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Will transmitting on this antenna cause problems?

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1

$begingroup$

Inspired by this discussion, I wanted to check if this is something I can get away with.

I've got an old FM Broadcast antenna that came with a home stereo receiver -- it's 75 ohm flat cable, in a T shape (not sure if it's a loop or folded dipole, but I'll check -- let's assume, for this question, that it's a folded dipole). It's close to resonant length for the 3m approximate center of the FM Broadcast band.

I've read that antennae often work well if they're on a third harmonic of the transmit frequency. This antenna is close to a third harmonic of the upper end of the 10m ham band (a little short, but not much).

I currently have it connected to a Heathkit SB-102; I was using it to test the receive functions. It occurred to me that I might be able to transmit meaningfully (possibly at reduced power) on the highest 10m segment supported by this transceiver (29.5-29.8 MHz), or, if there's enough matching capability, on the rest of 10m band. I'm concerned, however, not to damage anything while trying to adjust the transceiver's matching network.

Is that a realistic concern?

antenna-theory

share|improve this question

asked 8 hours ago

Zeiss IkonZeiss Ikon

1,29811 silver badge1616 bronze badges

$endgroup$

add a comment | 

1

$begingroup$

Inspired by this discussion, I wanted to check if this is something I can get away with.

I've got an old FM Broadcast antenna that came with a home stereo receiver -- it's 75 ohm flat cable, in a T shape (not sure if it's a loop or folded dipole, but I'll check -- let's assume, for this question, that it's a folded dipole). It's close to resonant length for the 3m approximate center of the FM Broadcast band.

I've read that antennae often work well if they're on a third harmonic of the transmit frequency. This antenna is close to a third harmonic of the upper end of the 10m ham band (a little short, but not much).

I currently have it connected to a Heathkit SB-102; I was using it to test the receive functions. It occurred to me that I might be able to transmit meaningfully (possibly at reduced power) on the highest 10m segment supported by this transceiver (29.5-29.8 MHz), or, if there's enough matching capability, on the rest of 10m band. I'm concerned, however, not to damage anything while trying to adjust the transceiver's matching network.

Is that a realistic concern?

antenna-theory

share|improve this question

asked 8 hours ago

Zeiss IkonZeiss Ikon

1,29811 silver badge1616 bronze badges

$endgroup$

add a comment | 

$begingroup$

Inspired by this discussion, I wanted to check if this is something I can get away with.

I've got an old FM Broadcast antenna that came with a home stereo receiver -- it's 75 ohm flat cable, in a T shape (not sure if it's a loop or folded dipole, but I'll check -- let's assume, for this question, that it's a folded dipole). It's close to resonant length for the 3m approximate center of the FM Broadcast band.

I've read that antennae often work well if they're on a third harmonic of the transmit frequency. This antenna is close to a third harmonic of the upper end of the 10m ham band (a little short, but not much).

I currently have it connected to a Heathkit SB-102; I was using it to test the receive functions. It occurred to me that I might be able to transmit meaningfully (possibly at reduced power) on the highest 10m segment supported by this transceiver (29.5-29.8 MHz), or, if there's enough matching capability, on the rest of 10m band. I'm concerned, however, not to damage anything while trying to adjust the transceiver's matching network.

Is that a realistic concern?

antenna-theory

share|improve this question

asked 8 hours ago

Zeiss IkonZeiss Ikon

1,29811 silver badge1616 bronze badges

$endgroup$

share|improve this question

asked 8 hours ago

Zeiss IkonZeiss Ikon

1,29811 silver badge1616 bronze badges

share|improve this question

asked 8 hours ago

Zeiss IkonZeiss Ikon

1,29811 silver badge1616 bronze badges

asked 8 hours ago

Zeiss IkonZeiss Ikon

1,29811 silver badge1616 bronze badges

asked 8 hours ago

Zeiss IkonZeiss Ikon

1,29811 silver badge1616 bronze badges

1 Answer
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6

$begingroup$

It won't work very well.

antennas often work well if they're on a third harmonic of the transmit frequency

This is calculating the third harmonic the wrong way around. A better statement would be something like

dipole antennas often work well at the third harmonic of the design frequency

A simple dipole antenna like an inverted-V, designed for 10 MHz will probably work at 30 MHz. The radiation pattern will be broken up into three beams, not one, but that doesn't matter much at HF. It might even help, as the middle lobe has a higher gain than a dipole.

