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Author Topic: Indoor Apartment Antenna and RF exposure  (Read 5365 times)
AD0AE
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Posts: 76




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« on: December 23, 2012, 09:14:11 AM »

Hi All-

I have been running an indoor loop antenna primarily on JT65, which of course has the key down for 40 seconds while transmitting.  The antenna has usually been located approximately 3+ feet from where I am while I am transmitting.  I have typically been on 40, 20, and 15 m not running more than 30 W of power into a loop antenna (I am figuring I am losing about 50% of the power due to loop inefficiencies).  I have occasionally adjusted the tuning while the radio has been transmitting about 20-30W, putting me within 1-2 feet of the antenna while it was "key down."  I usually have been tuning up using the RTTY mode on 5 watts and then take it up to 20-30 to get the SWR minimized.

I may be overly concerned about this, but I am wondering how close can you actually be to a HF antenna while it is transmitting?  Do the MPE tables change with a mode that has a long key down time, like JT65?  I understand that "all bets are off" if it is VHF/UHF because that is the ionizing radiation and can literally heat your tissue.  However, HF is such a long wavelength. 

I have also been considering building or purchasing a field strength meter just to get an idea of how significant the RF is at my operating station.  Can you actually make a measurement of the power density using a field-strength meter?  I found a schematic here on eham for what looked like a very simple field strength meter.

Ultimately I just want to be safe and perhaps not get my wife concerned she is going to get cancer because I am transmitting (Note: she is an oncology nurse).  That has a tendency to get ME concerned!

Thank you all and 73s,
Steve
AD0AE
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G3RZP
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Posts: 4391




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« Reply #1 on: December 23, 2012, 10:05:44 AM »

First, VHF/UHF is NOT ionising radiation. But it can cause heating. When I did my radhaz course some years back, I could find the leaks put in the waveguide joints for an exercise on an S band radar  with the back of my hand - you could feel the warming. Not recommended you look at them, though.

Cancer from radiated RF? Matter of argument: there hasn't been any real definitive evidence produced.

The limits are based on a 6 minute averaging period, and provided you are under the FCC limits, there is most unlikely to be a problem. The ICNIRP limit at HF is 60 V/m in a protected area and 30 V/m for general exposure.

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K7KBN
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Posts: 2764




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« Reply #2 on: December 23, 2012, 10:17:49 AM »

Steve - as a licensed amateur radio operator, you're required to determine the strength of RF fields that you create - primarily for your neighbors if you run on VHF and above, with really large, high-gain antennas.  However, even if you're QRP on 160 meters you're required - or USED to be required - to run this formula:

http://hintlink.com/power_density.htm
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73
Pat K7KBN
CWO4 USNR Ret.
AC5UP
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Posts: 3825




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« Reply #3 on: December 23, 2012, 11:17:09 AM »

As a cranky old bastard let me say that AD0AE has the right to operate within the restrictions presented by FCC Part 97 but also has an obligation to do so using good engineering practices. IMHO an indoor apartment antenna is not good engineering. Especially at anything above QRP. Not saying it can't be done, just saying that doesn't mean it should be done.

If AD0AE wants to expose himself to enough RF that his toes glow in the dark that's his business... But... He has NO right, zero, nada, zilch, to make a decision about how much RF exposure is " OK " for his immediate neighbors a few feet away.



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AC5UP
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« Reply #4 on: December 23, 2012, 11:52:56 AM »

Cancer from radiated RF? Matter of argument: there hasn't been any real definitive evidence produced.

Which brings up the question: Does a lack of apparent evidence mean there will be no evidence? Not now, not ever? We both know logic doesn't support that. Just because the Pope has never seen a mouse in the Vatican doesn't mean there are no mice in the Vatican... Despite the Pope being infallible and famous.

There was a time when this advert ran in popular magazines and was considered truthful.

Then there was this movie scene that suggested logical deduction could lead to knighthood.............................

The second link also shows that if you want something to be more compelling, say it with an accent. But that's for another thread.   Grin
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AD0AE
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Posts: 76




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« Reply #5 on: December 23, 2012, 12:14:56 PM »

Thank you for the replies!  I want to direct my reply especially toward AC5UP. 

First, I just want to emphasize that at the moment I cannot have an outdoor antenna.  Period.  So either I operate on an indoor antenna or nothing at all. [I am not going to go into the reasons, but if you want to know them send me a message].  This of course raises its own question about whether to operate or not.

I also agree with you that I do not have the right "to make a decision about how much RF exposure is " OK " for his immediate neighbors a few feet away."

To the point, are there good ways/good rules of thumb(some of which you pointed out) about how close you can be to an antenna if you're at low power?

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K7KBN
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« Reply #6 on: December 23, 2012, 03:29:05 PM »

47CFR97.13C
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73
Pat K7KBN
CWO4 USNR Ret.
K2OWK
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Posts: 1039




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« Reply #7 on: December 23, 2012, 04:54:32 PM »

How long do you people keep that cell phone running 900MHZ up against your head?

