Category: Amateur Radio

WSPR for A/B tests – a discussion – part 2

Continuing from WSPR for A/B tests – a discussion – part 1.

Above is a frequency histogram of the experiment log.

Approximately…

The histogram uses 1dB intervals for the bars, so it chunks the data into discrete bands, and that hides an important issue with WSPR SNR data, its granularity is 1dB, so it is a very coarse measure given the spread of the data.

Lets compare the probability distribution of the measured difference data with an ideal normal distribution.

Above is a quantile-quantile (Q-Q) plot of the raw data and an ideal response with the same standard deviation as the raw data. The data is for 4508 points, so these dots each typically represent a large number of observations, more so in the middle region. Continue reading WSPR for A/B tests – a discussion – part 2

Last update: 6th June, 2018, 8:54 PM

WSPR for A/B tests – a discussion – part 1

The WSPR  User Manual sets out the purpose of WSPR:

The WSPR software is designed for probing potential radio propagation paths using low power beacon-like transmissions.

Though that talks about measuring radio paths, it is often used to compare transmitters or receivers over radio paths.

WSPR SNR measurements include the end to end radio path, which on some bands is highly variable, so using WSPR reported SNR values to compare two transmitters can be quite challenging.

A/B tests

We are all familiar with ad-hoc tests where a station might switch between two antennas and ask for comparative reports from receiving stations. At time when the radio path characteristics change greatly, changes in transmitter are often masked or confused by path variation.

Of course some practitioners will conduct several so-called A/B changes, perhaps as many as five and someone (receiver or transmitter) makes an informal judgement of the central tendency of the observations. The observations might be given in quite subjective terms, or in quantitative terms, possibly from an S meter of unknown calibration.

Normal distribution

Repeated measurements of the same thing, or same type of thing (eg 10 measurements of 1 new dry cell, or one measurement each of 10 new dry cells) tend to yield a set of slightly different observations.

For a lot of common physical things, the distribution of repeated measurements follows a bell shaped probability curve.

Most things that we repeatedly measure will return slightly different results from observation to observation due to various contributions in an imperfect world.

Above is a plot of the probability distribution of a normally distributed random variable with mean=1 and variance=1 (standard deviation=1). Continue reading WSPR for A/B tests – a discussion – part 1

Last update: 4th June, 2018, 9:36 AM

RBN for antenna comparisons – Radcom 2018

There are a plethora of articles and presentations on the ‘net showing how to use the Reverse Beacon Network (RBN) to make quantitative antenna comparisons over real propagation paths.

It is certainly an interesting subject to most hams with a deep interest in antenna systems.

So called A/B comparisons of antennas are as old as ham radio itself, and experience hams know that they are quite flawed.

Because ionospheric propagation paths vary from moment to moment, the challenge is to make a measurement that is directly comparable with one made at a slightly different place, or frequency or time. Accuracy is improved by making several measurements, and finding a central value, more observations tends to reduce uncertainty in that estimate of the population central value.

The challenge is finding that central tendency.

Central tendency

There are three common methods of estimating the central tendency of a set of figures:

  • mean (or average);
  • median (or middle value); and
  • mode (or most common value).

The mean is a popular and well known measure of central tendency. It is a very good estimate of the central tendency of Normally distributed data, and in that case, we can compare means and calculate confidence levels for assertions about the difference between means. The mean is very susceptible to errors due to outliers, and skewed distributions.

The median is usually a better measure for skewed data.

The mode is if you like, the most frequent or popular value and has a great risk of being quite misleading on this type of data.

A recent article (Appleyard 2018) in Radcom provides a useful example for discussion.

Figure 3

Appleyard gives a summary table where he shows means of a set of RBN measurements of signals from two stations observed at 21 remote stations, and differences in those means.

There are some inconsistencies between the text and data recorded in the RBN database on the day. Continue reading RBN for antenna comparisons – Radcom 2018

Last update: 28th May, 2018, 10:45 AM

Small efficient matching transformer for an EFHW

At FT82-43 matching transformer for an EFHW I wrote about the likely losses at 3.6MHz of a common design using a FT82-43 ferrite core with a 3t primary. In that case, expected efficiency (meaning PowerOut/PowerIn) of the transformer was less than 65% at 3.6MHz.

I have been offered input VSWR curves for such a configuration, and they are impressive… but VSWR curves do not address the question of loss / efficiency.

Note that building loss into antenna system components is a legitimate and common method of taming VSWR excursions, eg TTFD, CHA250, many EFHW transformers, but in some applications, users may prioritise radiated power over VSWR.

Design context / objectives

Objectives are:

  • used with a load such that the input impedance Zin is approximately 50+j0Ω, Gin=0.02S;
  • broadband operation from 3.5-30MHz;
  • VSWR less than 2 with nominal 3200Ω load; and
  • transformer efficiency > 90% at 3.6MHz.

