G'day,
I am interested if a Faraday Cage needs an earth to prevent RF from escaping from it ?
Does a metal box, unearthed , with RF inside it constitute a Faraday cage ?
Does an antenna tuner in a metal box need to earthed to prevent RF from escaping from it ?
Example 2, an antenna tuner connected via coax to an earthed transceiver without a separate earth wire to the tuner.
Is an earth loop created by a separate earth wire to the earth connection on the tuner, if the tuner is already earthed by the coax shield to the transceiver ?
If using a balanced antenna such as a doublet with open wire feeder or dipole with a balun is an earth necessary if using a DC power supply that is earthed at the power point for protection.
Any comments will be appreciated
Andrew VK7CAV
RF Shielding
Re: RF Shielding
Hi Andrew,
If the RF source is fully contained within the box with no external connection into the box then it is technically a Faraday cage but in order to work effectively the cage must be grounded. I'm not sure that you can talk about ATU's and Faraday cages as the RF needs to leave the ATU at some stage. I think the unbalanced input should be grounded to the ATU chassis. This will ensure that if there "is" any RF on the outer of your coax it will be taken to ground.
To ensure there is no ground loop from your PS, check the resistance from the earth pin on the power lead to the chassis of your transceiver. I think you will find they are separate. In this case you might want to test for ground currents by putting a 1 ohm resistor (or whatever you have in your junkbox) in series with the ground lead of the ATU and 'scope the voltage across the resistor. You will get a fair idea of what's going on that way.
Perhaps you could give us a little more detail of what you are trying to achieve?
Lou.
If the RF source is fully contained within the box with no external connection into the box then it is technically a Faraday cage but in order to work effectively the cage must be grounded. I'm not sure that you can talk about ATU's and Faraday cages as the RF needs to leave the ATU at some stage. I think the unbalanced input should be grounded to the ATU chassis. This will ensure that if there "is" any RF on the outer of your coax it will be taken to ground.
To ensure there is no ground loop from your PS, check the resistance from the earth pin on the power lead to the chassis of your transceiver. I think you will find they are separate. In this case you might want to test for ground currents by putting a 1 ohm resistor (or whatever you have in your junkbox) in series with the ground lead of the ATU and 'scope the voltage across the resistor. You will get a fair idea of what's going on that way.
Perhaps you could give us a little more detail of what you are trying to achieve?
Lou.
Lou - VK3ALB
Being right doesn't excuse bad behaviour
Being right doesn't excuse bad behaviour
Re: RF Shielding
Andrew,
You are really interested in containment of electromagnetic radiation.
An effective screened box works by virtual of skin effect. Currents flowing on the inside of the screen (either a results of fields, or of direct conduction) flow mainly in the surface of the conducting material by virtue of skin effect. The current density on the outside of the screen due to currents on the inside of the screen is so low as to be negligible (by virtue of the meaning of "effective"). Equally, currents on the outside surface of the screen are isolated from the internal space of the box.
(I didn't mention earthing because it has nothing to do with the mechanism I have described, energy is contained inside the effectively screened box irrespective of whether the box is earthed.)
There is no such thing as a perfectly balanced antenna. There are imperfections in the symmetry of the antenna and its feedline, its environment, and baluns. The question of common mode current is one of not whether it exists, but the magnitude. Higher common mode current demands more attention to station RF earthing.
Keep in mind that in the scenario you describe, the common mode current that flows on the feedline just upstream of the ATU also flows on the feedline just downstream of the ATU. An ATU of itself does not block common mode current at a point as if it were an o/c switch. The situation is different if you have a path to earth (more) directly from the tuner. Kirchoff's current laws apply and will lead you to the solution.
On the matter of DC power supplies, do not assume that the -ve DC terminal is connected to the AC ground terminal. Do not assume that there is a low RF impedance connection between the transceiver chassis and the -ve terminal at the power supply.
The question you seem to be asking is whether the mains AC ground is a good RF ground. No, probably not for a host of reasons. Nevertheless, people do it every day and prove that it 'works' whatever 'works' means.
