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Old July 12th 17, 12:37 AM posted to rec.radio.amateur.antenna
Jeff Liebermann[_2_] Jeff Liebermann[_2_] is offline
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First recorded activity by RadioBanter: Jun 2007
Posts: 1,296
Default Magnetic receiving loop theory

On Tue, 11 Jul 2017 17:39:37 -0400, Pat wrote:

On Tue, 11 Jul 2017 10:11:11 -0700, Jeff Liebermann
wrote:

On Tue, 11 Jul 2017 07:35:35 -0400, Pat wrote:

Sounds good. As an aside, I just watched a youtube video of someone
trying out one of these magnetic loop antennas. With his particular
set of circumstances, it reduced the noise floor on 80 meters
significantly.


Compared to what other antenna? He probably reduced the received
signal strength by the same amount leaving the SNR unchanged.


Not really. Both signal and noise are reduced, but the SNR must be
better because you can hear an SSB conversation with the loop that is
not there with his sloper. Search youtube for MFJ-1886 and N9BC.


https://www.youtube.com/watch?v=ECDklLp2FOk (2:56)

Nice of him to crop off most of the spectrum analyzer display at the
top of the screen so that I couldn't see the SNR changes. Also, he
didn't indicate which antenna he was testing in the first video. He
mostly fixed those problems in the 2nd video.

https://www.youtube.com/watch?v=_-GPS1Kqfec (4:13)

Of course, there is no mention of antenna patterns. Maybe the QSO he
was listening to is in a null or his other antenna. Not enough info
to really tell.


Nice of him to compress the signal strength in the spectrum display so
that it's difficult to compare SNR between antennas. There might be a
difference between antennas, but my guess(tm) is that he has the AGC
turned off in his SDR receiver. If AGC were on, the base line noise
level would be about the same for both receivers, which would raise
some questions as to whether there really was a difference. With the
AGC turned off, the higher gain of the sloper antenna will show more
baseline noise, which is what your seeing on the spectrum display.

However, if I freeze the 2nd video for each antenna, and just look at
the RELATIVE levels of the signals to the base line noise levels for
each antenna, I think you'll see that they're fairly close[1]. That's
the SNR which is what's important, and not the absolute levels of the
noise and signal.

Please note that the MJF-1886 is an amplified broadband untuned loop
which means it has a rather low Q. It obtains no benefits from the
narrowing the receive bandwidth as would be found in a high-Q transmit
loop. Strong signals anywhere in the 1-30MHz amplifier bandwidth
will create intermod products which might land where you're listening.
Incidentally, if you disconnect the MJF-1886 amplifier, and use it
like the PA0RDT miniwhip, my guess(tm) is that the loop and the
miniwhip will work almost identically.

Ok, Methinks I see the problem. The MFJ-1886 looks too good. With
antennas, the uglier it looks, the better it works. Nice looking
antennas just don't seem to work well.


[1] I have a customer on the phone who wants my attention so this
will need to wait.

--
Jeff Liebermann
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