[TowerTalk] Full 40 mts dipole on side of a tower

Grant Saviers grants2 at pacbell.net
Mon Apr 2 00:24:59 EDT 2018

Another thought is the steel angle proximity to the dipole since 
ferromagnetic stuff and HF antennas are a bad mix.  I've never seen 
steel or stainless used as an element to boom mounting plate, probably 
for good reason other than the weight vs aluminum.  And the dipole 
feedpoint is a high current area.  Or maybe some combination of stray C 
and loss/loading is detuning the 40m dipole.

We can estimate the capacitance magnitude if we know the dimensions.  A 
calculator for a wire over an infinite ground plane might be helpful.  
With the element partially enclosed with the pair of steel angles,  this 
may be a sensible estimate. 

So a rationalized guess: 1.5" diameter element, spaced 0.2", pvc e=3 , 
pvc to angles length 15"  =  88pf.  So is that two 44pf capacitors, one 
from each 1/4wl half dipole to the plate, thus in series for 22pf total 
shunt at the feedpoint?  What is the correct analysis?  If 22pf is in 
the ballpark, I think it is more than enough to shift the 40m 
resonance.  btw what are the resonance numbers??

Lawson in Yagi Antenna Design discusses boom to element clamp correction 
factors and AC6LA added a nifty calculator widget in AutoEZ to estimate 
the corresponding diameter of an element clamped directly to the boom 
mount.  But (always one of those) even NEC4 has a hard time with big 
steps in diameter and very short segment lengths.  Lawson, referring to 
saddle mounted elements, "the small correction <increased center of 
element diameter> disappears as the element is spaced away from the boom 
(even by a small amount)."   (Missed that before.)  So another win for 
Stauff Clamp/split collar type insulated and uninsulated DX Eng saddle 
clamps for elements.  Maybe I'll add an Al spacer *under* the DXEng 
saddles on future antennas.  The u-bolts are usually long enough.

OTOH, I haven't found any discussion of the stray field issues for 
driven elements.

My F12 80m dipole had ABS drain pipe split insulators.  A really bad 
idea since the foam core compressed over time.

Grant KZ1W

On 4/1/2018 17:24 PM, Jim Thomson wrote:
> Date: Sun, 01 Apr 2018 03:03:32 +0000
> From: FERNANDO MUGUERZA VIDEGARAY <xe2fl.kf5mn at gmail.com>
> To: TowerTalk at contesting.com
> Subject: [TowerTalk] Full 40 mts dipole on side of a tower
> <I think the antenna is a big capacitor formed by the antenna as one of
> capacitor?s plate ,the big tower plus antenna clamps are the other
> capacitor?s plate,this gives a lot of capacitance that is changing the
> antenna?s frequency of resonance,it is better to separate the dipole from
> the tower which now is  only 1 inch.
>    Insulated plate and clamps are a must!
>         Fernando,XE2FL
> ## I find this hard to believe.  If there is any stray C  between  each ele half, and the al
> plate he has it mounted to,  you could easily measure it with any digital  LCR meter.
> ##  F-12  mounted their 40m and 80m rotary dipoles to large  17 inches wide x 8 inches tall
> al plates , using just split  PVC tube u find in any home depot electrical dept, the  grey  stuff,
> UV resistant.
> Something else is amiss here.
> Jim   VE7RF
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