MFJ amplified receiving loop antenna testing

So i finally got around to checking this thing out. A powered receive loop that covers 500khz to 30mhz, and has the same characteristics as their magnetic loop antenna, but no tuning required. Not designed for transmitting, however if you have an HF radio capable of transmitting on antenna A and receiving on antenna B, then you can open up your ears and null out the noise. You can see in this video how it eliminates the high level of interference from power and gives more received signal.

 

OP... nice video. You answered some questions of mine on 100w and a wire on FB, today.

I want to ask others on this .... how would this stack up against a simple, non-amped flag receive antenna? Any guesses?

I'd like to buy one of these, but shipping here, to Korea, would likely be pretty bad, as they are in a big box. Have been thinking of building and trying a simple flag antenna out, as soon as I can get around to finding the torrid, and resistor I need, here in Seoul.

 

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Thanks for this. I personally would rather have a passive, tuned loop even if it's just for listening, but that's just me.

 

The loop I use gives me anywhere from 2-4 S units in noise reduction and an S-Unit increase in signal over my doublet, depending on how it is orientated to the primary noise source.

 

What loop do you use?

 

The DX Engineering loop. https://www.dxengineering.com/parts...oDmChxWjGgudAImZqtSTv1Cqug-4TFtrr4xoCQtjw_wcB

 

I like the passive untuned loops [kk5jy.net], particularly on the lower bands. As long as the loop is large enough to set the noise floor in the receiver, the S/N can be even better than with a tuned loop. That's what the MFJ and DXE loops are, essentially, an untuned loop with a preamp. I just like to omit the preamp whenever possible to avoid the noise figure of the preamp.

That said, I'm also amazed that people tolerate power line noise, especially when it's obvious which pole is the problem. Before dropping a few hundred $$$ on an antenna, get them to fix the power pole. After all, you are paying the power provider to do maintenance, through the check you send them every month. They won't proactively search for noise, you have to find it and report it.

Once the power line noise is gone, you can really evaluate the difference between a vertical and a receiving loop.

 

That is easier said than done to get the power company to deal with the problem.

 

I agree. It took me 18 months, two visits from Mike Martin, and an FCC letter to the utility to get them to even admit they had a problem. However, once they got to that point, they became very good at resolving power line noise issues. If I can get them to the pole (an Arrow V/UHF antenna is your best friend when hunting power line noise), they can usually get it fixed in a day or two. So it's not difficult once they are motivated.

One thing is for sure, if you don't address the problem, it will never go away, and it will never get better.

 

Doesn't a loop that has a built-in preamp automatically qualify as an "active loop"?

I'm surprised that a loop like this one or the DXE loops are effective when their preamps are not in use. Interesting. I have a Wellbook loop here. I don't use it too often, but used it a lot in my SWL days. It's a fine loop, but without its preamp it would be no competition for a small tuned loop.

 

The DXE loop won't be very effective without that preamp. That preamp provides 30DB of gain.

 

Yes, you are right -- I was just making the point that the loop can be both not-tuned and not-active.

I can't speak for whether those devices can operate without their preamplifiers... I'm not sure if there is a way to bypass the preamplifier or not. When I build a receiving loop, I just use a loop that is well isolated from the feedline (to prevent common-mode pick-up) and hook it directly to the receiver. As long as the loop generates enough signal to raise the noise in the receiver when it is connected, it doesn't need a preamplifier. In fact, using a preamplifier when it is not needed is counterproductive to the S/N [wikipedia.org].

Those are nice devices. I don't know if you can bypass the amplifier in it or not. But it is another of the untuned loops with a preamplifier.

What I discovered is that using a tuned loop, while providing more signal, doesn't provide the best S/N. If you take a tuned loop, and center it up on a signal, then tune the capacitor off-frequency a bit so that the desired signal is well down in the "skirts" of the resonant peak of the loop, the S/N actually improves, even though the signal level falls a bit. That's what got me started using simple wire loops with no resonating capacitor for reception. I have a rather large one that I use for 160m, and it generates beautiful S/N, even though I'm in a rather busy residential neighborhood.

 

Do you know if the DXE loop automatically bypasses the preamplifier if the DC is removed from the coax? Their in-line preamplifier has that feature, but their active loop was originally someone else's design, so I don't know about it.

 

No it does not and the preamplifier is designed to have a huge dynamic range.
They use noiseless feedback design and high level low IMD CATV transistors.
There is also a low pass filter that cuts off just above 30MHZ.

bypassing the loop would be counterproductive.