The article nanoVNA – measuring cable velocity factor discussed ways of measuring the velocity factor of common coax cable. This article is a demonstration of one of the methods, 2: measure velocity factor with your nanoVNA then cut the cable.
Two lengths of the same cable were selected to measure with the nanoVNA and calculate using Velocity factor solver. The cables are actually patch cables of nominally 1m and 2.5m length. Importantly they are identical in EVERY respect except the length, same cable off the same roll, same connectors, same temperature etc.
Above is the test setup. The nanoVNA is OSL calibrated at the external side of the SMA saver (the gold coloured thing on the SMA port), then an SMA(M)-N(F) adapter and the test cable. The other end of the test cable is left open (which is fine for N type male connectors). Continue reading nanoVNA – measuring cable velocity factor – demonstration
With the popularity of the nanoVNA, one of the applications that is coming up regularly in online discussion is the use to measure velocity factor of cable and / or tuning of phasing sections in antenna feeds.
‘Tuning’ electrical lengths of transmission line sections
Online experts offer a range of advice including:
- use the datasheet velocity factor;
- measure velocity factor with your nanoVNA then cut the cable;
- measure the ‘tuned’ length observing input impedance of the section with the nanoVNA; and
- measure the ‘tuned’ length using the nanoVNA TDR facility.
All of these have advantages and pitfalls in some ways, some are better suited to some applications, others may be quite unsuitable.
Let’s make the point that these sections are often not highly critical in length, especially considering that in actual use, the loads are not perfect. One application where they are quite critical is the tuned interconnections in a typical repeater duplexer where the best response depends on quite exact tuning of lengths. Continue reading nanoVNA – measuring cable velocity factor
Deepelec store on Aliexpress sells a small test jig for use with their nanoVNA..
Above is the top view of the test jig mounted on a DIY PVC plinth. The test jig alone cost $17 on Aliexpress and took three months to arrive. Continue reading nanoVNA-H – Deepelec test jig
At AIM4170 – de-embedding the feed line in remote measurement a set of measurements of a monoband antenna looking from the transmitter were analysed to de-embed the feed line and arrive at the indicated feed point impedance.
This article explores a simple series match to improve the load seen by the transmitter.
In the Simsmith model above, the estimated feed point impedance is imported into element L, then a series section of lossless 50Ω line to represent the coax in the common mode choke (balun), then a series section of lossless 75Ω to perform the impedance transformation, then a section of 50Ω lossless line to make up the required length to the transmitter. Continue reading AIM4170 – de-embedding the feed line in remote measurement – a simple match
At nanoVNA-H – de-embedding the feed line in remote measurement I recently wrote on a procedure that can be very useful to refer measurements made at the transmitter end of a feed line to the antenna feed point.
A correspondent recently shared an AIM 4170 scan file of his 40m half wave dipole antenna system taken from the transmitter end of the coax and maintaining the common mode current path by bonding the shield of the coax connector to normal connection point on the transmitter.
Above is his graphic of the measurement looking into around 23m of RG58 feed line.
It shows the VSWR curve is quite classic in shape, the frequency of minimum VSWR is a little low, and the minimum VSWR is 1.478 which is quite within expectations of such an antenna. Continue reading AIM4170 – de-embedding the feed line in remote measurement
At nanoVNA-H – measure ferrite transformer I gave an example of using a nanovna to measure loss of a ferrite cored transformer.
Noelec makes a small transformer, the Balun One Nine, pictured above and they offer a set of |s11| and |s12| curves. Continue reading nanoVNA-H – measure ferrite transformer – Noelec balun
I have a nanoVNA-H which has had many hardware problems, some designed in,but mostly sub-standard / faulty components.
Above, the latest repair. A new battery socket to replace the original that crumbled apart… sub-standard plastic from all appearances. This was from a reputable supplier, so it is probably a genuine JST part rather than some cheap Chinese knock off.
The blue wire is part of a mod to invoke the bootloader on power up, R5 was also changed to something small, 1k IIRC.
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