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Author Topic: antenna safety questions  (Read 640 times)
KG6WHN
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« on: August 20, 2009, 02:10:51 PM »

I read another post recently and it got me thinking...
What are the practical dangers of an HF vertical? Or even a dipole for that matter?  I realize that the dangers increase with TX power, but...from a practical point of view..  Suppose you have a ~30' trapped HF vertical running =< 100W SSB (or possibly PSK31)
at a freq =< ~14.4MHz.  What are the risks of RF radiation or high voltage at the antenna?  I've read that the antennas do develop reasonably high voltages but nowhere I've read has actually put a number to that voltage.  Is it Kv, Mv, or volts?  Now I am very careful when I run my transmitter and make sure noone is even close to my antennas, but just out of curiosity....what are the details?  Another unrelated question...what is the best lightning protection for an antenna?  I typically unplug my transceiver from the electric outlet and disconnect the antenna from my ATU when not in use, but I'm curious if there's a way to protect my antenna?  My antenna switch also grounds my unused antenna too, is this enough?
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WB2WIK
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« Reply #1 on: August 20, 2009, 02:36:35 PM »

>antenna safety questions       Reply
by KG6WHN on August 20, 2009    Mail this to a friend!
I read another post recently and it got me thinking...
What are the practical dangers of an HF vertical? Or even a dipole for that matter? I realize that the dangers increase with TX power, but...from a practical point of view.. Suppose you have a ~30' trapped HF vertical running =< 100W SSB (or possibly PSK31)
at a freq =< ~14.4MHz. What are the risks of RF radiation or high voltage at the antenna?<

::It's all based on distance and operating frequency.  At 14 MHz with 100W, you just about have to physically touch the antenna to be presented with a hazard.  At 10' distance from the antenna, you're well into the "safe" zone, even for an uncontrolled environment, per the FCC formula.  With a 1/4-wave vertical, the highest voltage point appears right at the very top (end) of the antenna and the voltage at the feedpoint is quite low, about 71Vrms or 100Vpk when running 100W.  However there is energy there and you can get an RF burn by direct contact.  With a 1/2-wave vertical, the highest voltage point is at the base and with 100W the voltage can be in the 500Vrms range, obviously much higher; however the main hazard would still be a burn by direct contact.

>I've read that the antennas do develop reasonably high voltages but nowhere I've read has actually put a number to that voltage. Is it Kv, Mv, or volts?<

::Depends on the antenna design and its environment, including relative humidity of the air around it.  At 100W, though, the voltage would rarely be higher than 1kVpk (707Vrms) anywhere.  This isn't exactly "lightning bolt" type voltage, and it's not going to jump from the top of the antenna to somebody walking by unless they're awfully close.

>Now I am very careful when I run my transmitter and make sure noone is even close to my antennas, but just out of curiosity....what are the details? Another unrelated question...what is the best lightning protection for an antenna? I typically unplug my transceiver from the electric outlet and disconnect the antenna from my ATU when not in use, but I'm curious if there's a way to protect my antenna? My antenna switch also grounds my unused antenna too, is this enough?<

::The problem with grounding through a switch in the hamshack is that the switch is in the hamshack.  If lightning energy makes it that far (inside the house), that's not good.  You want to contain it *outside.*  A proper outdoor antenna installation includes grounding the antenna and/or its support directly to earth, right under the antenna, and also using a lightning arrestor in the transmission line, also installed and grounded *outside* the house, not inside.  There's a lot to this, to do it properly.  Thankfully in Sunnyvale, CA you're in a very low lightning incidence area (I'm in Los Angeles, which I think is even lower) so your risks are much smaller than in places where lightning is a daily occurrance.

WB2WIK/6
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KG6WHN
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« Reply #2 on: August 20, 2009, 02:51:49 PM »

Yep, I know that we rarely get lightning storms out here, but I was mostly curious.  I've seen these lightning arrestors and wasn't positive if they mostly protect the antenna, or the radio/downstream electronics.  I wasn't sure if the antenna is generally a lost cause if there is a direct lightning hit or not.  The antenna switch I currently use is also close to the antenna base and the antenna is mounted on a ground galvanized pipe, etc. so as far as grounding goes it should be decent....but I wasn't sure if that was enough.
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N3OX
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« Reply #3 on: August 20, 2009, 03:03:21 PM »

"What are the risks of RF radiation"

Well see ES1TU's thread below and use the RF safety calculator to make sure you would be within *guidelines.*

The fact of the matter is, no one actually knows what the risk is if you exceed those guidelines.  Being bathed in HF electromagnetic fields doesn't seem to do much to people.

"I've read that the antennas do develop reasonably high voltages but nowhere I've read has actually put a number to that voltage. Is it Kv, Mv, or volts?"

Well that question isn't very well defined, because the voltage is the gradient of the electric field, and the electric field pattern depends on what's near the tip of the antenna.

We can make a guess by looking at the feedpoint impedance of an end fed half wave fed against ground, and that's often what people will quote you.  Then the impedance is a few thousand ohms, and probably gives something like 500V-600V at 100W and maybe a couple thousand volts at 1500W.

This ballpark estimate would be consistent with something that seems to be observed frequently: wire antenna ends will arc through usual 600V rated THHN insulation to trees when full legal limit is used but not when 100W is applied.

Voltage has to be defined with respect to something, and it doesn't make sense to define the voltage at a dipole tip with respect to ground most of the time, because the details of the thing you're applying the voltage across matter a lot.  It would make sense to define the voltage between the dipole end and the tip of your finger as you approach it ;-)

How dangerous these voltages are is kind of a strange thing too.  Absolutely if you touch the end of a dipole that has 100W applied, you're going to regret it.  But you're not going to get flung across the room and probably die like if you put your finger on a 600V 60Hz AC line.  You're going to get a rather bad RF burn.  

