Vertical Receiving Loops

Discussion in 'Technical' started by W5HRO, Mar 6, 2017.

  1. W5HRO

    W5HRO Administrator

    This is a continuation from another topic, but here is a good article about vertical receiving loops...

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  2. W5HRO

    W5HRO Administrator

    I did some additional reading last night and it looks like most people make the 80-meter vertical loops around 20ft in circumference using coax. However, 0.1 wavelength would be exactly 25.13ft for 4.0Mhz and 27.92ft for 3.6MHz. I think people make them only 20ft because it's easier to cut everything so the loop is only 5ft on each side. However, I also read where if you make it double in size, like 40ft the gain comes way up like 8dB to 10dB higher and the sharp nulls are not much different than what you get with a 20ft vertical loop. The nulls can still be 40dB to 80dB broadside even when the loop has a circumference of 40ft because others have already tried and proved it. The only real issue with making it larger is the wind handling capability and etc. It's just big and a little hard to work with so you have to make it stronger using better construction techniques. I think the ideal loop size would be 50ft, but that would be pretty darn large, but I do think only 20ft is too short. What I would do is make it exactly 0.1 wavelength long for right at 4.0MHz then add the tuning to make it work all the way down. Doing that the loops circumference would be exactly 25.13ft so making it right at 25ft should work a little better than if it were only 20ft.

    In addition to what I found out is that if you just want the loop to cut down on noise to work local activity within a few hundred miles then mounting it close to the ground works best. However, if you want the long haul stuff say like from California or Arizona all the way to the east coast then it needs to be up in the air higher. Like at least 20ft or so off of the ground on a pole or short tower.

    Another thing is in one of the ARRL books they have the loops input impedance incorrect. A 20ft coax loop for example has an input Z closer to 5K-ohms and it's not lower than 50-ohms like the ARRL mistakenly calculated it to be. So what you want is at least a 50:5 turns ratio if using 50-ohm coax to the receiver. So 50 turns around a torrid on the loop's side with 5 turns wound over it on the feed-line side to the receiver. And, DO NOT CONNECT THE LOOP'S COAX SHIELD ITSELF TO THE COAX SHIELD OF THE FEED-LINE TO THE RECEIVER. They must be kept isolated via the transformer.

    Anyway, it looks like there is a lot of misinformation on the web about building and using these receiving loops or at least there was and some of that misinformation is still there so be careful.
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  3. WQ5Q

    WQ5Q Member

    Good info! And as you pointed out on the other thread, it wouldn't hurt to put some common mode choking on the coax to the shack to improve the S-N. I need to do that now with my current inverted V set up.
     
  4. W5HRO

    W5HRO Administrator

    Shouldn't need the common mode choking with the loop, but it won't hurt to add some maybe in one or two places on the coax run to the shack just to be safe.

    Anyway, below is an example box. See where the two connectors are together on the one end? Those short the shield of the loop part together so it goes all around and through the box. Then if you notice the feed-line connector at the other end is isolated from the box. You have common mode isolation doing that so noise would have to get on the feed-line coax run itself (not very likely) and the loop (antenna) would have nothing to do with it.

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  5. WQ5Q

    WQ5Q Member

    I see that...good design.
     
  6. WD5JKO

    WD5JKO Member

    Here is a good sight for all kinds of cool stuff:
    http://www.pa0fri.com/
    PA3GZK's WIDE BAND ACTIVE LOOP RECEIVING ANTENNA
    01-nov-2015 A 10 pF capacitance parallel at the input of IC INA02186.

    I also uploaded a PDF from Clifton labs. Here two commercial receive only magnetic loops are carefully compared. The US made Pixel seems to be the clear winner against the Wellbrook.

    Wellbrook ALA1530L
    Pixel PRO-1B

    Jim
    Wd5JKO
     

    Attached Files:

  7. W5HRO

    W5HRO Administrator

    I don't think you would want a wide-band receiving loop. In a way that would defeat the whole purpose of using one. You want something that has very narrow bandwidth and is single-band help to reduce the man-made or natural-made out-of-band noise as much as possible.

