Active loop build - construction questions

Useful information regarding antennas for SDR products.
rack201
Posts: 13
Joined: Mon Dec 26, 2016 12:39 am

Active loop build - construction questions

Postby rack201 » Mon Feb 06, 2017 4:08 pm

I'm about to attempt to build an active loop antenna and have a couple of questions re the loop.

I plan to use a 1m diameter loop - either aluminium tube if i can get it shaped, or the PE-Al-PE tubing used for hot water systems or a/c plumbing, and 1.5 or 2" PVC for the 'mast' which will run up the center of the loop.

First question is re the size of the gap at the base of the loop. Is this size critical? I have two ideas for construction Easiest for strength would be to use a PVC joiner/coupler inside the PVC mast but this would make the gap very small.

The second is on the location of feed points to the amplifier. It would be simpler for me to feed on the outside of the vertical PVC mast but this will be some distance (say 1 inch) from the ends of the loop? If necessary I could feed right that the ends of the loop inside the PVC mast but it will be very fiddly.

Cheers

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g1hbe
Posts: 217
Joined: Sat Jan 17, 2015 3:28 pm
Location: Cheshire

Re: Active loop build - construction questions

Postby g1hbe » Mon Feb 06, 2017 4:33 pm

Hello!

If it's a broadband untuned loop the gap is not important as the loop is low impedance. The feed should be taken near the ends of the loop but again this is not critical.
In my experimental loops I use plastic conduit boxes. I'm sure you know the kind of thing - about 3 inches diameter and an inch or two thick with 3 'spouts'. The ends of the loop can go in the 2 opposing spouts and the coax connector in the remaining one. I find that an SO239 fits pretty well in the hole. The loop can be secured by drilling through the spouts and driving screws through. The loop amplifier can easily fit in side the box. Once you are satisfied that it's all working as expected, you can fill the box and the spouts with potting compound and if you are a belt-and-braces man like me you can also wrap over the ends with self-amalg tape.
Have fun :mrgreen:

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Andy

DanubeBCL
Posts: 170
Joined: Sat Jul 02, 2016 12:39 pm

Re: Active loop build - construction questions

Postby DanubeBCL » Mon Feb 06, 2017 4:33 pm

The PE-Al tube is OK and a good idea. Important (for shortwaves) is big cross-section of the tube ( e. g. 1" or even more if possible).
The gap at the "base" is not critical. Connect the ends of the loop and the amplifier with cables as short and thick as possible to avoid additional inductance. Your 1 inch length is excellent. My constructions have more than 5 inches of additional cable. Ideal would of course be mounting the loop directly on thick bolts coming out of the amplifier case. But I know this is not always possible. And protect the connections against corrosion. Use stainless steel bolts and apply silicone coating or similar.
73, Heinrich

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rack201
Posts: 13
Joined: Mon Dec 26, 2016 12:39 am

Re: Active loop build - construction questions

Postby rack201 » Sun Feb 12, 2017 4:21 pm

Had a go this weekend making a single loop. I have not received the amplifier kit yet, but hooked it to the SDRPlay briefly and can certainly see some the sensitivity change when I rotate it by hand. Lower noise level is giving better digital mode decoding and HF fax reception.

Two questions: I would like to measure the inductance of the loop - i understand this is a critical dimension. I have a meter that measures inductance, but when I put the probes across the loop is drops down to zero on any setting. How can i measure this properly (am a novice)..

I found the Al in the Pe-Al-Pe pipe very thin and soft and think the quality of the connections may not be very good (its difficult to remove the Pe layers without damaging the Al). I am thinking about having some copper tube/piping shaped to a perfect circle - would be easier to connect and may give better results? Challenge is that the 'thin' copper tubing doesn't come in 1" around here. So my options would be heavier 1" copper pipe, or lighter 1/2" copper tubing (roll). Thoughts?

loop1.jpg
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DanubeBCL
Posts: 170
Joined: Sat Jul 02, 2016 12:39 pm

Re: Active loop build - construction questions

Postby DanubeBCL » Sun Feb 12, 2017 6:09 pm

A loop with this size should have appr. an inductance of 1.5...2.2 µH. I have a simple meter from http://www.morsex.com/aade/lcmeter.htm which does the job fine. (Unfortunately the creator of the kit has passed away.)
Alternatively you could put a known capacitor in parallel to the loop and measure its resonance frequency "somehow" ;-) and calculate the inductance. But I would not worry too much about this value with a loop of that size. In my experience the inductance slowly begins to be a limiting factor when the loop circumference exceeds roughly 1/5 of the wavelength.

