Modern hams live busy lives and it is difficult to fit everything in to the available time / resources etc. So, there is an appetite for the skinny on some key topics, the inside info that took the wise a long time to learn.
This article discusses one of those articles containing the skinny on VSWR, What is VSWR: Voltage Standing Wave Ratio, it takes only a minute or two to read and there is a six minute video for those who prefer that.
The issues discussed here are common in the ham world explanation of VSWR and analysing them provides a learning opportunity. The video contains the issues mentioned below… and some.
Right up front, eager readers are given a take home message. If something prevents them finishing the article they have learned something they can repeat as pros. So satisfying!
In order to obtain the maximum power transfer from the source to the transmission line, or the transmission line to the load, be it a resistor, an input to another system, or an antenna, the impedance levels must match.
In other words for a 50Ω system the source or signal generator must have a source impedance of 50Ω, the transmission line must be 50Ω and so must the load
Ok, it states clearly and unequivocally that a necessary condition for maximum power transfer it that source must match line and line must match load.
We will test that proposition, but firstly the detailed explanation follows… Continue reading When simple explanations target a simple audience
Recent discussion online of a purported commercial HF small transmitting loop (STL) was challenged in analysing the structure, questioning whether such a connection was ‘correct’.
The STL used a main loop resonator and a separate small auxiliary loop for the 50Ω feed, a very common arrangement.
The main loop is a coaxial cable with, in this case, a tuning capacitor inserted between the inner conductors at each end. Above is a diagram of the main loop. Continue reading Single turn coaxial loop resonator analysis
A new release, AIM914 appeared recently.
In the common theme of one step forward, two steps backwards, this version has defects that were not present in AIM910B.
Let’s review the internal consistency of this part of the display screen.
Most of the values given above are calculated from a single measurement value, and should be internally consistent. That measurement value is translated to different quantities, many based on the stated Zref (75Ω in this case). Continue reading AIM 914 produces internally inconsistent results
This article documents a measurement of Matched Line Loss (MLL) of a 35m test section of generic RG6/U Quad Shield CCS.
It has become impossible in recent years to buy low cost RG6/U with solid centre conductor locally, and the imported product with solid copper conductor is prohibitively expensive (~$6/m as against $0.35/m for the CCS).
The CCS cable does have near copper like performance at UHF and above, but what is its behavior at HF?
Above is calculated MLL from a S11 scan of the test section with S/C and O/C termination. There is a little ripple on the response due to measurement error. The graph also has a curve fit, MLL=0.0285f^0.1506 (F in MHz). Continue reading Matched Line Loss of generic RG6/U Quad Shield CCS
John, KN5L, published some interesting measurements he made of a recent purchase of JSC 1318 windowed ladder line. JSC Wire & Cable is now known as Seminole Wire & Cable, and this is their 1318 product.
Product with apparently similar specifications are sold by many ham retailers, they may or may not be sourced from Seminole.
Some sellers specify the % IACS rating of the copper clad conductor, usually 30%, some just don’t mention it.
John carefully measured the DC resistance of his line section, and found that it reconciled well with the Copperweld datasheet for 21% CCS.
He also used a VNA to measure S11 of the line section with S/C and O/C terminations, and he gives links to the Touchstone files at the top of his page.
The O/C Touchstone file allows calculation of Zin. The O/C line exhibits resonance at 4.2MHz, at Zin=3.7Ω. His fuller set of measurements showed that Zo at 4.2MHz is very close to 400Ω. We can use those measurements to calculate Matched Line Loss (MLL).
Above, MLL is 0.50852dB/100m.
Continue reading KN5L measurement of JSC 1318 windowed ladder line – MLL @ 4.2MHz
A recent article questioned the accuracy of measurement of Matched Line Loss (MLL) for a modified commercial transmission line. The published results were less than half the loss of an equivalent line in air using copper conductors and lossless dielectric, when in fact there would be good reason to expect that the line modification would probably increase loss.
How do you avoid the pitfalls of using analysers and VNAs to measure line loss?
