Published on: 2018/11/20, Rev. 1.0 July 2018, Rev. 1.1 Nov 2018
Author: Chavdar Levkov LZ1AQ
Setup
Two identical small loops were placed one above the other according to Fig.1. One of the loops is very low – almost on the ground. The other one is placed at height which usually is used by the loop users. Two AAA-1C wideband active antenna amplifiers were used. Their gain difference was not more than 0.3 dB. The feeder was FTP cable each 20 m long. No cable baluns were used. The outputs were connected through two way antenna switch to a SD RX (Perseus). I used a measurement technique described in - A Periodic Switching Technique to Compare Receiving Antenna Performance in the Presence of Strong Fading. This is a precise method to compare two receiving antennas with real sky wave signals and the resolution can be less than a decibel. The idea is to switch periodically between two antennas and to estimate their difference on a calibrated graphic strength meter of a SD radio.
Fig.1 Experimental setup. Dimensions are in meters. The loops are made of 16 mm PE/Alum. tube. The loops are placed vertically in one plane.
Environment
The loops were placed in a grass yard about 5 to 6 m from a house (Fig.2) in semi-urban location (in a valley). The soil is a bit sandy. The humidity of the soil is not known but the weather in the last several days was cloudy with occasional rain. Both loops are in one plane and the direction is N&S.
Fig.2
Experiment
The loop performance was tested on real signals in very wide frequency range from 77KHz to 28 MHz. Also listening on the bands for very weak DX stations was performed with manual switching to estimate for differences in signal strength. The band noise level was also measured at several frequencies as a power in 1KHz bandwidth at spots where there are no signals.
Results
The signal power difference between high and low loop Phigh - Plow is plotted on Fig.3. It must be mentioned that this difference is not constant and might be very unstable. Changes in the sign is common especially at higher frequencies. Fig.4 shows a case where the signal difference changes rapidly in time.
Table 1 shows the noise level difference PNhigh - PNlow
Fig.3 Signal strength difference Phigh - Plow for 108 measurements in the spectrum 0.07 to 28 MHz. For all points above 0 dB the signal in high loop is stronger and vice versa. As it can be seen, above 5 MHz the difference is unstable with strong fading affects. For some signals the temporal difference can reach 10 – 15 dB. Two points are plotted for the unstable signals – the minimal and maximal difference value. E.g. for certain signal the difference is floating from -5 to +10 dB – that means that sometimes low loop is better with 5 dB and then high loop is better with 10 dB. 0 dB means that the loops are identical in performance.
Table.1 Band noise level measured at some frequencies
Fig.4 A typical S-meter plot of an unstable signal at 15.6 MHz with a strong fading. Periodically the high loop is connected for 400 ms and the low loop for 200 ms. At the left side low loop is better with 7- 8 dB and then the high loop is prevailing with 10 dB. The signal is a carrier of a broadcasting station. The S-meter plot is made with Linrad SDR program.
Comments
There was almost no difference in signal strength up to 5 MHz. On higher frequencies the difference is floating and often can change the sign. These floating differences can reach a values up to + - 10 dB but it can not be drawn that one antenna has any advantages compared to the other. For several seconds the one antenna is better but then the other one is ahead. It is not clear why this dispersion begins so abruptly at frequencies above 5 MHz. The distance between loops at this experiment for 5 MHz is 0.05 lambda reaching 0.3 at 30 MHz. They are too close to expect such a differences in the case of plane wave. Obviously at these frequencies , the incoming electromagnetic wave is not homogeneous and can not be assumed as a plane wave. Additionally the interference picture is probably very “twisted” temporarily and spatially due to reflections from ground, nearby objects and hills. The distance between loops in this experiment is 3 m and at frequencies above 10 MHz they can be used even for diversity reception.
A side conclusion: the accepted assumptions for the e.m. field in antenna modeling might not be valid and the real life performance e.g. of a receiving phased array might be quite different to the model performance.
There is a slight advantage of the higher loop as can be seen on Fig.3 but more experiments must be performed to have statistically reliable results. Interestingly, the model calculations in NEC give slightly higher gain for the lower loop.
There is a difference of band noise levels at frequencies between 7 and 25 MHz and also at very low frequencies. It is also not clear what is the reason for that. This might be a local noise effects or a conducted noise since no special measures were taken to avoid the local man made noise.
Conclusions
The two antennas are with identical performance up to 5 MHz. At higher frequencies the wave interference picture around the two loops is not uniform (for this distance between loops) which manifests as unstable difference. This does not leads to marked superior performance of one of the antennas. The higher the frequency the stronger is this effect.
Practically we can mount a small vertical wideband loop at any height which is convenient for our local requirements without deteriorating its performance. An invisible loop almost on the ground level can be used with success for those who have a restrictions or inquisitive neighbors.