I was asked whether the nanoVNA can display VSWR in terms of 400Ω or some other arbitrary impedance.
Some antenna analysers and VNAs support display of results in terms of some specified impedance other than 50Ω, sometimes only a limited fixed set.
The direct answer to the question is “probably no, not directly on the ‘original’ nanoVNA today”, there are many firmware forks and many PC clients for nanovna, and now many significantly hardware versions appearing. Things may change.
However, if you can save a set of impedance measurements, they can be converted to VSWR relative to some other impedance reference.
At Implementation of G5RV inverted V using high strength aluminium MIG wire – impedance measurements a set of impedance measurements made with a Rigexpert AA600 is presented.
A similar set of measurements could be made with a standalone nanoVNA and saved, but given that it is such a clumsy device and its USB socket has become so unreliable, I will not repeat the measurement but use the data used for the article above.
So we have a table of frequency and (R,X) measured looking into a ‘real’ 450Ω feed line, so we will calculate wrt 450Ω but we could use any reference.
Above is a plot or (R,X) looking into the feed line. Continue reading nanoVNA – VSWR in terms of 400Ω
During annual inspection of my HF antenna system, I removed the lid from the common mode choke.
The choke is housed in a (German) Hensel PS electrical terminal box. Hairline cracks were observed radiating out from one of the terminal screws, and adjacent to both screws in the lid. The lid is obviously not strong enough to be secured by two screws in the way done. Continue reading Annual inspection of antenna reveals balun box issue
(Purdum 2020) describes a small transmitting loop (STL) which is a little novel in that it uses an arrangement of four circular conductor loops, two in parallel in series with the other two in parallel.
The article goes on to claim some pretty extraordinary efficiency calculated from radiation resistance for a loop structure that is shown at a height of perhaps 2m above natural ground. Continue reading Review of W8TEE, AC8GY STL (Radcom Feb 2020)
An article by K2PO in QST Feb 2020 entitled
An SWR shifting T illustrates the pitfalls in naive design and implementation of transmission line matching systems. I say naive because the article does not address the matter of loss, yet QST publishes it as an example. Continue reading Stub matching loss can bite you
The article A thinking exercise on Jacobi Maximum Power Transfer #3 discussed Kurokawa’s power reflection coefficient as in indicator of mismatch at a system node.
Above is a demonstration circuit in Simsmith, a linear source with Thevenin equivalent impedance of 50-j5Ω. The equivalent voltage is specified by useZo, which like much of Simsmith is counter intuitive (as you are not actually directly specifying generator impedance):
Vthev and Zthev are chosen so that ‘useZo’ will deliver 1 watt to a circuit impedance that equals the G.Zo. Zthev will be Zo*.
Continue reading A thinking exercise on Jacobi Maximum Power Transfer #4
In the article The system wide conjugate match stuff crashes out again I worked through an example proffered in an online discussion to show that Walter Maxwell’s teachings on system wide simultaneous conjugate match do not tend to occur in practical systems.
Why are hams so obsessed with conjugate matching?
The answer is on the face of it quite simple. Continue reading The transmitter matching problem
At A thinking exercise on Jacobi Maximum Power Transfer #2 I posed the question of a metric for the mismatch at the L2L1 junction in the following network where the calculated values L2L1_lZ is the load impedance at the L2L1 junction (looking left as Simsmith is unconventional), and L2L1_sZ is the source impedance at the L2L1 junction (looking right). The left three components are the fixed antenna representation.
Common practice is to speak of a “source VSWR” to mean the VSWR calculated or measured looking towards the source, and very commonly this is taken wrt 50+j0Ω which may be neither the source or load impedance but an arbitrary reference. Continue reading A thinking exercise on Jacobi Maximum Power Transfer #3
At A thinking exercise on Jacobi Maximum Power Transfer I posed an unanswered Q2:
Keeping in mind that C2 and L2 are an adjustable matching network, usually adjusted for minimum VSWR as seen at the source G. So, the questions are:
Does the system take maximum available power from the source G when the load impedance seen by source G is equal to the conjugate of its Thevenin equivalent source impedance (ie C2.Z=G.Zo in Simsmith speak)?
Does that ‘matched’ condition result in maximum power in the load L?
Above for reader’s convenience is the model conjugate matched at the GC2 interface. The calculated Po figure (lower right) is the power in the load L to high resolution. Continue reading A thinking exercise on Jacobi Maximum Power Transfer #2
nanoVNA-H v3.4 is out, and I don’t yet see significant problem reports.
When I compare the circuit with v3.3 (which I have), apart from new battery charger IC etc, the changes are in three areas:
- decoupling power to the mixers;
- increasing the drive to the mixers; and
- higher attenuation of input on the rx port. Continue reading nanoVNA-H – rework of v3.3 PCB to v3.4?
At The system wide conjugate match stuff crashes out again I discussed the failure of Walt Maxwell’s teachings on system wide simultaneous conjugate match using an example drawn from an online expert’s posting.
The replicated scenario with matching with an L network where the inductor has a Q of 100, no other loss elements is shown below. (Quality real capacitor losses are very small, and the behavior will not change much, the inductor loss dominates.)
Above is a model in Simsmith where I have adjusted the lossy L network for a near perfect match. I have used a facility in Simsmith to calculate the impedance looking back from L1, often known as the source impedance at a node but in Simsmith speak the calculated L1_revZ on the form, (ie back into the L network) from the equivalent load. Continue reading A thinking exercise on Jacobi Maximum Power Transfer