Tx antenna and high voltage - Page 1
Forum section: electronics / rf-microwave

author: ice595
author_rank: Regular Contributor
timestamp: April 26, 2020, 03:37:34 am
content: Noob question here. I've always been told that transmitting antennas are dangerous to touch but why?lets say if an antenna is connected to a 100W transmitter, 100 watts into 50 ohms is only about 70 volts RMS, but is 70 volts enough to give RF burns?

author: 0culus
author_rank: Super Contributor
timestamp: April 26, 2020, 03:51:06 am
content: I believe, if I recall correctly, RF burns have more to do with tissue heating than anything else, which higher frequency AC is good at (your microwave is a practical application of this...it's actually a quite powerful—but well shielded—2.45 GHz radio transmitter!). Granted, touching an operation broadcast tower, to give a more extreme example, is still gonna be dangerous if you are the thing completing the circuit to ground!Another practical, controlled application is diathermy:https://en.wikipedia.org/wiki/Diathermy

author: ice595
author_rank: Regular Contributor
timestamp: April 26, 2020, 04:04:32 am
content: well... touching a multi kW broadcast antenna is a bad idea of course.but how dangerous are transmitting antennas that operate in the 100W range?

author: 0culus
author_rank: Super Contributor
timestamp: April 26, 2020, 04:05:07 am
content: I haven't tried touching one personally, but I wouldn't be surprised if you get an RF burn for your trouble.

author: bob91343
author_rank: Super Contributor
timestamp: April 26, 2020, 04:07:52 am
content: While there is only 70 volts at the 50 Ohm point, there is much higher voltage at the ends of dipoles. In fact I have an insulator that was punched right through the porcelain by a few hundred watts of signal.

author: ice595
author_rank: Regular Contributor
timestamp: April 26, 2020, 04:18:27 am
content: but where are the high voltages coming from? the loading coil?I still remember the time where I got a shock from a solenoid coil, I ripped out the secondary coil from a microwave oven transformer and applied 12VDC to make an electromagnet and the back EMF gave me pretty nasty shock.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 26, 2020, 05:49:47 am
content: Well, depends on antenna type...A resonant type like a dipole may have some thousands of volts at the tips. An electrically short, narrow band antenna might be many times higher still (consider the Tesla Coil). A wideband type like a bicon, won't have much more than the feedpoint (which being closer to 200 ohms, will be about double the 50 ohm line voltage). A terminated TEM horn/stripline (used in EMC work), may not even have that (a combination of the design having a matched impedance, and the ~3dB losses from the termination).Tim

author: srb1954
author_rank: Super Contributor
timestamp: April 26, 2020, 06:21:57 am
content: I once got a RF burn while adjusting a 25W transmitter.I don't what voltage there was on the part of the circuit I accidentally touched but the burn hurt like hell for quite a few days.

author: radiolistener
author_rank: Super Contributor
timestamp: April 26, 2020, 08:57:40 am
content: Quote from: ice595 on April 26, 2020, 03:37:34 amjust try to touch 24V 100W soldering iron and you will realize why

author: radiolistener
author_rank: Super Contributor
timestamp: April 26, 2020, 09:03:44 am
content: Quote from: ice595 on April 26, 2020, 04:04:32 amI had very painful burns from a 10W transmitter when experimenting with high Q antenna. My finger skin becomes white at the lightning discharge point, and it really really hurts for several hours. Such a burn is more painful than from open fire. And such damage was caused with just 10W transmitter.

author: ice595
author_rank: Regular Contributor
timestamp: April 26, 2020, 11:16:06 am
content: Quote from: T3sl4co1l on April 26, 2020, 05:49:47 amBut where are the high voltages coming from?

author: David Hess
author_rank: Super Contributor
timestamp: April 26, 2020, 03:04:09 pm
content: The worst RF burn I ever got was with only 5 watts but it involved a helical resonator which allows very focused coupling. With dipole antennas, you can feel the heat with your hand around the element at 20 watts and greater but the impedance mismatch usually limits the power to relatively safe levels. Do not get your eyes close though.Quote from: ice595 on April 26, 2020, 11:16:06 amThe antenna does an impedance transformation between the feedline impedance, typically 50 ohms, and the impedance of free space which is 377 ohms.The high voltage at the ends of dipole elements can be high enough to cause corona discharge which erodes the ends of the elements. Solutions for this include removing sharp corners from the ends and using antenna elements which have no ends like quads.

author: ocw
author_rank: Regular Contributor
timestamp: April 26, 2020, 03:35:19 pm
content: It is not unusual for the first sign of an RF burn being the smell of burning flesh. After moving to stop that is when you think, "Hey, that was my flesh burning!" The pain usually comes later.

