HFU HF Underground
Technical Topics => Equipment => Topic started by: ChrisSmolinski on July 23, 2019, 1610 UTC
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Impact of the Height Above the Ground to the Receiving Performance of a Small Wideband Magnetic Loop
Full article: http://www.lz1aq.signacor.com/docs/loop-height-for-small-magnetic-loop.php
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.
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Odd how the loops responded differently to the same sig when they were mounted so close, wonder what caused that.
Also, the author has other interesting pieces such as ;
http://lz1aq.signacor.com/docs/std/Measurements_of_Signal_Time_Delays_in_Several_SDR.htm