In the same way, the mains AC ground is not a good means of lightning earthing.
Owen
The basic concept of a Faraday Cage is a structure to protect the inside from electric fields, and has its origins in Faraday's work relating to the location of (static) charge on conductors.VK7CAV wrote:G'day,
I am interested if a Faraday Cage needs an earth to prevent RF from escaping from it ?
Does a metal box, unearthed , with RF inside it constitute a Faraday cage ?
Does an antenna tuner in a metal box need to earthed to prevent RF from escaping from it ?
You are really interested in containment of electromagnetic radiation.
An effective screened box works by virtual of skin effect. Currents flowing on the inside of the screen (either a results of fields, or of direct conduction) flow mainly in the surface of the conducting material by virtue of skin effect. The current density on the outside of the screen due to currents on the inside of the screen is so low as to be negligible (by virtue of the meaning of "effective"). Equally, currents on the outside surface of the screen are isolated from the internal space of the box.
(I didn't mention earthing because it has nothing to do with the mechanism I have described, energy is contained inside the effectively screened box irrespective of whether the box is earthed.)
This example seems to hint that you are interested in an RF ground rather than lightning earthing or power protective earthing.VK7CAV wrote: Example 2, an antenna tuner connected via coax to an earthed transceiver without a separate earth wire to the tuner.
Is an earth loop created by a separate earth wire to the earth connection on the tuner, if the tuner is already earthed by the coax shield to the transceiver ?
If using a balanced antenna such as a doublet with open wire feeder or dipole with a balun is an earth necessary if using a DC power supply that is earthed at the power point for protection.
There is no such thing as a perfectly balanced antenna. There are imperfections in the symmetry of the antenna and its feedline, its environment, and baluns. The question of common mode current is one of not whether it exists, but the magnitude. Higher common mode current demands more attention to station RF earthing.
Keep in mind that in the scenario you describe, the common mode current that flows on the feedline just upstream of the ATU also flows on the feedline just downstream of the ATU. An ATU of itself does not block common mode current at a point as if it were an o/c switch. The situation is different if you have a path to earth (more) directly from the tuner. Kirchoff's current laws apply and will lead you to the solution.
On the matter of DC power supplies, do not assume that the -ve DC terminal is connected to the AC ground terminal. Do not assume that there is a low RF impedance connection between the transceiver chassis and the -ve terminal at the power supply.
The question you seem to be asking is whether the mains AC ground is a good RF ground. No, probably not for a host of reasons. Nevertheless, people do it every day and prove that it 'works' whatever 'works' means.
In the same way, the mains AC ground is not a good means of lightning earthing.
Owen
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VK7CAV
Re: RF Shielding
Thankyou Lou and Owen,
my main purpose of the post is to improve my understanding of earthing and how it relates to RF shielding.
I have the ARRL Handbook and ARRL Antenna handbook and I am seekinng more information.
I have researched information on the internet and various opinions seem to question the need for earthing when using a DC supply(3 pin) and a balanced antenna such as a dipole.
My interest in the Faraday cage is my belief that an ungrounded metal box contains RF.
It sounds like the box doesnt need to be earthed to contain the RF, and likewise to stop RF getting in.
I have recently bought a balanced antenna tuner and have grounded it with a single copper earth rod but I dont think its a good earth.
In the shack the open wire feeder comes through my brick wall (insulated wire) and terminates on banana plug sockets.
I understand that if the antenna isn't balanced,etc the feeder may radiate.
From the antenna tuner to radio via coax will a common mode current exist , when using a balanced T tuner ?
Regarding RF safety will the problem be outside the shack in my instance if an unbalance exists.
Andrew
my main purpose of the post is to improve my understanding of earthing and how it relates to RF shielding.
I have the ARRL Handbook and ARRL Antenna handbook and I am seekinng more information.
I have researched information on the internet and various opinions seem to question the need for earthing when using a DC supply(3 pin) and a balanced antenna such as a dipole.
