开云体育

25 February 2022


I had a play with my ten or so year old Wellbrook wide aperture loop size today. For a couple of years I had it near the house and other wire antennas, but it did excellent service on the 136 kHz LF ham digi band, a frequency I am rather interested in. It wasn't very good at all at higher frequencies, noise predominated, and it was deaf above Top Band. I think it was too big, and too near the house and its noise sources.

I then moved it about 20 meters or so further from the house and other HF wire antennas. At the same time I made it about half the circumference.

It generally worked very well right up to 10 meters and the noise issue was gone, as far as I was concerned, but the LF performance degraded markedly.

Today I made it double the size again and higher. The KiwiSDR I use it with again showed a hell of a lot of noise and the overload signal warning showed very very frequently. I will leave it running overnight to see if LF has perked up again, but generally its pretty much unusable.

To my questions at last! If the loop is "too big" and the Kiwi shows noise and overload, can I tame it without other consequences with a 50 Ohm adjustable attenuator in the feed from the Wellbrook voltage injector RF output to the Kiwi, or is the the Wellbrook amplifier box at the loop antenna corner being overloaded as well as the Kiwi? In other words is too big a loop circumference detrimental to the Wellbrook amplifier's performance, the Kiwi's performance, or both? How do I find the sweet spot? For the Wellbrook and active loops generally?

This current loop is about 10 meters long. 3 meters high, rectangular and the bottom wire is about 4 meters above the ground in a wood behind the house.

The "quiet" loop I took down earlier today was lower but still rectangular, about 3.5 meters long and 2.5 meters high, lowest point about 3 meters above the ground. Location and orientation remained the same.

Thanks for any comments, I am slowly learning but looking for some short cuts as the wife wants me to be, err, more productive with more important things...(in her humble opinion...) ;)


The Kiwi can be seen at The noise lines across the lower frequencies and noise in general are terrible, presumably due to overloading? I don't consider this a high noise location at all, most of the time, I am pretty much out in the sticks...
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Best regards,
Chris mailto:chris@...

Can a a signal be too strong ?

Yes sometimes.

The KiWi has a limited dynamic range, and is being presented with just about every signal you could throw at it over the 0-30MHz instantaneous bandwidth it is capable of receiving.

As a rule of thumb if the noise floor on a quiet bit of spectrum increases by 6 to 10dB when a antenna is connected, then that is the optimum level.

With active antennas the test is a bit different as the electronics will also add to the noise floor.? So ideally you would check the increase in noise floor by connecting a screened inductor, with the same value of inductance as the actual loop, in place of the loop itself. Then measure the noise floor, reconnect the loop and check to see by how much the noise level increases.

The higher the noise floor is above the rule of thumb value of 6dB, the less dynamic range you have available, but the signal to noise performance hardly changes.

What you will probably find is that the noise floor on the HF bands increases by 6dB, but that when you check the LF bands it increases by 20dB or more, which is excessive.

The trick is to use an attenuator that has a decreasing value with increasing frequency. This is sometimes called a slope amplitude / frequency equaliser, and it's very simple to make.

Something like this circuit works well. The resistor values are chosen to provide the required attenuation on the lowest frequencies, in this case it's approximately 10dB.



Clint KA7OEI, has a good blog post on the subject, including some other circuits.



Regards,

Martin