Hallo Barry,
looking at the phase plot you see the phase of the complex reflection faktor and not the phase of impedance Z. Having a perfect match at a certain frequency, the reflection factor is going to 0 and the phase of it can not be determind. As you can see from smith chart, the curve, coming from lower frequencies, hit the center of the chart nearly in a right angel to the real axis. In the phase chart you can see this behavior in a jump of phase from -180¡ã to +180¡ã.
73, Guenter, dk5dn
Am 13.12.2020 um 15:45 schrieb Barry Feierman:
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Looking at the PHASE data on a single band 80m center fed dipole.
See the two attachments: one without antenna tuner, one with an antenna tunder
I've had so much fun and have learned so much about antenna parameters with this little Nano VNA device.
One of the parameters that I was not familiar with is to look at the PHASE data, say on an 80m dipole: a 130 ft long center-fed dipole with about 150 feet of low-loss RG213 50-ohm coax.
The antenna on 80m had a natural resonance around 3580 kHz (where I like to operate digi modes on NBEMS nets).
Remember at resonance, the reactance is zero (regardless of the SWR). The PHASE is zero degrees.
But that antenna had an awful SWR at the top of the band, where the phone nets play around 3990 kHz.
At 3990 kHz, the SWR was more like 4:1 and my rig was not happy.
So, how do I QSY to the top of the band with decent SWR without cutting 10 feet off my dipole?
Well, with an antenna "tuner" in the shack, I can null out the reactance of the antenna at 3990 kHz, and get an almost 1:1 SWR on the coax going to the transmitter.
My rig is "happy" at 3990 kHz with that low SWR. But the SWR is still 4:1 on my feed line.
The SWR is only reduced on the coax between the tuner and the rig.
How would a PHASE plot look for my antenna AFTER the RF passes thru my antenna tuner (T network)?
Well, a PHASE plot shows me the original resonance of the 80m dipole around 3600 kHz (without the tuner) and the SWR and RETURN LOSS graphs verify this as well. Now, post antenna tuner, matched to the TOP of the 75m band, there is a second NEW resonance at around 3900 kHz post antenna tuner. Thus, the tuner RE-REFLECTS the reflected wave at 3900 kHz back in the direction of the antenna. Something W2DU Walt Maxwell claimed was how it worked.
Fascinating stuff. The tuner (in my shack) provides a "conjugate" impedance to offset the reactance at the feed line/antenna boundary. Thus an impedance of R + jX (in my shack) combines with an impedance of R - jX in the tuner ==> resulting in an impedance of R + j0 for the radio, with R about 50 ohms. Beautiful.
And, my low-loss RG213 feed line at 4 MHz, even with a 4:1 SWR, has less than a 1 dB "loss" due to the 4:1 SWR.
A 1 dB loss is barely noticeable.
So why worry about broad-band 80m antennas (gazillion articles in QST)?
Just put a decent tuner in your shack and QSY where you want with very little attenuation in low-loss cable (on 80m).
There is also very little "loss" in the tuner itself with decent size inductors and capacitors.
So cool to see all of this concept verified on the NANO VNA PHASE plot using VNA SAVER software (Win 10).
de Barry k3eui
Phila
schweppe