[TowerTalk] F12 C19XR Rivets

[TexasRF at aol.com]

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 at 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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