Magnetic Loop Antenna with SDRPlay

Useful information regarding antennas for SDR products.
dc1rdb
Posts: 19
Joined: Mon Jul 27, 2015 5:57 am

Magnetic Loop Antenna with SDRPlay

Postby dc1rdb » Fri Nov 20, 2015 6:36 am

Being in a situation where I can't install any external antennas, I've been experimenting with a lot of different indoor antennas but was never really satisfied with the results.
I finally decided to buy a active magnetic loop antenna, the ML200 from hamradioshop.net. This antenna covers a frequency range from 9kHz to 200 MHz. I attached the flexible loop via suction cups on the inside of a window. Initial results are simply spectacular. The previously experienced noise floor caused by home electronics like switching power supplies, computers etc. is almost completely gone and the signals over the entire frequency range are coming in clearly. Even VLF time signals outside the specified frequency range of the SDRPlay can be received with great quality.
A detailed review of the antenna can be found at http://www.fenu-radio.ch/index1.htm#ML200 , just use Google translate.
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g1hbe
Posts: 214
Joined: Sat Jan 17, 2015 3:28 pm
Location: Cheshire

Re: Magnetic Loop Antenna with SDRPlay

Postby g1hbe » Fri Nov 20, 2015 9:46 am

Loops are a fantastic solution to local noise. I use the Wellbrook loop (ALA1530) and it has saved my bacon. Any attempt to use my vertical wire on RX just gives me S9 of noise on the lower bands, but the loop just ignores it. I can now operate on 160m again - vertical wire on TX, loop on RX. Bliss!
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Andy

Fones
Posts: 17
Joined: Mon Oct 19, 2015 3:42 pm

Re: Magnetic Loop Antenna with SDRPlay

Postby Fones » Wed Nov 25, 2015 8:12 pm

Same here. I use a ALA1530 most of the time - absolutely superb antenna. An LA390 also gets a lot of use. Plus I have several TX loops. I've always been a huge fan of mag loops.
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snovotill
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Joined: Wed Aug 03, 2016 12:28 am

Re: Magnetic Loop Antenna with SDRPlay

Postby snovotill » Wed Aug 03, 2016 1:25 am

Hey how come everyone is using such expensive active loops for LF? Couldn't a guy just use a 3 foot diameter loop of copper pipe and match the impedance to the SDRplay input with a balun of some description?
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AH6DL
Posts: 49
Joined: Sun Jun 14, 2015 7:24 am

Re: Magnetic Loop Antenna with SDRPlay

Postby AH6DL » Wed Aug 03, 2016 6:55 am

I think it's a bit more complicated than that, but I'd like to see someone try!

For a low cost indoor alternative, consider the Degen DE31MS indoor active loop antenna. The whole thing knocked down easily fits in a 25 x 15 cm padded postal envelope so is great for travel. Performance isn't fantastic -- eHam gives it a bit less than "4" rating, but for under $20 on eBay you can't complain. It is tuned with a small potentiometer in the 2x AAA battery pack at the radio end. There is a varicap in and a MOSFET preamp in a small box at the loop.

Lot of details and links at http://www.drmrx.org/forum/showthread.php?t=1210. Looking at the comments on other web sites, I think some of the units shipped were defective, like so many cheap electronics, so buyer beware. If you don't want to gamble the money, make sure the seller takes returns.

During a stop in Los Angeles, where I had trouble even picking up local AM stations let alone any SW stations with an indoor whip antenna and SDRPlay, the DE31 provided a full band of AM stations, including at least one from San Diego. I also was able to receive SW broadcasts -- Ham radio on 20 and 40 meters, and, of course, the WWV HF time signals. I wish I'd had it with me in when I was working in San Juan, Puerto Rico last year where the noise level at the Caribe Hilton, right on the ocean, was so high that even with a 15 foot wire extended strung/thrown outside the upper floor lanai I got nothing but noise.

In Los Angeles, for local AM and FM reception, just connecting the loop, without the battery pack, to the SDRPlay did a reasonable job. I made a 1/8" female phone to F-male adapter to connect to the F-male to SMA adapter connected to the SDRPlay. The major negative is the varicap tuning voltage is generated in a switching power supply that operates around 3 MHz. The harmonics are a significant source of interference, especially around 6 MHz. Mine seemed to hit around 6.3 MHz, fortunately outside the SW broadband band.

If I have the time and energy on some upcoming trips I may give it a try in some other cities. It certainly beats the whip for AM/HF reception and does as well or better than a whip for FM (without the tuning box). I'm not sure how well it will handle the strong signals in Europe, but I might give it a try the next time I'm in Munich.

