I understand we're llimited to 3 meters in length. I was just curious about what everyone is doing for antennas.
I understand we're llimited to 3 meters in length. I was just curious about what everyone is doing for antennas.
I know some are using the Rangemaster whip design and I've seen the other design for the SSTRAN.
What else are you folks using?
And for a quick clarification, that 3 meters is physical wire length right?
We used to make antennas for a section of the 160m (mil freqs) out of a plastic pipe and wrap about 400' of wire on them and tune 'em. They were probably about the right length physically but WAY long in wire length.
The 160 meter helical wound antenna you describe, I'm told, is not acceptable under the 3 meter rule as the "coil" itself is the radiator and exceeds the 3 meter lenght allowed even though the "coil" is 3 meter or less in lenght. I looked into trying it and was told it's not acceptable.
Anyone else?
The 160 meter helical wound antenna you describe, I'm told, is not acceptable under the 3 meter rule as the "coil" itself is the radiator and exceeds the 3 meter lenght allowed even though the "coil" is 3 meter or less in lenght. I looked into trying it and was told it's not acceptable.
Anyone else?
That has and always be debatable. Who told you it was unaccepable?
by MRAM 1500 kHz
The 160 meter helical wound antenna you describe, I'm told, is not acceptable under the 3 meter rule as the "coil" itself is the radiator and exceeds the 3 meter lenght allowed even though the "coil" is 3 meter or less in lenght. I looked into trying it and was told it's not acceptable...
Could you please post more detail about this? What are your sources, and what are the engineering justifications supporting this statement?
Not to deny or support what you've stated in your post, but probably many of us would like to know the basis for this evaluation.
//
I am also interested in the details about a helical wound antenna "legal" length. If true, this would eliminate the use of base loading coils on 3 meter antennae.
Neil
The article I read was found at random, online. It was a primer on various types of short verticals for Part 15 AM. I will attempt to locate it again if I didn't save it and make it available here.
The author felt that the helical wound antenna didn't meet the 3 meter rule as the "coil" is the radiator and the length of the wire in the coil counted as the radiator. He went on to refer to the infamous "grey area" and that he was aware of FCC inspectors that gave it a thumbs down. If he's wrong, there's lots of ways to put a quarter wavelength of wire on a holder less than 3 meters long.
As for a vertical radiator with "loading coil" his opinion was that the coil is not considered part of the radiator as the close-spaced coil didn't significantly add to the radiating surface. Something to do with the whole E/H field theory.
He also discussed "top hat" loading of a short vertical and stated that the radius of the top hat counted as part of the antenna overall length, even though a properly designed top hat doesn't radiate-only adds capacitance to the antenna to make it appear electrically longer.
Along those lines, a properly designed radial system improves antenna performance but doesn't radiate (not the same as a long, single, grounding wire which does radiate). The radial system is not considered part of the antenna overall length since it doesn't radiate but the top hat radius is. Go figure!
I believe that most of his statements were based on experiences with FCC enforcement more than engineering studies.
I can only relate to you what I remember reading. I remember that it was why I didn't pursue the helical wound antenna until I could find more info about it related to FCC acceptance. I'm neither agreeing or disagreeing with his statements, simply looking for more info.
Here is a quote from an article by North Country Radio:
"A helical whip using 200-300 ft of wire wound on a pole 8 to 10 feet long would physically appear to be a legal antenna especially if it has a plastic jacket However, these helical antennas commonly used for HF mobile SSB communication would likely be illegal unless the wire was wound in a specific configuration so as not to have more than 10 feet radiating. This is a continuously loaded antenna and you cannot easily separate the inductor from the antenna, so some kind of legal interpretation and clarification may be needed. Use of these antennas with their legality question is at the discretion of the experimenter."
I'm still looking for the original article I read but this guy has the same opinion.
Interesting.
What I had in mind to try was a center loaded antenna keyed for the 160m band but with a longer adjustable tip.
There used to be a company that made one that we field tested when I was in the Army. I thought it was called the "outbacker" or something like that but can't find it on the web. It was also center loaded and you had to manually move a banana plug on the mast to change the resonance point on the center coil.
I'm surprised that most of the antennas out there are homebrew and there's no real commerical market (yet) for Part 15 AM antennas. I'm sure there will be soon enough.
