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I made a small yagi, with the driven element being a coil loaded dipole. What is the effect of coil Q in this situation?

The coils I built are 18 turns of 1.2mm enameled wire wound tightly with a calculated Q of about 400 on 14mhz. What benefit would I get from going to, say, a 100mm coil with thicker wire, with a Q of about 2000?

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    $\begingroup$ How much shorter than a half wavelength is the dipole - and how many Ohms of reactance is the coil giving? $\endgroup$ – tomnexus Nov 29 '19 at 2:38
  • $\begingroup$ Q is the ratio of the energy stored in a reactance to the energy dissipated; ideal reactance dissipates no energy, so it has an infinite Q. Q=400 is already a very good value; increasing it to 2000 would probably not have a discernible effect. $\endgroup$ – Brian K1LI Nov 29 '19 at 2:48
  • $\begingroup$ @tomnexus the dipole is 6m long (two 3m elements) instead of 10m long, for the 20M band, and the coil is placed at the center $\endgroup$ – hjf Nov 29 '19 at 3:30
  • $\begingroup$ @hjf yes as Brian said, a Q of 50, 400 or 2000 will probably not make a big difference to a yagi driven element that's about 2/3 of full length. RRad will be many Ohms, so a few Ohms in the coil won't matter. I could do the sums easily for a simple dipole, in a Yagi it's more complicated... If the other elements are full length, it probably matters even less. $\endgroup$ – tomnexus Nov 29 '19 at 10:49
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    $\begingroup$ @hjf yes as Brian said, a Q of 50, 400 or 2000 will probably not make a big difference to a yagi driven element that's about 2/3 of full length. RRad will be many Ohms, so a few Ohms in the coil won't matter. I could do the sums easily for a simple dipole, in a Yagi it's more complicated... If the other elements are full length, it probably matters even less. $\endgroup$ – tomnexus Nov 29 '19 at 10:49
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Increased Q generally decreases both the losses and the usable bandwidth.

You can expect:

  • The coil not to get as warm at the design frequency at a given power
  • Your usable 2:1 VSWR bandwidth to be narrower

Exactly how much is another matter.

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    $\begingroup$ But if the Q is really high (like with a magnetic loop) you might get a usable band witch of a few kHz at 14MHz, which would require constant tuning if you want to move around the band $\endgroup$ – Scott Earle Nov 29 '19 at 3:04

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