[Amps] Shielding Effectiveness

[Dave M]

What I have been using is the scavenged shield from Microwave oven doors , it is sturdy, attractive, reasonably easy to cut and it is free, I strip back the paint on the edges and use rf gasket material sandwichid between the bare  metal screen and the chassis , seems to work OK for me and I see no reason it wouldn't work for others 


I figure if it will sheild 1250 watts od RF at 2.4ghz there is no reason it wont be fine for HF 


......................................................................................................
 
73     VE3DV , Dave



 

> Date: Wed, 6 Oct 2010 14:23:37 -0400
> From: sub1 at rogerhalstead.com
> To: amps at contesting.com
> Subject: Re: [Amps] Shielding Effectiveness
> 
> 
> 
> On 10/6/2010 1:03 PM, Sam Carpenter wrote:
> > So...The screen on this retrofit might not be the best:
> >
> > http://www.hamradiomarket.com/Articles/Ten%20Tec%20Centurion.htm
> >
> > Looks a little wide and not to well snugged down.
> What I'd do in that case is take a strips of 1/8" Aluminum about 1/2" 
> wide and sandwich the expanded SS between them and the cabinet.
> I'd probably use #8 or #10 pan head screws (plated Brass or SS) about 
> one per inch or two. What ever it'd take to make good, continuous contact.
> The size of the strips, or even material is not important as long as 
> it's strong enough to do the job and not magnetic like steel. 
> Appearance may or may not play a part in the selection.
> 
> OTOH I've seen commercial amps that only used a blade protector to keep 
> fingers out of the fan in that location.
> 
> 73
> 
> Roger (K8RI)
> 
> > N9FUT
> >
> > -----Original Message-----
> > From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> > Behalf Of Roger (K8RI)
> > Sent: Wednesday, October 06, 2010 12:48 PM
> > To: amps at contesting.com
> > Subject: Re: [Amps] Shielding Effectiveness
> >
> >
> >
> >
> > It's important to remember that hardware cloth which comes in many
> > sizes, or screen does not normally give a good contact around the edge.
> > If fender washers are used they basically need to be edge to edge.
> > Commercial equipment uses a bracket, or frame to hold the screen in
> > tight contact with the cover/chassis around the entire periphery.
> > Although screen can be a much better shield at VHF, that advantage is
> > lost if that contact is not continuous around the edge.
> >
> > 73
> >
> > Roger (K8RI)
> >
> > On 10/6/2010 11:13 AM, Ward Silver wrote:
> >> Per Ott's "Electromagnetic Compatibility Engineering", section 6.10 -
> > shielding effectiveness in dB is:
> >> S = 20 log (wavelength / 2xL) where L is the maximum linear dimension of
> > the aperture.
> >> If you have multiple apertures, such as an array of holes, the general
> > procedure is to calculate S for one hole and then reduce S as 10 log (n)
> > where n is the maximum number of holes in a straight line in the array.
> >> For example, if S for one hole is 52 dB (a 2.5 mm hole at 2 meters) and
> > there are at most 20 holes in a line, S would be reduced by 10 log (20) = 13
> > dB to S = 52 - 13 = 39 dB.
> >> This is an approximation, of course, plus there are seams and other things
> > to worry about. But it's a good way to at least compare types of apertures
> > in shields.
> >> 73, Ward N0AX
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