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Thread: Questions about Reactance

  1. #1

    Default Questions about Reactance

    Is using an antenna analyzer the only way to measure reactance in an antenna (X value)?

    I found this cheap Chinese Analyzer, but will it measure X, or just overall impedance (Z)? Has anyone tried using one of these (I think it's basically a more cheaply made Sark), and are they accurate?

  2. #2

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    Well, I guess it's a moot point now. I went ahead and bought the Sark.

    From what I've gathered, the only way to measure impedance/reactance other than an analyzer, is by using a signal generator, which I also don't have.

    Another question that I have, is why do so many Hams seem to use SWR meters only? I asked a neighbor who is a ham user if she had an antenna analyzer, and she said yes, but it turns out it's just a really nice and expensive SWR/Wattmeter - which cost her about the same as a MFJ-259c would cost! But from what I've been learning, SWR doesn't really mean much when matching an antenna - what you really need to know is impedance, and resonance, right?

    In my specific case, since I will be running strictly monoband, I want to try using a 1/4 shorted stub for antenna matching. But to set it up correctly, I need to know the impedance at resonance (If I'm understanding all of this correctly.). Once I know the antenna impedance, I can then properly calculate my stub distance and length values.

    One other question I have, is this: Is there any benefit to antenna matching, if your SWR is below 2:1 (assuming no tuner is involved)?

  3. #3

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    An SWR under 2:1 may be deceiving as transmission line loss could eat some of the reflected power giving an inaccurate result. 2:1 will keep your finals happy, however, that does not always mean power is efficiently transferred to the antenna. This is normally only a problem for VHF and up since most coax isn't too lossy at HF.

    If you monitor SWR while adjusting your matching section, you can get close. As for the stubs... a quarter-wave shorted stub has an infinite impedance (resembles open circuit) and will not help you in matching. For shorted stubs under a quarter wavelength, the input impedance will be inductive. For open ended stubs under a quarter wavelength, the reactance will be capacitive. For stubs RIGHT AT a quarter wavelength, shorted stubs resembles a parallel resonant circuit and open stubs resemble a series resonant circuit (no reactance in either case and they appear as an open circuit and short circuit respectively). If you message me privately with an email address, I can provide you with a pdf on the topic, however, i dare not post it here as I do not have permission to redistribute the literature and wish not to offend the forum (education is too important to copyright and I'm too broke to fear a lawsuit).

    Hams use SWR meters because antenna analyzers, until very recently, have been prohibitively expensive. Keep in mind that if you do obtain an analyzer, the coax will dramatically affect the impedance measurement. As it is often difficult to test the impedance at the feed point without coax (your body and ladder coupled to the antenna, etc etc) you will need to first use the analyzer to cut a coax at an exact electrical half wavelength or a multiple thereof to eliminate the impedance transformation effect of the coax. If done right, the only error in the measurement will be a result of coax loss and analyzer accuracy. Some analyzers have the ability to be calibrated with the coax attached using open, short, load methods...

    This is how I use an antenna analyzer to make a half wave coax. I first estimate a half wavelength using the documented velocity factor and cut it a bit long for the measurement. Then I connect one end to the analyzer using a T connector with a 50ohm dummy load also attached to the T (paralleling the dummy load and stub at the analyzer). The analyzer is then set to the frequency of interest, and, using a range of a few MHz, an SWR sweep is plotted. The far end of the stub is slowly trimmed off (ensuring the center conductor is not touching the shield braid) and re-scanned until the dip in swr is centered at the analyzer frequency. What is happening is at the frequency of interest, the open ended coax, being a half wavelength, appears as an open circuit (opposite to the quarter wavelength case), and the only thing the analyzer sees is the 50ohm dummy load when cut at the right length (an open circuit in parallel to a 50ohm dummy load is 50ohms). It appears as a perfect match. The coax then has its other connector put on the far end and used to connect the analyzer to the feed point (without the T connector and dummy load).

    Btw, If your analyzer reads something like 67+j33, the matching section should be designed to afford a conjugate match. What that means is if you took the transmitter end and terminated it into a dummy load and measured the impedance going into the matching section (antenna side looking down the coax), the reading should be 67-j33. Opposite signs mean reactance cancels out. Check out Iowa Hills Smith Chart software, its free to download and it allows you to graphically see how various components in the line affect the impedance, and you'll surely learn smith charts along the way and everything will suddenly "click".

    Hope this helps.
    73
    Brandon, KE0KOY
    Last edited by brandon lind; Wed 28th Nov 2018 at 02:56.

  4. #4

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    a tool that should be in everybody's box when dealing with antennas is a signal strength meter...nothing does a better job of telling you how well your antenna is working.

  5. #5

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    Quote Originally Posted by Obed View Post
    a tool that should be in everybody's box when dealing with antennas is a signal strength meter...nothing does a better job of telling you how well your antenna is working.
    A signal strength meter is great once you've matched the coax to the antenna and energy is efficiently transferred to it. It is very useful to see where your main lobes are and make fine tune adjustments in yagis etc.

    If a signal strength meter alone was used to attempt to match an antenna to a coax (as mobius1 is trying to do), im sure the radio will be fried from high swr long before a match is found. Signal strength does not give any indication of match quality (if the main lobe is out of reach) nor does it give any insight into the direction the adjustment needs to go. Trial and error with unknown swr is not very healthy for the radio.

