Hello,

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I have made some experiments to evaluate the performance of simple 2-element arrays with small wideband ?active elements? - dipoles or loops. ?

" Receiving Phased Array with Small Electric or Magnetic Active Wideband

Elements. Experimental Performance Evaluation"

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Chavdar LZ1AQ

lz1aq@...

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More great work Chavdar. Thank you for sharing your findings.?

I have some reading to do. :)

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Hello Andrew,

Thank you very much for your interest in? my? work.

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I have a few comments to make: There is no provision to amplitude balance the antennas or to compensate for minor delay-line loss. One is therefore assuming that the antennas are always?gain matched with frequency even when the presence of nearby objects could be affecting the gain. This will?in some circumstances reduce the F/B as nearby objects (metal fences, power/phone line etc) affect the antennas near-field response especially at LF/MF. My experience with MW active dipole, active loop and active K9AY arrays using variable delay-line and switched delay-line phasing indicate that the provision to vary the antennas gain is just as important as having control over the phasing.

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You are right - there is a gain variance. As shown on Fig.18 the signal difference between two antennas depends very much from electromagnetic environment ??it is up to + - 3 to 4 ?dB in bad conditions and less than 1 dB in clear place. And moreover it ?is frequency dependant. ?The loss in the delay line is usually less than 0.5 dB. Usually the gain of the amplifiers is not the problem (with 1% elements the difference can be set to be less than 0.2 dB). As you say the problem is the environment.? But any precise gain correction is frequency dependant so changing frequency you have to change the gain correction. Probably with analogue phasers ?you can set? precisely the best gain and phase and to increase the F/B. But this is time consuming , frequency dependant and for me not very practical.? I do not think that it is so important to have let say 13 dB instead of? 10 dB F/B on real signal.? We must swallow the reduction of the F/B for the price of increased functionality and real wideband performance.? In the case where the array is built in a clear environment such a correction is not needed. From the experiments you can see that without such corrections the F/B ratio can reach 45 dB in controlled conditions.

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I agree with your observation that a 1 to 10? frequency ratio is readily achievable. However, where the antenna spacing is very small, there is a further degrading in s/n due the amplifier noise not cancelling out in same way as the signal. This is more of an issue with small 1m diameter high gain loop antennas.

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Yes, I have pointed out that in very closely spaced elements you should have to use larger elements to improve the noise floor. See? 5.5.? in discussion? part.

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>Also with very close antenna spacing the array phasing and gain balance becomes very critical, >causing pattern degradation.

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?I do not think so. See 5.4,? In very closely spaced arrays the main problem is the reduction of the effective height and the noise floor limitation,? the optimal phasing? is easily obtained at least with controlled delay line. You can look at Fig.8???? where the sensitivity to 20% deviation from the optimal ?delay? is plotted.? The pattern, at least in models, is not degraded down to very small D/wl. I can not say how is influenced the pattern in real environment – this will be very difficult and time consuming experiment but in my controlled experiments the F/B is not much degraded even below 0.05 D/wl. Look at Fig. 16,? Experiment 3,? where F/B ratio is plotted. For 1.8 MHz? and? D= 6m ( 0.036 D/wl )? the F/B is between 25 and 35 dB for both loop and dipole arrays.

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>Therefore, it may be prudent for MW/LW use to allow for an increase in antenna spacing to over >30m.

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Yes, The optimal distance D is somewhere between? 0.1 to 0.25? D/wavelength.? That means for ?1MHz, ??D is ??between 30 and 75 m . But my main question is how to squeeze D in order to build very closely spaced arrays which are still usable. I am pretty sure that with careful? antenna design? and proper amplifiers and? matching? we can reach uncompromised performance with D/wl values equal or below 0.05. For spaces 0.01 wl? and below I suppose that? the additional problems will be common mode signal leakage and probably the reactive filed influence between elements.

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>Using the above arrays at HF can be quite disappointing due to signals having too much angular >variance. Loop arrays offer the worst performance because they offer little rejection of rear high >angle signals.

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I do not agree. If? we expect the behavior of several element yagi of course this is not the case. But the 2-el loop array has decent? directional pattern and it is the best RX antenna in my small yard. The 2el- short dipole mode is influenced too much by nearby cables? and the metal fence and has too high noise from nearby high voltage electrical line. Practical ?comparisons to? K9AY loop show that the directional characteristics of the 2el-loop array are? definitely better (at least in my yard).? The loop array has also narrower front lobe compared to the dipole (vertical) array. From the other hand in clear environment using loops and dipoles with the same switching scheme? and delay setup is an advantage – it is always better to have 2 antennas with different patterns.

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>Additional Reference:

?>Keen’s Wireless direction Finding, Third Edition 1938.

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Unfortunately I do not have an access to this paper.? Some times I have a feeling that I am inventing the wheel again. I am pretty sure that a lot of work has been done in this field in 30’s upto 60’s years of 20th century. I am not professional in this field and I have done this work? to satisfy my own curiosity.

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Kind Regards,

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Chavdar

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Lz1aq@...

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