[AMPS] RFI and windows

[John T. M. Lyles]

As for putting viewing windows in amplifiers:

Good old tube medium wave transmitters always had a nice hole to look 
into, to see the plate orange spot on the 833, 4-400, and larger 
bottles. The wavelengths were on the order of 300 meters.

At 28 Mhz, with 10 meter wavelengths, the holes are still fairly 
small, but I don't think they have zero leakage. In VHF and UHF rigs, 
this is certainly a major radiator of RF. At HF frequencies, it is an 
inefficient but real RF leak. IF there is RF wall current inside the 
enclosure, due to proximity to inductors, conductors, such as eddy 
currents, and IF the viewing hole happens to interfere with the 
normal current path, you can bet that there are some magnetic fields 
(RF) distorted around the hole. In that case, there is some leakage 
at the slot or hole too.

I always have put copper screen (1/4 to 1/2 inch mesh or even window 
screen mesh) in high power amplifiers at the viewing window. It is 
grounded to the enclosure, and can have Lexan or other transparent 
plastic there to prevent screwdrivers, fingers, or flying glass from 
coming through. (also for cooling air control)

In Donald White's Handbook on Electromagnetic Shielding Materials and 
Performance, a chart (pg 2.21) shows the shielding effectiveness (SE) 
for various meshes and hole spacings versus frequency:
At 3 MHz, chicken wire mesh offers 70 dB of attenuation (actually 
SE). At 30 MHz it is only 45 dB. 1/4 inch mesh screen is on the order 
of 12 dB better than these numbers.

Most of this refers to radio interference suppression. When you are 
using an amplifier to drive an antenna, then so what if a little of 
your signal gets out downstairs. However, keeping it off the phone 
line, your other equipment, and your eyeballs is another question. 
Better put some screening in your amplifiers.

Heres some fun background stuff on RF heating effects, FYI:

Tesla, D'Arsonval, and others talked about the effects of high 
frequency fields on humans before 1900. Two interesting papers are in 
a special centennial issue of the IEEE transactions on Microwave 
Theory and Techniques, September 1984, on Historical Perspectives of 
Microwave Technology. One is "History of Biological Effects and 
Medical Applications of Microwave Energy", by Arthur Guy. It has 
photos of D'Arsonval experimenting on himself, standing inside large 
RF coils. The other is "A History of Microwave Heating Applications" 
by John Osepchuk of Raytheon. To quote from that paper, "The 
legendary story in a celebrated Readers Digest article is that Percy 
[Spencer of Raytheon] accidentally leaned against an open waveguide 
and noticed a candy bar in his pocket melt...... They all remember 
the discovery of a gradual process involving chance and deliberate 
observations by many individuals, e.g. , feelings of warmth near 
radiating tubes......"

Recently I found an earlier reference to RF effects on the body and 
on food! In a Proceedings of the IRE paper by Acheson and Dart of GE 
and Westinghouse (Characteristics of the UV-858 Power Tube for High 
Frequency Operation) dated 1932, reference to the RF fields around a 
transmitter are discussed. It may be the first time that the power 
levels and frequencies with tubes were high enough for some serious 
RF effects. Back then, most transmitting equipment was built open, 
with a pipe framework to hold the components in the air. The 
principle of Leecher Lines was just beginning to be applied (to use 
open wire transmission lines to force RF grounds at the filaments of 
higher frequency tubes, for example). Sometimes a screen door was 
included to keep electrocutions from happening. The paper closes with 
this paragraph:

"In tests where 5 to 10 kilowatts were radiated at frequencies from 
40,000 to 60,000 kilocycles it was noted that large space currents 
were circulated through all dielectrics within several feet of the 
set. These space currents were great enough to cause hot arcs between 
small rods of isolated metal, to illuminate isolated neon lamps at a 
distance of several feet and cause similar effects. More interesting 
was that parts of the human body, even though insulated, would heat 
to a fever heat at a distance of as much as 10 feet from the 
transmitter. When closer to the transmitter or when in contact with 
conductors it was possible to heat the entire body with ease. Later 
tests, in which the field was concentrated between plates, indicated 
that isolated wood blocks could be set on fire by dielectric loss, 
and that pop corn could be popped by dielectric loss. In all these 
heating tests it was noted that heat was generated internally in the 
dielectric, thus opening a speculative field as to what purposes such 
a novel method of heating might be applied."

73
K5PRO



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