Just another brainstorm.. 😉
Just another brainstorm.. 😉
Being that the current antenna design(s) for Part 15 usage are so inefficient even with a great grounding system, would there be much more of a loss if an antenna was constructed in a "dipole" fashion with (2) somewhat resonant elements? (Yagi style in mind) Staying within the 3m length rule, feeding the elements directly in the center by placing the xmtr between the elements. (an SStran unit comes to mind in this case) Maybe design the "elements" as center loaded or an end loaded to increase the efficiency?
The huge ground system seems to be a big problem with most installs. Why not have an element to replace the ground? Resonance is the key for best range, isn't it?
~Don
Brainstorming is always good, and it is true that doing as you suggest would make the antenna independent of an r-f ground.
But unfortunately the side effect to it is that the reactance of this even shorter driven element will be extremely high.
Concerns will be the construction difficulty and power loss of a coil needed to resonate this configuration, along with the narrow r-f bandwidth that would result, and the difficulty in preventing radiation/detuning from the power and audio cabling to the transmitter.
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Great idea, but at these frequencies the wavelength is so long that efficiency is terrible.
If you wish, give it a try though... you can isolate the audio and power cables on the SSTRAN with ferrite cores (recommend 2" #77 material). Connect a 5 foot antenna and a 5 foot ground. Spread apart; tune transmitter, and enjoy!
It will go about 100'... depending on your interpretation of what constitutes acceptable signal level...
WEAK-AM
Classical Music and More!
Quote..
" Connect a 5 foot antenna and a 5 foot ground. Spread apart; tune transmitter, and enjoy!"
I was thinking more on the terms of the elements having a loading coil, somewhat like the coil at the base of the antenna that can be built from the SStran site. I understand that losing element length is bad but isn't having a bad, unbalanced, non resonant grounding system almost as bad?
Actually the SStran type antenna is only 1% at it's best efficiency with any kind of ground. The ground is the unknown most of the time. My thought is designing a ground "element" that is somewhere close to resonance per frequency. This could be calculated by designing a coil and a radiating element of whatever diameter. (I'm thinking BIG.. Like 55 gal. drum BIG!) The larger diameter element/coil would help with the bandwidth issue and the vast surface area would help with the reactance somewhat..
It's a fact that a center/end loaded antenna has a higher efficiency than a base loaded unit. Resonance is a key to get the best efficiency from any antenna. The ground is the unknown in almost all cases in a Part 15 set up. Wouldn't it be better to know that the ground is somewhere near resonance than playing midnight baseball? I know we have the rules to play by but I feel that within the 3 meter law a somewhat resonant antenna could be constructed with possibly a little more efficiency than 1% if the ground that the radiating element has to work with is somewhere in the ballpark of resonance per frequency..
And yes.. I agree with installing chokes on the feedlines but if you think about it, if the ground "radial" is near resonance, the system will not look at the feedlines as much..
Just my thought..
~Don
Well, I'd wonder if a (3 m total length including any loading coils) dipole would actually amount to making the driven element shorter, since the "ground" side of the dipole also radiates as has been established many times in tech discussions here.
Now, something as simple as a dipole might not be more advantageous for a ground mounted system, considering the small fraction of a wavelength there is to work with inside the 3M rule.. In fact, as I recall, Rich did some work emulating a loaded dipole close to or directly on the ground and it actually came out as somewhat less efficient than a loaded vertical with a good ground.
Not to say there might not be room for improvement with the loaded vertical antenna. I've noted some QRP ham operators seem to swear by having a separate tuner for the ground, for example. A loading coil on the ground needn't take much length away from the 3M total, and it might or might not help. No way to know for certain until someone tries it and makes some experiments and reports some observations. Most of the loading coils I've seen people speak of using have been simple linear air core coils, and it's possible that something like a "pancake" coil or a conical coil might have some advantages, or they might not work as well..
I'd say that antenna development for part15 AM is a long way from "there is nothing new under the sun". One advantage (sorta) of the 3M rule is that with such a physically small antenna, the cost of experimentation is considerably less than if (for example) 100 ft antennas were allowed.
Emulation is a valuable tool, but it shouldn't discourage experimentation. Often the emulation is based on extrapolation of results from things like a 1kw xmitter at a distance of 1 km, and I feel that in many cases with part15 AM, the specific technology may have been less fully explored than other frequency ranges and physical limitation simply because the focus was always on large transmitters with antennas that could much more closely approach physical resonance because they did *not* have a 3M limit they had to be developed in. Emulations and equations certainly have their place and are useful tools for exploration/development, and they make a great place to start.. But they shouldn't ideally be the place it ends.
With your idea of a loaded dipole with large diameter coils.. Well, it'd be easy enough to try and see, wouldn't it? A bit of wire, a plastic barrel and maybe a bit of pipe and you could tinker with it and see how it compares to some fairly simple standard like a 3M wire with a simple ground.
Daniel