The following describes such a transformer using a Fair-rite 2643625002 core (16.25×7.29×14.3mm #43).

I mentioned in the reference article that the metric ΣA/l captures the geometry, the larger it is, the fewer turns for same inductance / impedance. ΣA/l for the chosen core is 3.5 times that of a FT82-43 yet it is only 1.6 times the mass.

The transformer is wound as an autotransformer, 3+21 turns, ie 1:8 turns ratio. Continue reading Small efficient matching transformer for an EFHW

Last update: 14th April, 2024, 1:35 PM

80m voltage fed Half Square matching workup

A correspondent wrote asking about the design of a matching network for a Half Square antenna for 80m, voltage fed at one end.

Above is the current distribution on the half square voltage fed. It is essentially two in-phase vertical quarter waves separated a half wavelength, a broadside array.

Feed point impedance at resonance is very high 5700Ω, and being a high Q antenna, they are very sensitive to dimensions, nearby clutter etc. Note that this is calculated for an antenna in the clear, it will be different where trees or conductive mast exist nearby. Continue reading 80m voltage fed Half Square matching workup

Last update: 11th November, 2018, 7:06 AM

Checkout of SimSmith v16.3 – spot check of transmission line database – further discussion

The article Checkout of SimSmith v16.3 – spot check of transmission line database raises an issue with SimSmith’s modelling of transmission lines.

The case chosen was Belden 8216, a RG174 type line with silver clad steel stranded inner conductor.

Fully developed skin effect

Most practical transmission lines used for HF and above have fully developed skin effect above some frequency, and are well represented by the loss model MLL=k1*f^0.5+k2*f. For an RLGC model, the R is given by the first term and with fully developed skin effect, it is proportional to square root of f. The loss of good dielectrics is usually simply proportional to f and indicated by the second term.

Under this model, L and C are independent of frequency.

Many calculators use this model, and it works fine where skin effect is fully, or even well developed. The model coefficients are commonly discovered by performing a regression on measured matched loss at a range of frequencies, and the quality of the regression fit is a good indicator of the quality of the model for that particular line. Continue reading Checkout of SimSmith v16.3 – spot check of transmission line database – further discussion

Last update: 27th June, 2019, 5:11 PM

FT82-43 matching transformer for an EFHW

A published design for an EFHW matching device from 80-10m uses the following circuit.

Like almost all such ‘designs’, they are published without supporting measurements or simulations.

The transformer is intended to be used with a load such that the input impedance Zin is approximately 50+j0Ω, Gin=0.02S.

Analysis of a simple model of the transformer with a load such that input impedance is 50+j0Ω gives insight into likely core losses.
Continue reading FT82-43 matching transformer for an EFHW

Last update: 14th April, 2024, 1:35 PM

Ham grade analysers and VNAs often use unconventional meanings for well known terms

Lets use a simple test circuit to review the meaning of some oft misused terms associated with VNA and antenna analyser measurements.

Above, the test circuit is a nominally 220pF COG capacitor connected between tx and rx ports of a two port VNA. An extra 1Ω series resistance is included to model the likely effect of capacitor ESR. Continue reading Ham grade analysers and VNAs often use unconventional meanings for well known terms

Last update: 6th May, 2018, 12:00 PM

Checkout of SimSmith v16.3 – spot check of transmission line database

I am not a SimSmith user, and with upgrade of my desktop computer, I have lost access to the Smith chart application I have used for 20+ years. That has given me reason to evaluate various Smith chart applications for a replacement.

Smith charts are about modelling problems in transmission line terms, and what better test than a simple transmission line problem.

Above, a model of Belden 8216 (an RG-174 type cable) picked from SimSmith’s library of transmission line data (source KN5L). The model is at 1MHz and essentially indicates the Matched Line Loss of 100m by deducting the left hand dBW figure from the next one to the right, -5.88228e-3–2.33357=2.33dB. (Duh, I could not copy and paste these values, they had to be read and typed in by hand which is not only laborious but more importantly gives scope for error.)

Lets check the Manufacturer’s data sheet. Continue reading Checkout of SimSmith v16.3 – spot check of transmission line database

Last update: 6th May, 2018, 11:40 AM

Measuring ambient noise level using a spectrum analyser

The external noise figure Fa is defined (from ITU P.372-13) as:

I have taken a sweep of the 40m band when this is a little activity, but little enough to see the ambient noise floor at the time. It is raining and it is relatively noisy.

Above, the noise floor in 9kHz bandwidth with a CISPR quasi peak detector is about -78dBm. This is 12dB above the instrument noise floor, sufficient to not bother making a correction and we can take the external noise to be -78dBm (see below for correction calculation if needed). Lets allow 1dB loss in the antenna system, and call it -77dBm at the air interface.
Continue reading Measuring ambient noise level using a spectrum analyser

Last update: 29th April, 2018, 8:49 AM