adjustable screws are useful.  I first modeled the antenna with the measurement from the above formula.  You can see from the SWR curves below that it can be improved.  The resonant is above 148 MHz (curve is still going down - haven’t reached resonance yet).  I changed the model and added some adjustment screws and guessed at what length (1/2”) they should be.  I then ran another SWR curve.  It improved (SWR 2) curve without moving the coax up and down tto find a better match.  The resonant point now shifted to just above 146 MHz and it covers the whole 2M band with less than a 1.6:1 SWR.  thus it makes it easier to adjust the J-Pole to cover the whole band. 
SWR Curve 1
The curve above shows that the resonant frequency is high for the 2M band (antenna too short).  To get the antenna to resonant at a better frequency then the elements needs to be lengthened.  
SWR Curve 2 - J - Pole with adjustments