Standing waves change the distribution of voltage and current on a transmission line, and that results in a change in attenuation from that published for a matched load (ie Zl=Zo).
There are many formulas given in various places for calculating the loss under mismatch, almost all of them depend on either ρ (the magnitude of the complex reflection coefficient Γ) or VSWR. Since these are simply related (VSWR=(1+ρ)/(1-ρ)), they have the same dependency. In fact, there is not enough information in ρ (or VSWR) to calculate loss exactly, so they are approximations with underlying assumptions that are rarely exposed.
This article compares the calculation using two common formulas which related loss under mismatch with Matched Line Loss (MLL) with the exact solution using lengths of RG58 terminated with two different VSWR=10 loads at a range of frequencies from 1-30MHz. Continue reading Graphic demonstration of loss under standing waves
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
Measure MLL using the Rin where X=0
Another method of estimating Matched Line Loss (MLL) from measurement is using the input resistance of a section that is an odd or even number of quarter waves in electrical length.
I say estimate because this method depend on an assumption of the value of Zo, and using purely real nominal Zo introduces some error.
The required length can be approximated by fining a frequency where X passes through zero. Again, this method is an approximation.
There is a simple formula published in many ham handbooks:
It is, a discussed at Measuring matched line loss, a crude approximation (and should be written with ≈ rather than =).
A better formula is one I developed though it may not be novel:
It is exact, but there is error introduced in using nominal Zo.
Low Z measurement
Lets measure Zin of our 4m o/c line section, and find the lowest frequency where X passes through zero, and note the value of Rin.
Above is a wide sweep, the frequency we want to focus on is around 13MHz. Continue reading Exploiting your antenna analyser #5
Measure MLL using the half ReturnLoss method
Again in the theme of measuring something known, let us determine the matched line loss (or normally quoted attenuation) of our cable at 3.5MHz.
To make the measurement, just connect the two line sections in cascade with a joiner, and one end on the instrument, other end open circuit, and measure ReturnLoss.
Most analyser manuals and lots of helpful articles in journals and handbooks will tell you that MLL=RL/(2*length) where RL is the ReturnLoss of an open circuit or short circuit line section (the only requirement is that the ρ=1 at the line end).
Wow, that is so low, and using the traditional formula:
Of course we are measuring way low in the instrument’s capability and there is some considerable uncertainty… but when we consult a good transmission line loss calculator, we expect around 0.029dB/m… that is 12 times what we measured. Continue reading Exploiting your antenna analyser #4
This blog was started two years ago when VK1OD.net was closed due to forced change of call sign.
In that two years, more than 450 blog articles have been published here and there are more ‘in the wings’, around 10 articles ready for release as I write this.
Though I set out to write mostly about electronics and less about Amateur Radio application of technology, that clearly hasn’t happened as the majority of articles are about Amateur Radio related technology.
Interestingly, the most popular articles are about technology, but not about Amateur Radio technology.
Amateur Radio has relegated itself to a backwater in electronics and technology hobbies. Relegated from the proud tradition that saw many of the significant contributions to electronics and communications technology from people who were also radio amateurs, and that had help shape their development. Amateur radio has transformed itself under the active guidance of its peak bodies into a contesting activity, train-spotting if you like and promoted as “radio sport” and the ‘original’ social network, a pursuit where technology is no longer the main shared interest (or much interest at all) and bad behaviour is rewarded. Progress!
Overall, hit rates on the new site are much lower than for VK1OD.net which was quite expected. Factors, positive and negative include:
- + shorter articles intended to be more digestible to modern audiences with busier lives and shorter attention spans;
- + format that presents well on smartphones and tablets for the mobile user; and
- – diminishing interest in the underlying technology in amateur radio.
There a many current projects that will give rise to blog articles, after all, the blog is my experimental notebook. Most current projects have some Amateur Radio element, but they apply wider current technologies to the experiments and are relevant not only to Amateur Radio but to radiocommunications more generally and general applications of technology to discovery.
The sign of reactance
At Exploiting your antenna analyser #2, the matter of determining sign of reactance was mentioned.
If you have an analyser that does not measure the sign of reactance, this information should be important to you.
Above is a Smith chart plot of measurements from 15MHz to 25MHz.
One can see that the locus of Zin on the Smith chart forms an arc, and the points on the arc rotate clockwise about the arc centre with increasing frequency. Continue reading Exploiting your antenna analyser #3
Reconciling the single stub tuner results
So having found that the length of the RG58 lines sections are both 1.98m (approximately 2m), let’s try to reconcile measurement and prediction of Zin at 9MHz.
The examples discussed in this series of articles are designed for the test jig as described at Exploiting your antenna analyser #1 with a pair of nominally 2m length RG58 patch leads, a pair of 50Ω loads and some tee pieces and adapters to connect it all up. If you duplicate it, you will experience the same learning opportunities (the examples are structured). If you presume to redesign the experiment, your outcome will probably be different.
Before you read on, take a measurement of Zin at 20MHz and write down the impedance value. Do whatever you do to determine the sign of the reactance. If your instrument displays the sign properly, use it, otherwise use the method in your user manual or whatever wisdom you trust.
Done that? If so, read on… Continue reading Exploiting your antenna analyser #2
I often see posts in online fora by people struggling to make sense of measurements made with their antenna analyser.
This article is about exploitation of a modern antenna analyser beyond its capability as a self excited VSWR meter. The latter is fine, and it is often not only all you need, but the best tool in optimising some antenna systems… but if you want to exploit the other capabilities of the instrument, read on.
Great benefit can be obtained by measuring some known loads, and reconciling the measurement with the known.
A quite simple set of equipment can be used to create a scenario rich with opportunity to prove your understanding of the basics of complex impedance, transmission lines, and measurement. Lets explore a simple example.
Above is a test jig. It is two equal lengths of identical coax connected to a tee piece on the analyser. The end of one piece of coax has a tee piece with two nominal 50+j0Ω loads as used on Ethernet 10base2 networks. The analyser is a Rigexpert AA-600. Continue reading Exploiting your antenna analyser #1
I have recently installed the Threshold 2 update to Win 10, and apart from a number of small issues, it seriously broke my grabber environment on a 64 bit machine.
The error messages:
5 [main] lftp 2944 C:\bin\lftp\bin\lftp.exe: *** fatal error in forked process - fork: can't reserve memory for parent stack 0x600000 - 0x800000, (child has 0x400000 - 0x600000), Win32 error 487
1481 [main] lftp 2944 cygwin_exception::open_stackdumpfile: Dumping stack trace to lftp.exe.stackdump
6 [main] lftp 5008 fork: child -1 - forked process 2944 died unexpectedly, retry 0, exit code 0x100, errno 11
For that environment, I use an lftp port to Windows (https://nwgat.ninja/lftp-for-windows/), and had installed the 64bit verion which is essentially a cygwin compilation packaged with the necessary DLLs. (This issue probably more widely affects cygwin, or perhaps just the bash shell under cygwin.) Continue reading Win 10 Threshold 2 issues
Voltage Standing Wave Ratio is the ratio of the voltage maximum (antinode) to the adjacent voltage minimum (node) on a transmission line. (This assumes a fully developed standing wave, that the maximum and minimum are not forced by end of line). Continue reading Expression of VSWR as a simple decimal real number