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Author Topic: How accurate are RF safety calculations for in attic antennas?  (Read 1737 times)
WXSHAM
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Posts: 77




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« on: October 23, 2018, 06:37:17 AM »

I put a homemade fan dipole 40 meters and 20 meters in my attic at about 5 feet above the attic floor.
It's fed with about 30 meters of lmr-400 coax to a 1:1 balun.

Using this calculator
http://hintlink.com/power_density.htm

Even uncontrolled should be safe and at .9234 mw/cm2 at 2.48 feet @14MHZ @ 15 watts
According to the calculations it should be safe even to over 50 watts output at 14mhz. 

But even running 30 watts I've measured with this meter close to over 40mw/cm2 or more (at certain spots in the floor below the attic) using this meter
https://www.dxengineering.com/parts/hgs-he-em2

According to the calculations I shouldn't even be able to exceed 1.5mw/cm2 at the full 100watt output power of my transmitter @ 5 feet! And I'm measuring about head height or lower below the ceiling of the floor below so it's closer to 7 feet or more from the antenna.

I'm trusting the meter more than the calculations and only running 15-20 watts if the hot spots in the house would be occupied in any way to avoid exceeding the limit.

Are higher levels caused by resonance and re-radiation of the metal throughout the attic/house?

Have other folks seen this as well?





« Last Edit: October 23, 2018, 06:55:54 AM by WXSHAM » Logged
K5LXP
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« Reply #1 on: October 23, 2018, 07:27:33 AM »


Per the disclaimer at the calculator web page (emphasis mine):

This program is intended for approximate far-field calculations. It may overestimate the actual field strength of high-gain antennas in the near field (within several wavelengths of the antenna.) However, it may also underestimate the strength of fields that may be encountered in hot spots in the near field. No computer program can predict where wiring or reflective objects may create hot spots in your particular installation.


If you were in free space your results might be a bit more predictable.  Your house is not free space.  You're in the near field and are as much coupling to the antenna as seeing incident RF.  If you can accurately measure RF fields then go with that, since the calculator cannot take into account the variables in your installation.


Mark K5LXP
Albuquerque, NM
 
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WXSHAM
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Posts: 77




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« Reply #2 on: October 23, 2018, 07:47:56 AM »


Per the disclaimer at the calculator web page (emphasis mine):

This program is intended for approximate far-field calculations. It may overestimate the actual field strength of high-gain antennas in the near field (within several wavelengths of the antenna.) However, it may also underestimate the strength of fields that may be encountered in hot spots in the near field. No computer program can predict where wiring or reflective objects may create hot spots in your particular installation.


If you were in free space your results might be a bit more predictable.  Your house is not free space.  You're in the near field and are as much coupling to the antenna as seeing incident RF.  If you can accurately measure RF fields then go with that, since the calculator cannot take into account the variables in your installation.


Mark K5LXP
Albuquerque, NM
 

Yep guess I should always read the fine print.  I see a lot of people online mentioning using just the RF calculators for in attic antennas, they really should use a meter to be safe then...  I guess it's very installation dependent, but in my case at least there were quite a few hot spots exceeding calculations by 30-40x or more. (at least as far as this meter shows)


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ND6M
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« Reply #3 on: October 24, 2018, 06:53:21 AM »

Do you have a microwave oven?
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WXSHAM
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« Reply #4 on: October 24, 2018, 08:18:17 PM »

Do you have a microwave oven?

I do and I think I know where you are going with this. I tested RF for the antenna while the microwave was off.  I verified negligible signal when not transmitting many times at repeated locations including the hotspots.


Separately though I did notice when the microwave was on it could easily generate RF at 40mw/cm2 at 8+ feet!  Luckily I don't think it's microwaves (though that meter unfortunately doesn't tell.) I bought a microwave only meter and found out when operating my microwave may leak a little with 5mw- 9+ mw/cm2  only at pretty close range like a couple inches.

Having a meter I definitely learned some things about RF in my house.

