VNA fixture for measuring Zcm of a common mode choke – twisted pair wound

VNA fixture for measuring Zcm of a common mode choke – coax wound discussed issues with common ham practice for measuring coax wound common mode chokes.

The article left readers with some homework:

  • Does the same thing occur if the core is wound with twisted pair that is well represented as a uniform two wire transmission line?
  • Are the resistors beneficial?
  • Do they degrade fixture behavior?
  • Then, why are the used so often?

This article addresses those questions.

Does the same thing occur if the core is wound with twisted pair that is well represented as a uniform two wire transmission line?

Let’s treat the common mode choke as a black box with two input terminals at left and two output terminals at right with voltages as annotated above. Continue reading VNA fixture for measuring Zcm of a common mode choke – twisted pair wound

Receive only antenna for 160m – matching and performance discussion

This article explores the design / analysis of a passive receive only antenna for the 160m band (1.8MHz).

The example and calculations assume linear systems, if there is significant nonlinearity that gives rise to significant IMD, IMD noise is not captured by the analysis.

Results are for the scenarios calculated and may not be extensible to different scenarios.

Another caveat: I have reservations about transmission line modelling in SimNEC, especially for composite conductors, but for the purposes of the discussion, assume that it is reasonably correct.

The example antenna is a K6SE 14’x29′ Pennant optimised for 160m.

Design objective

The objective here is to design a receive only antenna system that can be relatively remote from local noise sources (like house wiring), and captures enough external signal and noise that the receiver internal noise does not degrade S/N too much. Continue reading Receive only antenna for 160m – matching and performance discussion

VNA fixture for measuring Zcm of a common mode choke – coax wound

A common online question is what sort of fixture is appropriate to measure the common mode impedance of a common mode choke.

Above is a screenshot from a Youtube video by Trx Lab, probably about 2016 vintage. I see many problems with the fixture, lets start with the resistors. Continue reading VNA fixture for measuring Zcm of a common mode choke – coax wound

Does RBN give a reliable metric for comparing antennas – more detail

Does RBN give a reliable metric for comparing antennas? gave an example of signal strength measurement and the effect of fading over time.

This article goes into a little more depth on the subject using a further data capture of 600 measurements 10s apart.

Above is a plot of signal strength of an 80m A1 Morse (CW) beacon measured in 20Hz bandwidth over 100min (a terrestrial path of length 105km). Continue reading Does RBN give a reliable metric for comparing antennas – more detail

Reconciliation of transmitter power, EIRP, received signal strength, antenna factor, ground wave propagation etc @ 576kHz

This article reconciles measurements with path predictions for a MW AM transmitter on 576kHz. The techniques used could be used to validate / assess the performance of a transmitter.

Source

The source is a MF AM transmitter on 576kHz located about 74km distant.

Above is the station data from the ACMA licence register. Conveniently it gives the EIRP as 132kW, we would expect something a little less than 150kW from the nominal 50kW transmitter, system efficiency calculates to 80%.

The EIRP would have been calculated from a set of field strength measurements at the time of commissioning. Continue reading Reconciliation of transmitter power, EIRP, received signal strength, antenna factor, ground wave propagation etc @ 576kHz

Does RBN give a reliable metric for comparing antennas?

I see that lots of hams depend on HF RBN to compare to antennas, or to compare before and after a change.

Experience says that A/B comparisons on HF are subject to variation in Ionospheric propagation paths, and that variation can be wide in range and rapid.

An example

Above is a plot of signal strength of an 80m A1 Morse (CW) beacon measured in 20Hz bandwidth over 15min snapshot (a terrestrial path of length 105km). Continue reading Does RBN give a reliable metric for comparing antennas?

The need for infinite preamplifier gain?

A recent posting on social media kicked off some discussion about infinite gain preamplifiers, mostly in the context of an unloaded very short vertical.

Over the past couple of years I’ve had a number of comments and questions about active antennas, instigated by my ARRL book, Receiving Antennas for the Radio Amateur.

The “main ingredient” of an active antenna (in this discussion, we’ll center on the very short WHIP), is the preamplifier, which generally takes the form of an FET source follower.

A true source follower (or ideal cathode follower) is theoretically capable of INFINITE power gain). In practice, modern FET input op-amps have an input resistance on the order of a teraohm or so, and an input capacitance of about a picofarad.

Although we can’t QUITE get to infinite power gain with a real FET (or FET input op amp), we can get EXTREMELY high power gains. Assuming an output (source) resistance of 1Kohm and an input resistance of 1 teraohm, a voltage follower will have a power gain of 10^21:1…..not too shabby. (This is assuming essentially a DC signal, where the input parallel capacitance can be ignored).

At this point in time, there has been no mention of noise… but it is key to the problem. Continue reading The need for infinite preamplifier gain?

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

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