This article reports an experiment to compare two adjacent antenna systems on 40m over a 14,600km path using QRSS.
The experiment is to compare a 40m quarter wave vertical with an an OCF wire dipole antenna of complex shape.
The receiving station was W4HBK at Pensacola, FL (USA).
The transmitting station was VK2DVK located about 100km S/SW of Sydney (Australia), and the antenna systems were :
At the time of the tests, the path elevation angle at the transmitter is very low, in the range 2° to 6°, and bearing is 78°.
A QRSS keyer that facilitates antenna switching based on special characters embedded in the message was constructed. It was used with a quite standard Icom IC7410 in CW mode adjusted for 5W output.
Fig 1 shows the internals of the QRSS keyer. The keyer is described in detail at Another Morse beacon keyer - A/B RF switching.
The message is structured to send a period of key-down for 20s, then VK2DVK at QRSS6. Antenna B is used to send DVK, the rest of the message is sent on Antenna A.
The receiver used Spectrum Lab to gather and present a view of received signal and noise.
Two kinds of charts are used for analysis:
Data is presented below for the hour from 08:00UTC on 23/12/12.
Fig 2 shows a set of six graphics cropped from the full waterfall charts. They show VK2DVK's signal at the top of each chart over the hour.
The watch plots below plot the calculated Signal/Noise ratio in blue, and this is the best indicator of the relative performance of each transmitting antenna at the time. The green and red lines are Signal and Noise respectively.
Some interference can be seen as vertical lines on the waterfall charts and elevated noise (red) on the watch plots below. Best inferences can be made at times when the interference is absent.
Fig 3 shows a set of six graphics cropped from the full watch plots. They show VK2DVK's signal over the hour.
Fig 4 shows a capture of VK2DVK at VK1OD, 18km distant and an NVIS path.
The first question is whether one antenna is significantly better than the other?
The existence of statistical noise due to fading constrains the meaning of significant given the extent of the data. Fading is always present in Fig 3, and ranging from about 5 to 12dB in roughly 10s cycles, superimposed on a slower more random fading phenomena.
Indeed the nature of the fading disclosed in Fig 3 suggests that the common practice of A/B tests by switching between antennas for a quick report, perhaps even a few times, is very likely to be exposed to large error.
So, working with an hour's data and concentrating on the S/N plots when a dah element is being sent, on the 10min plots in Fig 3, it would seem that Antenna B S/N tends to be 5 to 10dB lower than Antenna A.
The situation is reversed in Fig 4 where Antenna B is perhaps 5 to 7dB better than Antenna A. Fig 4 is presented to show that an antenna that is optimal for low angle paths may not be so good for NVIS paths.
So, in answer to the question as to whether one antenna is significantly better than the other, it would seem that on the Robertson-Pensacola path, Antenna A is around 5 to 10dB better, but Fig 4 shows that it is quite different for an NVIS path.
The final detail, Antenna B is (obviously) the horizontal dipole, and Antenna A is the quarter wave vertical over elevated radials.
© Copyright: Owen Duffy 1995, 2021. All rights reserved. Disclaimer.