Antenna Analyze, or System Analyze?

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

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  1. KE0NSK

    KE0NSK Ham Member QRZ Page

    Nobody is disputing post #48, it's just not relevant. The fact you believe it is puts us at a fundamental disagreement. I don't accept the responsibility of convincing random people of things they don't want to consider. I've been down that road and it's a waste of my time.

    You are free to analyze antennas and feedline separately, makes no difference to me.
  2. 2E0VSS

    2E0VSS Ham Member QRZ Page

    Id be very wary taking Glenn W9IQ on re transmission lines
    Hes a very sharp cooky and many "Hammy hams" except for the truly enlightened will get thier fingers badly burnt in doing so too, and rightly so!

  3. KE0NSK

    KE0NSK Ham Member QRZ Page

    I have no desire to take anyone on. I knew he was intelligent and knew a lot about antenna theory. I was hoping he had a solid argument against walt maxwells claim. He didn't have any answers that were directly related. Politics, religion, and antenna theory are things I don't debate on the internet. It benefits no one.
  4. 2E0VSS

    2E0VSS Ham Member QRZ Page

    You did Sir..



    Ok, thread is interesting, a bit left and right of the tracks, but that's ok, all good info.

    So, is there any reason SWR would change if the only thing changing was Tx power? I was testing a few uhf/vhf whips via a MFJ-842 and the only thing I change is Tx power, and the SWR changes quite a bit much higher. If all else remains exactly the same why would SWR change? Is there something not linear somewhere?
  6. NG1H

    NG1H XML Subscriber QRZ Page

    Your last sentence is a likely cause. From what I've seen a lot of affordable meters that can handle higher power are not accurate at low power. There are cases where it is a zebra(*) such as a damaged feedline/balun/antenna that allows arcing at higher power but that can be easily tested. Try different feedlines and different antennas. If you see the same thing it is the meter.

    (*) Ok, that one may be a little obscure. There a saying in medicine (and probably other fields): "When you see hoofprints, think horses, not zebras."
  7. K9AXN

    K9AXN Premium Subscriber QRZ Page

    Glen or anyone,

    "Then recall that the radiation resistance, Rr, of an ideal 1/4 wavelength antenna is ~35 ohms"

    I cannot drag that one from memory, maybe too long ago. It seems mixed up with the "Infinitely thin wire and 1/2wl and 1/4wl antennas". Are these referring to the same or different definitions? In either case, what are the parameters of the antennas? Thanks

    Regards Jim K9AXN
  8. W9XMT

    W9XMT Ham Member QRZ Page

    It wasn't too long ago that Glenn, W9IQ posted these accurate values (from Reply 51 of this thread):

    "We only need to look at the impedance of a few lossless, free space antennas to see that the feedpoint reactance of an antenna has nothing to do with the efficiency of the antenna.
    1. A lossless, freespace, center fed, 1/2 λ dipole has an impedance of 73 + j42.5 ohms
    2. A lossless, freespace, 1/4 λ monopole has an impedance of 36.5 + j21.25 ohms."
    Note that the R and j(X) of antenna system 2 listed above is exactly half that of system 1, as expected because the radiating aperture of system 2 is exactly half that of system 1.

    An edit to clarify the text of item 2 would address the point that the monopole needs to be driven against a perfect ground plane, which could not be true for a "freespace" environment.

    For technical support of this, consult Kraus' ANTENNAS for all applications, 3rd Edition.
  9. W9IQ

    W9IQ Ham Member QRZ Page

    Changing the transmitter power does not change the SWR of the antenna system. So if you are seeing changes in SWR, you need to look elsewhere.

    Many SWR meters are very simple devices that can introduce errors due to the diodes that are used in the meter, poor construction techniques in the meters, or poor calibration of the meter. Most of these issues become more problematic at VHF and UHF frequencies. MFJ is frequently cited for marginal construction quality.

    If you have common mode current on the coax, this can result in changing SWR values since the coax is then acting as part of the antenna. Sometimes you can detect common mode current by sliding your hand along the coax to see if the SWR or power readings change. If they do, you probably are in need of a common mode choke or two. If not, the test is not conclusive. If the whips that you are testing have no "ground plane" then you are almost certainly experiencing some common mode current issues.

    Defective cables and connectors can cause all sorts of strange issues. Even a fraction of an ohm due to a marginal solder joint/crimp connection or a loose connector can wreak havoc.

    How much of a change in SWR are you experiencing? How much of a power change causes this change in SWR?

    - Glenn W9IQ
  10. W9IQ

    W9IQ Ham Member QRZ Page

    I thought I presented a solid engineering description of Walt's error but I obviously didn't do a good enough job to satisfy you. If you can tell me where my description doesn't "click" for you, I would be happy to try a different approach. Would it be more helpful if I avoided the use of complex numbers or would you rather see more complete calculations?

    - Glenn W9IQ
    Last edited: Oct 14, 2021

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