Measuring coaxial cable loss by reflection with a directional wattmeter

At Measuring coaxial cable loss by reflection with a VNA I discussed measuring terminated coax cable loss by reflection with an VNA, and you might ask the question can it be done with a scalar network analyser, return loss bridge, or directional wattmeter, all of which provide a measure of the amplitude of reflection wrt some reference impedance.

This article explores using a Bird 43 directional wattmeter to measure line loss in a similar scenario. We will use 6m of Belden 8359 (RG58A/U) @ 3.6MHz.

Expectation

A short digression, what is the specification Matched Line Loss (MLL) at 3.6MHz? Using TLLC we get 0.171dB, that is our expectation.

Return Loss of SC section

(Bird 2004) gives the following advice.

Line loss using open circuit calibration: The high directivity of elements can be exploited in line loss measurements, because of the equality of forward and reflected power with the load connector open or short circuited. In this state the forward and reflected waves have equal power, so that φ = 100% and ρ = ∞.
Open circuit testing is preferred to short circuit, because a high quality open circuit is easier to create than a high quality short. To measure insertion loss, use a high quality open circuit to check forward and reverse power equality, then connect an open-circuited, unknown line to the wattmeter. The measured φ is the attenuation for two passes along the line (down and back). The attenuation can then be compared with published data for line type and length (remember to halve Ndb or double the line length to account for the measurement technique).

This also contains the hoary old chestnut that a good OC termination is hard to achieve, but this author’s experience of measurement with modern VNAs is not consistent with Bird’s assertion.

So lets do a theoretical simulation of the Bird 43 applied to this problem.

Lets say we connect a source to the line section with a short circuit (SC) termination, and that the Bird 43 reads Pfwd=90W, and we read Pref=78W, we can calculate return loss \(RL=10 \cdot log_{10}\frac{P_{fwd}}{P_{ref}}=0.65dB\), so RL/2=0.65/2=0.325dB.

Continue reading Measuring coaxial cable loss by reflection with a directional wattmeter

Measuring coaxial cable loss by reflection with a VNA

At Measuring coaxial cable loss with a voltmeter I discussed measuring terminated coax cable loss with an RF voltmeter, and it had some real practical limitations.

This article explores using a nanoVNA to measure line loss in a similar scenario. We will use 6m of Belden 8359 (RG58A/U) @ 3.6MHz.

The same technique could be used with a quality antenna analyser.

Expectation

A short digression, what is the specification Matched Line Loss (MLL) at 3.6MHz? Using TLLC we get 0.171dB, that is our expectation.

Return Loss of SC section

A common method proposed is to measure Return Loss (RL) of a section with load end RL=0dB and halve it. Many experts advise that the section should be terminated in a short circuit (S) because short circuits are more reliable than open circuits. So let’s get cracking.

Above is measured |s11| using a nanoVNA with recent OSL calibration from 1-30MHz. |s11| @ 3.6MHz is by eye -0.651dB, RL=-|S11|, so RL/2=0.651/2=0.325dB. Continue reading Measuring coaxial cable loss by reflection with a VNA

Measuring coaxial cable loss by transmission measurement with a directional wattmeter

The article Measuring coaxial cable loss with a voltmeter discussed some pitfalls of that measurement method, starting with the influence of theoretical error in actual Zo at lower frequencies.

You might expect that using a directional wattmeter has exactly the same problems because as many online experts advise, at the end of the day they are just a voltmeter.

They are wrong, a Bird 43 might use a half wave detector driving a d’Arsonval meter and you might regard that to be a voltmeter, but the RF signal it measures is a combination of samples of forward and reflected waves wrt to its calibration impedance (usually 50+j0Ω) and we will see that makes a difference.

Where a directional wattmeter is calibrated for a purely real impedance (ie X=0), then the relationship \(P=P_{fwd}-P_{ref}\) holds true (On the concept of that P=Pfwd-Prev).

Lets take an example to explore the theoretical answer. We will use 10m of Belden 8359 (RG58A/U) @ 3.6MHz.

Lets model the scenario in TLLC. We will select the “Use Lint” switch for a better model of this specific cable at 3.6MHz and take the “Long” output.

Above is the input form. Continue reading Measuring coaxial cable loss by transmission measurement with a directional wattmeter

Measuring coaxial cable loss with a voltmeter

A question asked online about measuring terminated coax cable loss with an RF voltmeter and whether to condemn it based on comparison with specs raises an interesting case to discuss.

