There are plenty of reasons that this capacitance scenario is bogus. It
would be nearly impossible to maintain a spacing of 1 mil or 3 mils between
conductors with the pressure created by the element weight and wind forces.
Then there is the ever changing dielectric constant of joint compound as
moisture comes and goes with rain. There is the loss tangent of the dielectric
as well.
More likely that a joint like this looks more like a low value resistance.
With a few screws or rivets in place even that would be modified to a very
low resistance (negligible).
Granted, we have all seen intermittent connections due to corrosion and
other causes but invariably they are caused by lack of attention to the
proper use of joint compound and/or joining hardware.
The concern about stainless steel connecting hardware seems over blown.
After all, mobile antennas have been made with stainless steel for many
decades and you never see one that has turned black due to rf heating.
Aluminum vs copper conductors also seems over blown. It is very well known
that aluminum resistivity is about 28% more than copper. While this at
first sounds like a lot, one can see that 28% more of something that is near
zero loss is still near zero loss.
Making low loss connections is another matter though. One can easily make
soldered connections with copper; not so much so with aluminum. Perhaps use
of joint compound is advisable in all applications with connections to
aluminum.
My opinion of course, yours may be different.
73,
Gerald K5GW
In a message dated 3/9/2016 7:51:01 A.M. Central Standard Time,
jimlux@earthlink.net writes:
On 3/8/16 9:09 PM, Bert Almemo wrote:
Jim,
While I agree with most of you're writing I think you'll be hard pressed
to
get a solid 9 square inches of surface contact in your 1 inch tube with 3
inches of overlap. Maybe if you put in a lot of SS screws or rivets at
the
joint. If you're using any kind of joint compound, like Penetrox, you
need a
certain pressure to make a good contact, as I'm sure you know. SS hose
clamp
+ SS screws has been a good combination for me.
Don't forget the capacitance. 9 square inches separated by 1 mil is
about 2000 pF
at 14 MHz, that's an impedance of about 5 ohms. Even if there's a bigger
gap (say, 3 mils) the impedance goes up to 15 ohms, and in any case
capacitance is lossless: you just make the element a bit longer to
cancel the series C.
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