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Acceptable indoor antenna?
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by KG4DGF on September 3, 2009
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My question is 2 fold. Firstly i am an apt based op, currently with an indoor antenna. It is 33' long plugged directly into the back of my LDG auto tuner. Another 33 foot piece is attached to the ground of the tuner. I am in the 3rd floor, so it is probably 20-25 feet above the ground. I have made a few contacts (CW only) and was wondering if there was a better antenna i could construct indoors? Perhaps a loop or something. Preferably for 40M. Also i gave been wondering what a safe power to run would be? I have been sticking to nothing over 30 watts, with the norm being 20.
Thanks for any feedback,
73 de KG4DGF
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RE: Acceptable indoor antenna?
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by N3OX on September 3, 2009
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Play around with it... what works best inside your apartment is really too variable to give any advice.
"Also i gave been wondering what a safe power to run would be? I have been sticking to nothing over 30 watts, with the norm being 20"
That's kind of a loaded question ;-)
There's an RF Safety calculator at:
http://hintlink.com/power_density.php
Make sure you understand the difference between "Average Power" and the power you have set your radio to before you worry about any results... the "Average Power" even if you run CW transmissions 24/7 is not more than 50% of the PEP... and if I recall correctly (check in FCC OET Bulletin 65 as referenced on that page) the average is calculated over 6 minutes, so DXing style CW would have an even lower average power (though ragchewing might not)
20-30W PEP is probably a good choice for being in compliance "no matter what" ... controlled, uncontrolled, etc.
As far as the actual safe power goes? Just don't set anything on fire... ;-) The regulations are pretty arbitrary and conservative... they took some previous guidelines, made 'em a factor of 10 more conservative, and made that the law if I remember correctly. There's not much evidence that HF EM radiation is at all dangerous to human beings until it starts heating 'em up, and that takes a HUGE field at HF.
73
Dan
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RE: Acceptable indoor antenna?
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by WB2WIK on September 3, 2009
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It's true that at 7 MHz, you'd have to touch the antenna (literally) to be exposed to any RF hazard, even at 100W output power. Per the FCC formula, 100W output to a 0 dB gain antenna is perfectly safe at a distance of five feet. There's no hazard until you get above 20 MHz, and even then you could run 100W output ten feet from a very good antenna and be well within the "safe" zone.
The real hazards occur at VHF-UHF.
As for a better antenna, you might try a full-wave loop tacked up (stapled, taped or whatever) along the ceiling. If you can lay up 140' or so of wire to form a loop, you hardly need a tuner to make that work on 40m...its impedance should be about 100 Ohms or so, which is a pretty good match to coax all by itself.
Almost anything "outside" will be better.
WB2WIK/6
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RE: Acceptable indoor antenna?
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by KG4DGF on September 3, 2009
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I guess when i said safe what i meant to say was not to be the next biggest attraction on my neighbors TVs, though the RF safety information is quite helpful. I have never had problems with RFI at home (except the GFCI circuit breaker my sisters room is wired on, but thats just funny) so i do not know what it will take to mess up the neighbors reception in such close quarters.
Thanks again
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RE: Acceptable indoor antenna?
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by N3OX on September 3, 2009
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"i do not know what it will take to mess up the neighbors reception in such close quarters. "
Unfortunately, there's really no way to know. It's just totally dependent on how much coupling there is between your antenna and their electronic gear's wires, etc.
When I started out playing with radio gear it was on a CB, and I got into the phone at my parents' house with just 4W to a vertical dipole mounted on the house.
After I started hamming, I would totally shut off my sister's TV with 100W to an outdoor end-fed wire on 15m
But I also ran 100W to an antenna just outside my apartment building without causing any RFI to myself or, I think, anyone else. The neighbors across the hall would have been most affected and they actually caught me slinging wire one night, so I think they would have suspected me of being up to something.
The nice thing is that CW RFI is going to be hard for your neighbors to figure out unless it's really profound. It'll just be a weak thumping in their audio and it won't be correlated with any sound coming from your apartment.
If you hear your neighbors cursing at the TV every time you transmit, dial it back 10W ;-)
73
Dan
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RE: Acceptable indoor antenna?
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by AD5X on September 4, 2009
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"When I started out playing with radio gear it was on a CB, and I got into the phone at my parents' house with just 4W to a vertical dipole mounted on the house."
I can do one better. I was operating AM phone with my Knight T60 while my parents were hosting a square dance club at our home (this was 1964 and I was 15). They were recording the caller, and guess what? The recorder perfectly captured my voice whenever I was talking (good old recorder internal rectification of the AM signal). And guess what I was talking about? The silly square-dancing that was going on in our basement!
Phil - AD5X
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RE: Acceptable indoor antenna?
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by AA4PB on September 4, 2009
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the GFCI circuit breaker my sisters room is wired on, but thats just funny
-------------------------------------------------------
Actually thats just dangerous. The GFCI breaker is there to protect a life if someone gets connected between the power and a ground like a sink or a bathtub. Very often one breaker serves more than one bathroom so it could be putting numerous family members at risk.
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RE: Acceptable indoor antenna?
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by KF7CG on September 4, 2009
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I don't think that the breaker is "wired on" but rather that that was the type of breaker used for his sisters room.
By the way, certain of the older GFCI breakers were highly susceptible to RFI; shutting off at the first good sniff. The company I was with had problems with them in commercial applications, the early switching type supplies in the large monitors we were supplying would trip them. Absolutely now ground fault leakage from the monitors but the small amounts of switching signal on the lines would trip the breaker.
The interference to the GFCI was probably much more of a nuisance than a hazard, ala false alarms on CO detectors from RFI.
KF7CG
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RE: Acceptable indoor antenna?
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by N5LRZ on September 4, 2009
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Any antenna indoors within the confines of the electrical systems of a house or appartment, particularly in the HF Bands, is going to pick up all the RFI running thru the electrical wires AND IT will also insert RFI into the electrical wires as well.
Bottom line on receive all indoor antennas are going to be completely inferior due to environmetal factors such as brick, steel, electrical wireing ,etc etc etc.
You BEST BET and probably ONLY good bet is to get permission to put up a nice tribander or good commercial multi-band vertical on your roof.
Your next best bet is to join a local amateur club that has a club station that does have a good antenna on a roof.
and least best as far as actual useful performance is any kind of an indoor antenna--they are all going to suck.
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RE: Acceptable indoor antenna?
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by KG4DGF on September 4, 2009
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Thanks for the warning on the GFCI circuit. I think i have figured out why that is, since i am not hooked into that circuit. I believe the ground the GFCI is connected to is about 30 feet long, which explains why it only trips on 40 meters. GFCI work by looking for a small current in the ground. and by small i mean small enough so they will trip before the hairdryer in your tub will kill you. So i believe the ground rod is resonate at 40 meters, and the slight excitation of electrons is enough to get the GFCI to flip, since i am not hooked into that circuit that is my only guess. Though if someone has a better theory i am all ears.
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