Last time I touched many hundreds-of-volts RF (on an open frame antenna tuner on 28MHz) I got a searing pain and a little hard black spot on my finger that took weeks to go away, like an RF arc had drilled and cauterized a very tiny hole through my finger.

The way RF is conducted across the human body seems to limit the damage mostly to nasty burns, at least at amateur power levels.   "RF Electrocution" gets very few google hits ;-)

73
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WB2WIK
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Posts: 20635




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« Reply #4 on: August 20, 2009, 03:17:19 PM »

>RE: antenna safety questions       Reply
by KG6WHN on August 20, 2009    Mail this to a friend!
Yep, I know that we rarely get lightning storms out here, but I was mostly curious. I've seen these lightning arrestors and wasn't positive if they mostly protect the antenna, or the radio/downstream electronics.<

::They're intended to protect everything, from the antenna to all downstream.  If done very well, they can.  If done poorly, they don't do much.

>I wasn't sure if the antenna is generally a lost cause if there is a direct lightning hit or not. The antenna switch I currently use is also close to the antenna base and the antenna is mounted on a ground galvanized pipe, etc. so as far as grounding goes it should be decent....but I wasn't sure if that was enough.<

::The galvanized pipe in the ground probably is not sufficient.  But then, again, if nothing in your neighborhood has ever taken a lightning strike, probably your antenna won't, either.  Those who take strikes live in places where other stuff also takes strikes.  In 21 years living in L.A., I've only seen lightning once that was close enough to actually confirm it was lightning.  There are occasional flashes up in the mountains, miles away...but that's where they always are, not down here where the people live.  ;-)

WB2WIK/6
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KI4SDY
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« Reply #5 on: August 21, 2009, 06:38:04 AM »

Who would be transmitting in your shack while you were outside in close proximity to your antenna? If you don't have another ham in the family, I wouldn't worry about radiation as long as you stay within legal limits and your antennas are properly erected.

As far as lightning, you can do all the "grounding" you want, but nothing will save you from a direct hit. Disconnect everything when not in use and during storms.
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W6RMK
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Posts: 660




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« Reply #6 on: August 21, 2009, 07:05:20 AM »

"If you don't have another ham in the family, I wouldn't worry about radiation as long as you stay within legal limits and your antennas are properly erected."

would those be the legal limits for RF exposure? Or are you talking about running less than 1500W, etc.

Part 97 says you DO have to worry. You can't say "oh, I read on eham that I don't have a problem".. you actually have to make that assessment, and provide it to the FCC is asked. (The FCC has sent letters to hams asking them to provide their field strength assessments, for a vertical antenna installation, no less.)  

It is true that for the vast majority of hams, running 100W or so, and using typical duty cycles, they're well below the limit. But, you do have to document your assumptions (for instance, if you're running RTTY contesting, you can't use the SSB duty cycle assumption when doing the exposure averaging)

The concern about controlled/uncontrolled exposure (which, by the way, isn't the term used in the standard anymore) is not that the ham will be standing next to the antenna while someone else operates.  It's whether the ham has adequately controlled access to the high field area near the antenna while they are operating.  If your antenna is a dipole 70 feet in the air, probably no problem.  If your antenna is a vertical in your backyard, and your kid is out there mowing the lawn, you have a potential problem. (In the commercial broadcast world, which is overkill for hams, they usually put a fence at the point where the exposure limit is reached.  Outside the fence, guaranteed under the limit.  Likewise, locked doors to get to rooftop installations, etc.)

Anyway, it's good to be concerned. Read the OET bulletin (linked from the ARRL RF safety page), do the calculations on the form in the back, be safe.
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WS3N
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« Reply #7 on: August 22, 2009, 02:33:52 PM »

"voltage is the gradient of the electric field"


Actually, it's the other way 'round.
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KB9CRY
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« Reply #8 on: August 22, 2009, 04:17:44 PM »

what is the best lightning protection for an antenna?


Why not try the Search feature here on eHam?
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K4DPK
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« Reply #9 on: August 22, 2009, 05:37:58 PM »

At the points along an antenna where voltage is highest, contact with it can be quite painful, even at very low power levels.

Years ago, when fine-tuning the last little bit on a pair of voltage-fed half-squares for 80 meters, I had turned the power down to minimum on the transmitter, about a watt, thinking I could move coil taps with impunity.  

Wrong!

RF at any power level will burn and ruin your day.

On all the antennas I've had with vertical wire elements, they've been voltage fed with the feed points at the bottom.  These have been half-squares, bobtail curtains, that sort of thing.  In all of these, I've used 40 kv insulated wire for the bottom ten feet or so of the "hang-me-downs", and very often slid Tygon tubing over that, to keep it safe for the neighborhood kids.

I don't worry about harm from radiation.  I figure if that would hurt, I'd have died long ago.

Back in the days when they had the X-Ray machines in shoe stores, my sister and I would cram the family cat in the machine just to look at his insides.  Didn't hurt him a bit.

Phil C. Sr.
k4dpk
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N3OX
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« Reply #10 on: August 22, 2009, 06:23:26 PM »

"voltage is the gradient of the electric field"


Actually, it's the other way 'round."

Yessir it is.  Embarassing error considering my line of work.  Apologies.  Not in the habit of writing down the math for my forum posts... probably should be.  (Dimensional analyis woulda been good enough)

Anyway, the point was you still have to know the geometry to know the voltage.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
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