Ask Michael, KB9VBR: Help me understand R and X values on an antenna analyzer
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- Опубликовано: 5 фев 2025
- We frequently talk about reactance and resonance when describing antenna efficiency. But what does it mean? Dave and I answer that viewer's question.
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Ok guys. Z= impedance. R = resistance and X= recatance. It should be remembered that the Rig Expert measures the values at the connector on the device by default. The R and X values at the antenna end and the RigExpert end of the coax are different. If one connects an antenna with a piece of coax then one must recalibrate the RigExpert using open, short, load standards (OSL) placed at the far end of the coax. Now you will measure Z, X, and R at the antenna. The nominal impedence, 50 ohms, is only need to calculate SWR. Resonance occurs when X =0. The impedance of a vertical with a good ground system is about 35 ohms and dipole at 1/4 lamda height is about 72 ohms. For short verticals the capacitive reactance is canceled out by adding an inductor (coil) to achieve resonance (X=0).
Good summation. A key point that's often confused is that In the presence of standing waves (i.e., mismatch load), a transmission line (e.g., coax) behaves like an impedance transformer. Thus, at any given point along a transmission line Z will be different from Z at the load, with the exception of a transmission line point that's electrically 1/2 wavelength or a multiple thereof in length from the load.
We know 0-1.5 swr is fine and 1.5-2 is ok. And 2+ should be worked on or watched, but what is a similar mindset on reactance? 0 being good, what range is fine and what range is ok and over what reactance should you stop and fix something? Obviously the relationship to swr comes into play, I am trying to “color” in a smith chart to show green ok operating areas, yellow acceptable areas and red, stop and fix something areas on the chart.
Appreciated this one. Thanks for the work.
Great video and topic!
Great job ! 75 ohm coax will work great on center fed mono band dipole. Just push the ATU on radio. Tune antenna for lowest reactance . even though SWR might climb like 1. 6 or so. it will work better than a tune of 1.1 and higher reactance. Let the ATU fix the 1.6 or so SWR. Their is 2 parts to reactence. capacitive and inductance. I just made a T2LT antenna for a friend. I tuned the center conductor for lowest X and tuned the coil for Z to close to 50 ohm. works super great. SWR worked out 1.2 to 1.3 in the end. And working skip during poor band conditions. I used a VNA to tune. and I calibrated at the end of feed line at antenna . You can do this with RigExpert as well. Takes your feed line out of play. 73
What about when the X shows ups as a negative number? Doing some experiments and impedance was 44.5 ohms swe was great about 1.2:1 RL was 25.8 dB. X was a large negative number
Lots of good info in comments. Doesn't the velocity factor of coax needs to be input to the rig expert to get true readings on all parameters? Most is 0.66 but i have Messi & Paolni that is 0.87 so just curious how much that plays into true readings .
Dipoles closer to the ground than a 1/4 wave see feed point impedance fall to nearer 50 ohms.
Your idea of running 75ohm cable to the rig (for a mono band dipole), would that work better (than with 50ohm coax to the dipole) when using a rig with an internal matching unit in it and matching to 75ohm at the SO239? If matched at 75, wouldn't that mean that there should be minimal losses in the coax?
Yes there will be minimal losses in the coax due to it matching with the antenna. One thing I neglected to say is that if you can cut your coax so that its total length is a half wave multiple, you will most likely have a near perfect 50 ohm impedance at the feed point. (This is an old CB radio trick, but that doesn't mean it won't work with the amateur bands, too).
Is this an instance of too much information? If hams will still pursue minimum SWR as a goal, is there a practical concern over Reactance and Resistance that can be mitigated to improve signal strength? Should Reactance meters replace SWR meters? Linda Blair is envious of my rate of cranial rotation at the moment.
So we all know that swr is ideal below a 1:2.1 what is the limit for the reactance?
Is it acceptable to note the reactance when swr is at 1:2.1 and note the reactance as the benchmark?
There are many R and X values that will give the same SWR. X=0 has the greatest efficiency.
This discussion begs the question then also, that while constructing antennas, (especially center fed dipoles) to target the 1.6 swr, or more precisely, ignore the swr and target the reactance, then load the antenna with a transformer to make the finals happy. Am I off base with this?
No you're not. When I am cutting a dipole antenna (or 1/4 wave vertical for that matter), I'll aim for resonance if at all possible. If I see on the analyzer that SWR is a little high, but reactance (Z) is close to zero, I'll stop and leave it be. Low SWR is good, and it is an easy concept to grab onto. But taking a more wholistic view of what reactance is and its relationship to resonance (and SWR for that matter) takes your antenna skills to a new level.
Is there a procedure for recalibrating a RigExpert analyzer?
Typically RigExpert meters don't require recalibration, but most models do have instructions in their manual on how to perform one.
Yes, there is for my AA230 it is described in the manual but it is the same as any VNA. You need an OSL (open, short, load) set. The Rig Expert will guide you on the order which is expected. The OSL standards are placed where you want to make a measurement (e.g. at the end of a piece of coax that will be attached to the antenna of interest). If you change the frequency range the unit will return to factory defaults. The default calibration places the reference point at connector on the RigExpert. Incidentally, the SWR measured at the RigExpert will be lower than that of the antenna due to cable losses. All these measurements can be made using a NanoVNA as well. The only difference is the user interface.
DX Engineering sells a set of OSL attachments for calibrating. Search “DX Engineering OSL Calibration Set” on their website and you’ll find it.
We know 0-1.5 swr is fine and 1.5-2 is ok. And 2+ should be worked on or watched, but what is a similar mindset on reactance? 0 being good, what range is fine and what range is ok and over what reactance should you stop and fix something? Obviously the relationship to swr comes into play, I am trying to “color” in a smith chart to show green ok operating areas, yellow acceptable areas and red, stop and fix something areas on the chart.