I'll keep hunting for that antenna and post it up. I think the actual length was right at 10' too.
Bingo! http://www.alphadeltacom.com/pg9.htm
Looks like the civilian version only goes down to 75m though...
Hi,
Northcountry's article mentions hundreds of feet of wire on a short pole and questions the legality unless it is specially wound so as not to radiate. We may be in a gray area here but it appears to me that the rules do not specify a radiating length, just a physical length. The purpose of a helical winding on a short antenna is to cancel the capacitive reactance and I am not aware that the helix increases the radiation for a short antenna with a small diameter other than by providing a better match to the transmitter; the same advantage you get from a base loading coil.
Considering that 15.219 is an alternative to 15.209 and the intent is to limit field strength by limiting the antenna length (and power) and if a helical antenna does not have gain above a whip (my presumption) then there would be not legal problem since the intent to limit field strength would be satisfied. So the question is does a helically wound antenna provide gain above a straight conductor? I'll research a bit more and post if I find a good answer.
Neil
Anyone care to comment about the Isotron antenna. They make a 3 meter version, model 200B, which they claim is legal for part 15 and have told me over the phone that they've supplied to numerous part 15 broadcasters. I was told that one part 15'er working with 100mw reported a range of 7 miles with one of these antennas. Jim B
Totally avoiding the aspect of FCC compliance to Part 15 AM when using a helical antenna, I thought I'd model one using Numerical Electromagnetics Code to see how it compared to a linear radiator of the same overall length.
Both antennas are mounted with their base at ground level, with 10 ohms of resistance in their r-f grounds.
The helical antenna uses 1/4 wavelength of wire arranged in 100 layers over a 3-meter length. This configuration does not produce the gain of a linear 1/4-wave radiator, because the radiation from the various layers does not add in space in the same manner as if the conductor was linear. It also has a horizontal component.
The helical arrangement does not produce a resonant radiator, either -- a loading coil is still needed, although it can be smaller. It does have higher radiation resistance than a linear 3-meter radiator, however, so its system efficiency is higher than the 3-meter linear (other things equal).
As to the FCC issues, you are on your own.
The comparison is shown in the graphic at the following link.
Nice info Rich!
Surprised at the results.
EDIT The 5 dB advantage I mention in the next paragraph is for the modeled helical 3 meter antenna compared to a modeled 3 meter whip and not compared to an isotropic radiator or a 1/4 wave ground plane antenna. I should have made that clearer. Both of these 3 meter antennas have very low efficiency at AM frequencies. ***
I too am surprised by the apparent 5 dB advantage (from the plot) of the helical wound antenna over the wire antenna. Since the patterns are about the same it looks as if the higher field strength results from the higher radiation resistance.
Earlier in the thread I asked " So the question is does a helically wound antenna provide gain above a straight conductor? ". It looks as if the answer is yes. Therefore my argument about equivalent field strength from these antennae satisfying the legal intent is not valid. As Rich says "you are on your own" with the legal issues.
I spent over an hour searching for actual gain numbers for a short, small diameter helical antenna and the closest I came was with a VHF "rubber duck" which has a reported gain around -6 to -10 dBi. I did not extrapolate this to the 3 m antenna because of the vast difference in dimensions in terms of wavelength. Rich's work fills a gap in knowledge regarding extremely short antennae. Thanks Rich.
Neil
I revived this old thread to illustrate a point I recently made on another website: NEC does not work for analyzing helical monopoles over ground that have very small diameter compared to wavelength.
The statement made on the first September 7, 2006 post in this thread, "The helical arrangement does not produce a resonant radiator...a[n] [additional] loading coil is still needed..." is absurd. The writer claims that a a helical antenna cannot be resonant, and an additional loading coil is needed to achieve resonance. Of course, in reality, you don't need an additional loading coil because the helical antenna itself serves as its own loading coil for achieving resonance.
I know exactly what the writer is talking about. When modeling a small-diameter helical monopole with NEC, there is always a large capacitive reactance in the simulation of the input impedance that does not actually exist in reality. This is because NEC does not work when analyzing a small-diameter helix.
Now, nearly five years later, nothing new has been learned, and the same misinformation is being promulgated.
This is a warning that running a computer simulation program is not a substitute for thinking. The results should always be examined to see if they make sense.