But it doesn't work the other way around. An antenna designed for 100 MHz will be extremely short for 30 MHz, and hardly radiate at all.

I'm not certain that a folded dipole will resonate nicely again at three times the design frequency, because of the interplay of the transmission line and antenna modes. This needs to be simulated to be sure.

share|improve this answer

answered 7 hours ago

tomnexustomnexus

2,93577 silver badges1515 bronze badges

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Sigh. I knew it couldn't be that easy...
  $endgroup$
  – Zeiss Ikon
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  @ZeissIkon Tom answered nicely. Sub-harmonics are what you are thinking of; unfortunately, they don't exist.
  $endgroup$
  – Mike Waters♦
  4 hours ago
  

  
  
  
  

add a comment | 

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6

$begingroup$

It won't work very well.

antennas often work well if they're on a third harmonic of the transmit frequency

This is calculating the third harmonic the wrong way around. A better statement would be something like

dipole antennas often work well at the third harmonic of the design frequency

A simple dipole antenna like an inverted-V, designed for 10 MHz will probably work at 30 MHz. The radiation pattern will be broken up into three beams, not one, but that doesn't matter much at HF. It might even help, as the middle lobe has a higher gain than a dipole.

But it doesn't work the other way around. An antenna designed for 100 MHz will be extremely short for 30 MHz, and hardly radiate at all.

I'm not certain that a folded dipole will resonate nicely again at three times the design frequency, because of the interplay of the transmission line and antenna modes. This needs to be simulated to be sure.

share|improve this answer

answered 7 hours ago

tomnexustomnexus

2,93577 silver badges1515 bronze badges

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Sigh. I knew it couldn't be that easy...
  $endgroup$
  – Zeiss Ikon
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  @ZeissIkon Tom answered nicely. Sub-harmonics are what you are thinking of; unfortunately, they don't exist.
  $endgroup$
  – Mike Waters♦
  4 hours ago
  

  
  
  
  

add a comment | 

6

$begingroup$

It won't work very well.

antennas often work well if they're on a third harmonic of the transmit frequency

This is calculating the third harmonic the wrong way around. A better statement would be something like

dipole antennas often work well at the third harmonic of the design frequency

A simple dipole antenna like an inverted-V, designed for 10 MHz will probably work at 30 MHz. The radiation pattern will be broken up into three beams, not one, but that doesn't matter much at HF. It might even help, as the middle lobe has a higher gain than a dipole.

But it doesn't work the other way around. An antenna designed for 100 MHz will be extremely short for 30 MHz, and hardly radiate at all.

I'm not certain that a folded dipole will resonate nicely again at three times the design frequency, because of the interplay of the transmission line and antenna modes. This needs to be simulated to be sure.

share|improve this answer

answered 7 hours ago

tomnexustomnexus

2,93577 silver badges1515 bronze badges

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Sigh. I knew it couldn't be that easy...
  $endgroup$
  – Zeiss Ikon
  4 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  @ZeissIkon Tom answered nicely. Sub-harmonics are what you are thinking of; unfortunately, they don't exist.
  $endgroup$
  – Mike Waters♦
  4 hours ago
  

  
  
  
  

add a comment | 

$begingroup$

It won't work very well.

antennas often work well if they're on a third harmonic of the transmit frequency

This is calculating the third harmonic the wrong way around. A better statement would be something like

dipole antennas often work well at the third harmonic of the design frequency

A simple dipole antenna like an inverted-V, designed for 10 MHz will probably work at 30 MHz. The radiation pattern will be broken up into three beams, not one, but that doesn't matter much at HF. It might even help, as the middle lobe has a higher gain than a dipole.

But it doesn't work the other way around. An antenna designed for 100 MHz will be extremely short for 30 MHz, and hardly radiate at all.

I'm not certain that a folded dipole will resonate nicely again at three times the design frequency, because of the interplay of the transmission line and antenna modes. This needs to be simulated to be sure.

share|improve this answer

answered 7 hours ago

tomnexustomnexus

2,93577 silver badges1515 bronze badges

$endgroup$

share|improve this answer

answered 7 hours ago

tomnexustomnexus

2,93577 silver badges1515 bronze badges

answered 7 hours ago

tomnexustomnexus

2,93577 silver badges1515 bronze badges

answered 7 hours ago

tomnexustomnexus

2,93577 silver badges1515 bronze badges