ad: w5yi


Discussion in 'Antennas, Feedlines, Towers & Rotors' started by WA6MHZ, Jan 4, 2012.

Thread Status:
Not open for further replies.
ad: L-HROutlet
ad: l-rl
ad: MessiPaoloni-1
ad: Left-2
ad: L-MFJ
ad: abrind-2
ad: L-Geochron
ad: HRDLLC-2
ad: Left-3
  1. W8JI

    W8JI Ham Member QRZ Page

    You should measure a real vehicle, with all the breaks and loops in current paths instead of a uniform thin screen, especially when the path length starts to get longer in terms of wavelength.
    Things get worse in a hurry.
  2. K1DNR

    K1DNR Ham Member QRZ Page

    I'm following this with a great deal of interest. Good thread.

    What I gather so far - (this is my understanding so far - corrections are quite welcome)

    1 - An assumption is often made that the vehicle itself is the primary and/or only source of ground losses.
    2 - Due to that assumption, another assumption is made in regard to ground losses inferred from feed point impedance measurements.
    3 - Actual measured impedance changes are more likely the result of capacitive effects of the mounting irt to the vehicle and have little to do with actual ground losses.
    4 - The vehicle is only an intermediate part between Earth and the antenna. Any calculation that assumes otherwise is inherently flawed.
    5 - Shape and size of the vehicle and mount location (usually/may generally) change capacitive effects on the antenna more than ground effects, which makes any generalization about ground loss relative to measured feed point impedance on a vehicle antenna a guess at best. (especially at low frequencies)
  3. K0BG

    K0BG Ham Member QRZ Page

    You're correct Charlie.

    If you believe in such things, the often quoted number for ground losses in a vehicle varies between 10Ω on 80, to about 2Ω on 10 meters. Charts depicting that data go back as far as the first Mobile Antenna Book from the ARRL. The truth lies elsewhere.

    It is not uncommon for the ground loss to be higher on say 40 meters, than it is on 80 meters. As mentioned here, it is the standing waves between the body of the vehicle, and the under under it that determines ground loss. This said...

    When you mount an antenna high on a vehicle, the coupling between the antenna, and the surface under the vehicle (the true ground plane as it were), is lessoned. Whether the overall efficiency is increased, remains to be seen unless (as Tom pointed out), you measure the field strength. And this leads to another problem.

    The reason I do not put much stock in antenna shootouts has more to do with changing climatic changes (and standing wave differences) than it does anything else. Anyone with a VNA or antenna analyzer can measure their antenna's input impedance over time, while parked over the same spot. One might argue that the field strength doesn't change, but that would be an incorrect assumption as it may indeed change (or maybe not). The same phenomena can be seen by looking at the reflective power while driving down any street.

    The other issue, already spoken of, is standing waves. It is indeed possible for any two comparative antenna installations, to have opposing field strength measurements when parked over any two given surfaces. Again, this is a result of standing waves between the vehicle, and the surface under it. I believe Tom will agree—I defy anyone to measure the standing waves with accuracy! Fact is, I don't think you can compute them either!
  4. W5DXP

    W5DXP Ham Member QRZ Page

    "Only" is an exclusive term but standing waves can exist in free space devoid of a feedline. Standing waves obviously exist on standing wave antennas. I once calculated the SWR on a 1/2WL dipole to be in the ballpark of 20:1, i.e. 90% of the current on a 1/2WL dipole is standing wave current. The current envelopes for a 1/2WL thin-wire dipole published by Kraus are obviously standing waves. Kraus' phase graphs for the standing waves on a 1/2WL dipole indicate that the phase shifts on a standing wave antenna cannot be used to calculate the delay through the wire (or through a loading coil) in a standing wave antenna and EZNEC simulations agree.

    Since the standing wave antenna is a distributed network and not a lumped circuit, the loading coil required to make the antenna resonant is the degrees of delay that will put the reflected wave in phase with the forward wave at the feedpoint.

    Actually, the feedpoint impedance for a 90 degree standing wave monopole is (Vfor-Vref)/(Ifor+Iref). Lots of things can happen to Vref and Iref on that round trip. For instance, does folding a 1/4WL monopole increase the radiation resistance by 4 times or does it merely decrease the magnitudes of Vref and Iref because the reflected wave has to travel 4 times the distance of a non-folded monopole?
  5. K0BG

    K0BG Ham Member QRZ Page

    This is wrong:
  6. G3TXQ

    G3TXQ Ham Member QRZ Page

    There you go, Cecil; someone took the bait!

    Steve G3TXQ
  7. W5DXP

    W5DXP Ham Member QRZ Page

    Actually, it cannot be any other way. A loaded shortened standing wave mobile antenna with a purely resistive feedpoint resistance must necessarily have the magnetic field of the reflected wave in phase with the forward wave at the feedpoint or else the feedpoint resistance would not be purely resistive. That means the forward wave must traverse 90 degrees on its way to the tip of the stinger and the reflected wave must traverse 90 degrees on its way back to the feedpoint. Because of the 180 degree phase shift at the tip of the stinger, the magnetic fields are in phase at the feedpoint. Of course, the electric fields are then 180 degrees out of phase which gives us a low purely resistive feedpoint resistance.

    The typical loaded mobile antenna is electrically (not physically) 90 degrees long. This is all explained in some articles on my web page:

    If one can understand this shortened stub, one can understand shortened loaded mobile antennas.

    Steve, if you could prove me wrong, wouldn't you have done it by now?:)
  8. G7DIE

    G7DIE Ham Member QRZ Page

    Better than wire, try tubing:


    It's what a hamstick was meant to be :D
  9. W8JI

    W8JI Ham Member QRZ Page

    There is a way to accurately use base impedance, or feedpoint impedance, to determine losses.

    We simply enclose the entire system in a highly conductive enclosure that is out in the far field of the antenna, and compare the feed resistance there with the feedpoint resistance without the dome.

    The dome stops radiation, so it forces radiation resistance to zero.

    .....or we can use a field strength meter.......
  10. KF5FEI

    KF5FEI Ham Member QRZ Page

    Heh. Looks like some hillbilly is missing part of his still. Good idea, though. Probably holds it shape better than smaller wire.
Thread Status:
Not open for further replies.

Share This Page