On testing two wire line loss with an analyser / VNA – part 4

This article calculates and compares three models for matched line loss (MLL) based on measurement of a transmission line section with short and open termination.

This article follows on from:

Measurements

The measurements permitted calculation of MLL vs frequency over the measurement frequency range of 10-200MHz.

The measurement frequency range was chosen as appropriate to the intended application range and the available / manageable sample length. To make measurements down to 100kHz with similar measurement noise would have required a test length of hundreds of metres.

Curve fitting

The measurement data was fitted to three popular models for MLL.

Above is a plot of MLL (dB/m) calculated from the measurements saved as s1p files (raw), and fits to three models: Continue reading On testing two wire line loss with an analyser / VNA – part 4

Last update: 20th February, 2022, 8:36 AM

The oft asked question of how much an LNA improves a 70cm weak signal station – Rules of Thumb

The article The oft asked question of how much an LNA improves a 70cm weak signal station solicited some comment on optimal configurations. This article deals with the notion / Rule of Thumb that optimal LNA gain is just sufficient to offset line losses.

This article explains with graphs the relationship between Signal / Noise degradation (see Signal to noise degradation (SND) concept) and LNA gain in the configurations discussed in the original article. See The oft asked question of how much an LNA improves a 70cm weak signal station for documentation of the scenario assumptions.

The critical value for SND is a personal choice, but for the purpose of this discussion, let’s choose 1dB. That is to say that the S/N at the receiver output is less than 1dB lower than the ultimate that could achieved with the antenna system given the external noise environment.

The total line loss in the example configurations was 2.6dB. The model assumes that LNA Noise Figure is independent of LNA Gain, though in the real world, there is typically some small dependence.

Often the choice of LNA Gain drives the choice of a single stage or two stage LNA, which has cost implications.

Terrestrial external noise – 495+5K

Above is a chart showing SND vs LNA Gain. It can be seen that as LNA gain is increased, SND improves rapidly with a knee around 15dB LNA gain above which SND improvement is slower. Continue reading The oft asked question of how much an LNA improves a 70cm weak signal station – Rules of Thumb

Last update: 15th February, 2022, 7:33 AM

The oft asked question of how much an LNA improves a 70cm weak signal station

A recent online post seeking opinions on the chap’s 70cm weak signal configuration is an interesting subject for study, and one that should be of interest to many weak signal DXers.

This article focusses on just one question in a quite similar configuration, what is the advantage given by the LNA?

Study configuration

The scenario will be evaluated for both terrestrial and satellite paths.

Above is the assumed ambient noise environment, it has great bearing on the results. More on that later. Continue reading The oft asked question of how much an LNA improves a 70cm weak signal station

Last update: 15th February, 2022, 4:53 AM

An admittance graph for NanoVNA-App

Often one finds that a cartesian plot of the components of admittance (conductance and susceptance) would be a convenient plot in understanding / solving a problem.

Let’s work through an example of designing an antenna shunt match to illustrate.

The example is based on measurement of the feed point impedance of a real antenna, an M40 1.2m long vehicle mounted helical whip for the 40m band.

Above is a plot of the VSWR. The minimum VSWR is a bit high, let’s drill down on it. Continue reading An admittance graph for NanoVNA-App

Last update: 12th February, 2022, 6:14 PM

A desk study of the Swan 500CX PA

I recall the arrival of the Swan 500CX in Australia, it was regarded highly and talked up quite forcibly on air by the local agent.

At the time, I was still acquiring the knowledge and skills to analyse the PA design in the 500CX, but I recall lots of on air discussions that were disparaging, but were not convincing.

More recently, I have had occasion to perform a desk study of the 500CX PA.

The Swan 500CX used a pair of 6LQ6, low cost TV sweep valves. From the GE datasheet, the valves are rated at 30 W plate (anode) dissipation. No safe grid 1 current or dissipation is given, so the safe approach is to regard that they must be operated with zero grid current, Class AB1 in this case. Continue reading A desk study of the Swan 500CX PA

Last update: 12th March, 2022, 9:16 AM

Measuring the gain of an antenna by the three antenna method

There are many methods of measuring the gain of an antenna, most of them call for a reference antenna of known gain. This method requires three antennas and does not require knowledge of the gain of any of them, but will find the gain of each of them.

Explanation

Harald Friis gave us the familiar transmission equation: \(\frac{P_r}{P_t}=\frac{A_r A_t}{r^2 \lambda^2}\\\). Continue reading Measuring the gain of an antenna by the three antenna method

Last update: 1st February, 2022, 7:25 AM

On ferrite cored RF broadband transformers and leakage inductance

By broadband transformer, I mean a transformer intended to have nearly nominal impedance transformation over a wide frequency range. That objective might be stated as a given InsertionVSWR over a given frequency range for a stated impedance. eg InsertionVSWR<2 from 3-30MHz with 3200(+j0)Ω load.

These are used in many things, including medium to high power applications such as EFHW matching transformers.

Leakage inductance is the equivalent series inductance due to flux that cuts one winding and not the other, and vice versa. For most simple transformers, the total primary referred leakage inductance is twice the primary leakage inductance. Since the leakage inductance appears in series with the signal path, it causes degradation of nominal impedance transformation, the very simplest approximation of the frequency response is that of a LR circuit.

Above is a Simsmith model of a 1µH total leakage inductance in series with a 50+j0Ω load, the InsertionVSWR is greater than 1.5 above 3MHz.

Is this a common scenario? Continue reading On ferrite cored RF broadband transformers and leakage inductance

Last update: 14th April, 2024, 1:36 PM

Tips and techniques for measuring small RF inductors and transformers

Over more than 50 years, I have measured literally thousands of RF inductors and transformers. This article gives some hints and techniques for making / preparing prototypes for measurement, and measurement.

RF inductors and transformers will often use enameled copper wire (ECW) or some form of insulated wire or coax.

Solid core LAN cables are a good source of small insulated wire for prototyping. The conductor is around 0.5mm, and overall about 0.9mm.

Handy tools

Above, from left to right: Continue reading Tips and techniques for measuring small RF inductors and transformers

Last update: 21st July, 2024, 8:41 AM

A common scheme for narrow band match of an end fed high Z antenna – further explanation

A common scheme for narrow band match of an end fed high Z antenna gives a Simsmith model for the matching arrangement that follows.

The tapped coil could also be considered an autotransformer.

Simsmith model

Continue reading A common scheme for narrow band match of an end fed high Z antenna – further explanation

Last update: 27th January, 2022, 7:01 PM