Lets walk through a simple exercise that you can try at home with a good one port analyser (or VNA). Measuring something that is totally unknown does not provide an external reference point for judging the reasonableness of the results, so will use something that is known to a fair extent,
For this exercise, we will measure the Matched Line Loss (MLL) of a 6m length of uniform transmission line, RG58C/U cable, using an AIMUHF analyser. The AIM manual describes the method.
If you need to know the cable loss at other frequencies, enable the Return Loss display using the Setup menu and click Plot Parameters -> Return Loss and then do a regular scan of the cable over the desired frequency range with the far end of the cable open. Move the blue vertical cursor along the scan and the cable loss will be displayed on the right side of the graph for each frequency point
Note the one-way cable loss is numerically equal to one-half of the return loss. The return loss is the loss that the signal experiences in two passes, down and back along the open cable.
Our measurements will show that this is a naively simple explanation, and to take it literally as complete may lead to serious errors. Yes, it IS the equipment manual, but it is my experience that the designers of equipment, and writers of the manuals often show only a superficial knowledge of the relevant material.
Above is an extract of the datasheet for Belden 8262 RG58C/U type cable, our test cable should have similar characteristics. Continue reading Transmission line measurements – learning from failure
At On Witt’s calculation of Matched Line Loss from Return Loss I discussed the common but flawed thinking that Matched Line Loss (MLL) can be calculated as half of the Return Loss of either a S/C or O/C section of transmission line.
The article discusses Witt’s calculation (half the average of Return Loss for S/C and O/C conditions) and notes that it can be a good approximation where the actual Zo is very close to the Zo on which the Return Loss measurement is based, and that the line loss is low.
This article looks at a case study of a section of low loss nominally 75Ω line is measured on a 50Ω instrument to illustrate sensitivity to Zo error.
A 3.1m section of RG6 was measured with O/C then S/C termination using a 50Ω VNA, and HalfReturnLoss (HRL), |S11| and phase of S11 is plotted.
Above, the O/C termination. Continue reading Inferring Matched Line Loss from Half Return Loss measurements – Zo error
From time to time one sees discussion online about consistency of ‘measured’ VSWR at different power levels (on the same instrument).
A question often asked is:
I tune up at 10W and achieve VSWR=1.5, and when I increase power to 100W, the VSWR increases. Which should I believe?
The first thing to note is that good antenna systems SHOULD be linear, VSWR should be independent of power, it is if the system IS linear.
For the most part they are linear, even though many antenna systems contain elements such as ferrite cored inductors that may exhibit some small level of non-linearity in ‘normal’ operation.
Non-linearity caused by for instance saturation of magnetic materials, loss of permeability where the temperature of ferrite cores reaches Curie point, arcing of capacitors or other insulating materials is NOT normal linear operation of a GOOD antenna system. If high indicated VSWR at high power is caused by any of these effects, it is flagging a problem that requires attention.
That said, a significant non-linear element may be the VSWR meter itself.
A common, if not most common way to make these meters is to use a half wave detector to convert the direction coupler RF outputs into DC to drive an ordinary moving coil meter. These meters commonly assume that the detector DC output voltage is exactly proportional to the RF input voltage.
Lets look at the accuracy of that process.
Above is a plot of the detector output vs RF input voltage for a commercial 200W VSWR meter. The measurements cover input power from 10 to 100W.
Continue reading VSWR meter trap for the unwary
In Aug 2018 QST John Portune (W6NBC) described an improvement he made to ordinary windowed ladder line.
Above is his improved line, it is ordinary windowed ladder line fitted into black foam water pipe lagging.
Continue reading Portune’s magic windowed ladder line
A correspondent wrote asking about the design of a matching network for a Half Square antenna for 80m, voltage fed at one end.
Above is the current distribution on the half square voltage fed. It is essentially two in-phase vertical quarter waves separated a half wavelength, a broadside array.
Feed point impedance at resonance is very high 5700Ω, and being a high Q antenna, they are very sensitive to dimensions, nearby clutter etc. Note that this is calculated for an antenna in the clear, it will be different where trees or conductive mast exist nearby. Continue reading 80m voltage fed Half Square matching workup