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.
https://www.emisoftware.com/calculator/wire-over-ground-plane-capacitance/
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@gmail.com>
To: TowerTalk@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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