    Anyway, that's just my opinion. Most people use them to null out a nearby noise source or during the summer months when you have atmospheric static coming from a certain direction.
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  8. WD5JKO

    WD5JKO Member

    Brain,

    You bring up a good point. The receiver used has a lot to do with the need to tighten up the front end bandpass. Consider most any receiver that makes AGC fronm the narrow IF amplifier. If the IF bandpass is 6KHZ wide, and there is a big signal 50 Khz away, that signal isn't seen by the detector, or AGC circuit. The RF front end however has to handle that big signal, and if it overloads, IMD results. This is where the receiver 2 tone tests come in. Some space 2 signals 2Khz apart, and some do it 20 Khz apart. The folks that really look at the 2 Khz 2 tone tests are the CW guys who often have a receiver IF bandpass at 500hz or less.

    The better receivers developed over the last 10 years have a dynamic range of 100db or more. Consider a 1uv signal ( -107dbm -- about an S2 on many receivers), that is 20 Khz from a S9+40/ signal (-33dbm).* To keep the receiver from overloading when tuned to the weaker signal, the receiver needs to have a dynamic range of at least 107-33 or 75db.

    * S9 @ 50uv = -73dbm

    A high dynamic range receiver that can tolerate those conditions can easily handle the output from a broadband antenna. I attach a good info source on this from NC0B at Sherwood Engineering.

    Here is another broad band vertical loop:
    https://www.w6lvp.com/
    http://www.eham.net/reviews/detail/13215
    http://www.ebay.com/itm/222414399775?

    Jim
    Wd5JKO
     

    Attached Files:

  9. W5HRO

    W5HRO Administrator

    I cannot find any pattern information about that magnetic loop antenna. With a built-in broadband amplifier and it's size there is now telling what it looks like say on 75-meters and at the different elevations above ground. It may be almost completely omnidirectional. It would be interesting to try, but for $300 it's a pretty big gamble and using it with an old boatanchor receiver.

    Here's the typical pattern for a narrow band 75-meter loop antenna with no amplifier.

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  10. WD5JKO

    WD5JKO Member

    The chart below shows about an S9 noise floor on 160m in a noisy urban environment. In a case like this, every 20db of attenuation added up front of the receiver will drop the S meter 4 units (assuming 5db/S unit). If the background drops from S9 to S5, then you lose nothing. If fact you shift the available dynamic of a given receiver UP 20 db, and that might move the off frequency crud down enough to prevent IMD.

    Jim
    Wd5JKO

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  11. W5HRO

    W5HRO Administrator

    But what about the sharp nulls? Does it have any? I looked around on that website, but the data (or pattern) is not present. How many dB of attenuation will it have when it's turned broadside to a signal?

    What I'm getting at is what if you live here in California and you want to work an Am'er in Pennsylvania, but there is some dam group of slopbuckets in Texas that are so strong you cannot hear the guy in Pennsylvania. In that case you need a loop you can turn to null the Texas slopbuckets out. Well, if the loop is pretty much omnidirectional then it won't do a dam bit of good. Good signal to noise ratio is one thing, but what I'm talking about is something else entirely.

    With a homemade single band 75-meter loop you will still have a null as long as it's no more than around 20ft or so above the ground. The lower to the ground it is the better the null will be, but if you want the distance then you have to raise it a little. You can use a built in preamp, but it better be narrow-band or tuned for a specific frequency otherwise the null will widen.

    I have a hunch those small magnets loops are fairly omnidirectional. They may have some directivity but probably not as much as a single band one made 0.1 wavelengths long for a specific band. I could be wrong, but I'd hate to spend $300 and find out the hard way.

    P.S. I did find in the white paper linked on that site where it says a loop can null out a signal by 30dB which is not correct. If done correctly it can be as high as 80dB. Nevertheless he has not published any data for it. All that white paper says is "a loop".
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  12. WD5JKO

    WD5JKO Member

  13. W5HRO

    W5HRO Administrator

    Ok, that's better... it looks like the null is a bit more narrow than the typical single-band loop, but at least that's something. The only thing I question is what's the real attenuation in dB of that other loop? That white paper said 30dB and you can get a whole lot better than that.

    Anyway, the guy should post all of that data on his website including the pattern.

