# Tag Info

13

There's a lot that can be said about lightning protection, but it usually boils down to doing all of these things: make good, low impedance connections to Earth bond all Earth connections together with a low impedance path have only a single point of entry to protected equipment A real problem when discussing lightning protection is that people will ...

12

If you want to be certain, you will need calibrated electric and magnetic field probes and a spectrum analyzer. If you had that equipment, I doubt you would ask this question, so let's assume you don't have it, and you don't care to drop thousands of dollars to get it. How can you be pretty sure that exposure is safe, without expensive test equipment? First ...

12

The direct effects of RF on people are: Tissue Heating Electric shock (shocks and burns) and electrocution (death) Interference with implanted medical devices The General test question quoted is about evaluating the exposure for the purposes of tissue heating. I assume everyone "gets" not grabbing a wire you are pumping 1500W into. Tissue heating is a ...

8

Magnet mounts can come off, and they do fall occasionally. I can't quote any statistics, but it has happened to me twice in the past - tall HF antenna combined with too small magnet mount and high speed. Luckily the cable held the thing on the roof. Short VHF/UHF sticks (think 1/4 wave for 2 meters) would never fall with the same mount - the wind load is ...

6

QST magazine printed a comprehensive three part article in 2002 addressing lighting: Lightning Protection for the Amateur Radio Station. They are republished by the ARRL. QST June 2002, pp. 56-59 QST July 2002, pp. 48-52 QST August 2002, pp. 53-55 The surest way to protect your radio gear is to disconnect it from power, from the antenna, from your computer ...

6

Many companies make lightning surge protectors. the install in the coax to your radio and divert the strike to ground. NOTE: THERE IS NO SURE WAY TO PROTECT YOUR RADIO EXCEPT DISCONNECTING IT FROM POWER AND THE ANTENNA. Search for polyphaser, they are well respected, MFJ also makes some lower cost options..

6

The peak voltage at the tip of the antenna is a function of the input power and the field impedance at that tip. The impedance varies based on the geometry of the antenna, but for a typical mag-mount whip it's probably not more than 5000Ω. Voltage $E$, power $P$, and resistance $R$ are related by $$P = {E^2 \over R}$$ $$E = \sqrt{PR}$$ With the worst-...

5

Sure, there are plenty. Unfortunately, they all seem to be selling something. The scientific consensus is quite clear: no known risk, beyond the obvious risk of being cooked which MPE limits are set to avoid.

5

If the receiver is close to the transmitter, and you are transmitting with more than a few milliwatts, just fire up the receiver with no antenna attached. Not having an antenna makes the receiver far less sensitive, but with the path loss being only some feet, it really doesn't need to be very sensitive.

5

I'd recommend one of two solutions: 1. Don't throw it out the window. Most lightning damage comes not from direct strikes but nearby strikes, which can still induce large-ish voltages on the feedline: not enough to make lightning, but enough to damage things. A couple feet of air between the feedline and the tranciever will protect against this. If you do ...

4

Heating; it can burn you the same way a microwave cooks food, or UV from the sun gives you sun burn. The wikipedia article about radiation burns generally covers RF a bit: http://en.wikipedia.org/wiki/Radiation_burn

4

According to Alan K0BG's page about wiring, the negative lead is fused because of the possibility of a faulty ground elsewhere in the vehicle, which could cause excessive current to flow in the negative lead to the radio. Here's a diagram from Alan, showing how to properly wire power to the radio: Alan's page about wiring, and the rest of his site, are ...

4

Also, there's a lot of attenuators out there. Just do the math: If your transmitter is capable of thransmitting let's say 20 dBm, and your receiver has a sensitivity of -90 dBm and a max input of 10 dBm, then your maximum attenuation you could use is ( 20 - (-90) ) dB = 110 dB, and the mininimum necessary is ( 20 - 10 ) dB = 10 dB. Always go for the higher ...

4

Regarding the overlap of the mast to the antenna, there should be about 1 foot of overlap as shown in this picture: The top of the pipe must stay below the decoupling radials in order for the antenna to function properly. This puts the top clamp within an inch or so of the top of the pipe. Keep in mind that on VHF and UHF FM, line of site is key to ...

4

Various organizations have issued guidelines on maximum permissible exposure. Below a certain power, the station can be assumed harmless. Above that power, some more careful design is prudent. Your station is mostly below that threshold, except possibly on the upper HF bands. Thresholds below which a station does not need to be evaluated, according to the ...

