RF Radiation Protection from a PA
I am designing a power amplifier at 27.5MHz output power is 1.2kW. now there are couple of issue but most important at the moment is how to protect myself and the rest of the LAB testing from radiation of this amplifier.
So does anyone have any suggestions or ideas...
thanks in advance for your help..
best regards,
Malik
Hello Malik,
I assume that during testing the power will be dissipated in a dummy load.
You may download the ICNIRP guidelines (this is a free download), then you know the E- and H-field levels for your frequency band of interest.
If first assessment shows that you may exceed the levels (for example because of malfunction), you may install some E-field detectors (based on simple rectification and comparator with alarm device). This can be used as a form of space supervision.
when you are very close to the amplifier (with open case, during measuring or trouble shooting), you could measure the fieldstrength (from distance) and determine the area that may result in instant thermal damage. When levels are high, you should have a provision that one cannot operate the PA when near to it. Think of a (plastic) casing with safety interlocks.
Additional feature can be an SWR bridge that monitors the load VSWR and shuts down in case of bad VSWR. In such case you can be sure something is wrong in the cabling from PA to load. Bad cabling may lead to increased radiation due to common mode current. Very likely that you have such feature allready to protect the active devices.
Hello WimRFP,
yes power will be dissipated into dummy load and yes you are right i am planing to put VSWR protection circuit in place but before i put protection circuit in place i need to test the amplifier for it's RF characteristics.
Now my question was lets say that there is radiation and its at harmful level then do you know any thing that can protect surroundings from it i.e. Faraday cage etc. but i know farday cage is used to protect devices from external electrostatics charges. But some thing like that if you know about it.
Hello Malik,
Depending on the physical size of the PA and layout, the zone that may exceed ICNIRP levels may be less then 1m. If you use the PA always closed (metallic case, wire mesh shield), you may not even exceed ICNIRP levels at all. Strength of reactive field decays rapidly with distance.
Large loops (think of common mode current) are more of a problem, but when you have a metallic PA case, power supply is decoupled, tight fit connecters are used, high levels due to common mode issues are very unlikely.
Simple metallic mesh directly connected to the metal PA housing can be used to shield static fields (for example the anode region of valve PA).
Wire mesh will give some magnetic shielding also, but this depends on the vicinity of the coils that generate the magnetic field.
If you use the PA without case and is unbalanced (single valve design), the E-field that leaves the PA will introduce common mode current/voltage. When touching metal parts, you may exceed the levels for contact current. A compact solid state push pull type PA produces less E-field when used without its casing.
In my opinion, you don't need a faraday cage for protection. You may need it to avoid unintentional radiation (EMC issue).
Did you know that a faraday cage could make things even worse. When the cage becomes in resonance for the operating frequency, E-, H-field levels may rise significantly. A faraday cage isn't an anechoic chamber.
Is this a project for a thesis? Will you use mosfet or valve?
To be honest, I never worried about exceeding EM safety from radiation from PA's. The first time I did an evaluation was for > 10 kW HF PA (long pulsed operation). Even there we were below the reference levels for the general public. Only when you would reach within the PA's casing, levels increased dramatically.
thanks WimRFP I think i have got some starting knowledge and i should bit more research in this as well. If i still feel like need some answers then i know where to come :)
thanks for your help...
you can get a shield box for bench top testing, like this one:
EMI shielded box | Faraday cages | EMI/RFI shielding | Holland Shielding Systems BV
Sometimes these come up on ebay for S200 to S500 range.
You can also get a hand held radiation monitor to see if the Volts/meter of the radiated field strength exceeds OSHA limits:
Narda Safety Test Solutions
OET -- RF Safety FAQ's