Attic antenna safety

Discussion in 'General Technical Questions and Answers' started by W9MBT, Feb 29, 2012.

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  1. W9MBT

    W9MBT Ham Member QRZ Page

    I live in a condo and have absolutely no choice but to have my antenna farm in the attic and have had success on 10 through 80. Not like an antenna outside but OK for WAS in a year, Long way toward DXCC and have lots of fun contesting.

    I have a "what if" question that I'd like an answer to based on facts and not just opinion.

    Presently I run 200 watts in order to make up for some of my disadvantaged antennas.

    Now for my "what if". What if I added an amplifier with 400 or 600 watts output?

    The closest physically I am to the the antennas is 20 feet. Would that be an actual RF danger.

    The 40/75m antenna runs parallel to the main roof rafter and about 6 inches away. The big question that I haven't been able to get answered is, is that a danger from heat with higher power? What king of temperature does a properly tuned antenna see? Is there a fire hazard?

    Again, please keep answers to facts and not just opinions.:)

    Many thanks,
  2. KB4QAA

    KB4QAA XML Subscriber QRZ Page

    You are in no danger healthwise from RF.

    On the other hand, your health may be seriously endangered when your neighbors figure out you are the culprit for overloading every consumer device within a block!
  3. W9MBT

    W9MBT Ham Member QRZ Page

    That's a good one!:) Thanks
  4. KB5HAB

    KB5HAB Ham Member QRZ Page

    From what I've read the ends are the biggest danger as they'd be "hot" with rf. As long as the wire and especially the ends are not close to anything conductive you should be good. I can't quote scripture and verse here but some on here have talked about antennas not getting hot from transmitting. I never ran more than 100 watts but have had insulated wire antennas laying on rafters and when they were removed there was no melting of insulation on the wire and of course no burn marks anywhere. Look at some cable charts, they show breakdown voltages and power ratings. If it don't hurt coax I'd think wire 6 inches from anything would be good to go.
  5. W9MBT

    W9MBT Ham Member QRZ Page

    That's what I've been kinda guessing, Thanks for that!
  6. W9GB

    W9GB Ham Member QRZ Page

    Electromagnetic Radiation: Field Memo == OSHA

    Figure 2. Plot Showing RF Levels for EMS Concern (Pd is Power Density in mW/cm²)
    This OSHA Field Note, based on science and math, graphically provides the information that you should be aware of.

    BTW, most amateur radio clubs in Chicago area that hold training classes for new amateur radio operators
    cover this specific topic (Appendix B in OSHA doucment) ... to minimize confusion and for new radio amateur to be aware of:

    ... the strength of a field due to a point source or the irradiance from a point source decreases as the square of the distance from the source.

    73, greg
  7. K9ZMD

    K9ZMD Subscriber QRZ Page

    From long-past experience, I can tell you that 500-600 watts into the 20 meter dipole I had in a condo attic did not fry me or set fire to the building. However, my transmissions definitely put a crimp in the family TV viewing, stereo listening, and telephone talking. Gas stove igniters also went haywire. Other condo occupants probably experienced the same sort of interference, but the only complaints I ever heard were from my wife . . . mainly because she was the only one in the building who knew I was an amateur radio operator.
  8. K0RGR

    K0RGR Premium Subscriber QRZ Page

    Well, if you're getting away with 200 watts now, 500 might not be that big a jump. But, you also have to consider the benefits of the higher power. To get one S unit higher performance, you'd need to go to 800 watts (6db).

    You must do an MPE calculation, or study, if you run over 500 watts on any band. The power level decreases from 425 watts on 30 meters, down to 50 watts on 10 meters. On ten meters, at 500 watts, you need a separation of 24.5 feet to be in compliance, but on 40 meters, a distance of 3 feet is OK. Sadly, you are probably more likely to have RFI issues on the lower HF bands, because RFI proofing measures tend to be less effective there.
  9. NM7G

    NM7G Ham Member QRZ Page

    I'll leave it to those more knowledgeable about Maximum Permissible Exposure and SAR to respond on physiological effects. The ARRL Antenna Book covers it.

    Fire is a real concern at the power you're contemplating. As one who was RF burned many times when running 80W to 100W out as a teen (and being stupid), I'm very mindful of the voltages that may be present. Multiband antennas can have more than two high voltage points and multiple current loops on the highest freqs. Since you don't describe your antennas, we don't know if this is an issue. I encourage you to see that no antenna can arc, and that includes eliminating sharp wire ends or sharp fasteners which facilitate corona. It's not difficult to create several hundred volts on an antenna. Chronic high voltage arcing can char wood as easily as heat from high current. See ARRL Antenna Book, 20th Ed., page 4-30.
  10. K8JD

    K8JD Ham Member QRZ Page

    The problem is not so much heat as RF leakage at the high voltage points of the antenna. RF arcing to wooden rafters could be a big problem if the ends come loose from insulators and touch anything.
    Many years ago I ran a KW on 80M. My dipole ran through trees and was blown around by wind and came into contact with a tree limb near one end.
    One afternoon, while I was on the air, pounding brass with the amp running, my XYL ran into the radio lroom and told me hot embers and smoking ashes were falling out of the tree.
    I had to move and re secure the end of that antenna to prevent a mini forest fire in my front yard.
    Just think of what could happen in the unseen confines of an attic !!!

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