Backing out transmission line
Often we make measurements through a section of transmission line, and the measurements are wrt the reference plane, which for many analysers is the connector on the instrument.
Some analysers, or their associated software allow the effects of the transmission line to be backed out.
Above is a Smith chart view of measurement of a test antenna through some length of RG58. The antenna will have R<50Ω at minimum VSWR, so the angle of the complex reflection coefficient Γ will be close to 180° at the feed point. Antscope uses a different notation, but shows here the angle at the point of measurement to be -15.1°, so we need to increase it by 180–15.1=195.1°, which will take about half that electrical length of line, 97.6°. From TLLC, I calculate the length involved is 7.6m of RG58, which is an estimate that gives a starting point for backing out the cable. Continue reading Exploiting your antenna analyser #11
A correspondent has suggested to me that my practice of giving Return Loss as a positive dB value is wrong, citing US FS-1037C.
US FS-1037C has been superseded by ANS T1.523-2001, Telecom Glossary 2000, and the wording in the latter is identical to the former, so let’s discuss the more current document. Continue reading ANS T1.523-2001, Telecom Glossary 2000 and Return Loss
FSM has been updated to v1.11.0.
The update adds an export to Gnuplot file of the wave file to allow visual examination of the recording on which the measurements is based.
This replicates the utility of the existing Dplot export, but with the freely available Gnuplot package.
Above is an example of the receiver noise recording, and whilst it might not seem very interesting, it is interesting that it is of the character of white noise.
Of more interest are cases where there is a distant cyclic pattern at twice the AC power frequency which hints insulator failures on each half cycle. Some other types of periodic modulation are helpful in identifying possible sources of emissions.
- Duffy, O. 2005. Field Strength Meter software (NFM). http://owenduffy.net/software/fsm/index.htm (accessed 11/01/2016).
(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
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
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
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
Now one of the methods that is often used to transform the impedance of an antenna to suit a 50Ω feed line is the shunt match.
Lets explore that with our test jig reconfigured.
Connect up the two line sections in cascade from the analyser, and terminate it with the two 50Ω loads on the tee piece. Don’t worry too much about what we have in terms of implementation, it provides a load to the analyser that presents a similar scenario to shunt matching a loaded short monopole.
So, measure the input impedance around 21MHz.
Above is a scan with the Rigexpert AA-600 from around 21MHz. Ignore the |Z| line, it is irrelevant and confusing but I cannot switch it off, a shortcoming of the software.
What we are exploring is that as we change frequency, the parallel equivalent resistance changes at 21.275MHz above, it equals 50Ω. The full parallel equivalent is 50Ω//-j77.3. So, if we were to make a small inductor of 77.3Ω reactance (L=X/(2*pi*f)=580nH) and connect it in shunt, the resulting impedance will be 50+j0Ω. Continue reading Exploiting your antenna analyser #6