[TowerTalk] Change in Frequency As Antenna Height Rises

[Jim Thomson]

Date: Tue, 31 May 2016 12:32:51 -0400
From: "john at kk9a.com" <john at kk9a.com>
To: towertalk at contesting.com
Subject: Re: [TowerTalk] Change in Frequency As Antenna Height Rises

Force12 sure has complicated way to assemble a standard 40m shorty beam!

I agree that is sounds strange that a dipole would shift 200+Khz moving it
from 30' to 60' however my quick EZNEC+ model shows an even bigger
frequency change when moving full sized wire dipole.

John KK9A

##  The f12  240N, and 340N are assembled to the diagrams included.  However,
they suggest to verify each individual ele for resonance at a specific freq.   Now most
folks just assemble per the instructions, and since everything is pre-drilled etc, there
is nothing to adjust anyway...except for the linear loading.  The linear loading is not adjusted
either, and installed with max linear loading in each half of each ele.  They say that for each
6 inch adjustment of both tips, that resonance will shift  exactly 100 khz.  The linear loading rods
on the N style els  are made of  solid  3/16 AL rod.  Whats unique is they  extend outwards 
towards the tips.   The linear loading starts  7 ft  9 inches out from the boom...so its  not 
half way out each side of the ele.  Done that way so you dont require  any where near as much
loading. 

##  In my case,  I replaced the solid LL  rods with silver plated  .375 inch OD  copper tubing.
Flattened on each end, so I can get a 1/4-20  bolt through each end....and then right through
the AL tubing + solid fiberglass insulator.   I need at least  .375 inch tubing...in order to handle a 1/4-20 bolt.
Q of each coil is sky high, like > 1750.  ESR of each coil is just .07 ohm.   Each coil is 2.79 uh. 
I worked the coils out on software such that  I could re-use the oem solid fiberglass insulators. 
I can only wind a coil in full turn increments, so have to also juggle the diameter and also the
spacing between turns to achieve the desired uh.   Even then, I could easily be off  by a tiny bit. 
So each ele has to be checked at a moderate height, like  35-40 ft,  and tips  adjusted if required,
to resonate each ele  at the desired freq. 

##  The  240N and 340N  are already designed on software..and both work superb. 

##  I still contend that your EZNEC+  is out to lunch.  Install a full sized 40m dipole at 30 ft.....
then move it up to 60 ft..... and keep the feedline the same length in both cases.  Resonance barely shifts
higher in freq.    Ask anybody who has a motorized crank up tower  + loaded rotary dipole.  Put the
test gear  right at the DE, with a double male connector. 

Jim   VE7RF