73....
....Doug AH6DL
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snovotill
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Joined: Wed Aug 03, 2016 12:28 am

Re: Magnetic Loop Antenna with SDRPlay

Postby snovotill » Wed Aug 03, 2016 2:38 pm

I was looking at that Degan thing but passed it up because it isn't broadband.
This looks absolutely fantastic though, so I bought one tonight:
http://active-antenna.eu/amplifier-kit/
And the full write-up with theory is here:
http://lz1aq.signacor.com/docs/wsml/wideband-active-sm-loop-antenna.htm
S

AH6DL
Posts: 49
Joined: Sun Jun 14, 2015 7:24 am

Re: Magnetic Loop Antenna with SDRPlay

Postby AH6DL » Wed Aug 03, 2016 5:50 pm

> snovotill :

I've been looking at that amp/antenna design for use at home. I like the design and the balanced twisted pair feeder. Please let us know how it works!

The Degen wouldn't be my first choice for a fixed antenna. It's main benefits are its size, making it a good alternative to the typical telescoping whip antenna for AM, SW and FM band reception (sans amp) when traveling. There is a switch on the amplifier at the loop that has to be changed for MW or SW reception, so in addition to the tuning knob, changing bands requires going to the antenna. Not a problem in a hotel room (at least the little ones I get) but would likely be an issue in a permanent setup.

73...
...Doug AH6DL
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DanubeBCL
Posts: 168
Joined: Sat Jul 02, 2016 12:39 pm

Re: Magnetic Loop Antenna with SDRPlay

Postby DanubeBCL » Thu Aug 04, 2016 9:16 am

"Degen style" antennas (or tuned high impedance loops) are certainly not a bad choice for portable operation at low power (batteries). For fixed installation a wideband loop covering 10 kHz ... 30/60 MHz should be preferred. You can put it in your garden (hopefully) as far away from neighbouring noise sources as possible. The LZ1AQ amplifiers are one of the best designed (and cheapest available) amplifiers for this purpose. LZ1AQ has theoretical background and practical experience and his descriptions make sense.

But one should not forget that magnetic loops are not the remedy against all plagues in a modern urban environment. The actual home "helpers" create high amounts of electrical and magnetic broadband noise. Switching power supplies and the worst invention of mankind ever, those "homeplug powerline modems" PLC/PLA/PLT, spread noise over hundreds of meters and make magnetic loops helpless just like any other type of antenna.

Well designed magnetic loop antennas can help against electrical fields in the immediate vicinity. But they are helpless against the high level noise coming from neighbouring homes in a modern urban environment. Unfortunately in many countries the spectrum authorities and law makers do not care much about high frequency pollution and have surrendered to the lobbyists of the industry.

73, Heinrich
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DaveB
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Re: Magnetic Loop Antenna with SDRPlay

Postby DaveB » Mon Oct 17, 2016 7:17 pm

Couldn't a guy just use a 3 foot diameter loop of copper pipe and match the impedance to the SDRplay input with a balun of some description?


Yes - I've tried it - it works. Not having any copper tube to hand I used a 10 ft length of RG6 U - fitted f-connectors at each end and used a barrel connector to join the ends together to form a loop. A 20 turn balun was wound on a large diameter toroid and this was threaded onto the loop of RG6. So the outer conductor and shielding acts as the magnetic loop. This loop was hung up near the SDR and connection to the balun via coax and thence to the SDRPlay.

I found this idea from the internet - on a document discussing the principles of magnetic loops - it was not being recommended btw - it was more a case of the simplest loop - from which to launch the discussion of loops.

I certainly got a lot more signal with the loop plugged in that without it - but nothing like the amount of signal I get with a 2 m long dipole fed through coax that is mounted outside and is primarily for the OIRT ~70 MHz)band. The checks were carried out on MW and the main broadcast SW bands.

It was an interesting experiment though.

David

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F1BJB
Posts: 108
Joined: Wed Jul 15, 2015 4:24 pm
Location: Beauvais France

Re: Magnetic Loop Antenna with SDRPlay

Postby F1BJB » Wed Oct 19, 2016 10:36 am

DaveB wrote:I certainly got a lot more signal with the loop plugged in that without it - but nothing like the amount of signal I get with a 2 m long dipole fed through coax that is mounted outside and is primarily for the OIRT ~70 MHz)band. The checks were carried out on MW and the main broadcast SW bands.
It was an interesting experiment though.

Hi
I made a few ,very small, magnetic loops for the FM broadcast band.
Sure they need to raise the gain of the SDR near maximum but they are wonderful at
rejecting vertically polarised local stations either by directivity or polarisation.
The trick is to use a stable set up. I used a cheap microphone stand from LIDL.
Selectivity of the loop is useful too specially with RTL SDR.
Sometimes you get better results with them than with a 5 elements yagi. :D

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