My interest in the Faraday cage is my belief that an ungrounded metal box contains RF.
It sounds like the box doesnt need to be earthed to contain the RF, and likewise to stop RF getting in.
I have recently bought a balanced antenna tuner and have grounded it with a single copper earth rod but I dont think its a good earth.
In the shack the open wire feeder comes through my brick wall (insulated wire) and terminates on banana plug sockets.
I understand that if the antenna isn't balanced,etc the feeder may radiate.
From the antenna tuner to radio via coax will a common mode current exist , when using a balanced T tuner ?
Regarding RF safety will the problem be outside the shack in my instance if an unbalance exists.
Andrew
Re: RF Shielding
1. If your ATU is small, and the only connections to it are a transmission line on the tx side and a transmission line on the antenna side, the common mode current adjacent to the tuner on the feedlines on each side are approximately equal.VK7CAV wrote: From the antenna tuner to radio via coax will a common mode current exist , when using a balanced T tuner ?
2. It does not matter whether the feedlines are coaxial or open wire type, they both are able to carry common mode current, just the way in which it is distributed on the conductor surfaces is different.
3. The contents of the ATU box (eg a balun) might influence the amplitude of common mode current that may flow, but that does not alter 1 above.
4. There is merit in ensuring that all equipment in the shack is at the same RF potential, and that it is all close to ground RF potential.
5. There is benefit in minimising common mode current that is routed into the shack (eg symmetric transmission line routing, baluns, common mode chokes, bonding coax shields to external ground connections etc).
6. None of the above addresses mains earthing or lightning protection.
Owen
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VK7CAV
Re: RF Shielding
Thanks Owen for your reply.
1)Would it be correct to say that a metal box doesn't have to be earthed to shield RF.
2)If the feeder wires are symmetrical and the antenna has the same capacitance to earth,each leg, there should be no radiation from the feeder when using a dipole.
3)If the balun in a tuner is doing its job there should be no RF on the coax shield (common mode current) between the TX and tuner.
4) Earthing is mainly for safety purposes with regards to RF if a fault condition occurs, when using balanced antennas.
5)All equipment should be earthed to the one point, at the same potential.
5) A good RF ground is mainly needed for unbalanced antennas such as verticals and end-fed zepps/longwires.
Please correct anything you disagree with.
Regards,
Andrew
1)Would it be correct to say that a metal box doesn't have to be earthed to shield RF.
2)If the feeder wires are symmetrical and the antenna has the same capacitance to earth,each leg, there should be no radiation from the feeder when using a dipole.
3)If the balun in a tuner is doing its job there should be no RF on the coax shield (common mode current) between the TX and tuner.
4) Earthing is mainly for safety purposes with regards to RF if a fault condition occurs, when using balanced antennas.
5)All equipment should be earthed to the one point, at the same potential.
5) A good RF ground is mainly needed for unbalanced antennas such as verticals and end-fed zepps/longwires.
Please correct anything you disagree with.
Regards,
Andrew
Re: RF Shielding
I recognise that you are looking for one word answers...however...VK7CAV wrote: 1)Would it be correct to say that a metal box doesn't have to be earthed to shield RF.
I explained how shielding works in an earlier response.
If you have an exposed earth conductor that carries RF current, the RF is 'out of the box' and there will be radiation as a result of the RF current on that conductor.
If the antenna SYSTEM is PERFECTLY balanced, then there WILL NOT be radiation from the feedline.VK7CAV wrote: 2)If the feeder wires are symmetrical and the antenna has the same capacitance to earth,each leg, there should be no radiation from the feeder when using a dipole.
To repeat: "There is no such thing as a perfectly balanced antenna. There are imperfections in the symmetry of the antenna and its feedline, its environment, and baluns. The question of common mode current is one of not whether it exists, but the magnitude."
There is no such thing as a perfect balun.VK7CAV wrote: 3)If the balun in a tuner is doing its job there should be no RF on the coax shield (common mode current) between the TX and tuner.
