Loop in ground (LiG) – #4 – SDR for measurement?

A series of recent articles developed a Loop In Ground antenna system design.

To test the prototype, I thought it an interesting exercise to use a low end rx only SDR for the instrumentation, providing a graphic quantitative measure of performance that is within the reach of most hams.

The first device trialled was a RTL-SDR v3 dongle with Sdrsharp (SDR#) software under windows, a very low cost option ($40). I was unable to find meaningful NF specifications or end user measurements for the thing in direct sampling mode. Continue reading Loop in ground (LiG) – #4 – SDR for measurement?

Loop in ground (LiG) – #2 – trial topology selection

* * * D R A F T * * * – a working document.

This article documents the selection of the trial loop in ground configuration as a development from the loop on ground antenna (KK5JY).

Baseline

The baseline is a minor variation of a design by KK5JY, a 15′ square loop 20mm above average ground, with 9:1 transformer and 50Ω load middle of one side.

Above is a plot of feed point impedance when the loop is driven. At 3.6MHz, the source impedance for a rx system is 43+j852Ω, and the mismatch loss to a 450Ω load is 11.0dB, a direct contribution to Antenna Factor (AF). Continue reading Loop in ground (LiG) – #2 – trial topology selection

Loop in ground (LiG) – #1 Feasibility study for rx only on low HF

* * * D R A F T * * * – a working document.

This article documents a feasibility study of a smallish loop on or in ground as a rx only antenna for 160-40m, possibly with advantage in high noise environments.

Various ‘on ground’ antennas are discussed online etc, but there is a distinct lack of supporting scientific evidence though subjective anecdotal evidence abounds.

The approach used here is to determine the degradation of S/N resulting from a low gain antenna system in the context of expected ambient noise as per ITU P.372-13. The analysis leans to the conservative side. Continue reading Loop in ground (LiG) – #1 Feasibility study for rx only on low HF

Implementation of G5RV inverted V using high strength aluminium MIG wire – 12 month review

This article continues on from Implementation of G5RV inverted V using high strength aluminium MIG wire documenting review after 12 months operation under a wide range of temperature, humidity and wind conditions.

Above is a view of the steel mast with the Inverted V G5RV rigged from the top of the 11m mast using a halyard though a purchase on a small gibbet to offset the antenna and feed line from the mast. There are lateral guys at 7m height, and the left hand one is non-conductive synthetic fibre rope. Atop the mast is a 2m/70cm vertical. Continue reading Implementation of G5RV inverted V using high strength aluminium MIG wire – 12 month review

40m filter for WSPRlite flex

The WSPRlite flex requires external low pass filters for each band of operation.

Since my experiments will be conducted on the 40m band, the following low pass filter meets the requirement. The inductors and capacitors make a seven element Chebyshev filter as designed by G3CWI for use in a 50Ω system.

Implementation

Above, the filter is assembled on a piece of matrix board with two BNC connectors. The inductors are fixed with hot melt adhesive, and the whole thing served over with heatshrink tube. It is not waterproof. Continue reading 40m filter for WSPRlite flex

Another RFI mod of a speaker mic for DMR use

I bought a remote speaker-microphone (RSM) for a DMR portable from eBay (~$12 posted). Experience says that these suffer RF ingress which is seriously bad in DMR due to the amplitude modulation of the transmitted signal.

This RSM had somewhat improved filtering around the electret compared to others I have purchased. Continue reading Another RFI mod of a speaker mic for DMR use

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

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

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

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