Re: RF Conductivity of thin Mo film

Peter Simon wrote:
> "Mark Fergerson" <[EMAIL PROTECTED]> wrote in message
> news:[EMAIL PROTECTED]
>
> >   Right. Is the physical layout absolutely fixed? Can you
> > relocate the radome so it's out of the exhaust plume(s)?
>
> I'm afraid not.  The radome covers the feed horn for a very large reflector
> antenna made of Mo mesh.
> The thruster plume will hit the reflector and dislodge Mo particles.  The
> feed and radome must be
> located very precisely wrt the reflector.  Various problems arise when
> considering repositioning the thruster, or deflecting its plume, though the
> latter may be the best solution so far (but see Uncle Al's suggestion about
> a sheet of mylar being continously rolled up to remove the contaminants).

  Ugh. I assume there's an array of thrusters; can they be 
rearranged such that none of their plumes reach the antenna 
structure or aren't hot enough to sputter bits off it by the 
time they do reach it? By rearranged I mean "detaching", 
rotating, and "reattaching" the entire thruster array so to 
speak without disturbing the relationship to the craft's 
C.G. It should "just" be a matter of relocating the thruster 
attachment points and rewriting some software. Or the 
thrusters could be boomed out farther if launch volume and 
weight allow. Are folding booms considered reliable these days?

  Failing that Unc's sheet could be fitted over the horn. 
That way it'd be smaller and lighter than putting it between 
the thruster and the antenna structure (ISTM that could act 
as a sail, doing bad things to the thrust vector). The 
transport mechanism would produce its own spurious 
reflections though.

  OTOH let's back up a step; Mo is fairly refractory. Who 
determined what the off-sputter rate will be? Was it 
extrapolated from ground-based industrial stuff, or known 
from spacecraft experience?

> >   Well, I meant try to duplicate just the conditions of
> > deposition, do a worst-case buildup, and then do your
> > transmissivity tests without worrying about other elements'
> > possible inclusion in the film (due to outgassing etc). All
> > the rest shouldn't have much effect as I can't see
> > vacuum-sintering and annealing altering the film's
> > properties much. But I could be wrong.
>
> My reading (and the post from Joseph D. Warner) lead me to believe that the
> character of the
> resulting film is strongly dependent on the temperature and other factors
> present during the
> deposition process.

  Your "unacceptable" definitions below seem to mean that 
you could say with some confidence that the merest amount of 
even pure metal plated onto the radome is an adequate 
approximation to reality.

> > > Great reference! Thanks!  How'd you find it?
> >
> >   I Googled for
> >
> >  Molybdenum +"electron mean free path"
>
> I thought that I had done the same thing, but perhaps without the quotes.

  Google sees words in quotes as a block and returns 
documents containing them as a block. Leaving off the quotes 
gives you everything including any of the words.

> >   First I'd find out what "unacceptable" means. Is this an
> > Earth-communication antenna? What range, ERP, etc?

> Range and EIRP are not really relevant here, because the radome is located
> in close proximity to the feed horn.

  I was worrying about power reflected back into the feed 
guide.

> In this case, "unacceptable" means any of the following:  radome return loss
> less than 20 dB, insertion loss increased by more than 0.1 dB, power
> dissipated in the contamination layer greater than about 0.5% of incident
> power density (it's a high-power, transmit feed antenna).  My preliminary
> calculations using the Hansen-Pawlewicz formula show that a thickness of
> even 10 Angstrom will be unacceptable.  However, I have my doubts
> about their validity when the layer thickness is this small.

  Then you may just have to plate up a few radomes to 
various thicknesses (corresponding to various aging rates) 
and see. Your prelim figures (and the lack of available 
data) ought to justify the time and expense; they would make 
me sufficiently nervous to want it checked out if I were 
project management.

> Thanks for the inputs

  You're welcome. Please let us know how it turns out.

  Mark L. Fergerson