AA-600 Antenna Analyzer with 600 ohm ladder line

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WA7F, Apr 5, 2018.

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  1. K8JD

    K8JD Ham Member QRZ Page

    If you have a 600 Ohm system and a 50 Ohm analyzer I would think you need a 12:1 BalUn between them.
    BUT, you don't expect to have a 600 ohm load on the other end if it is a "NON RESONANT" antenna like a G5RV !
    Can't see how an analyzer would help. Maybe adjusting a tuner with an analyzer is one use.
     
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Actually it can help quite a bit. The SWR50 of something like a G5RV or ZS6BKW is well known and published so comparing actual field measurements to expected values can tell you quite a bit about whether the antenna and or matching lines are trimmed properly.

    Scalar and vector network analyzers including consumer versions like antenna analyzers(which focus on S11) have a defined system impedance but that does not limit their usefulness to measuring systems at only that impedance.
     
  3. W2WDX

    W2WDX Subscriber QRZ Page

    I can see how an analyzer is useful for measuring one parameter at one impedance, however using one does not tell you if the line is balanced and other factors for instance. I can see how you can measure an antenna like a G5RV, but using it on something like a doublet it seems that it would only glean useful and accurate information in relation to a system at 50 unbalanced. SWR on a balanced line is less important than issues like balance (equal current). I think this emphasis on SWR is more a byproduct of the use of coaxial over the years, where SWR is relevant to performance, than it being a real useful approach to open feeder performance measurement.

    The nature of open feeder is such that if you get a reading on an unbalanced analyzer, the question has to be asked where in the system that figure is relevant; at the test point, or the feed-point, or somewhere else. I would think that measuring an open wire line with an oscilloscope, measuring each leg separately on a 2ch scope with an "add" function, gleans more useful information as to current transfer and overall performance than the information from an analyzer designed for a different purpose. My understanding is if you have equal current of max amplitude and opposing phase on each leg you're golden, and all other factors are moot. Impedance is an issue dealt with by a balanced tuner and only as a means to match the impedance required by the transmitter. And when using the tuner, measuring current on the output of the tuner for maximum is all that is necessary on a truly balanced line.

    The reason I say this is that it seems to me that what people are trying to do is force the antenna system to be resonant at 50 ohms. Why? There is no magic about that impedance, especially if the antenna is being fed with open-line and a balanced tuner is used. Also it can be argued there is no magic about resonance either. Radiation is more important than resonance. The antenna system could be as efficient as it can be at any impedance, assuming the transmitter is matched to the impedance presented and the current is balanced on the line. And if the approach is to measure the parameters of the antenna system in-situ then sure the information is now known, but what does that really tell you?

    Measuring a G5RV is interesting and I can see how using an analyzer of this type could be useful in its construction. Especially since this type of antenna is designed to present a SWR50 unbalanced load to a coaxial feedline; exactly what these analyzers are designed to test. However, I would think that's about it.

    I have to say now that I could be totally wrong here. I'm just going on an intuitive gut feeling, so everything I just surmised could be wrong. And if it is please educate me and others. Tell me if I'm wrong that if you can match the feeder so the transmitter is transfering 100% of its rated power, and the tuner is showing 100% of the current at its output (subtracting losses of the tuner), and the line is balanced in terms of current and phase, then isn't the antenna system performing as good as it can in-situ? Why all the extraneous measurement?
     
    Last edited: Apr 5, 2018
  4. K7TRF

    K7TRF Premium Subscriber QRZ Page

    If the antenna is working as expected and the tuner isn't struggling to achieve a match there's no real reason for making analyzer measurements. It's the cases where the antenna doesn't seem to be performing as expected and or the tuner has trouble finding a good tuning solution or arcs or other situations where it's very handy to run an SWR sweep on the natural antenna without the tuner in line. As SWR curves are published for common antennas are can be modeled a field sweep of the SWR at the base of the balanced feed line can be very useful as a sanity check and to support troubleshooting.

