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 temporary 40m quarter wave vertical with a more permanent structure with more radials at greater height.
The receiving station was W4HBK at Pensacola, FL (USA).
The transmitting station was VK2DVK located about 100km S/SW of Sydney (Australia), and the antennas were two adjacent quarter wave verticals over elevated ground planes. The two antenna systems are:
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 07:40UTC on 22/12/12.
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.
Fig 3 shows a set of six graphics cropped from the full watch plots. They show VK2DVK's signal over the hour.
The first question is whether one antenna is significantly better than the other?
Concentrating on the S/N plots when a dah element is being sent, on some of the 10min plots, it is difficult visually to assert that one antenna is better than the other. There do seem to be more instances when Antenna B is poorer than Antenna A, but without some statistical analysis of the results, the difference would seem to be less than 2dB and quite small in terms of the variation in S/N ratio due to normal propagation fading.
|Height of radials||Gain (dBi)|
Table 1 shows the results of an NEC4 model of the maximum gain of a 40m quarter wave with 3, 4, 8, and 32 radials at 0.005m, 1m and 3.5m above average ground.
It can be seen that NEC4 would suggest there will not be much difference between an antenna with 3 radials at 1m and another with 4 radials at 3.5m.
The experimental results don't question the model.
© Copyright: Owen Duffy 1995, 2017. All rights reserved. Disclaimer.