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Antenna Analyze, or System Analyze?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by CHUCKSTEIN, Oct 8, 2021.

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  1. K9AXN

    K9AXN Premium Subscriber QRZ Page

    Good morning Richard,
    #1: I did not mention or refer to the efficiency of the ideal antenna in free space. I am interested in the physical parameters of the ideal antenna referred to; diameter of radiator specifically.

    #2: Also, is it the often referred to antenna "The impedance of an infinitely thin 1/2wl wire in free space"?

    Thanks for the response and have a good day
    Regards Jim K9AXN
  2. KE0NSK

    KE0NSK Ham Member QRZ Page

    You are attempting to answer a feedline question, yet won't even acknowledge the feedline. Feedlines and antennas are not the same. You know this. You have either failed to comprehend walt's statement, or you are playing games because you can't justify your beliefs. Either way, it's still not my problem.


    Up on gmrs, 2.5 to 6w, there is no feedline, antenna is directly attached to meter. A 6" line from Tx to meter, all sma, all new stuff.
    On 2.5w the meter shows under 3 swr , but just change power to 6w and swr shows above 5. The only change was Tx power, all else (connections, physical position, etc) stayed the same.

    I know whips are a pita w/o having a good ground plane, but I don't understand (yet) why I get higher swr simply by changing Tx power. There's plenty of online chatter about same subject, but in most cases it had something to do with a bad connection or spurs from the Tx. In same chatter some talk about the diodes used in the meter, but this MFJ is being used within its' specifications. The Tx in my case are handhelds. So perhaps it's just an issue with the Tx when upping the power.
  4. WE4E

    WE4E Premium Subscriber QRZ Page

    Both of these are easily suspect.

    It's not uncommon at all for a transmitter to produce more harmonic and spurious content with an increase in output power. Driving the output stage closer to it's limits, closer to the max DC voltage available, saturating the device. If there's already an antenna mismatch, there will be power being rejected by the antenna system. If the ratio between the power being delivered to and rejected from the antenna changes, so will the SWR reading.

    Same is true with a typical meter. It's not a lab grade instrument, and the particular design and the particular parts in the particular meter you have in the particular application will have a tolerance on the readings it can provide, and although you're using the device in it's intended application, some variation is inevitable.
  5. W9XMT

    W9XMT Ham Member QRZ Page

    However, when that change is related to a new set of operating conditions such as an increase in the r-f bandwidth of the applied waveform, then the change seen in measured SWR may be useful -- even if the measured value is inaccurate.


    If the meter parts allow for wild swings in measurements, then what good is the meter?

    fwd and rev (rev = Tx - fwd) power (their ratio) defines the swr, yes? Why does this ratio change when I increase Tx power? Whatever mismatch there is in feed+antenna, is a constant (less heating and such). Tx=12 10fwd and 2rev on wire, is same swr as Tx=24 20fwd and 4rev on wire. Should be same swr unless the "rev" part of the equation is not a linear term, or some spurs and harmonics on output of Tx.

    I understand the basics, but what I saw was not my expectations, and I don't have a good enough scope to see what the signal looks like to find harmonics and such.

    I put antenna on VNA and see some #'s, fairly flat swr across UHF band, but then try to "analyze" the "system" with a Tx and mechanical swr meter, wildly different #'s.
    K9AXN likes this.
  7. WE4E

    WE4E Premium Subscriber QRZ Page

    Depends on what you're measuring and how critical you want to be. A Bird 43 is commonly thought of as a good standard rf meter. It's accuracy is listed as ±5% of full scale, so a 100w radio measured using a 100w slug, in an otherwise perfect world, could read 95 to 105 watts. Most applications don't require a calorimiter to measure power. Depends on whether you're measuring feet or inches.

    In the same perfect world, the ratio between forward and reflected power should not change. The change is often related to an increase in reflected power due to higher levels of harmonics or spurs, and as XMT pointed out, the bandwidth of the transmitted waveform if it encroaches on the bandwidth of the load. If you're measuring with a pure tone, the latter should not be the case.

    You'd need a spectrum analyzer to see them. Even a TinySA would likely have the resolution to show them. You have to take care to sufficiently isolate the signal source from the spec an's input stages so you don't create spurs in the mixer.

    Well sure, they're two different sources with potentially two different ground references. It's not uncommon at all to see a difference between a radio and a handheld analyzer.


    Which boils back down to my thread title, where to analyze it? If antenna on VNA looks ok, but then the system #'s look bad, do I remove the antenna from possibilities of being a component contributing to the bad #'s? If when mounted on a VNA it's not really the same as when mounted in the system, then why should I lean on VNA #'s? The comms really only work when put into the "system". Seems like a bad conundrum to me.
  9. W9IQ

    W9IQ Ham Member QRZ Page

    May I ask what models of GMRS radios you are using?

    - Glenn W9IQ
  10. W9IQ

    W9IQ Ham Member QRZ Page

    I don't understand the problem. I used the term "transmission line" at least three times in the post and the majority of the post related to the characteristic impedance of the transmission line. So to say that I didn't even "acknowledge the feedline" is disingenuous at best.

    I do fully understand Walt's statement, I have a very comprehensive understanding of transmission lines and I am certainly not "playing games". But if you don't want to discuss it, I will move on.

    - Glenn W9IQ

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