    In the case of a new design like a doublet of arbitrary dimensions and or feed line lengths an SWR sweep without the tuner in line can again help and if you can get an actual complex impedance sweep (vector measurements) it can help determine things like whether a 1:1 or 4:1 balun is better suited to the antenna as built. Again modeling can tell you what to expect but an analyzer sweep can tell you what you've actually built and deployed which can be useful information.
     
  5. W2WDX

    W2WDX Subscriber QRZ Page

    OK ... I get that. However, what assurance is there that the measurement is accurate, considering these devices are design for a specific targeted impedance range with a specific type of load, that being unbalanced? Furthermore, assuming the measurement result is accurate, is knowing the complex impedance actually useful? I mean in a true doublet with open feeder and a balanced tuner does not require a balun. The "bal to un" occurs in the tuner (inductor) and the complex impedance is tuned out by the caps (simply put). This is the point of a balanced antenna system, isn't it? The ability to run an antenna that radiates well regardless of complex impedance, impedance or SWR.

    I can see this useful if the range of impedance falls outside the tuning range of the "matchbox", such as the case of an impedance below 40 ohms on a Johnson KW Matchbox. This is more of a hardware issue related to the tuner, and a not a true indication of the antenna performance. Additionally, it seems that for overall design and installation all these measurements seem extraneous and the measurements themselves seem to have the possibility of being inaccurate since the measuring device is designed for a different set of circumstances. Kinda like using a DMM where a VTVM is required. Or using the wrong probe on an oscilloscope. In those cases the measurement is questionable.

    Am I wrong in that analyzers of this type reference the "negative" side to ground, where in a balanced system the negative and positive should be referenced equally above and below ground (or close to). The question is does this have an effect on the measurement? The other point I ask, is this reciprocal; does the device impact the parameters of the system due to the unbalanced nature of the test device, leading to erroneous data? Does not anything physically attached to a system impart an effect on the overall? Or are these devices somehow magically neutral?

    I'm not arguing ... I'm trying to understand. I still have a lot to learn about antennas. :)
     
    Last edited: Apr 5, 2018
  6. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    I hope these analyzers can measure impedance. No reference to any specific impedance. They are calibrated at 50 ohms (open/short/load) but this is to establish a measurement plane (the point where the impedance needs to be measured). I also hope they can measure impedance over a very wide range. Ideally 0 to infinity would be nice:) I repaired and calibrated about 3 or 4 generations of HP vector network analyzers from about '78 until the late 90's and all of these could plot the impedance on a Smith Chart except for the earliest ones that required a plastic overlay on the CRT. During testing I could see the cursor move to any part of the Smith Chart from 9 o'clock (0 ) to 3 o'clock (infinity and anywhere in between. The standards were a 50 ohm load and air lines and line stretchers etc. As technology improved the measurement uncertainty improved like anything else. Now these so called SWR or Antenna analyzers for the most part are optimized right near 50 ohms as Zack mentioned earlier. Move off 50 and your uncertainty degraded pretty rapidly. When I first tried to use the old Palomar noise bridge I was very dissappointed. The dial was pretty much meaningless except you could tell if the Z was inductive or capacitive. Then you have the MFJ-259 (no suffix). What the heck does the "Resistance" meter indicate??? 73, Pete
     
  7. W2WDX

    W2WDX Subscriber QRZ Page

    That's exactly my point Pete. At least from an intuitive perspective with my limited knowledge. Data is only useful if there is a high confidence in it's accuracy. The old "sigma" thing in science. And if you do make decisions based on erroneous data, what is the result? Unknown. So the test could be pointless, and in fact lead to poor conclusions. I think simple approaches are more elegant, like simply measuring current and balance, all things that can be accomplished with a high amount of assurance; a high sigma.