Not only do baluns not offer infinite impedance to common mode current, but most baluns unbalance the load as they shunt it with unequal impedances on each side of the 'balanced' terminals.
To repeat: "The question of common mode current is one of not whether it exists, but the magnitude."
Effective RF earthing of the equipment may not be needed if feed line common mode current is sufficiently low.VK7CAV wrote: 4) Earthing is mainly for safety purposes with regards to RF if a fault condition occurs, when using balanced antennas.
Single point earthing is a good topology for avoiding voltage differences between equipment interfaces due to currents flowing on earthing conductors.VK7CAV wrote: 5)All equipment should be earthed to the one point, at the same potential.
Marconi type antennas (where one wire of the feedline connects to the dirt) need an effective ground system to achieve good efficiency. That effective ground system should not be delivered from / via the equipment room because it means that there are significant RF currents flowing through the equipment room.VK7CAV wrote: 5) (sic) A good RF ground is mainly needed for unbalanced antennas such as verticals and end-fed zepps/longwires.
This point is not made in the current 'Foundation Manual', but in my experience, it is common practice for our six hour hams to use an end fed wire connected to an ATU, the ATU connected to the tx by a short coax run, and the tx connected to 'ground' by some accidental / unplanned route, possibly via the power supply and mains wiring. My recollection is that in the first year of the FL, the majority of FLs that I worked were using such an antenna, and I got tired of hearing 'expert' FLs advising others that the best antenna they could use is an end fed long wire and they should buy the very best ATU that they can afford. (Of course, the 10m to 20m length 'long wire' many were using isn't a long wire in terms of having an end-fire pattern and a higher feed impedance, it is just a end fed unknown / unplanned / random length of wire with unknown earthing requirements.) IMHO, (and I argued this when working on the manual), FLs should not be encouraged to use random end-fed wires.
Owen
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VK7CAV
Re: RF Shielding
So Owen ,
with a balanced antenna tuner , dipole, earthed at ATU, if I see RF coming from the earth wire does this indicate an unbalanced situation.
BTU
with a balanced antenna tuner , dipole, earthed at ATU, if I see RF coming from the earth wire does this indicate an unbalanced situation.
BTU
Re: RF Shielding
Andrew,VK7CAV wrote:So Owen ,
with a balanced antenna tuner , dipole, earthed at ATU, if I see RF coming from the earth wire does this indicate an unbalanced situation.
BTU
If you are describing a small ATU that has three connections, one from the transceiver, one from the antenna, and one to ground, the ATU can be thought of as a circuit node and Kirchoff's Current Law applies, the sum (having regard for phase) of the common mode currents adjacent to the ATU is zero.
If you were to measure RF current on the grounding wire, it would probably be as a consequence of of common mode current on the feedline (though not equal to it), and that could be ascertained by direct measurement.
The use of open wire feedline and a 'balanced ATU' does not assure zero common mode current or perfect balance. You should regard perfect balance as unlikely. To repeat from an earlier post "The question of common mode current is one of not whether it exists, but the magnitude. Higher common mode current demands more attention to station RF earthing.".
Owen
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VK7CAV
Re: RF Shielding
Owen,
my main concern is RF safety. I have borrowed a calibrated Radiation Hazard Meter and tested on 28 Mhz.
On the main external earth wire there is about 0.01- 0.02 mW/cm^2 when the probe is positioned very close to it.
Most of the RF is at the back of the tuner and above 0.2 mW/cm^2 public exposure levels.
Outside the shack, underneath the feeder at its lowest point , with the probe about 2m high it is 0.02 mW/cm^2 when power out is 10W.
At 100W the figures are about 10 X these.
50 cm from the front cover of the ATU the power flux density is about 0.01 mW/cm^2 when 100W carrier is transmitted.
It is about the same 15 cm from the front panel of the transceiver at 100 W, dropping to 0.002 mW/cm^2 at 50 cm from the front panel.
I haven't got the equipment to measure RF current but I think RF would be at an acceptable level in the shack or for a member of the public walking briefly underneath the main feeder at its lowest point.
