mr2channel, I really really appreciate you quoting me in full, but please heed Daniel's advice and edit out whatever part of the quote you are not really referring to.
On 11/10/04 18:44 ET, mr2channel said...
One question though Ernie if the antenna is not
grounded (for better performance) how can you
stay up to code (NEC) since all "antenna's" are
required to be grounded?
First, the antenna is not grounded for better performance, it is grounded because the radio is trying to receive voltages between the air and the earth. If you are not connected to the earth, you will not be connected right. Now, because there is capacitance between the receiver's ground and the power transformer and the power line, which is connected to ground, the antenna will still work somewhat, but that capacitance lowers the signal that the receiver sees, and since you will, in effect, be using the house electrical circuit as your ground, EMI will be higher.
Second, the ACTIVE part of the antenna is not what is to be grounded. The mast is to be grounded. On a dish, for instance, the ground goes to the mount. The idea is to try to discharge static in the air, or even receive lightning, directly to ground, not directly to your A/V system.
If you were to ground the long wire antenna (near the receiver end), that would be similar to shorting an LNB output to ground. The long wire does not have two conductors itself, an inner one and an outer (grounded) one, so I was pointing out that the long part itself just hangs up there.
I suppose that, if a long wire were long enough, grounding the end away from the lead going to the receiver could still work, making the antenna look like one humongous loop. I haven't tried this.
If you look at a TV antenna and check it with an ohmmeter, you will find that most of the elements are connected to the "backbone," which is connected to ground, but the ones that the antenna cable connect to are often insulated from the mast. There are exceptions having to do with balanced antenna outputs, but the active parts are usually isolated.
Then Theaterworks asked:
So, a follow-on question. Can I use the center conductor for an RG-6 going up to
the attic, and then connect that to some uninsulated wire that would be laced back
& forth through the attic to add up to 100+ feet? And send another wire out of the
house to ground to the earth? Sounds like, from you description, that this would be
the ticket.
There are two issues here: impedance and just how flaky the whole AM antenna scene is.
RG6 is 75 ohm cable, and I have no clue what the characteristic impedance of a long-wire antenna is. Or, for that matter, the input impedance of an AM tuner. Or if they are all alike.
Remember CB? Those antennas, and most ham antennas, are 50 ohm. You never use 75 ohm cable for that, to avoid signal loss. If you use the wrong impedance cable, you will lose signal on the way down. Ham operators, when they build circuits to connect their antennas to their receivers, often include tuning circuits to "match," which means adding inductance and/or capacitance to optimize the transfer of energy both down from and up to the antenna. I have seen projects that include matching for AM, but it has been at least twenty years....
Which brings us to flakiness. If you don't know the antenna impedance or the radio's input impedance, you can't possibly bring the signal down with full efficiency. On the other hand, if the wire is really really long, you probably have signal to spare, so what the heck? Use 75 ohm cable! Or 50 ohm! Whatever you have on hand!
When I was a kid, I just continued the antenna wire down and into the window with no shielding at all. Since there are so many more sources of interference now, shielding might help. It might not. That is why I use the term flakiness.
I've also thought about making my own dual loop, using old 5 gal paint buckets.
Wrap each with a loooong length of wire, orient them 90 degrees off from one
another, and locate them in the attic. Would that work? Quicker than stapling up
long wire runs after the house is complete.
I LOVE THIS IDEA. But, again, the bugaboo is the impedance of the resulting coils -- will it match the radio? Will 50% of your received signal be lost due to impedance mismatch?
I once built an AM transmitting antenna. Keep in mind that when I say AM, I don't actually mean "amplitude modulation." I mean "540 to 1600 kHz" or whatever they have expanded the band to this week. Anyway, that antenna was about an eight foot whip with a loading coil at its base around ten inches in diameter and a foot high, with some forty turns of wire. What was the impedance? Who knew? I built the antenna, built the transmitter, then removed turns from the loading coil until the signal peaked. That meant that whatever the antenna impedance was, it was the same as the transmitter impedance.
Yeah, flakiness.
This message was edited by Ernie Bornn-Gilman on 11/15/04 13:12 ET.