4

The frequencies your neighbor would be transmitting are different than the ones your own communications would use. They should not interfere with your utilities. Occasionally, funny things like old metal connections can end up re-broadcasting related frequencies instead of the original one, but this is usually accounted for as well when the allocations are ...

4

The FCC has published a bulletin OET 65B that has a table showing for VHF you don't need to evaluate the RF hazard if your effective power is 50 watts or below. For 70 cm UHF the limit is 70 watts. This does not mean it is not safe above those limits, it just means you must do an evaluation. https://transition.fcc.gov/bureaus/oet/info/documents/bulletins/...

3

What I would like to know is; how much EMF/RF does an average scanner emit? No more than your TV, electric lights, or phone charger. In fact, quite possibly less. Certainly far less than your cell phone, laptop, or Xbox controller, all of which contain intentional transmitters. A scanner does not emit anything intentionally because it does not transmit. ...

3

Cellular networks are heavily regulated all over the world. Part of these regulations is that there's no amount of power radiated in any inhabitable direction that would pose a risk to the health of any human being. Yes, there are ways to measure the transmit power, but you'd need to measure that in the antenna cable (and there's no way they'd let you do ...

3

What you need to do is follow the requirements of the National Electrical Code (assuming USA). This article covers it well. (ps. Failure to comply with NEC 810 is the #1 reason ham radio operators in the US have fire insurance claims denied, and why they have to buy ARRL's silly little insurance policy. If you follow NEC 810 and get permits for your work, ...

3

A discharge tube would serve to protect your rig from a close strike. Apart from that, make sure you have a decent ground for your shack. Having said that, it's probably not a good idea to operate in heavy weather. Unless absolutely necessary to continue operation, I'd go QRT, and disconnect the coax from the rig.

3

RF burns aside (these often occur without physical contact), the voltage on the antenna can reach very high levels. There is a good discussion of this at http://forums.qrz.com/showthread.php?243998-Voltage-at-antenna. Paraphrasing the linked discussion, Ohm's law applies, so if you're dumping 50W into a 35 ohm load: $P = I^2R$, so \$I = \sqrt{\frac{50}{35}} ...

3

Touching a "live" (transmitting) antenna could impart a serious RF burn if the power level was high enough. This is different from the classic "electric shock" obtained by putting a finger into a live wall socket. Radio frequencies heat tissue (non-ionizing radiation), proximity and contact with high RF fields will cause a RF burn. There is a risk of ...

3

There is a government doctrine on this very subject entitled OET-65. You must remember that RF exposure is cumulative and does the most damage over a long period of time. The symptoms of over-exposure to RF are stomach pains, scratchy/sandy feeling eyes, the internal feeling of overheating (possibly followed by stroke). The most effective ways of avoiding ...

3

One enthusiast reports, ".029" spacing for a KW station, and .045" spacing for 2.5 KWs" Keep in mind that a lightning arrestor doesn't stop an electrical discharge event, it merely shunts most of the energy to ground. There's still a lot of damaging current that ends up in the wire which will damage attached equipment. See How can I protect equipment ...

3

The ARRL RF Exposure page has a lot of useful resources. It has links to a number of articles and external websites that provide some good info. If you happen to have a copy of the ARRL Handbook, the Safety chapter has a section that covers RF safety. Between 1.34-30MHz, the FCC maximum permissible exposure (uncontrolled exposures/general population) for ...

3

There's no easy way to weatherproof it, but only you can know how bad the weather is likely to get, and balance the effort and cost of providing protection against the actual needed protection. If you do want to throw it outside and protect it, though, one method that works fairly well is to take a piece of capped PVC mounted so the cap is up, forming a ...

3

800 volts is far more likely to burn you. Just doubling the voltage from 120 to 240 will quadruple the power heating your flesh. This is just basic Ohm's Law: [P=E*I] DC is one nasty customer: It is easy to get complacent after spending a youth and a career working with docile, harmless 5-24 volts DC, or well-behaved 100 - 240 V AC voltages because of ...

3

You can buy copper strap at Georgia Copper. There are other sources, such as DX Engineering's Georgia Copper page. For RF, it is better to use this copper sheet metal strap wherever possible. The flexible braided stuff eventually becomes more lossy as corrosion and oxidation sets in and the small wires become significantly insulated from each other. From ...

3

Estimating the near field is best done with modeling software or empirical measurement. The details of the antenna construction and environment (the ground and any nearby conductive objects in particular) can significantly impact the near field. That said, 100W is not much, and often regulations specify that no particular evaluation need be done if the ...

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