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  14. KD6VXI

    KD6VXI Member

    This fact of the narrow band loop on attenuating signals off center would really make SDR radios perform.

    You'd need another sdr on a wide band antenna as well, me thinks..... If you do any search and pounce.

    --Shane
    KD6VXI
     
  15. WD5JKO

    WD5JKO Member

    Here is a narrow band loop that can take up to 10 watts transmit. On 80m, the 2:1 SWR BW is around +/- 3 Khz.
    Tunes 80-10m.

    http://www.btv.cz/en/mla-m


    HAM Radio > Transverters > MLA Magnetic Loop Antenna > MLA-M Magnetic Loop Antenna
    MLA-M Magnetic Loop Antenna
    At indoor desktop antenna design is special tuning by two knobs without any problems and by use of two variable capacitors which works in similar way as standard Π unit. Perfect impedance optimization in 3,5 MHz - 28 MHz is achieved by OK2ER and B PLUS TV a.s. patented design.

    MLA-M antenna is suitable for 8 HF bands : 3,5 MHz; 7 MHz; 10 MHz; 14 MHz; 18 MHz, 21 MHz, 24 MHz and 28 MHz.

    Technical specifications:
    Loop diameter: 62 cm
    Dimensions: 63 x 75 x 18 cm
    Weight: 2,6 kg
    Input impedance : 50 Ohm
    Input connector: PL
    Maximum input power (according to indoor use): 10 W
    Frequency range: 3,5 to 28 MHz
    SWR (tuned): 1:1 max 1:1,2
    2-zav-3_5-mhz_50_swr_resize_1342609816.jpg MLA-M Magnetic Loop.jpg

    Jim
    Wd5JKO
     
  16. WD5JKO

    WD5JKO Member

    Here is the big brother of the one posted above. This one can be placed outside, and tuned remotely. Adds 160m, and covers 160-40m. Power Max is up to 100 watts. On 80m, the 2:1 SWR BW is +/- 2 khz.

    Jim
    Wd5JKO

    http://www.btv.cz/en/mla-t

    Screenshot from 2017-03-14 05:55:56.png
     
  17. WD9GNG

    WD9GNG Member

     
  18. WD9GNG

    WD9GNG Member

    One Friday evening I contacted Larry W6LVP and asked a question about the kind of coax that he would recommend for his Magnetic Loop antenna. I was surprised to get a response with technical information within an hour or so. With this kind of customer service I decided to go ahead and order one of his antennas later that evening. I actually received the antenna on Monday afternoon within a couple days.
    I have extreme electrical noise that was S-8 most of the time on my Carolina Windom and made my radio almost unusable. I temporarily installed the Magnetic Loop antenna on a short 5 FT pole in the backyard. With the XYL as the null monitor at the radio I called her on my cell phone and rotated the antenna by hand and was able to get a sharp noise null of about S-1. Very tight null when rotating just a few degrees one way or the other. Went in the house and couldn't believe the clear signals that were hidden by the previous high noise level. It reminded me of SWLing 50 years ago as a kid back in the good old days before the electrical noise environment turned so bad.
    As I mentioned, the antenna arrived within a couple days and was of high quality construction and packed extremely well for shipment. I had read the previous reviews about Larry's product quality and customer service and my experience was also very good!!
    I am planning to mount the antenna on a Channel Master rotator one of these days to get the full effect of the excellent directionally of this Magnetic Loop antenna. I even read where Broadcast Band Listeners use this antenna to pick up and select between multiple stations on the exact same AM frequencies.
    I highly recommend Larry W6LVP and his Magnetic Loop antenna to other Hams and SWL listeners. He responds personally to emails within a business day usually just an hour or two. What more could a customer ask for?
     
