Topband: MFJ Receive Loop

[Andrew Ikin]

Bill,

I Bought one of these loops to evaluate. Without going into an in-depth 
critique. There are a few things that are worth noting.

The loop  amp. has an input z of 140 Ohms. Hence, there is loop/amp. 
miss-match at 1.8MHz i.e. the amp. input z should be approx. 20 Ohms.

The amp. gain is low at just 19dB.

Notwithstanding MFJ claims that MMIC amplifiers afford low noise. The 
reality is that the measured noise is approx. 6dB higher than thermal noise 
at 290 K.

Thus we are looking at amp. + the loop NF of approx. 7dB. At 1.8MHz there is 
also a 4dB, loss due to the loop/amp/ miss-match.

Therefore, the MFJ loop is only really suitable where the local noise is 
high.

The W6LVP loop has a similar high noise figure at MF, but it does reduce to 
approx. 3dB above 4MHz. Also it provides a better loop/amp. match at MF 
compared to the MFJ. However, there maybe potential issue with IMD.

The W6LVP amplifier third order OIP3 is low at +25dBm.  The OIP2 is a 
respectable; +70/80dBm. The Low OIP3 may not seem a problem at first sight.
But what it does; it reduces the overall power bandwidth of the amp. i.e. 
lets say you have 2 AM stations at about –10dBm output, the IMD will be
–80dBm. This is still too high. If there are other lower level signals these 
will be affected because of the amp. is being driven into non-linearity.

The Wellbrook ALA1530LN has an OIP3 of +50dBm. Therefore, with the same 
signal level compared to the W6LVP, the 3rd order IMD will 50dB lower 
compared to the W6LVP Loop.

What is also interesting; neither MFJ or W6LVP provide any loop/amp. 
performance data.

1m dia. loops manufactured by Wellbrook and Pixel/Dx engineering have low 
IMD and much lower amp. noise figures; ranging from 0.2dB to 2dB 
respectively; thus making these antennas much more suitable for Dxing.


73

Andrew Ikin

-----Original Message----- 
From: Bill Gillenwater
Sent: Tuesday, September 25, 2018 5:31 PM
To: topband at contesting.com
Subject: Topband: MFJ Receive Loop

Anyone have the MFJ 1886 receive loop? Any reviews on 160?

Thanks,
Bill K3SV
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