author: cvanc
author_rank: Frequent Contributor
timestamp: April 26, 2020, 04:29:27 pm
content: RF burns hurt like hell. I got one thru my fingernail once and it left a small crater. Took a looonng time to heal.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 26, 2020, 07:44:54 pm
content: Quote from: David Hess on April 26, 2020, 03:04:09 pmAnd for small resonant antennas, only a small fraction of the impedance of free space can couple to the antenna, because it's much smaller than a wavelength; this means its impedance is very low (resonant loop) or very high (resonant stub). The same amount of power into such impedances gives much higher currents or voltages, respectively, than on the feedline. Which also means there's big opportunities for losses in the circuit, due to resistance of the wire and loss of insulators -- making a small and efficient antenna is a challenge.Even resonant dipoles -- a full half wave long -- don't couple perfectly to space, because they are thin. Well, okay let me specify, resonant wire dipoles, the most common case. The conical dipole holds the advantage that it's... wide at the end... It has a self-similarity over many wavelengths, which is where its wideband response comes from, and necessarily also its lower voltage ratio. It's also self-dual (at a certain cone angle?), which gives a feedpoint impedance of Zo/2 ohms, or a bit under 200 ohms.I'm not actually sure offhand what the peak voltage ratio is, at the end of a resonant wire dipole; I'm just assuming this is correct for now.For antennas with loading coils, you can think of them as an intermediate case between electrically short and naturally resonant antennas, so the voltage ratio will also be intermediate between them.Tim

author: M0HZH
author_rank: Regular Contributor
timestamp: April 26, 2020, 08:52:11 pm
content: Dipoles have theoretically around 73ohm at the center and 5000ohm at the ends; practical antennas the size of a dipole might achieve slightly different figures.All* antennas have points where the feed impedance is high (low current & high voltage) and others where the feed impedance is low (high current & low voltage). We usually feed them where impedance is low (or close to 50ohm) because it minimizes losses and/or it's convenient. "100W in 50ohm" is only at the feed point, at the tip it's probably "100W in 3000ohm".

author: ice595
author_rank: Regular Contributor
timestamp: April 26, 2020, 11:13:25 pm
content: so the antenna itself acts like a transformer?

author: TheMG
author_rank: Frequent Contributor
timestamp: April 26, 2020, 11:46:18 pm
content: More like a complex L-C resonant circuit.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 27, 2020, 01:15:28 am
content: In narrowband circuits, LC and transmission line networks are often used as (and called) transformers, always specifying the type of course so as not to cause confusion. "Matching networks" being a more common (and less confusing?) term.Using transmission lines, there's the 1/4 wave impedance transformer for example, which inverts a low impedance at one port, to a high impedance at the other. Same thing works with an LC tank, which a transmission line can be approximated as under suitable assumptions (there is an equivalence between transmission lines of specified length and impedance, and an LC tank).Worked example: say you want to convert 50 to 75 ohms. Use a transmission line with an impedance the geometric average of the two, and 1/4 wave length at the center frequency. Or sqrt(50 * 75) ~= 61.2 ohms. This will work over a modest bandwidth (say 10-20%), limited in this case by the transmission line length (which only looks 1/4-wave-ish for a modest range, obviously). For larger ratios, the bandwidth is correspondingly narrower. So a 5 to 500 ohm transformation would need 1/4 wave of 50 ohm line, and have a Q around 10, or a bandwidth of less than 10%. I forget offhand, actually, if the bandwidth of a 1/4 wave transformer is less than this because reasons; anyway, it can't be more.So, in effect, the dipole's element acts as a 1/4 wave transformer between the feedline and free space; it's not a proper, shielded transmission line geometry, so it's not simply a transformation between the impedances at the feedpoint and tip, but rather there is a loss coupled into the whole length of the element (primarily magnetic towards the feedpoint, primarily electric towards the tip). It's more of a distributed transformer, of sorts. And what it's transforming into, is free propagating wave modes. Hence, that resistive loss element -- as it appears to the circuit -- is not really loss in a global sense, but radiated (or received) power. Which we call the radiation resistance.Tim

author: CaptDon
author_rank: Super Contributor
timestamp: April 27, 2020, 02:43:25 am
content: A simplified way to view a quarter wave vertical is that the bottom or 'ground end' is at a verylow impedance therefore low voltage and high circulating current. At the top of the 1/4 waveyou would expect extremely high impedance. Calculate 100 watts at say 5k to 10k ohms.They used to make a little neon lamp 'lightning bug' that attached to the tip of a 102" CBmobile whip antenna that would light from about 3.5 watts of power into the antenna. Thelamp wasn't even a 'complete' circuit. It simply pressed onto the tip of the antenna. Therf voltage was high enough to ionize the neon gas just by proximity!!! Neon tends to gloworange and purple in the presence of RF. I used dead Nixie tubes to test radar transmitterson ships. Just hold it in front of the scanner and see if it flashes. Quickest way in the worldto tell if your dead radar has a transmit or receive problem!!!

