(Terman 1955) gives a meaning for the term SWR (or VSWR).
The character of the voltage (or current) distribution on a transmission line can be conveniently described in terms of the ratio of the maximum amplitude to minimum amplitude possessed by the distribution. This quantity is termed the standing wave ratio (often abbreviated SWR)…
Note that the use of capital E implies the magnitude of voltage, so Emax/Emin must always be a positive number.
Lossless line example
Let’s look at an example of a 5Ω load on a line with Zo=50+j0Ω at 0.1MHz.
The standing wave is observable, the expression VSWR=Emax/Emin seems straight forward enough. The voltage along the line could be sampled and VSWR determined, seems all very practical. Continue reading On negative VSWR
Let’s examine a number of transmission line loss calculators on the following scenario:
- line type Belden 8259 (RG58A);
- length 1m; and
- frequency 0.1MHz,
Continue reading Can the magnitude of the complex reflection coefficient (ρ) be greater than 1
Fractional G5RV antennas seem very popular in the US market, and they appeal to hams wanting multi band performance in small space.
One of the offerings is the quarter size G5RV, commonly marketed as the G5RV Mini.
The original concept set out by G5RV was a combination of a centre fed dipole and open wire transformation section to successfully deliver a lowish VSWR(50) on several of the pre-WARC bands. This enabled arbitrary length low Z feed extension to the transmitter, and allowed direct attachment to transmitters of the common design of the day (1950s). Continue reading NEC model of the quarter size G5RV
A reader has asked the question in a transmission line context after reading Walter Maxwell’s teachings on system wide conjugate matching.
In the real world, transmission lines have loss and almost always, the nature of that loss will mean that Zo is not purely real.
The answer to the question depends on whether or not there are standing waves on the transmission line.
Nothing in this article is to imply that a transmitter is well represented by a Thevenin equivalent source. Continue reading Is maximum power transfer and conjugate matching simultaneously possible
Measuring an RF inductor
This article walks through practical measurement of a ferrite toroidal inductor using an antenna analyser.
To be relevant practically, lets use an example from N4SPP’s end fed wire antenna on 3.6MHz. His coupling transformer uses a two turn winding on an FT240-43 core for the nominal 50Ω connection to the antenna system.
We could calculate the impedance of this winding using one of the plethora of online and desktop inductance calculators, but lets first fetch the data from the manufacturer.
A simple statistic that is widely used is Al, and above, Fair-rite gives Al=1075nH +/-20%. Note that although they give a tolerance of +/-20%, it is not uncommon that manufactured product has greater error, they may have optimistically quoted the standard deviation and it is easy to fall outside that (37% chance). Continue reading Exploiting your antenna analyser #10
Disturbing the thing you are measuring
In all measurements, we need to be careful that the measurement does not disturb the thing being measured.
This article explores an example where the instrument measurements appear wrong.
The story starts with a mobile antenna that the transceiver indicates has very high VSWR over the 40m band, though starts to decrease towards 7.350MHz.
To assist in problem identification / tuning, the antenna connector is disconnected from the radio and connected to the AA-600 analyser and a sweep taken.
Above is the sweep, but it is quite inconsistent with the transceiver’s VSWR meter readings. The plot above looks good, a little adjustment of the tip would get it down to 7.060… but the transceiver does not see it that way. Continue reading Exploiting your antenna analyser #9
Having recently published Graphic demonstration of loss under standing waves, I was interested in a thread running on QRZ.com which offered solutions to some mismatched scenarios. Continue reading Graphic demonstration of loss under standing waves #2
Finding resistance and reactance with some low end analysers
There are some analysers on the market that do not display reactance X or even magnitude of reactance |X| and possibly resistance, but do display VSWR and magnitude of impedance |Z|. Continue reading Exploiting your antenna analyser #8
Walt Maxwell (W2DU) made much of conjugate matching in antenna systems, he wrote of his volume in the preface to (Maxwell 2001 24.5):
It explains in great detail how the antenna tuner at the input terminals of the feed line provides a conjugate match at the antenna terminals, and tunes a non-resonant antenna to resonance while also providing an impedance match for the output of the transceiver.
Walt Maxwell made much of conjugate matching, and wrote often of it as though at some optimal adjustment of an ATU there was a system wide state of conjugate match conferred, that at each and every point in an antenna system the impedance looking towards the source was the conjugate of the impedance looking towards the load.
This is popularly held to be some nirvana, a heavenly state where transmitters are “happy” and all is good. Happiness of transmitters is often given in online discussion by hams as the raison d’être for ATUs . Continue reading Walter Maxwell’s teachings on system wide conjugate matching
Application to a loaded mobile HF whip
This article explores application of an antenna analyser to a helically loaded 7MHz mobile whip that has an adjustable length tip for tuning.
The task at hand is to ‘tune’ the antenna to a desired operating frequency.
The analyser used is a Rigexpert AA-600, but the article deals more generally with analyser features.
Initial measurement and interpretation
Above is a plot of R, X, and |Z| measured at the cable connector that plugs onto the transmitter. Ignore |Z|, it is irrelevant and confusing but unfortunately a ‘feature’ of the Rigexpert software that cannot be disabled. Continue reading Exploiting your antenna analyser #7