 

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JAZZMAN
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« Reply #5 on: October 25, 2018, 05:04:34 PM »

Years ago I lived in a apartment and it was pretty big but I could not put a antenna outside.  I had 2 spare rooms and one room I used for my radio area.  I put up a 20 meter dipole like Christmas lights inside the farthest  empty room from the radio.  At that time I had a TS-830, MN2000 tuner and a Drake L-7.  Everything loaded up good no problems so I started making contacts.  I couldn't believe it I was making contacts from my apartment.  Well it went bad on me real quick.  My closest apartment neighbor had a mild seizure and the rest of the residents of the apartment complex had extreme electronic problems. 

Please don't make that STUPID type of mistake.  BE SAFE and have the common sense I should have had.   

In your attic it could cause a fire.  Please be safe.   
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K5LXP
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« Reply #6 on: October 26, 2018, 04:22:14 AM »


Ham radio has been known to cause all sorts of maladies...

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NE1U
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Posts: 122




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« Reply #7 on: October 26, 2018, 02:49:56 PM »

about the K5LXP post ... there is the commercial radio site matra, "last one in owns all the problems!"
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JAZZMAN
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« Reply #8 on: October 26, 2018, 06:15:55 PM »

Are you for real?

Who?
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JAZZMAN
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« Reply #9 on: October 26, 2018, 08:57:06 PM »

Years ago I lived in a apartment and it was pretty big but I could not put a antenna outside.  I had 2 spare rooms and one room I used for my radio area.  I put up a 20 meter dipole like Christmas lights inside the farthest  empty room from the radio.  At that time I had a TS-830, MN2000 tuner and a Drake L-7.  Everything loaded up good no problems so I started making contacts.  I couldn't believe it I was making contacts from my apartment.  Well it went bad on me real quick.  My closest apartment neighbor had a mild seizure and the rest of the residents of the apartment complex had extreme electronic problems. 

Please don't make that STUPID type of mistake.  BE SAFE and have the common sense I should have had.   

In your attic it could cause a fire.  Please be safe.   
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KB4QAA
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« Reply #10 on: October 27, 2018, 09:02:23 PM »

Don't worry about the RF unless you cup of coffee gets warmer!  Wink
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KA0HCP, ex-KB4QAA Relocated to Ks. April 2019.
SM0AOM
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Posts: 258




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« Reply #11 on: October 28, 2018, 07:12:42 AM »

Both the calculations for close-in EMF exposure and the indications of inexpensive (and some expensive) EMF meters are notoriously inaccurate, as the assume far-field or plane-wave conditions.

On HF and lower VHF, these conditions are very seldom fulfilled.

To make things worse, some EMF software also use an isotropic or point-source model for its calculations. Isotropic radiators on HF only exist in text-books...

I do a lot of calculations, simulations and actual measurements on HF and VHF system installations on small naval platforms and armoured vehicles.

It turns out that clearly un-physical indications can be obtained in the near-fields around the ends and feed points of especially shortened antennas.

As the limits for EMF exposure are derived from an energy absorption point-of-view, where it is assumed that the body is reached by plane waves, and an E-field indication can be directly translated to a power density, a calculated power density indication may be off by orders of magnitude in the reactive near-fields.

There are currently not any physically good models of how to handle the EMF problem around small antennas or in the near-field region.

My general advice is always: "Do not get so close so you can touch the antenna during transmissions".


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N9CM
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« Reply #12 on: November 01, 2018, 10:44:01 AM »

Who knows, they can't even come to grips on harmful cell phone radiation (right next to your skull) and yet folks use them and the feds say little to nothing....I would worry more about that and radiation from TVs the an attic antenna....
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JAZZMAN
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« Reply #13 on: November 08, 2018, 01:43:54 PM »

Who knows, they can't even come to grips on harmful cell phone radiation (right next to your skull) and yet folks use them and the feds say little to nothing....I would worry more about that and radiation from TVs the an attic antenna....

I agree!!!
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