|To:||Bill Coleman <firstname.lastname@example.org>, Red <RedHaines@centurytel.net>|
|Subject:||Re: [TowerTalk] Radials Questions - 270 or 360 degrees.|
|From:||Jim Lux <email@example.com>|
|Date:||Wed, 05 Jan 2005 16:55:14 -0800|
At 04:08 PM 1/5/2005, Bill Coleman wrote:|
On Jan 5, 2005, at 11:50 AM, Red wrote:
1.1 dB is very slight in the context of using EZNEC to model ground planes, something it is not particularly good at (especially if the wires are buried, where it probably doesn't work at all). The error in the model is probably a lot bigger, as Red said. Upshot is, don't trust the 1.1dB number. You could have 1 dB more gain, or 1 dB less gain, or no change at all.
The assymetry, 1.1 dB, can be reduced to 0.2 dB by removing the opposing 90 degrees of radials, at the cost of about 0.5 dB of maximum gain.
I would maintain that this change is insignificant compared to the other uncertainties in the model.
Modeling buried radials is exceedingly non-trivial. Even with a code like NEC4, which does handle buried wires correctly, would need a good model of your soil properties, with a resolution comparable to the other stuff you're modeling.
Somehow, I doubt any of the readily available inexpensive modeling codes would adequately model the effects of your house, which will dominate any directive effects of your asymmetrical radial field.
The performance will benefit some from using more radials, even if, in the interest of limiting the amount of wire used, some are shorter than 60 feet. However, variables associated with modeling errors, assumptions of ground characteristics, terrain features, wires and pipes in the structure, etc. are generally greater than the calculated differences in performance. Put down as many radials as you can and don't worry much about the 90 degree gap. If you can fill the gap without introducing excess interference or complexity, do it.
There you go.. the real useful advice.. put down as many radials as you have time/money/patience for. Don't worry about the length (well... extending past, say, 1/2 wavelength might be a diminishing returns)
Well, that about answers my first question -- how about the other three questions?
No way to predict. One would probably be safe in saying that there would be "some change", but it might not be detectable. That wire is presumably next to something (earth, concrete, interior structural members, etc. All of that will have an effect, and one that is basically impossible to predict (at least without spending a lot more time and money).
Question 3: Is there any coupling or danger associated with having these radials inside the house? Is there a potential for high voltages to appear, or to have the elements radiating RF inside the structure? Is this different from option A or B?
Yes.. and this is probably the best reason to NOT do it. Putting antenna components (of any kind, connected anywhere in the system, ground radial, radiating element, whatever) in close proximity to people and/or flammable stuff is not a good idea. If nothing else, since you couldn't do a credible model, and I assume you intend to use it for transmitting more than a trivial amount of power (e.g. a few milliwatts), you'd have real trouble complying with the FCC RF safety regulations (47 CFR 97.13). How would you assure that you're not exceeding the RF exposure limits? (And, no, I don't think the categorical power exemption for amateur stations would apply here. Take a look at page 9 of OET Bulletin 65, Supplement B... it's on the web)
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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