Author: Owen

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: 4th February, 2022, 8:28 AM

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: 30th January, 2022, 5:09 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

NanoVNA-H4 v4.3 – initial impressions

I have owned a NanoVNA-H v3.3 for more than two years now. It required some modification to fix a power supply decoupling problem on the mixers, reinforcement of the SMA connectors, replacement of the USB socket, rework of the case so the touch screen worked properly / reliably, and some minor works (eg battery charger chip, bad patch cables, faulty USB cable).

With recent enhancement of firmware to support an SD card, the prospect of stand alone use becomes more practical, so I set about researching and purchase.

It seemed the best option was to buy a ‘genuine’ NanoVNA-H4 v4.3, and I started the search at the recommended (by Hugen) store, Zeenko… but whilst there was a listing for v4.2, there was no v4.3 listing (perhaps it is out of stock). I did find another store selling what they described as a ‘genuine’ NanoVNA-H4 v4.3, but this is a high risk transaction, experience is that Chinese sellers are not to be trusted, and Aliexpress is an unsafe buying platform.

This is one of those concerning transactions where the seller notifies shipment and gives a tracking number hours before the deadline, then a week later change the tracking number (the ‘real’ shipment).

Above, the promo image from the listing. Continue reading NanoVNA-H4 v4.3 – initial impressions

Last update: 18th March, 2022, 1:30 PM

Ambient noise measurement using whip on vehicle – #2 – active antenna electronics

This article continues from Ambient noise measurement using whip on vehicle – #1 – estimate Antenna Factor with a case study for the active antenna electronics.

For this discussion, I will use the amplifier developed at A high performance active antenna for the high frequency band, but applied to the antenna described at Ambient noise measurement using whip on vehicle – #1 – estimate Antenna Factor.

Let’s assume that the antenna + amplifier will be used with a HF receiver with Noise Figure 6dB, Teq=864.5K.

From (Martinsen 2018) Fig 3.8, the amplifier internal noise at the output terminals is -118dBm in 100kHz @ 3.5MHz. That implies that the amplifier Noise Temperature is 857.93K. The amplifier has 6.4dB voltage gain which needs to be subtracted from the AF calculated for unity gain (at the amplifier input terminals). Continue reading Ambient noise measurement using whip on vehicle – #2 – active antenna electronics

Last update: 21st January, 2022, 7:38 AM