Thanks for your information on common mode current Owen and I have since tried to improve my knowledge in the area.
Whilst the ARRL Handbook and ARRL Antenna Handbook are great references they don't answer all the questions.
Thanks again ,
Andrew
my main concern is RF safety. I have borrowed a calibrated Radiation Hazard Meter and tested on 28 Mhz.
On the main external earth wire there is about 0.01- 0.02 mW/cm^2 when the probe is positioned very close to it.
Most of the RF is at the back of the tuner and above 0.2 mW/cm^2 public exposure levels.
Outside the shack, underneath the feeder at its lowest point , with the probe about 2m high it is 0.02 mW/cm^2 when power out is 10W.
At 100W the figures are about 10 X these.
50 cm from the front cover of the ATU the power flux density is about 0.01 mW/cm^2 when 100W carrier is transmitted.
It is about the same 15 cm from the front panel of the transceiver at 100 W, dropping to 0.002 mW/cm^2 at 50 cm from the front panel.
I haven't got the equipment to measure RF current but I think RF would be at an acceptable level in the shack or for a member of the public walking briefly underneath the main feeder at its lowest point.
Thanks for your information on common mode current Owen and I have since tried to improve my knowledge in the area.
Whilst the ARRL Handbook and ARRL Antenna Handbook are great references they don't answer all the questions.
Thanks again ,
Andrew
Re: RF Shielding
I have come across ATUs where the protruding shaft of the tuning caps is not grounded, so they act like a small antenna. MFJ are infamous for this shortcut.VK7CAV wrote: It is about the same 15 cm from the front panel of the transceiver at 100 W, dropping to 0.002 mW/cm^2 at 50 cm from the front panel.
Is that a Mighty Fine Junk tuner?
Regardless of brand, is there any protruding metal shafting that is not bonded to the box?
Owen
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VK7CAV
Re: RF Shielding
Owen,
the transceiver and ATU are about 3m apart , ATU on wall , 2.5m behind where I sit, transceiver about 50 cm in front of me.
RF comes out of both boxes but dissipates within a short distance.
Both boxes are grounded.
I don't think RF safety is addressed properly in any of the amateur radio studies, Foundation,standard or advanced. A bit amateur.
I havent got a problem with FL's , we all have to start somewhere.
It is not much better in the real world where big companies think of the dollar first and don't invest in training to the extent that public owned companies such as Telecom Australia and the public service once did.
Technical training is at the lowest level ever and driven by industry who want quick results.
I suppose the focus in education has always been in passing exams rather than ensuring students really understand what they are learning.
A 75 % pass mark means a student didn't understand 25 % of the course or other factors such as exam nerves and silly mistakes,etc.
I am not sure what your qualifications are but obviously from an electrical engineering background.
I find that practice makes perfect whether practical or theory.
I am not an expert in the RF field and I am quite receptive to advice and ideas.
I will keep an eye on this forum and learn a bit more.
Andrew
the transceiver and ATU are about 3m apart , ATU on wall , 2.5m behind where I sit, transceiver about 50 cm in front of me.
RF comes out of both boxes but dissipates within a short distance.
Both boxes are grounded.
I don't think RF safety is addressed properly in any of the amateur radio studies, Foundation,standard or advanced. A bit amateur.
I havent got a problem with FL's , we all have to start somewhere.
It is not much better in the real world where big companies think of the dollar first and don't invest in training to the extent that public owned companies such as Telecom Australia and the public service once did.
Technical training is at the lowest level ever and driven by industry who want quick results.
I suppose the focus in education has always been in passing exams rather than ensuring students really understand what they are learning.
A 75 % pass mark means a student didn't understand 25 % of the course or other factors such as exam nerves and silly mistakes,etc.
I am not sure what your qualifications are but obviously from an electrical engineering background.
I find that practice makes perfect whether practical or theory.
I am not an expert in the RF field and I am quite receptive to advice and ideas.
I will keep an eye on this forum and learn a bit more.
Andrew