I have written several articles on untuned loops for receiving, as have others. A diversity of opinions abounds over several aspects, probably none more than the idea of an optimal load impedance for the loop.
This article analyses a simple untuned / unmatched loop in the context of a linear receive system (ie no IMD) of known Noise Figure. Continue reading Small untuned loop for receiving – simple model with transformer
At nanoVNA-H – Port 2 attenuator for improved Return Loss I explained the reasons for essentially permanent attachment of a 10dB attenuator to Port 2 (Ch 1 in nanoVNA speak).
Above, the 10dB attenuator is semi permanently attached to Port 2 principally to improve the Return Loss (or impedance match) of Port 2, a parameter that becomes quite important when testing some types of networks than depend on proper termination (eg many filters). I should remind readers that the improvement in Port 2 Return Loss comes at a cost, the dynamic range of Port 2 is reduced by 10dB. Continue reading nanoVNA-H – Port 1 attenuator for improved what???
A friend wrote saying “I thought the nanoVNA display was smaller than this”.
I make the index finger nail width exactly the same as the round part of the SMA nut which is 7.6mm. That is a very tiny hand… or the image is a composite fraudulently not to scale. Continue reading nanoVNA – promotion by cheats
There is little doubt that the nanoVNA has made VNAs very popular in the ham community, possibly more so that any other device.
Eager owners are trying to apply them to solve lots of problems, often without sufficient knowledge or experience to properly inform the measurements.
An example that has a appeared a few times on online forums in the last weeks is measuring the matched line loss (MLL) of a section of RG6 coax… to inform a decision to discard it or keep it.
The common approach is to use a measurement of |s11| and to calculate Return Loss and infer the MLL.
For discussion, lets consider an example of 30′ of Belden 1694A RG6 solved in Simsmith. We should note that unlike most RG6 in the market today, this uses a solid copper centre conductor.
Short circuit termination
Some authors insist that the half return loss method is to be performed using a short circuit test section. Bird does this in their Bird 43 manual.
Above is a plot of calculated |s11| (-ReturnLoss) from 1 to 20MHz for the test section. The three plots are of |s11| wrt 50Ω, 75Ω and frequency dependent actual Zo (as calculated for the model). The cursor shows that the actual |s11| is -0.37474dB (ReturnLoss=0.37474dB). Using the half return loss method MLL=ReturnLoss/2=0.37474=0.187dB/m. Continue reading nanoVNA-H – woolly thinking on MLL measurement
A question was asked in an online forum specific to nanoVNA as to how the use the nanoVNA to
check the attenuation loss in some old & weathered RG-6 (75 ohm) cables for the TV signal frequencies. Excuse the term
attenuation loss, lets assume the poster is asking for matched line loss (MLL).
The assembled experts are offering solutions to transform the ports to 75Ω and make a measurement, deducting the loss of the transformation (minimum loss pads were suggested).
There is a very simple solution that should be quite practical for the scenario described. Let’s work through two examples using 35.5m of unbranded quad shield RG6 with CCS centre conductor (of unknown quality) for the DUT. Continue reading nanoVNA-H – thinking laterally
Users of some ATUs may have noticed particular sensitivity to hands on the capacitor adjustment knobs. It is a common problem with cheap implementations of the T match as the capacitor rotor is usually at high RF voltage and if that shaft is extended to the adjustment knob, under certain circumstances tuning becomes very sensitive to hands on the knobs.
In some of these implementations, if the users hand touches the metal grub screw in the knob, or the metal panel bushing behind the knob they may get a significant RF burn.
Let’s use the MFJ-949E as a discussion example. It is a T match, and the metal capacitor shafts in the knobs and panel bushings carry RF voltages.
So why is this only sometimes a problem?
The RF voltage across the coil, and impressed on the capacitor shafts can be extremely high when using loads with small resistance and large negative reactance, more so on the lower bands. Continue reading MFJ ATU hand effects on capacitor knobs
There are often times when it would be useful to transform measurements made looking into a feed line to the other end of the feed line.
Common advice given by online ham experts include:
- it just cannot be done, the best (only) point to measure an antenna is at the feed point;
- it can be done, but only with an integral number of half waves of feed line;
- use the port extension facility in your software;
- use software package x;
- do an OSL cal with the feed line being part of the fixture.
Continue reading nanoVNA-H – de-embedding the feed line in remote measurement
I mentioned in my article WIA 4:1 current balun that the use of a single toroidal core in the above graphic compromises the balun. This article presents some simple measurements and analysis that question whether the balun works as so many users think.
The popularity of the balun derives from the work of VK2DQ and is often known as the VK2DQ 4:1 current balun (though probably not his invention).
Analysis at the limits
Often, analysis of a network as frequency approaches zero or infinity can simplify the analysis whilst allowing a reasonable test of the sanity of the design.
Above is a conventional transformer schematic of the WIA 4:1 current balun on a perfectly symmetric (balanced) load. At frequencies where the electrical length of each winding is very short, we can assume negligible phase delay along or between windings, simplifying analysis greatly. Continue reading WIA 4:1 current balun – further measurements
Correspondents have informed me that the balun dealt with in article 4:1 current balun – review and fix and variants are very common. This article gives a checklist of common issues and some basic measurements using an antenna analyser that should reveal some issues without breaking into a sealed assembly.
Baluns are commonly employed to obtain nearly balanced feed line currents (ie negligible common mode current) in two wire lines or negligible common mode current on coaxial feed lines. This article discusses baluns for that application.
A very common 4:1 current balun is Guanella’s 4:1 current balun, but there are others including pretenders.
Three common 4:1 current baluns
Guanella 4:1 current balun
(Guanella 1944) described a 4:1 current balun in his 1944 article, he did not show the winding pairs coupled by a magnetic core as shown above.
Above is Guanella’s circuit, and he does not show coupling between the two winding pairs.
Properly implemented, this device is known to work very well.
Sevick’s single core 4:1 current balun
Let us look at Sevick’s device because it underlies so many failures.
If you look at it very carefully, you will see that the two output wires emerge from opposite sides of the core, the left hand wire exiting under the core was wound from front to back of the core and the right hand wire exiting on top of the core was wound from back to front of the core. Continue reading 4:1 current balun – identifying bad ones
This article reports tests on two 4:1 current balun configurations – a collaboration between Bruce, VK4MQ, and myself.
Purported current balun on a single magnetic core
Above is an attempt at a 4:1 current balun on a single core. Note that this is NOT wired in the insane series opposed connection of the WIA 4:1 current balun. Note also that this is NOT a Guanella 4:1 current balun (see below).
Lets measure the Insertion VSWR by placing a good 200+j0Ω load on the output terminals and measuring input VSWR over the range 1-30MHz. This load is what we will call an Isolated Load meaning it has only two terminals, and the current that flows into one terminal must flow out of the other terminal… in other words, the current MUST be balanced (ie equal magnitude but opposite phase currents in the two terminals)… we will come back to the Isolated Load later.
Above, measured InsertionVSWR. It is not too good, but not very bad either. Broadly the balun gives an almost reasonable 4:1 impedance transformation from load to input. Continue reading 4:1 current balun – review and fix