    Now if these analyzers do indeed give accurate figures in this application, that being measurements made outside the targeted plane ... well that's cool. But does anyone really know for sure that they do?

    Don't get me wrong. I own an AA-600 and use it often. However, I only do so on systems where measurement is made within the specified target plane of the analyzer.
     
    Last edited: Apr 6, 2018
  8. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Most ham grade antenna analyzers are battery powered and float with respect to any external ground. If you keep your hands off such an analyzer during measurement to minimize capacitive body coupling they have no reference to any external systems including ground and read very accurately regardless of whether the load is balanced or unbalanced.

    True, but the accuracy of typical antenna analyzers is pretty good across a range from SWR50 of 1:1 up to around 25:1 and both real and reactive impedances in from around 5 ohms to around 600 ohms though their accuracy is best across a somewhat more limited range as in 15 to 400 or 20 to 200 ohms depending on the quality of the analyzer in question. It's very easy to validate the range over which they're reasonably accurate by testing against known resistive or reactive loads just as you might validate an ohm meter using known value resistors.

    No test instrument is 100% accurate across an infinite measurement range but that doesn't typically diminish the usefulness of test equipment. The trick is to know the capabilities of the instrument and use it accordingly. To suggest antenna analyzers might not be accurate enough to measure something like a typical ladder line fed doublet and thus not use them at all seems like an all or nothing approach to test and measurement.

    I've used my RigExpert and MFJ analyzers many times on antenna systems fed by coax and by balanced feed lines and they've helped quite a bit during the tuning or troubleshooting of various antenna systems. They're typically far more accurate than most SWR meters and give information like complex impedances which can be very handy when subsequently designing matching networks like L networks or even simple Beta match, shunt inductor matching. SWR alone can leave out some useful information when it comes to matching. For instance you might see a 2:1 SWR but if you don't know what the impedance is you can't really know if a shunt inductor would improve or degrade the matching, an analyzer can tell you that.
     
  9. W5DXP

    W5DXP Ham Member QRZ Page

    At system resonance in a standing wave antenna system, current is at a maximum value which results in radiation being at a maximum value. A Z0-match to 50+j0 ohms is close to a conjugate match in a low-loss antenna system and also close to maximum available current at the antenna feedpoint.
     
  10. W2WDX

    W2WDX Subscriber QRZ Page

    See here's the thing. I don't care about SWR on balanced systems, especially on HF. It's a non-determinative factor on antenna system performance. On unbalanced systems this is an important consideration, especially at higher frequencies. And even on my AA-600 the calibration should be done at specific resistive impedances, 25, 50, 75 or 100. Anything outside those measurement planes are uncalibrated measurements.

    As far as match goes on an unbalanced system, that's a factor of designing a device with a wide inherent range to begin with. One of the reasons I mentioned the Johnson KW Matchbox, which has a known deficiency at very low impedances. This is all independant of the antenna/feedline system. Since SWR is a simple ratio, it's easy to determine the range for the device involved in matching impedance, especially for a balanced system like a true multi-band doublet. And as far as tuning out the complex parameters, again it's a matter of range. The balanced system's parameters are not fixed, nor should the devices used to tune it. It must be variable in a doublet. So using anything with a fixed value is less than optimal anyway, such as a balun.

    Bear in mind all of this is constrained by the fact we are talking about a balanced doublet in this case. A situation where complex impedance is all over the place anyway if multi-band operation is the purpose. The matching devices must be designed to have a very broad range as a matter of necessity, independent of the actual parameters of the antenna. If we are talking about a simple dipole tuned for a specific frequency of operation, then sure, an analyzer will give you good information, especially if the dipole is cut to length and configured as an inverted-V for 50 ohm coax, or flat-top for 75 ohm coaxial (given a proper environment).

    I am glad to know that you feel these analyzers are accurate, however once complex impedances are presented outside the calibrated resistive impedance reference plane, I do not have that level of confidence.
     
    Last edited: Apr 6, 2018

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