ad: AlphaRF-1

AA-600 Antenna Analyzer with 600 ohm ladder line

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

ad: L-HROutlet
ad: l-rl
ad: Subscribe
ad: L-MFJ
ad: Left-3
ad: Left-2
ad: abrind-2
  1. W2WDX

    W2WDX Premium Subscriber QRZ Page

    Yes ... when the required impedance of the transmitter is 50 ohms. However, in a multi-band doublet with 600 ohm open feedline, there is a need to have a variable match capability to assure the transmitter is satisfied. The impedance of the doublet may have wildly different impedance than 50 ohms, either inductive or capacitive in nature and in fact may not be resonant. However, it will radiate fine regardless of any arbitrarily set impedance, like 50 ohms. There is nothing magical about 50 ohms. It depends on the overall system in question.

    Like a 134" doublet fed with 600 ohm feeder, the impedances the tuner will "see" will be anywhere from ~20 to 3000 ohms +/- some amount of reactance.

    And I am not sure what you mean by a "standing wave antenna system". I know some automatic ATU and SWR meter manufacturers have used that strange terminology in their manuals, but I still don't know what it means or what it refers to.
    Last edited: Apr 6, 2018
  2. W5DXP

    W5DXP Ham Member QRZ Page

    Yes, and when the tuner is adjusted for a Z0-match to 50 ohms at its input, it comes close to resonating the entire system if the system is low loss.

    There are standing wave antenna systems which we tune to resonance, a dipole is an example. It has standing waves on the antenna. If someone measured the SWR on a resonant dipole, it would be in the ballpark of 20:1.

    There are traveling wave antenna systems which don't require tuning to resonance, a terminated rhombic is an example. There are negligible reflected waves on a traveling wave antenna system.
  3. W2WDX

    W2WDX Premium Subscriber QRZ Page

    Am I missing something, Cecil? 20:1 ratio on what, the antenna itself? Between the two elements? Where is that ratio derived from? I always thought a flat-top dipole tuned to resonance in free space would have a theoretical feed-point impedance of somewhere around 72 ohms, or there about. That wouldn't be a 20:1 ratio with common coaxial feedlines. Are you saying with 600 ohm open feed? Sure that would be closer at ~9:1, but 20:1? Not sure what your point is here or am I misunderstanding?

    Obviously, we are talking about measuring a specific antenna type; a non-resonant doublet with 600 ohm open-feed of a proper length. So of course you are going to tune/match at the transmitter to achieve the Zo-match to 50 ohms for the transmitter (if it requires that, not all do); notwithstanding, resonance of the antenna itself in isolation. In this case the doublet, feedline and tuner are all one entity. That being the antenna; with one part (doublet) doing the radiating. The overall system is in resonance, while the doublet in isolation may not be. Resonance is not a prerequisite for propagating a signal, and resonance is more an issue of efficient current transfer across a system. SWR is only a measure of a ratio of impedances at a given point, and also has little to do with propagation of the radiating elements. Both element resonance nor low SWR are prerequisites for efficient propagation. This is why doublets work well and are generally considered efficient, given the right environment.

    BTW for reference, I use the term "doublet" to mean: A dipole of random length, fed with open-wire feeder used with a balanced tuner.
    Last edited: Apr 6, 2018
  4. W5DXP

    W5DXP Ham Member QRZ Page

    How about a brainteaser? Here's a graphic from EZNEC of a center-fed dipole at 30 ft. using #14 wire. The source is in the middle of the wire so there is no feedline. The wire size and height above ground result in a characteristic impedance (Z0) of the antenna wire of 600 ohms. The violet curve is the standing wave current envelope. There is a forward wave traveling from the feedpoint to the open end where it is reflected and becomes a reflected wave. The forward current and the reflected current interfere constructively (add) at the feedpoint. The forward voltage and the reflected voltage interfere destructively (subtract) at the feedpoint. Assuming that the steady state feedpoint voltage is 100 volts and the steady state feedpoint current is 2 amps, what is the standing wave ratio at the feedpoint on the standing wave antenna?

    Vtot = 100v = Vfor-Vref
    Itot = 2a = Ifor+Iref
    Vfor/Ifor = Vref/Iref = 600 ohms
  5. WA7F

    WA7F Premium Subscriber QRZ Page

    Thanks, Pete. Good info.

    Dave, after spending close to $600 on the AA-600 I was a little disappointed with with documentation and not all that happy with the build quality. Haven't tried their customer support yet but, I don't find your comments a surprise.

    The display on mine recently tilted and I've seen the same on another unit that HRO tried to sell me. I caught that one before leaving the store and thought that someone must have dropped it. It turns out that the case tolerance is a bit loose and this allows the display to move around. It's an easy fix with a bit of double sided tape but, I'm disappointed that a fix was necessary on a releatively new and unabused antenna analyzerI thhghgthgtgg. Also, I've never been able to get mine to connect with a PC.
  6. WA7F

    WA7F Premium Subscriber QRZ Page

    zw2wdx, Even non-resonant antennas resonate somewhere. This one was designed for just under 80 meters at around 3.455 MHz. The 600 ohm ladder line is 85 to 86' so I don't expect any issues there. The analyzer was measuring around 10:1 or higher at the design frequency. I'll try again this weekend after a calibration and with a better lead-in set-up.
  7. W5DXP

    W5DXP Ham Member QRZ Page

    A 135 ft dipole is resonant around 3.455 MHz. 85 ft. of 600 ohm tuned feeder shifts the system resonant frequency to around 4.33 MHz. It would take around 116 ft of 600 ohm tuned feeder to resonate the antenna system on 3.75 MHz.
  8. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    If I understand your question and the rest, I'd say the SWR on the antenna itself is infinity (max current at center /zero at the ends) but at the feed point with respect to 50 ohm transmission line the SWR is 1:1 (100/2 = 50) How'd I do?
  9. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    I guess I don't know what I am doing:) I am not sure where Vfor and Iref are. If they are on the transmisson line, Zo = 600 ohms (V/I is the same for the signal traveling up and down if there was a transmission line). 100V and 2A could be multiplied to get the Apparent Power of 100W but they are out of phase so the power factor is < 1.
  10. WA7F

    WA7F Premium Subscriber QRZ Page

    Good point, I failed to consider the effect of ladder line length on the resonant frequency. Honestly, I don't remember how to calculate that and I'll need to dust off some books to do some homework. After a little wiring in the shack I'll give this a test run this evening. I originally put the analyzer on the doublet out of curiosity and posted here when I didn't understand the results. You guys have all made valid points and your discussion has increased my understanding of both analyzer calibration & use as well as doublet operation. Thanks for the detailed discussion.

Share This Page