yes antennas. but then again as out Part 15 consultant has pointed out, ground leads can become antennas.
Who is the consultants actual employer?
Self. 73 years old.
What is the fellow's self-job-description?
Reply to Rich and his helix efficiency run.
2.24 percent efficiency, that sounds really good on the helix. The antenna's bandwidth looks really low though, like it would have lots of sideband roll-off and be hard to keep tuned to max.
I found a helical CB antenna on the road this week, it must have broken off of someone's car. It's about 3/8 inch whip, with about #18 wire in a helix, and the winding pitch varies. It's wide at the bottom, then at the center there's a half inch section where the wire goes close-wound, then wide spaced again, and for about the last 4 inches to the top the wire is close-wound, with a rubber boot covering the end.
Nate Crime wrote (in part): 2.24 percent efficiency, that sounds really good on the helix ...
Note that this helix model used a perfect (zero-resistance) ground plane, in order to show its maximum performance.
ALL Part 15 AM antenna systems have a ground plane loss significantly higher than that.
The bottom line here is that the radiation resistance of a normal-mode helix is ~ identical to that of a linear conductor having the same end-to-end length, or height.
Therefore it doesn't matter to system radiation efficiency if the radiator is in the form of a linear conductor, or a normal-mode helix of the same end-to-end length -- other things equal.
I know I am going to regret this but...
Yes, Rich, but you can fit a quarter wave length of wire (or half wave, if you believe the real world results) wound into a helix on a 5 foot PVC form 2 inches in diameter (insulated 14 guage wire - I did the math); two of them comprise a helical dipole. There is nothing in the rules that talkings about conducting antenna length - it is the physical length that the rules state has to be less than 3 meters in length.
And if you add another 10 foot helix, parallel to the first, connecting the two end points of the dipole, you end up with a helical loaded dipole antenna. Loading the dipole increases the radiation resistance, making it more efficient.
Ignoring the legality arguments of this antenna, I wonder if it can be modelled using NEC.
Here is another approach to a helical antenna, this time fashioned into a loop. http://www.antennastealth.com/ (and the basic material was also featured in an eham article). He claims to have resonated a 6 foot diameter helical loop at 3.8Mhz - perhaps this could be extended to 1.7Mhz or so.
And again, I am deliberately ignoring the legality of a 10 foot helical loop (which, after all, would only be 10 feet in length).
Researching this antenna stuff is fun. Both the theory and practical experience is fascinating (as the two do not always match). That is why, if you have an idea, you actually have to try it out (as, obviously, did Station8). It remains to be seen exactly what his antenna is.
... if you add another 10 foot helix, parallel to the first, connecting the two end points of the dipole, you end up with a helical loaded dipole antenna. ...
That physical configuration does not comprise a conventional dipole antenna, but rather a single, 10-foot, MONOPOLE radiator having an effective diameter greater than that of either of the two radiators arrayed parallel to each other -- and regardless of their physical forms (whether helical, or linear).
That U-shaped configuration may have greater SWR bandwidth than when using a single 10-ft conductor as the radiator, but at MW frequencies would not produce greater field strength at a given distance than a single, Z-matched, 10-ft antenna conductor, other things equal.
An inquiry was made about the expected performance of a dipole antenna with its arms physically parallel to each other. The graphics and text below show a NEC analysis of this configuration.
This configuration has no other conducting wires such as a "ground lead," power and audio wires.
I didn't bother modeling helical arms for this dipole because that takes a lot more effort, and the radiation pattern when using them essentially is the same as for the linear wires of this model.
PS: I have no idea if this configuration relates to anything that Station8 is doing.
Lost some where in the old threads here at part15.us there was a post about someone mounting their AM transmitter on a metal roof, but i cannot remember all the details.
For instance, i don't know if the antenna was using the metal roof for a ground or if the antenna had ground radials spread out on the roof and grounded to the metal roof?? I don't know, if i knew what to look for then maybe I can do a search here on part15.us to re-visit that thread.
During our last TS meeting, my wife mentioned putting the SSTran and antenna system up on our roof. It just so happens we have a section of the roof that is flat and is made of metal covering the back porch. It is wide enough to lay some ground radials out but i don't think that will be necessary.
I wouldn't attach the ground to the roof either, i feel like making the roof a part of the circuit on the antenna system is asking for trouble, one would be the ground length would be exceeded and the chance of lightning strikes damaging the transmitter are two real reasons not to ground the antenna system to the roof.
I think just elevating the antenna could be enough to send a signal out and possibly use the roof as a counterpoise. It's all in my head at this point and I am still leaning towards building the antenna on the hill behind the house as I orginally had planned.
This old goat has no business climbing on roof's and i have seen how well Timinboveys transmitter does hanging off the attic window that i think the hill will be more than adequate.
The ground radials are going to be a bit difficult since the slope will put them at an odd angle. Not really sure how that is going to play out.
Barry opens an interest area for discussion: "The ground radials are going to be a bit difficult since the slope will put them at an odd angle. Not really sure how that is going to play out."
There are many threads about ground radials both in and above the ground, but I don't recall ever seeing a technical conversation about ground radials installed at a tilted angle due to a hillside or sloping roof.
I am guessing that it will change the pattern or angle of transmission of the entire antenna system, but have no clue what the resulting signal will look like.
Grounding Systems - NOT Radial Related
ok none of you have mentioned these but they are around i figuerd to put my my range master 20 t0 25 feet in the have it grounded and still part 15 leagle i whould use 4 8 foot grounding rods one 8 ft in the ground and 3 above ground you see they sell grounding rods that are threaded on both ends and couplings for them to connect then just bring the coupled grounding rods up to the bottom of the rangemaster on a telescopic mast like the ones from the amature radio company mfj they sell the telescoping masts in their website google mfj the are a ham radio company you can get in the air this way and stay in regs with part 1g by extending the ground rods with couplinhgs :} ill provide a company link
http://www.electrical-insulators-and-copper-ground-bars.com/gvn-6258s.html threaded rods
http://www.mfjenterprises.com/ look for telescoping antenna mast :}
This scheme may or may not be legal as viewed by an FCC inspector. Try it at your own risk.