    Edit: An apology to Obed. It is not my intention to put down your response regarding the signal strength meter. I just like giving useful, but more importantly, relevant advice. I wouldn't go on a car forum, click on a question regarding an oil filter question, then proceed to mention how useful a torque wrench is... that's just the sort of thing that makes forums absolutely useless. I merely commented as I did so that one might not be mislead into thinking that's the best tool for that particular job.
    Last edited by brandon lind; Wed 28th Nov 2018 at 17:01.

  6. #6

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    Thanks for the advice.

    In the case of my mobile setup, I would have about 2-3 feet of RG-58 between the antenna and the analyzer. I was actually wondering if that would throw off the reading.

    I'll have to look into making a 50 ohm dummy load.

  7. #7

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    you can measure the impedance through the coax as long as you account for the length of the coax. The ARRL has a program called TLW which will allow you to adjust for the coax, otherwise, a good old smith chart should do fine. if you told me what the reading was through the coax and told me the exact length of the cable and what type of cable it is, i could tell you a rough estimate of the actual feed point impedance.tlw example.png

    if you make a dummy load, make sure to use non-inductive resistors (avoid wire wound types) with the shortest leads possible and ensure the power dissipation of the resistors can handle the power you plan to put into them!
    Attached Files Attached Files
    Last edited by brandon lind; Thu 29th Nov 2018 at 01:31.

  8. #8

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    You can also obtain a close match without knowing the impedance values. If you set your analyzer to sweep and note the frequency of the dip in swr, you will know what direction to start trimming/adding to the whip. If the swr dip is lower in frequency than you desire, you simply need to trim a bit off the tip or otherwise make it shorter little by little until the dip matches the frequency you want. If the observed dip is higher in frequency than the frequency you are after, the whip is too short and it must either be lengthened somehow or have some type of inductive loading coil in the antenna. This method can be done with a simple swr meter. although you don't get the nice graphical display. In that situation, you need to get creative to find out which way to go (longer or sorter) and this can be done several ways. I personally like to attach an alligator clip with an inch or two of wire soldered to it on the antenna tip and re-scan. If the swr went down, you need to make it longer. if the swr goes up, you need to make it sorter. Ive even heard of some people using their hand to capacitively couple to the antenna as a means to determine direction, but that has risks and considerations with it.

  9. #9

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    I will hopefully have a more accurate measurement tomorrow. Honestly, I will have to solder on a cable to the NMO mount just for measurement, so it can really be any distance I choose as long as it clears the roof liner - 3 feet would easily clear it and give me room to connect the analyzer.

    I'm not sure if I mentioned it, but this will be for a 6 meter 1/4 wave antenna. Likely using both FM and SSB. I was inclined to try a stub match, because I heard it was good for monoband use.

    I don't really care to buy an antenna tuner since I have no plans of going with any other bands in this vehicle.

  10. #10

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    So I originally wanted to write a step by step instruction on how to measure and tune the antenna using the existing coax but had great difficulty uploading images. I made a youtube video demonstrating the process for you.

    https://www.youtube.com/watch?v=W4u9wbvaBlw

    Not sure if the forum has an issue with youtube links, if so, someone should inform me... Anyhow, hope it helps!

  11. #11

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    So you think that using an inductor is better than a shorted stub?

    Honestly, I was leaning towards using a stub mostly because it seemed easier to implement, and according to K0BG's site (https://k0bg.com/match.html), has greater bandwidth.

    Also, thanks a bunch for the awesome video. I will definitely put it to use.
    Last edited by Mobius1; Fri 30th Nov 2018 at 18:35.

  12. #12

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    To be honest, I don't have hands-on experience with stubs. I could, however, calculate the length of the stub and distance from the feed point based on published formulas if you gave me the impedance value at the antenna input. From what ive read, they work. My concern with them has always been a fear of common mode currents due to the break in the shield where the stub attaches. I would guess that a proper coax tee would be needed to mitigate the escape of common mode currents at the junction.

    The loading coil would not be very effective further down the coax for multiple reasons. One reason is the coax after it would transform the impedance, another is the leakage of radiation inside the vehicle and more common mode currents escaping (it would all need to be shielded like a tuner unit is). If the transmitter is 50ohm and the coax is 50ohm, the only mismatch is at the antenna feed point, making that the best place for matching. There is nothing gained by disturbing that nice 50ohm path to the antenna!

    Is the antenna a straight whip or is there already a coil at the base of it?

    EDIT: The ARRL Antenna Book states that stub length and distance may be calculated via SWR, but the load must be purely resistive for the formulas to work. In your case, this fact is unknown.
    Last edited by brandon lind; Fri 30th Nov 2018 at 18:52.

  13. #13

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    My antenna analyzer was supposed to get here this week, but sadly hasn't, so I haven't been able to do any measurements yet.

    Here is the antenna I went with: https://www.diamondantenna.net/m685snmo.html

    It's a 6m whip.

    I actually found a website that will do the stub calculation, including factoring in the velocity factor (http://www.arcticpeak.com/antennapag...tub_match.html).

    As far as the t-connector, I was looking at something like this: https://www.amazon.com/Dual-Cable-Co.../dp/B00CJ8M8SS

  14. #14

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    Looks like a good plan to me! All that remains is waiting on the analyzer. There is a good chance you would be able to simply install the antenna and make a minor length adjustment for minimum SWR and be just fine. I would still wait for the analyzer just to be sure though as it beats the "trim trim trim" method. Its always better to keep as much antenna as you can and match the impedance as trimming takes away from radiation resistance (the good stuff).

    Remember to account for the coax connectors and tee length when calculating the stubs. I doubt an inch error would hurt the SWR much at 6m but its better to keep it in mind.

    I am curious as to the model, frequency range and features of that analyzer. Does it do time-domain reflectometry (or connect to a computer)?

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