[Amps] nylons

Karl-Arne Markström sm0aom at telia.com
Wed Mar 16 02:09:44 EST 2005


It is most unlikely that any differences will be noted between nylon (or other "lossy" plastics) 
or high-grade dielectrics in these applications, as the E-field intensities around the components will be quite low, 
compared to the intensities found between i.a. the tuning capacitor plates.

When you are building a "balanced tuner" or other network that has to be insulated from ground potential,
the E-field distribution around the circuit would be quite benign, and the supports and enclosures would not
be very stressed RF-potential-wise.

My point about avoiding "lossy" plastics such as Nylon in high RF field intensity places
is that they can form the dielectric of a capacitor (stray or intended)
with high losses and consequently low Q, which may be disastrous 
when you have a lot of circulating RF current concentrated in or around a component.

>From the thermal design and structural point of view, the softening point of the materials may be of concern if you intend to
run "amateur ratings" on the components in the networks.

However, it appears that the "pros" are taking this seriously, and in modern high-power commercial PA's and tuning 
units you only find PTFE, fiberglass and ceramic insulation and supports.

73/

Karl-Arne
SM0AOM



----- Original Message ----- 
From: "k0dan" <k0dan at comcast.net>
To: <amps at contesting.com>; "Karl-Arne Markström" <sm0aom at telia.com>
Sent: Wednesday, March 16, 2005 3:09 AM
Subject: Re: [Amps] nylons


> Suppose a ham was building a balanced matching network, consisting of
> variable inductors and vacuum caps. It would be designed for full legal
> limit on any mode (AM, SSB, CW), from 160M-10M.
> 
> To keep it balanced and safe, it would be enclosed in an acrylic or
> polycarbonate case (say, 1/2" stock), and might measure approx. 18"x12"x6".
> If ceramic insulators were not available to "float" the components above the
> (plastic) chassis, nylon standoffs or polycarbonate/acrylic stock would be
> used as standoffs.
> 
> Depending on its tuning condition, this device could have rather large RF
> currents or voltages. Are you saying the standoffs, and plastic case itself,
> would be unsuitable? And what would be the result? HV breakdown and arc
> over? RF coupling and "unbalancing" of the network? Meltdown of the
> standoffs?
> 
> What would be the "best" materials to use in this scenario?
> 
> 73
> 
> Dan
> K0DAN
> 
> 
> ----- Original Message -----
> From: "Karl-Arne Markström" <sm0aom at telia.com>
> To: <amps at contesting.com>
> Sent: Tuesday, March 15, 2005 3:31 PM
> Subject: Re: [Amps] nylons
> 
> 
> > Nylon should probably be avoided entirely in strong RF fields.
> >
> > The dielectric material property list in ITT Reference Data for Radio
> Engineers
> > lists the dissipation factor of nylon as about 0.022 at 1 MHz , 0.02 at
> 100 MHz and 0.012 at 3 GHz
> > (actually decreasing with frequency).
> > This is about two orders of magnitude worse than the best plastics and
> ceramics.
> >
> > Coupled with its low softening point and dielectric strength, it seems
> that nylon is a
> > low-priority choice as an RF dielectric.
> > Much of the same also goes for acrylic plastics (Plexiglas and Lucite).
> >
> > 73/
> >
> > Karl-Arne
> > SM0AOM
> >
> > ----- Original Message -----
> > From: "David Kirkby" <david.kirkby at onetel.net>
> > To: "John Irwin" <crazytvjohn at yahoo.com>
> > Cc: <amps at contesting.com>
> > Sent: Tuesday, March 15, 2005 9:06 PM
> > Subject: Re: [Amps] nylons
> >
> >
> > > John Irwin wrote:
> > >
> > > >dudes
> > > >
> > > >a large ace hardware store opened here last week , One block away.  I
> have noticed a lot of nylon items that look a lot like the ceramic standoff
> insulators, they come with a hole in the center,and some are taped for 6-32
> or other simular sizes.  lenght every 1/8 inch or so,diameter 1/4 to 1/2
> inch
> > > >
> > > >How does nylon act in rf?    Like in a amplifier as used to support
> variable capacitors or inductors? Next to the plate choke.
> > > >
> > > >Anyone put some in the pi coil and see if it holds up. or does it go up
> in smoke?
> > > >
> > > >thanks
> > > >john kb9tc
> > > >
> > >
> > > One other thing I forgot to add is that I have here some Ross HyperSPARC
> > > CPUs,  which were from a Sun workstation. (Sun did not make this CPU,
> > > but a 3rd party did). I know this particular machine had been run 24/7
> > > for many years, and never broke down.
> > >
> > > The heat sinks were attached using some sort of black plastic screws.
> > > When I moved the computer, several snapped. The material (probably a
> > > nylon) had become very brittle indeed. But these HyperSPARC processors
> > > were known to run very hot and were subjected to this for many years
> > > indeed. So a short-term test does not give the whole story.
> > >
> > >
> > > --
> > > Dr. David Kirkby,
> > > G8WRB
> > >
> > > Please check out http://www.g8wrb.org/
> > > of if you live in Essex http://www.southminster-branch-line.org.uk/
> > >
> > >
> > >
> > > _______________________________________________
> > > Amps mailing list
> > > Amps at contesting.com
> > > http://lists.contesting.com/mailman/listinfo/amps
> > >
> > >
> > > --
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> >
> >
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> >
> 
> 
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