Which antenna is better?
How to compare two antennas using Weak Signal Propagation Reporter (WSPR).
The paper below describes the way that WSPR data can be used to study radio wave propagation.
You are welcome to download the pdf report.
This page also describes the antennas used at my Ocean View radio station.
Antenna performance measurement
Weak radio signal levels can be measured and reported via the internet, using a software application named WSJT-X. The reports are forwarded to a central database, updated daily and publicly available for analysis.
In order to measure two antennas, two identical receivers are required.
My configuration uses two identical Software Defined Radios (SDRs) contained in one ‘black-box’ named SDR-Duo. The SDRs convert two radio frequencies (RF) to two audio frequencies (AF). The two audio signals are mixed and forwarded to the WSJT-X application.
All signal transfer and mixing is done in software – no audio cables required.
For accurate results, signal processing pathways must to be carefully calibrated to ensure that differences in signal levels are attributable to antennas under test, and not attributable to differences in two signal processing pathways. Accuracy better than +/- 0.5dB is achievable as shown in image 1.
The calibration method in image 1 uses one vertical antenna to feed two SDRs via an RF splitter.
In this example, the first SDR is tuned 10Hz above the nominated central radio frequency of 14 095 600 MHz, and the second SDR is tuned 10Hz below the central frequency. The 20Hz frequency separation is enough to identify which SDR and which antenna is being measured.
All signals under measurement fit inside a 200Hz bandwidth. At the audio spectrum, the 200Hz bandwidth is from 1400 Hz to 1600 Hz. This system gets accurate results with inexpensive hardware and free software.
The rural station location is ideally suited for weak signal analysis. The spectrum analyser confirms a noise floor better than -140 dBm with a bandwidth of 200 Hz, on the 20m band using a vertical antenna. That is the total system noise level of external environment plus the internal signal processing pathway. WSPR transmitting stations normally radiate less than 5 Watts of RF power. That is enough power to hear weak signals from all continents over a low noise floor.
Contest antennas at the Ocean View farm
The Ocean View station has a growing collection of radio antennas, including:
- 160m Inverted vee, 20m high supported by a tree branch (lobe favouring Asia);
- 80m wire dipole, 18m high between trees (lobes favouring North America and Africa);
- 40m full-size 3-element beam, 15m high, on telescopic tower with remote controlled rotator;
- 40m wire dipole, 13m high (lobes favouring North America and Africa);
- 20m dipole, 18m high on pole rotated by hand;
- 15m 4-element beam, 18m high on pole rotated by hand;
- 10m 4-element beam, 15m high on pole rotated by hand;
- Multi-band vertical dedicated to three SDRs;
- Low-band vertical phased array for directional reception.
A new 20m 4-element beam is currently being built and assembled in my workshop.
Work is underway to install a 30m tower to carry the 40m 3-element beam. The tower is telescopic and tilts for easy antenna installation – with two feet on the ground. The tower will also support three inverted V wire antennas: 80m for Europe, 80m for North America, and 160m with broad directional coverage favouring Asia.
The 40m beam and 40m dipole compliment each other – particularly in my afternoon when Europe is open on the long-path, (azimuth 140 degrees) while North America is open on the short-path, (azimuth of 50 degrees) side-on to the beam. Either direction is instantly selected with a switched coax stack-match.
I enjoy designing and building antennas.
Why do I use dipoles in a contest?
Dipoles are often underrated – particularly on the 20m band.
Dipoles have a wide capture area. The difference between a dipole and a multi-element beam is like comparing a spear gun and a fishing net. The spear gun is good to catch the rare fish. The net is good to catch the school of fish.
A beam may be useful on 20m when trying to penetrate QRM in Europe. However, in my experience, nothing penetrates QRM in Europe from the East coast of Australia. Time is often better spent moving to a higher band – from 20m to 15m.
Dipoles work on both the short-path and the long-path at the same time. The 20m dipole is useful in my early morning when North America is loud on both paths at the same time. Again in my afternoon Europe opens on both the short and long paths. Echo effects only become a problem when both stations use dipoles.
So, I say the humble wire dipole is an awesome contest performer.
Beginners can get an awesome start with a half-wave dipole and a single band entry in any contest. The next step is to build two dipoles to cover all continents on the same band.
If you find this article or paper useful, feel free to send to your friends: https://johnloftus.com.au/build-a-better-antenna/