This is good Dave. Those charts at the beginning look useful. I found this out years ago when I had a long wire, roller coaster and variable capacitor to tune using an r.f. pickup meter at the aerial. When I made my resistive SWR meter I did find the maximum r.f.out did not necessarily coincide with the lowest SWR. A tiny fraction of an increase and won't be even noticeable on a distant receiver. My attic dipoles are pretty good and can get below 1.5:1 on most frequencies so brickwork and only about 26 feet above ground I guess gets them close to 50 Ω. G4GHB
Many thanks for the clarification! It is what it is. But an impedance transformation network can help in case the antenna impedance is too far from 50 Ohms.
Thank you for the explanations. I have a few questions. When the reactance is tens of Khz from the lowest SWR is it worthwhile to work toward adding some coil / capacitor combination to set the reactance to zero on the frequency of lowest SWR? IF so, what calculations should be done to accomplish that? Maybe you've guessed I have an interest in QRP. Thank you again for your time.
Hi Dave, I'm a bit unclear what the resonant frequency of the antenna means here. In another video you said it means when the reactive part of the impedance is zero. Here you suggest it means at a half wavelength size even if there is some reactance.
Hi Dave, do you perhaps have a link to the slides you're reading from? I've checked your website but can only find the (excellent) presentation video. The slides themselves would be super helpful!
1. Would the tuning strategy change for a listening-only antenna? Tune for resonance in that case? 2. Could you get resonance & SWR to align better by changing the antenna height, or perhaps just raising the center some amount (like an inverted V), to adjust the impedance?
I can answer #2 because I went through this. When erecting my homebuilt 20/40 meter fan dipole (which is now a 20/40/80 dipole with coax traps in the 80 meter legs for 20 meters, but that's a different story), impedance of this inverted V dipole 28 feet off the ground was too low. So yes, antennas that are closer to the ground (or large metallic objects) or that have high angle in the "V" will show lower impedance than antennas higher off the ground or that have less angle in the "V". My solution? Feed the thing slightly off center to raise the impedance to a more radio-friendly range.
I think this raises more questions than it answers, in my opinion
This is good Dave. Those charts at the beginning look useful.
I found this out years ago when I had a long wire, roller coaster and variable capacitor to tune using an r.f. pickup meter at the aerial.
When I made my resistive SWR meter I did find the maximum r.f.out did not necessarily coincide with the lowest SWR.
A tiny fraction of an increase and won't be even noticeable on a distant receiver.
My attic dipoles are pretty good and can get below 1.5:1 on most frequencies so brickwork and only about 26 feet above ground I guess gets them close to 50 Ω.
G4GHB
I noticed while operating on 2m while the antenna is resonant, the SWR is high like 4/1 or even 5/1
it gets more higher as power increased.
Many thanks for the clarification! It is what it is. But an impedance transformation network can help in case the antenna impedance is too far from 50 Ohms.
Thanks, Dave. Even after 30 years in the hobby, radio always amazes me. Great video explaining the often times unexplainable. 73, AC3HT
Thank you for the explanations. I have a few questions. When the reactance is tens of Khz from the lowest SWR is it worthwhile to work toward adding some coil / capacitor combination to set the reactance to zero on the frequency of lowest SWR? IF so, what calculations should be done to accomplish that?
Maybe you've guessed I have an interest in QRP.
Thank you again for your time.
Hi Dave, I'm a bit unclear what the resonant frequency of the antenna means here. In another video you said it means when the reactive part of the impedance is zero. Here you suggest it means at a half wavelength size even if there is some reactance.
Hi Dave, do you perhaps have a link to the slides you're reading from? I've checked your website but can only find the (excellent) presentation video. The slides themselves would be super helpful!
Tunes off.... ?
True dummy load is 1.1or 0
Resonant is not allways wht we think x close to 0
If you have an impedance load of 50+j0, your swr will be 1:1 by definition.
1. Would the tuning strategy change for a listening-only antenna? Tune for resonance in that case?
2. Could you get resonance & SWR to align better by changing the antenna height, or perhaps just raising the center some amount (like an inverted V), to adjust the impedance?
I can answer #2 because I went through this. When erecting my homebuilt 20/40 meter fan dipole (which is now a 20/40/80 dipole with coax traps in the 80 meter legs for 20 meters, but that's a different story), impedance of this inverted V dipole 28 feet off the ground was too low. So yes, antennas that are closer to the ground (or large metallic objects) or that have high angle in the "V" will show lower impedance than antennas higher off the ground or that have less angle in the "V". My solution? Feed the thing slightly off center to raise the impedance to a more radio-friendly range.
Another great video. Thanks Dave!
👍👍