  19. W5HRO

    W5HRO Administrator

    Get a 20ft push-up mast and try it at different heights. At only 5ft from the ground the null should be really tight, but not a lot of distance. Usually with the bigger homemade loops you need to keep it below 20ft. 10ft to 15ft is usually the right height for those before you lose all of its nulling capability. I'd get a 20ft push-up mast and play around with the height above ground to find the optimum height then lock it down. That’s a small size loop so there is no telling at what height will work best, but you have to start somewhere.
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  20. WD9GNG

    WD9GNG Member

    Thanks for the suggestion. I haven't put up the rotator yet and had planned to just use a 10 Ft metal pipe from Lowes. But I like your idea of a 20 Ft pushup mast to adjust and optimize the height allowing greater receive capability while not eliminating needed null directionally and pinpointing stations that I want to pick up. Thank you for taking the time to respond! I also picked up a Timewave ANC-4 noise reducing device that I am planning to put an outside noise antenna on to eliminate any remaining electrical noise from power lines, plasma TVs and other electrical trash around. I also have been going over to HF Underground website in order to find and figure out what some of the weird stations I hear are such as number stations, pirate stations, as well as monitoring military traffic. Really enjoying the Magnetic Loop and looking forward to it warming up here near St. Louis so that I can spend sufficient time outside to improve the setup. I am older now and have been retired for several years and that cold weather just gets to my bones if I am outside in the cold too much.
     
  21. W5HRO

    W5HRO Administrator

    What you do is tune into some AM station on 160 or 75 meters late at night with the loop close to the ground first. Tune to someone with good audio who you can hear that's far enough away and then go outside and put the loop on the pushup pole or mast. Then just raise it up and down until you bring that AM stations signal up, but to where your noise level stays low enough. Just find the right balance and then lock down the height.
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  22. WD9GNG

    WD9GNG Member

    Thank you for the additional information on how to optimize the height! I have really enjoyed listening to fellow hams on the 160 meter band. Used to have too much noise to do so before picking up the W6LVP Magnetic Loop. My ham rig and SWL radio is a Yaesu FT-857 that I have had for a long time now. I don't have anything for an antenna that will resonate on 160 meter for xmit now. But I am good on xmit from 80-6 Meter with my Carolina Windom. About 40 years ago I did fire up on 160 Meters with a Yaesu FT-101EE I had and a decent 160 meter dipole when I lived out more in the country. just getting the 80-6 Meter Window to fit in my city lot is difficult now days.

    I have been reading about magnetic loops for the last couple years. I had looked at the expensive Italian Baby Loop but it was just too much money for me to do. I have pondered building one with vacuum capacitor etc but most likely would never ever got around to it. I really enjoy jumping around on this forum! There is a lot of information that is shared and very valuable! Really appreciate folks like you that take the time to Administer these sites!
     
  23. WD9GNG

    WD9GNG Member

    W5HRO,
    I took your advice but instead of a slip up mast I used a 10 ft upper mast that slipped into a lower 10 ft mast and slid the top part up and down and compared 160 meter signals. 15 ft was the ideal happy medium for signal, directionality, and nulling. I locked the height at 15 ft and it is working great with the Channel Master rotator.

    I seldom need it but I also installed a 12 ft noise dipole 2 ft off the ground directly under my power lines. I connected this to my Timewave ANC-4 which is connected in line to both the 132 ft Carolina Windom and the W6LVP Magnetic Loop Antenna. The W6LVP Loop has an incredible noise null which is only a few degrees wide. With W6LVP noise nulled I don't need or mess with the ANC-4. But when I want to separate BCB, Beacons, or different hams on the same frequency I rotate to maximum signal on the station I want. When I do that sometimes I will still pick up some electrical noise which is eliminated with the ANC-4 noise phase reversal system.

    I find myself flipping the switch between the W6VLP loop and the Windom a lot to compare the signal and readability. I still can't believe how incredible the improvement in signal to noise ratio is with the loop. There are times when the Windom has a better signal but when I flip to the loop the background noise floor just falls away allowing good copy. My XYL who is not a ham complained she couldn't understand people on my radio on SSB because of static and hiss. When I flipped to the loop she commented that she could actually understand the people I am talking to on SSB.

    I just got everything hooked up recently and have truly enjoyed the better operating environment. I still can't believe that 80 meter SSB QSOs almost sound like 2 Meters. I actually run the squelch now on HF and it is dead quiet when folks are VOXing. Of course BCB, beacons, and SWLing has become a real joy now!

    Thanks again for the suggestions! I see that you take a lot of time as an Elmer helping folks make best use of their ham and SWL equipment! Your effort is appreciated!!!