73s

K2OWK
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K5LXP
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Posts: 4448


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« Reply #8 on: December 23, 2012, 05:38:28 PM »

Run the numbers and see what you get.

http://hintlink.com/power_density.htm


Mark K5LXP
Albuquerque, NM
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K4SAV
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Posts: 1823




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« Reply #9 on: December 23, 2012, 06:04:46 PM »

As an amateur radio operator you are required by the FCC to do an RF survey and abide by the standards stated in this document:
http://www.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet65/oet65b.pdf
There is no requirement that you determine what levels of RF are safe, or not safe.

You can find general rules for most common antennas, but you won't find any for a mag loop.  RF fields around a small antenna are always much greater than for a large antenna like a dipole, and the details can vary the fields a lot.  The only way I know to determine the fields from a mag loop is to model the antenna in NEC.  It's actually very easy to do.  Most of the NEC programs will let you calculate the E and H fields at any position relative to the antenna.  You can then compare the field levels to the limits specified in the document above.  That document also gives the derating factors for modes of operation.

If you use EZNEC, the program will have a warning message in the near field window declaring that EZNEC knows nothing about RF safety either.  It can calculate the fields, but it can't tell you what is safe or not safe.

Jerry, K4SAV
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AC2EU
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Posts: 339


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« Reply #10 on: December 23, 2012, 07:58:39 PM »

here is an article from QST:
http://www.arrl.org/the-fcc-s-new-rf-exposure-regulations
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KB4QAA
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Posts: 2269




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« Reply #11 on: December 23, 2012, 08:23:05 PM »

-Comply with FCC exposure regs
-Keep in mind that there is no proven cause between RF exposure and cancer
-The FCC chose to implement industry voluntary standards which are based on an abundance of caution.

-Cancer is caused by damaging DNA.  It takes ionizing radiation to do this. RF is NOT ionizing radiation.

-Millions of Radiomen, Technicians and Sailors have been exposed to RF for over 110 years.  If RF caused cancer or made people sick, these men would have the highest rates of cancer and be the sickest people on earth.  They are not.

-Cataracts of the cornea can be caused by high level exposure of UHF/SHF frequencies.  This has been known since the 1950's.   Out of caution I recommend never standing in front/behind of a gain VHF/UHF antenna.

-Never put your tongue on an antenna.  Especially in the winter!  Smiley

bill
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KE3WD
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Posts: 5694




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« Reply #12 on: December 23, 2012, 08:38:01 PM »

Also there are the properties of the small magnetic loop antenna to consider.  Likely one of the safest antennas as far as RF exposure goes. 

And, you are not running full power of 100W and not running an amp.

Driving your car is a risk that can have dire consequences, yet most people don't think twice about doing that every day...


73
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AC5UP
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Posts: 3825




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« Reply #13 on: December 24, 2012, 02:40:06 PM »

To the point, are there good ways/good rules of thumb(some of which you pointed out) about how close you can be to an antenna if you're at low power?

Part of my caution comes from a broadcast background... Transmitter sites for AM BCB stations often have a copper screen lined interior which makes it far easier to keep RF out of the audio chain and monitoring gear. A TV station I worked for had the TX 16 miles north and one mile up from the service area with a 34' tower under the antenna bays. Video levels are commonly monitored by oscilloscope and the copper screen Faraday shield was demonstrated to me by opening the steel door on the blockhouse... All the test gear went bonkers with RF. Tower crews routinely wear 'space suits' when they can't shut down all the services on a tower.

Point being there are both technical and legal reasons to guard against excessive RF exposure. No employer wants any hint of liability for injury or illness from an employee or contractor. The electric power lobby routinely defends against any suggestion that living near high tension lines is bad for your health just as the cell phone lobby 'knows' there is no conclusive proof that brain tumors are caused by you-know-what. Yet the rumors persist and studies have suggested that exposure to a 60 Hz or 900 MHz field of sufficient intensity for prolonged periods is not a good idea...... Near me is a string of houses almost directly under 38 kV power lines and even though the field strength on the ground might be relatively low, living there offers 14 or more hours a day seven days a week of exposure. I'd say that's worth a field measurement and research into what the risks might be - before - there is a medical issue.

Total exposure should be considered in terms of intensity and duration. In your case at HF I don't consider 30 Watts as particularly intense, even though you're sitting within a few feet of the antenna. From what I can tell from your posts the loop is probably compromised enough that 30 Watts in might equal 10 - 15 Watts out and you're QRP without knowing it. Consider also that to the loop you're just one large bag of saline solution and it's entirely possible you're doing more harm to the radiation efficiency of the loop than the loop is doing to your keester, and that brings us back to the question of duration. If you play radio a few hours a week that's better than a few hours a day, but not as good as an outdoor antenna in the clear for the same amount of air time........

Use your best judgement about what's a reasonable power setting and balance the need to radiate against band conditions. If the bands are dead you're better to wait for another day because a little help from 'Ol Sol goes a long way with a modest signal. Likewise, there's little to be gained by cranking up the RF pollution on a day when you're going nowhere at any power level............   Wink
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KE3WD
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« Reply #14 on: December 24, 2012, 05:53:37 PM »

Small Magnetic Loop = no electrical field.  Well, minimal.

I consider the small magnetic loop to be one of the "safer" radiators as far as exposure concerns.


74
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