- March 9, 2022 at 5:16 pm #119342MarkModerator
Total posts : 711
From Carl Blare’s blog I thought may get some discussion here….
Is there a relationship between a wire cut to wavelength and a length of wire wound on a loop antenna to reach resonance?
I think yes as the end result is common to both methods.
Any other ideas?
March 10, 2022 at 12:21 am #119346RichParticipant
- This topic was modified 2 months, 2 weeks ago by Mark.
Total posts : 206
RE: Is there a relationship between a wire cut to wavelength and a length of wire wound on a loop antenna to reach resonance?
There is a relationship as far as the reactive terms of their feedpoint terminal impedance both approaching “resonance” (jX = 0) at some given lengths of straight or coiled transmit antenna conductors, on a given frequency.
However the most important facts in the performance of a transmit antenna depend on its radiation resistance compared to its system losses, which is based on its dimensions, construction, installation, and its “aperture” exposed to free space.
As an example, a legal, 3-m-tall, Part 15 AM vertical monopole can be resonated at ~1600 kHz, but its radiation EFFICIENCY can be less than 1%.
If a vertical monopole having a physical height of 1/4-wavelength was used in that same situation, its naturally resonant (jX =0) length would have a radiation resistance of ~37 Ω, and its radiation efficiency can be 90% or better.March 18, 2022 at 5:02 pm #119366MarkModerator
Total posts : 711
From Carl’s blog on this topic he was really asking, as it was misunderstood, and he cleared it up as to what the real question is.
Carl asked “I was looking for a comparison between the lengths of wire to reach resonance. For example, if the long-wire approach was 100′ long, then how long would the wire for the loop version be? I still have no idea.”
The answer, or more importantly my opinion, A loop or coil depends on the number of windings and the diameter of the loop to get resonance at a given frequency in the MW band. If you look at the loop antennas used in tube radios made in the 20s 30s 40s 50s the diameter was about 4 to 6 inches usually oval shaped and about as I remember maybe 20 to 25 windings of wire which if unstretched would be no where near the length a single length of wire would have to be to have resonance at a frequency in the MW band. Going by the wave length.
I am looking now at my Grundig tunable AM loop antenna and the diameter is 9″ and the number of turns of wire is 28 turns. If I unstretched the wire it would be nowhere near the length of wire needed for resonance which would be over half a km at 530klz and at 1700klz would be 176 meters. The length of wire I’d have is 56 ft if I unravelled the wire from my tunable loop which is 17 meters roughly.
But maybe Rich can weigh in on this.March 18, 2022 at 10:28 pm #119368RichParticipant
Total posts : 206
RE: But maybe Rich can weigh in on this.
As usual in engineering matters, accurate answers to apparently simple questions can require a deep understanding of the Physics involved.
Correctly understanding these topics requires the careful study of appropriate antenna engineering textbooks, such as ANTENNAS For All Applications 3rd Edition (McGraw-Hill), by John D. Kraus.
One point that will emerge from doing so: there is much more involved in the performance of a multi-turn loop antenna used on a given frequency/band than the number of loops it contains.
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