[Harc] SWR VS Resonance

Wed Feb 3 15:13:57 CST 2021

Thanks Dan.

I don't think I have a meter placement issue, but some might have.  The "SWR Meter" reads the "SWR" of what is beyond it, not what is before it.

So if I put the SWR meter at the output of an "antenna tuner", it will (correctly) read the high SWR I thought I had "tuned out" using the tuner.  If placed between the "tuner" and the transmitter, it will read what the radio sees, as matched to 50 ohms (and a low SWR) by the "tuner".

Unless... Unless the radio has a built-in (built inside the radio) auto tuner, in which case, if placed at the output of the radio, the SWR meter will continue to correctly show a mismatched load.  The internal (inside the radio, before the built-in tuner) SWR meter in a radio with an internal auto tuner will read what the radio sees as matched by that internal tuner after a tune, or the mismatch if un-tuned or bypassed.

I like to picture boxes with labels and lines between boxes indicating coax from box to box.  I did not read the entire article and all the responses, but it seems the "issue" was with someone putting the SWR measuring device after the tuning device and complaining that they did not see the tuner do anything to the SWR.  Of course they didn't.  The SWR of the antenna system stayed the same.  The match to 50-ohms for the radio was actually accomplished, but the SWR meter didn't know it, because the SWR meter was after the tuner.  Make sense?

And, my post was primarily to clarify any misunderstanding about equating SWR and resonance of the antenna.  I state that in one sentence here:  Just because you have a resonant antenna doesn't mean you will have a low SWR.

I know that when we say "SWR" we mean vSWR, the ratio of instantaneous highest peak voltage to lowest at the nodes (nodes, which are formed by "reflections" caused by an impedance mismatch).

And I also wanted to state that an "antenna tuner" is only an impedance matching device (matchbox), that doesn't tune the antenna itself.

The matchbox tunes the impedance of what comes before it to what comes after it, radio to feed line, feed line to radio.  I think of it like I think of the output transformer of a tube-type audio amplifier, where the high impedance of the tube final is matched to the low impedance of the speaker.  In transistor audio gear, an output transformer is not needed because the output impedance of the transistor final amplifier is low (closer to the nominal 8 ohm speaker system).

In the case of impedance matching radio and antenna system, the mismatch is usually but not always that the load (antenna system) has a higher impedance than the source (the radio's 50 ohm antenna coax connection).

Impedance is a very important and very complex issue.  I used to know all the formulas and the why and how back in my early days of audio before I learned about radio.  The formulas have been overwritten by more data, but the principles are still there.  Maximum power transfer occurs only with matched impedance.

I find that I have made this reply longer than the original.  That's all for now...

Howard
--
KB6NN

On 2/3/2021 12:31 PM, Daniel Eaton via Harc wrote:
> Hi Howard:
> Check this out and let me know what you think. Do you have a VSWR meter placement issue?
> 
> This is a restatement of your paragraph 2.
> https://forums.qrz.com/index.php?threads/placement-of-swr-power-output-meter.608247/ <https://forums.qrz.com/index.php?threads/placement-of-swr-power-output-meter.608247/>
> 
> 
> 73,
> Dan KB6DE
> 
> 
> On Wed, Feb 3, 2021 at 11:31 AM Howard, KB6NN via Harc <harc at humboldt-arc.org <mailto:harc at humboldt-arc.org>> wrote:
> 
>     I have been trying to put into words why a dipole, resonant on a frequency we want to operate on may not show a 1:1 SWR.ï¿½ From experience I know that such an antenna does not always show a low SWR, and that the placement of the antenna is important (height above ground, nearby objects, etc.) in determining the SWR of an otherwise resonant antenna.
> 
>     Also important is the method of feeding the antenna and connecting it to the radio.ï¿½ I also have been saying that SWR, when measured at the transceiver, is an indication of the match between the radio and the antenna system, not [just] the antenna itself - the antenna system includes anything from the antenna jack on the back of the radio all the way up to and including the antenna itself.
> 
>     Another thing I say is that the matchboxes commonly called "antenna tuners" are matchboxes, not antenna tuners.ï¿½ They are impedance matching devices.ï¿½ If the radio wants a 50 ohm load, the matchbox makes sure that, whatever the actual impedance of the load (antenna system), a match to 50 ohms is obtained.
> 
>     I found an article that says it better than I can, and has examples: https://hamradioschool.com/good-swr-and-antenna-resonance/ <https://hamradioschool.com/good-swr-and-antenna-resonance/>
> 
>     Thanks for reading,
>     Howard
>     -- 
>     KB6NN
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