Resonant Frequency for a Coil-Only Antenna

  • #1
Summary:
To Decrease the Physical length of a Dipole antenna people use loading coils in series with antenna wire, But what if we use the coil(s) alone as our antenna as shown in figure, without any extra dipole length, what will be the Resonant Frequency of such an antenna ? will it be a function of wire length , coil inductance and coil capacitance ?
coil_antenna1.jpg
 

Answers and Replies

  • #2
Baluncore
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Quote; "will it be a function of wire length , coil inductance and coil capacitance ? "
Yes. The wire used to wind the coil has capacitance to itself and to the environment. It is like a leaky transmission line. The end capacitance will be important. You must specify the details and wavelength in order to know what will be important. Will you match the transmitter to the antenna coil? or just drive it with a fixed voltage RF.

Every inductor has multiple resonant frequencies for example the capacitance between the ends with the whole inductance, or the capacitance between two turns and the inductance of one turn, or every combination in between.

At the same time the wire used to wind the coil has a physical length and an average dielectric constant of the insulation. From that you can find a transmission line velocity factor, and determine the resonant frequency of the coil as a half wave dipole.
Coil self resonance is a "red herring" topic. If you engineer a rational resonant circuit with a capacitor, it will work better as an antenna than will a simple unterminated coil.
 
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  • #3
Quote; "will it be a function of wire length , coil inductance and coil capacitance ? "
Yes. The wire used to wind the coil has capacitance to itself and to the environment. It is like a leaky transmission line. The end capacitance will be important. You must specify the details and wavelength in order to know what will be important. Will you match the transmitter to the antenna coil? or just drive it with a fixed voltage RF.

Every inductor has multiple resonant frequencies for example the capacitance between the ends with the whole inductance, or the capacitance between two turns and the inductance of one turn, or every combination in between.

At the same time the wire used to wind the coil has a physical length and an average dielectric constant of the insulation. From that you can find a transmission line velocity factor, and determine the resonant frequency of the coil as a half wave dipole.
Coil self resonance is a "red herring" topic. If you engineer a rational resonant circuit with a capacitor, it will work better as an antenna than will a simple unterminated coil.
thank you for the suggestion, can you tell me a simple experiment to determine frequency of such a self resonant coil antenna ( in my case made out of iron wire without any insulation ) as a quarter or half wave dipole ?
 
  • #4
Baluncore
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can you tell me a simple experiment to determine frequency of such a self resonant coil antenna
Get an antenna analyser and test it with a swept RF signal. Look at the impedance.

With only air as the insulation the coil will be resonant when the wire is half a wavelength long. The antenna pattern will be that of a very short dipole.

Iron wire is too resistive due to skin effect, and so will not work well unless it is galvanised or plated. Any oxide on the wire will absorb moisture and lower the resonant frequency.

Avoid using simple coils as antennas. They are very inefficient.
 
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  • #5
Get an antenna analyser and test it with a swept RF signal. Look at the impedance.

With only air as the insulation the coil will be resonant when the wire is half a wavelength long. The antenna pattern will be that of a very short dipole.

Iron wire is too resistive due to skin effect, and so will not work well unless it is galvanised or plated. Any oxide on the wire will absorb moisture and lower the resonant frequency.

Avoid using simple coils as antennas. They are very inefficient.
Sir, I got your Point, Can you please answer another doubt I have in the same topic ? What if in the above Coil Antenna I add a series or a parallel capacitor (see figure below) ? will it help us to tune the frequency to a desired value ?
antenna2.jpg
 
  • #6
Baluncore
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will it help us to tune the frequency to a desired value ?
Yes. The parallel arrangement will become a resonant loop antenna. You need to drive it with a coupling loop. It will be efficient when it is tuned to the frequency of operation.
 
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  • #7
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Get an antenna analyser and test it with a swept RF signal. Look at the impedance.

With only air as the insulation the coil will be resonant when the wire is half a wavelength long. The antenna pattern will be that of a very short dipole.

Iron wire is too resistive due to skin effect, and so will not work well unless it is galvanised or plated. Any oxide on the wire will absorb moisture and lower the resonant frequency.

Avoid using simple coils as antennas. They are very inefficient.
They are often used at VHF and UHF and are usually called "rubber duckies".
800px-UHF_CB_with_rubber_ducky_exposed.jpg
 
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  • #8
Baluncore
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They are often used at VHF and UHF and are usually called "rubber duckies".
UHF handheld radios were more convenient because the antennas were smaller and more efficient.

A “rubber duckie” made it look like you had the latest UHF, when actually you had the previous generation VHF. Compact rubber covered antennas were stylish in black and didn't get tangled or bent when abused.

If you want compact convenience with HF or VHF radio you must compromise on efficiency. Any antenna will work where the range is short and there is excess power available.
 
  • #9
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However at 27 MHz frequently may be used coil antenna consisting of 6 mm elastic plastics rod with turned on 0.5 mm wire turns with gap between turns about 0.5 mm at lower third, about 2 mm in middle and 5 mm at top third. My antennoscope shows the perfectly active 50 Ohms with VSWR about 1.1 thus I believe it isnt very bad transmitter assymetric antenna for 25 W.
 
  • #10
Baluncore
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Welcome to PF.
How long was the wire?
How long was the rod?
How is it driven? Maybe from a coax as a λ/4 over a ground plane?

My antennoscope shows the perfectly active 50 Ohms with VSWR about 1.1 thus I believe it isnt very bad transmitter assymetric antenna for 25 W.
A low VSWR is not always good, it can be due to losses in the antenna or feedline.
Where is 25 W permitted on 27 MHz ? or is most of that lost in the antenna.
 
  • #11
tech99
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There is a tendency for monopole antennas to have 50 - 100 Ohms of loss resistance due to the ground, unless special measures are taken to avoid this. Such an antenna may be well matched to a 50 Ohm source but much of the power will be used warming the soil.
 
  • #12
Averagesupernova
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Not saying swr is something that should be ignored but there is so much made of getting the swr down as low as possible that people tend to forget it is only half the story, or even less. I went quite a few years as a young ham before I realized what all was involved in an adequate antenna system.
 
  • #13
Baluncore
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You cannot be a truly good person unless you have bad thoughts, and can resist them. Likewise, there is no advantage in a low VSWR antenna, unless you have a high VSWR feedline.
 
  • #14
tech99
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Sir, I got your Point, Can you please answer another doubt I have in the same topic ? What if in the above Coil Antenna I add a series or a parallel capacitor (see figure below) ? will it help us to tune the frequency to a desired value ?View attachment 288874
If you add a capacitor in parallel, as shown, you need a wire extending down the antenna to make the connection. This will carry a current opposing the antenna current. So to put it simply, it will reduce the radiation from the antenna.
 
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