Thursday, 30 October 2014

Antenna hints

Something I was searching for when building antennas is the difference between resonance and impedance. Really, I've learned it when I did my radio exams but when you learn a lot you also forget a lot. So a future reference for myself and others:

R=resistive load, X=reactance

1.) RESONANT FREQUENCY is where reactance is zero ohms, or in some cases as close to zero ohms. Since resistance has nothing to do with resonance, the resonant frequency is NOT
always at the point of lowest indicated SWR (although they certainly can be the same). The most desirable load is almost always the load with lowest SWR, even though it may not necessarily be the point of no reactance (resonance).

2.) An IMPEDANCE of 50 ohms can be composed both resistive and reactive components. If the impedance is 50 ohms, but the SWR is not 1.0 to 1, the likely cause is reactance makes up part or all of the impedance. Contrary to popular (but very incorrect) misconceptions, it is impossible to obtain a perfect 1 : 1 SWR when the load is reactive, even if the complex impedance is 50 ohms.
A good example is a 50 ohm nearly pure reactance load. R=0 X=50, while the impedance meter reads 50 ohms. The SWR would overflow (SWR>25), since the reactive 50 ohm impedance load absorbs almost no power from the source and has a nearly infinite SWR.

3.) Even if a perfect transmission line is cut to an exact electrical half-wave (or a multiple thereof ) it is a true half-wave multiple only on one frequency in that band. On a slightly different frequency the line will not represent the true feedpoint impedance of the antenna. The line is only “impedance transparent” when lossless and when an exact multiple of 1/2 wl. The longer the transmission line in wavelengths, the “more length critical” it becomes and the less accurate measurements become.

4.) If the feedline is not an exact multiple of 1/4 wl, the resonant frequency of the antenna might be shifted higher or lower by the transmission line. A mismatched non-quarter wave multiple feedline adds reactance that can cancel antenna reactance at frequencies where the antenna is not resonant. Multiple antenna and feedline combination resonances commonly occur with dipoles, where reactance crosses zero (indicating resonance) at some frequency other than the antenna’s actual resonant frequency. This is a normal effect.

5.) If the line is a 50 ohm line, has no radiation or parallel currents, and if the line has minimal loss, moving a analyzer to another point on the line will NOT change SWR reading. Impedance and resonant frequency might change from line transformation effects, but the SWR will not change.

6.) If SWR changes with coaxial line length, line placement, or line grounding (any distance away from the antenna) changes, the feedline has one or more of the following shortfalls:
 a.) The feedline is carrying common mode current and radiating.
 b.) The feedline is not a 50 ohm line.
 c.) The feedline has high loss.

We have a clubmeeting tomorrow evening. PA0HPG Harm is going to talk about using a FA-VA3 antenna analyzer and how it helps to make your antennas work. I really like this kind of talks and hope to learn a lot from it. If it reveals something important I will certainly update this post or post a new one about it.


4 comments:

  1. Hi Bas,

    Thanks for sharing this. I have recently built a VK5JST aerial analyser and am in the process of learning on how to make best use of it.

    73 Angel M0HDF

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    1. Your welcome, In fact this theory is always the same whatever analyser you use. It's the basic of antenna theory and you have to understand this before building or modifying antennas. 73, Bas

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  2. Hallo Bas, toch leuk zo'n lijstje om even het geheugen op te frissen. Antennes ontwerpen en bouwen vind ik nog steeds erg leuk. Mijn vertical met de inklapbare 3m straler en ba;lunbox is nu ook klaar. Ik zal komend weekend je de foto's sturen. Ik ben dinsdag begonnen met een nieuwe kruisyagi te maken. Berekend met het programma van VK5DJ maar dan met een gammamatch.
    Ik ben benieuwd naar de resultaten. Alle materialen heb ik vanmiddag al gezaagd, vanavond de gammamatchen gemaakt en hopelijk volgende week, als het weer toelaat, buiten in de tuin op de boom opbouwen en testen.
    Veel plezier bij de meeting. 73 Hans, PE1BVQ

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    Replies
    1. Hallo Hans, ben benieuwd naar de fotos. Ik vind het erg jammer dat jij nog niet met een blog gestart bent. Ik krijg van jou mooie fotos en grootse plannen en vind het erg leuk. Ik denk dat er nog veel meer mensen geïnteresseerd zijn in wat jij doet met de hobby. 73, Bas

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Thanks for your comment. Bedankt voor je reactie. 73, Bas