The subject raises some immediate concerns:

  • the accuracy of the termination;
  • the accuracy of the voltmeter;
  • the extent to which the voltmeter disturbs the thing being measured; and
  • assumptions about matched conditions.

Lets take an example to explore the theoretical answer. We will use 10m of Belden 8359 (RG58A/U) @ 3.6MHz.

Lets model the scenario in TLLC. We will select the “Use Lint” switch for a better model of this specific cable at 3.6MHz and take the “Long” output.

Above is the input form. Continue reading Measuring coaxial cable loss with a voltmeter

Chinese wattmeter / power analyser fix

I bought a little wattmeter / power analyser with SB50 style plugs on it on eBay for about $20.

These devices have been common in the RC market for many years, and I have found them useful for a number of things but note that the input -ve lead is NOT directly connected to the output -ve lead, you cannot use them where the input -ve and output -ve are common.

 

The problems

Above is the promo pic. Of course they are not Anderson plugs, but clones. Continue reading Chinese wattmeter / power analyser fix

Simsmith bimetal line type

This article discusses various measurements and models of Wireman 551 windowed ladder line, including adapting Simsmith’s bimetal line type to bear on the problem.

Measurements

A starting point for characterising the matched line loss (MLL) of the very popular Wireman 551 (W551) windowed ladder line is the extrapolation of measurements by (Stewart 1999) to 1.8MHz. Since the measurements were made at and above 50MHz where the W551 has copper like performance, this is likely to underestimate actual MLL and such wide extrapolation introduces its own uncertainty. Nevertheless, the datapoint is MLL=0.00227dB/m.

Dan Maquire recently posted a chart summarising measurements of these lines.

For the purposes of this article, let’s tabulate the MLL at 1.8MHz in dB/m. Continue reading Simsmith bimetal line type

A thinking exercise on Jacobi Maximum Power Transfer #4

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

The transmitter matching problem

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

Strength of reinforcement of nanoVNA-H connectors

The nanoVNA-H connectors are end launch PCB connectors and they have a decidedly spongy feel as 1Nm torque is approached. This was due to flexing of the PCB and was likely to lead to track cracks in the longer term.

Specs for SMA connectors range from minimum of 0.2Nm torque to maximum of 1.7Nm, but 0.6Nm and 1.0Nm are common commercial practice.

Some nanoVNA sellers state:

As the SMA ports are made of cast copper, please not connect hard 50-7 / RG213 and other cables directly to the SMA ports through M-to-SMA connector to avoid damaging the SMA ports. You can use the included SMA cable to connect to the SMA port as shown in the picture below, and then use M to SMA connector.

Clearly Chinese Cheats, they will say anything to make a sale and anything to avoid commitment to quality. These connectors are very unlikely to be copper, but are likely to be a copper alloy: brass. What they also avoid in the above statement is claim for PCB damage due to flexure of the SMA connectors torqued to accepted industry torque for reliable connections and measurement.

Above is a pic of a modification to reinforce the connectors. This article sets out the analysis of just the solder joint within the cross section of the brass pieces.

A side effect is that this modification bonds the ground planes for the input and output parts of the nanoVNA via the brass bar where they have been kept isolated to some extent.

I should note that there has been much discussion online as to whether the noise floor of the nanoVNA is degraded by the shields fitted to the board, and various modifications to the shields. Some of this discussion proposes that the issue is mostly around the mixers and noise loops, and I note that in -H designs prior to v3.3, the mixer power supply was not adequately decoupled. It is possible that electrical connection of the SMA connectors in this way degrades noise performance at some frequencies. No significant change was observed in the noise floor of s11 or s21 channels from 1 to 300MHz (I don’t regard instrument performance to be good above 300MHz). I have not seen credible evidence of degradation of the nanoVNA-H v3.3 build.

If indeed bonding the two SMA connectors close to the instrument increases the noise floor or has other performance impacts as suggested, it questions whether the currents on the exterior of the coax influence measurement (which it should not) and it questions whether two port measurement fixtures or adapters should  be attached close to the nanovna.

(See also Reinforcement of nanoVNA-H connectors – performance discussion.)

At first, the strength of the butt soldered joint might seem a simple case of beam analysis where the beam is of cast solder of the same cross section l x w as the soldered joint. Continue reading Strength of reinforcement of nanoVNA-H connectors