The loop size is a compromise between good VLF/LW and good SW reception. The bigger the loop the better VLF and LW, but the more the loop current drops for upper SW. I found that a loop circumference of 3...5 meters is a good compromise for all bands from VLF to 50 MHz. Important is the input impedance of the loop amplifier. It should be as low as possible (see LZ1AQ loops). To reduce the inductance you could switch two loops in parallel or use so called "crossed loops" which look like an infinite sign (two loops in parallel) or like a four-leaf clover (four loops in parallel). The latter are difficult to construct with ring shaped loops.

I also have a loop made from PE/Alu hot water pipes. I use stainless steel nuts, bolts and washers for the electrical connections and have additionally sealed the connections with silicone. Full copper material has no advantages. It brings more weight and the DC resistance of the material itself is not so important.

73, Heinrich

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rack201
Posts: 13
Joined: Mon Dec 26, 2016 12:39 am

Re: Active loop build - construction questions

Postby rack201 » Sun Mar 05, 2017 4:10 pm

I received the LZ1AQ amplifier kit and I decided to update from the PE-Al-PE material to plain 1inch Al tubing (1.5mm wall size). I am running two 1m loops in the same plane, but results in the 40m band where I do most of my listening are not as good as my randomly cut center fed dipole.

For power I am using an old style transformer 12V AC-DC power adapter supply (not switch mode) and voltages are within limits specified by the kit. I am using a 20m shielded and foiled Cat5e FTP cable between the control board and the amp, with the shield disconnected at amplifier end. Leads to both loops are the same length at 200mm as specified.

Results for loop A, loop B and crossed loops are almost identical. Dipole mode (using the loops as dipole arms) is worse. Right now I am getting a strong SW station on the LO frequency at 7Mhz which shouldnt be there. I've been playing with Gain/gain reduction, the external front end gain setting and Zero IF/Low IF without much success, but I'm very much a beginner so I'm shooting in the dark. Seems like only very strong signals are coming through.

Attached are some pics of both the build and what I see in SDRUno. Any suggestions appreciated!
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DanubeBCL
Posts: 170
Joined: Sat Jul 02, 2016 12:39 pm

Re: Active loop build - construction questions

Postby DanubeBCL » Sun Mar 05, 2017 5:47 pm

Nice circle shaped loops! How did you bend the material? I never get mine perfectly round shaped.
I think you should place the antenna higher above ground to get better results with the dipole.
The crossed loops should deliver appr. 2.5 dB more SNR for frequencies above appr. 14 MHz. For frequencies below the atmospheric noise is higher than the total noise figure of the whole construction. If you have very high local noise (friendly neighbours with all sorts of modern electronics in their houses) I think you cannot benefit from the crossed loops so much.
The ghost signal seems to be an overload effect in the SdrPlay. The loop amplifiers deliver high signal levels. But you only could rule out this when another receiver shows the same problem.
73s, Heinrich

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rack201
Posts: 13
Joined: Mon Dec 26, 2016 12:39 am

Re: Active loop build - construction questions

Postby rack201 » Mon Mar 06, 2017 2:01 am

I cheated and had them rolled at place nearby with a hydraulic pipe bender :)

I now have it mounted up on the 2nd story roof and results are the same - seems to be overloading the SDRPlay everywhere.

Would a simple external attenuator help if this is the case? Where would I insert it? If it needs to go before he antenna amplifier it will be tricky as there are 4 leads from the loops, not just coax. Can I put it after the amplifier before the SDRPlay?

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DanubeBCL
Posts: 170
Joined: Sat Jul 02, 2016 12:39 pm

Re: Active loop build - construction questions

Postby DanubeBCL » Mon Mar 06, 2017 8:38 am

rack201 wrote:I cheated and had them rolled at place nearby with a hydraulic pipe bender :)
Good work.
rack201 wrote:Would a simple external attenuator help if this is the case?
When overload is the cause a simple attenuator right at the receiver input should of course help. I often use a simple 470 Ohm pot for test purposes. This is not state of the art RF technology, but it works better than one might think.
(Of course I have calibrated switchable 50 Ohm attenuators, too. But for reception without necessity of calibrated measuring the pot "solution" is absolutely OK.)
73, Heinrich
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