Re: How do I ground a chassis with only a 2-conductor power cable?
From: Circuit Breaker (bagboy6437_at_ship.aol.com)
Date: 07/26/04
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Date: Mon, 26 Jul 2004 00:01:23 -0400
John Woodgate wrote:
> Circuit Breaker wrote:
>> all I really need it for is to transmit MP3s about 150 feet to our
>> hangar (we live on an airpark)
>
> STOP IMMEDIATELY! You could cause an air accident by interfering with
> ATC radio or navigational aids. Fancy 30 years in jail and dozens of
> deaths on your conscience?
I hope you're kidding, knowing somehow that the airpark upon which I live
is more than 20 statute miles from the nearest VOR station, more than
25 statute miles from the nearest controlled airspace, and more than 75
statute miles from the nearest airport that services commercial
traffic, but I suppose it's possible you're serious. In case you are
serious, your concern is noted.
To ease your concern, please take the following into account:
(1) It's not an airport. It's an airpark. May mean no difference to
you, but to most of us, it means that there are no facilities for fuel or
FBO, and that usually also indicates that the only traffic in or out is
private, small/light aircraft such as the well-known Piper Cub, Cessna
172, Grumman Tiger, Beechcraft Bonanza, and occasionally an original
Stearman (those things are no quiet birds), all of which are owned by the
residents or their guests. The largest plane to operate from here on any
regular basis is a single engine Beechcraft 6-seater (I forget its actual
name). Whereas these craft may have some sensitive avionics on them, I
feel they will not be adversely affected for the following reasons:
(2) The purpose is to transmit about 80 feet, give or take, so I don't
use an antenna (as I really don't need one for so short a distance), and
it *almost* reaches to the end of our hangar. So, the range is probably
only about 100 to 120 feet, give or take. It goes about 20 to 25 feet
farther than I need it to. My nearest neighbor's /house/ is only about
200 to 250 feet, and his plane and soon-to-be-hangar are about another 150
beyond that. Plus, the runway edge is about 250 to 300 feet away from our
house, give or take. So, the main frequency's range being at most 120
feet will prevent the main frequency from noticeably affecting any of my
neighbors, to say NOTHING of a jetliner at 30,000 feet.
(3) Spurious emissions and sidebands typically are weaker than the main
signal. Therefore, any harmonics or "birdies" etc. will affect an even
shorter range than my intended frequency.
(4) When I *do* use an antenna (rare), or when I experiment with
different coils or what-have-you, the best range I've ever gotten was
maybe a quarter mile. 1,320 feet, give or take. When I *do* experiment,
I do it at night, when nobody is flying (they're mostly weekend warriors
and usually only at daytime), for this very reason. Even still, for it to
affect air traffic, they'd have to be pretty much overflying our field
(pattern traffic on downwind/crosswind/base legs are too far away -- only
upwind/overflight traffic would be close enough), in which case they're
paying more attention to the pattern than they are to their VOR dial, if
their VOR could even be affected. Anyway, point is, when I do this,
nobody's around for it to affect, and I do keep tabs on the TV channels to
make sure I am not screwing up the neighbors' TV sets.
I do realize that if I go over a certain radiated power limit (which I
believe I read somewhere that it's 250 mW at the antenna) that I'll be
transmitting illegally, but I do believe I am well below that limit at all
times. My radiated power cannot be any more than the power the entire
circuit draws from the supply, which I don't recall ever seeing going
more than 150 milliwatts, and about 60 of those are between two LEDs that
are loading the circuit (one in my supply to show "power on" and one in
the radio to show it's powered up)... 90 mW total input power, probably
half that in output, after you go through the two transistors, 5
capacitors, 10 resistors, and an inductor. I think I'm safe legally, but
of course I have no way of truly knowing what my output power is as I do
not have a field strength meter that will work accurately on this band.
I figured there'd be someone sooner or later to tell me this was bad, but
I want to do this, and I'm taking every precaution I have in my power to
do this while not affecting anyone nearby. Of what research I've done,
and what information I've been able to get from the web, I've determined
that my transmitter is not likely to cause any negative effects in nearby
aircraft, or anything else other than TVs and radios within 200 feet.
This was a concern from the beginning, and I considered these
possibilities before, and I kept all transmissions to just a few seconds
in length when I started out "smoke-testing" the transmitter to ensure I
had a good frequency chosen and I wasn't interfering with TVs. I
have no avionics with which to test, but I do have a Radio Slack
scanner... I used the scanner to try to pick up the transmitter's FM
signal, and it only did if I was right on the base frequency --
meaning the scanner rejected the birdies and spurious emissions, and
if a cheapie R.S. scanner can reject this signal at 10 feet distance,
certainly instruments of much higher construction standards can
reject the signal, if it's even strong enough to reach that far. Plus,
the radio is of a schematic that can be found in any number of places
around the net, in many different variations, so I'm sure that if this
particular circuit was known for its glaring spurious emissions that there
would be note of it somewhere -- I recall none. I know my milliammeter,
which is inline and "built-in" with the power supply, is fairly accurate
as well, as I have had a light bulb of known wattage on at a known voltage
and did the math -- the calculations came out to within a few mW. The
voltmeter accuracy was also verified by a simple connection of a Craftsman
digital multimeter to the output leads. So, I'm fairly sure my scales are
calibrated well enough to know my input power is correctly displayed, and
of course, output must be less due to circuit loss.
I apologize for such a winded response, but I felt it necessary to say
what I've done so as to ease any serious concern for the lives and safety
of air passengers. I do not have all the equipment I really need for this
kind of playing around, but I make do with what I have, and I feel I have
sufficiently limited my explorations of the hobby to a safe level.
Further concerns welcome for discussion. If anyone knows of any other
tests or perhaps some test circuitry I can build from Radio Snatch parts,
feel free to let me know that as well, as I'm always willing to improve my
methods.
CJ
- Next message: Allan Herriman: "Re: Is ground the same thing as the black wire in DC?"
- Previous message: Ban: "Re: Any reason to build discrete OpAmps for High Quality Audio?"
- In reply to: John Woodgate: "Re: How do I ground a chassis with only a 2-conductor power cable?"
- Next in thread: John Woodgate: "Re: How do I ground a chassis with only a 2-conductor power cable?"
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