OK Grant -- very nice work. Those dimensions are pretty close to what is
up there. I have read the many inputs from various people -- all very
thoughtful and really informative. I have decided to modify the mount. If
I can describe this clearly .. here goes.
I will use the existing mount and insert a steel pipe or multilayered
aluminum tube that extends beyond the leg on the tower a couple of feet or
so. I will fabricate an aluminum plate to mount the antenna on. It will
be mounted with solid PVC insulating pipe sections of minimal width, just
wide enough to cover the saddle clamps. In order to do this, the antenna
will be rotated 90 degrees, making it point the same way as the rotator
says it is pointing. It was 90 degrees off with the other way it was
mounted.
The weight of the antenna will be supported by the truss ropes which
connect out 25 feet from the center. The stray capacitance and the effect
of the ferrite in the steel will be removed from the equation. I think
that should do it.
The weather is not going to be good for another week and that will give me
time to fabricate the new tube and plate.
Thanks to all who have contributed. I will report back as soon as i have
something to report.
73,
Dave
On Mon, Apr 2, 2018 at 12:24 AM, Grant Saviers <grants2@pacbell.net> wrote:
> 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/ca
> lculator/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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