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: April 27, 2020, 03:33:36 am
content: I can personally say that 1kW@1GHz delivers a substantially nasty bite. It keeps with you for days. 5% duty cycle are totally sufficient for that!

author: ice595
author_rank: Regular Contributor
timestamp: April 27, 2020, 06:55:35 pm
content: Quote from: Neomys Sapiens on April 27, 2020, 03:33:36 amIsn't it dangerous to even get near a microwave antenna let alone touching one?

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: April 27, 2020, 07:15:47 pm
content: Quote from: ice595 on April 27, 2020, 06:55:35 pmWasn't an antenna, but a open transmission line (center conductor accessible) at the PA output. Antenna radiation field is in that case above the legal limits, but not in an area where it is proven to be dangerous in the short term.This is of course completely different at other frequencies and in the case of higher power.

author: tkamiya
author_rank: Super Contributor
timestamp: April 27, 2020, 07:39:40 pm
content: Let's go back to antenna theory.Imagine a dipole. Fed in the middle by a feed line supplying 100 watts. Impedance there is 75 ohm. So voltage is low, current is high. As you go further away from that point toward to the end of the antenna, impedance goes up. When that happens, current drops but voltage rises. It won't go to infinity but it will go high enough to create dangerous voltage.At 100 watts, you feel a ZAP. You might even get a small black char spot on your finger. Is it that dangerous? No. Don't try this with 500 watts though. You will get a serious burn. Another thing with RF burn is, unlike regular burn, it's deep. It will hurt for a while.I've done this few times. When conditions are just right, you can draw an arc.

author: chris_leyson
author_rank: Super Contributor
timestamp: April 27, 2020, 08:31:01 pm
content: I once made the mistake of feeling around inside a Racal BCC540G antenna tuner trying to find a warm matching coil whilst is was still powered up. Tuning coils with poor Litz wire terminations would run warm. Oooh look tiny blue spark from tuning coil to knuckle followed by shit it's still powered up. Drive power was only about 20W or so and it left a tiny tiny black pin prick on my knuckle. Later that night I woke up wondering why my hand hurt. There was a blister the size of a marble, about 1cm in diameter and it took about 3 months to heal properly. That was HF at 30MHz and maybe less than 20W. RF burns and microwave heating are two completely different things, RF burns go deep, very deep and don't need a lot of power. EDIT: The load was a dummy whip antenna so the drive voltage at the output could have beenseveral hundered voltsif not more.

author: TheMG
author_rank: Frequent Contributor
timestamp: April 28, 2020, 01:32:28 am
content: I was once troubleshooting a 500W HF linear amplifier (solid state) and thought it would be a good idea to use my finger to touch various components to see if anything was getting excessively hot. Well, I found the ferrite toroid in the output combiner network was alarmingly hot to the touch, but was perplexed as to why it was cooling down virtually instantly the moment I unkeyed the transmitter.Then it dawned on me... the toroid wasn't actually getting hot at all, it was the RF heating my finger!!!

author: srb1954
author_rank: Super Contributor
timestamp: April 28, 2020, 04:22:09 am
content: That was exactly my experience.The first indication was a burning smell. After 5 seconds I felt the pain and realized that the burning smell came from my finger.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: April 28, 2020, 07:24:57 am
content: I have gotten small burns because I touched a CPW carrying about 34 dBm of 5.8 GHz CW. You could clearly see two lines of burnt flesh, exactly the shape of the two gaps between the center and grounds of the CPW.

author: fmkit
author_rank: Contributor
timestamp: April 28, 2020, 08:19:43 am
content: no pain

author: ice595
author_rank: Regular Contributor
timestamp: April 28, 2020, 12:40:21 pm
content: Quote from: fmkit on April 28, 2020, 08:19:43 amthis reminds me of the 1 wire light bulb trick where, no wonder RF is kinda like black magic

author: fmkit
author_rank: Contributor
timestamp: April 28, 2020, 01:36:25 pm
content: no finger

author: radiolistener
author_rank: Super Contributor
timestamp: April 28, 2020, 01:51:06 pm
content: Quote from: T3sl4co1l on April 26, 2020, 07:44:54 pmhalf wave dipole for short wave has Q about 12, so for 100 W input it should be about 850 Vrms or 1200 Vpk