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Thread: Building a Small Magnetic Loop Antenna (Part 2)

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    Default Building a Small Magnetic Loop Antenna (Part 2)

    In Part 1 we discussed construction of the antenna. This section will discuss computed vs. actual on-air performance


    COMPUTING PERFORMANCE


    I calculated the performance of the loop using the AA5TB spreadsheet available at Steve's website:

    http://www.aa5tb.com/loop.html



    The loop's performance is exactly what I was looking for in a compact package. The following graph shows the computed efficiency and bandwidth of the antenna:Here we can see the antenna really begins to function as an SMLA starting with the 30m band and moving down in frequency. The efficiency of the antenna is as expected, with the lowest usable band being 80m. As we move above 30m, the antenna still has excellent performance, however, its radiation patterns will start to change from those of an SMLA. Taken to the extreme, a full wavelength loop is really more like a "squished" folded dipole and it's radiation pattern looks quite similar.

    The following graph highlights the antenna's Q and capacitor parameters (assuming 100W transmit power):

    We can see that peak Q happens in a broad curve covering the 80, 60, and 40m bands. High Q gives the antenna sharp tuning and the resultant potential to reduce or eliminate noise adjacent to the desire operating frequency, thereby enhancing SNR performance. We can also see that below 80m, capacitor values jump into the mildly ludicrous range, indicating that this antenna effectively bottoms out at 80m. Computed efficiency of this antenna on 160m was approximately 0.5%! Efficiency on 80m was calculated to be around 5%, which seems low, but is actually quite suitable for NVIS operations. NVIS operations are completely doable with reliable circuits at QRP levels because there is relatively little path loss with an NVIS signal compared to lower angle multi-hop skywave propagation. US military personnel work NVIS operations on a regular basis with 15W manpack radios and low wire antennas.

    Also notice the yellow line in the second graph above, which is the voltage through the coax stub in our antenna. Loops are capable of tremendous voltages. If coax stubs are to be used to tune the loop, ensure the coax is something suited to high voltages. RG-8X is not going to cut it here, folks! RG213/U or RG8/U would be my minimum recommendation. Even with good coax stubs, I would still avoid more than about 50W using high duty-cycle digital modes or CW due to voltage through the stub. SAFETY NOTE: Due to the high voltages running through this antenna, mounting it at ground level can pose a hazard for severe RF burns and RF field exposure. Precautions should be taken to ensure personnel and pets are kept well away from the antenna during any operations.

    Summarizing the performance graphs, I expected the antenna to operate well as an NVIS antenna on 40-60-80m bands, and also enjoy moderate performance for DX on some of the higher bands. Anecdotally, upon connecting the antenna to a transceiver I was pleased to find that receive performance was excellent. The loop was noticeably quieter than my 43' vertical in an urban setting. SNR's were excellent and pulling in signals, even outside the transmit bandwidth of the antenna was very easy. On 40m during the daytime, the antenna has been a tremendous NVIS performer. I have made contacts with ops in Prescott, east Mesa, southern Utah, Kingman...all previously unreachable with my 43' vertical. The antenna seems fairly omni-directional as an NVIS antenna...in the far-field I didn't notice any major gaps in coverage due to sharp antenna lobes. I did attempt to work the Flying Pigs Club Run For The Bacon Monthly QRP Contest on 40m last night to see how weak signal performance stacked up. Due to the low mounting height, for DX work the house to the east of the antenna gets in the way. I did make a California contact (approximately 300 miles away) on 40m running only 3W. This was NVIS propagation at its best, and helped validate the theory that path losses over NVIS easily accomodate QRP operation.

    I also attempted some local NVIS work with Virgil on 40m with mixed results. Late one afternoon I tried Virgil on 40m while the sun was still up. His QTH is approximately 30 miles from mine as the crow flies; however, there are two 700 foot mountains in direct line of sight path between us. Virgil was able to copy me via LSB when I ran 50W or so. He was able to copy CW when I was running 5W. We tried an EasyPal picture running 75W (B Mode/2.2/QAM4/RS2), but it wasn't quite enough to get the signal to lock in. Overall, I'd call it mixed results, although once again I was impressed with the efficiency of CW for QRP NVIS ops.

    This afternoon I hooked up my 17m stub and figured I'd try out the antenna for some DX. Although I expected the loop to be efficient on 17m, I also realized that the TX pattern would not resemble that of a small loop because the loop is too large in circumference to be considered an SMTL on 17m. Over a casual 2 hour operating period I made SSB contact with S58N in Slovenia (6000+ miles contact), and CW contact with HA9RT, ZP6CW, HR5/F2JD, JQ2IQW, and XE2AI. On 17m the antenna seemed directional in the plane of the loop, but I was unable to assess any side lobes from transmissions. Coupled with my fancy new Flex3000 radio (thanks to W6SDM!) the loop seems to be a more than adequate performer for higher band DX, as well as local ops.

    CONCLUSION


    After initial testing, this loop has met and exceeded every expectation I had for a small antenna that would comply with HOA restrictions. I still have work to do on the antenna. My vision for the finished product would be a loop with a cleaned up gamma match, as well as a stepper motor driven vacuum variable capacitor to replace the coax stubs, and a TV rotor on the base to allow me to tune the loop and spin it from inside the house. As such, I learned an enormous amount about antenna construction through this project and have already started a list of "things I would do better next time." Again, I want to emphasize the need for appropriate antenna safety for any who may consider ground-mounting theirs, as I did. Voltages can exceed 5kV running as little as 100 watts. Please keep people and pets away from this antenna while in use! Overall, I think the small magnetic transmitting loop is an excellent answer to those who live in HOA-governed communities, and I have been impressed with its RX performance, small footprint, and low cost. It has proven itself to do everything I expected and a little more. I can't wait to put the finishing touches on this one and start on my mobile HF loop!

    73,
    Kurt
    KE7KUS

  2. #2
    4x4 Ham Member WB7X's Avatar
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    Excellent job Kurt.

    Maybe you can reach me in Peoria.


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    Super write up!
    Steve - KY7K
    That's the thoughts of a fool, thinking out loud.

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    Quote Originally Posted by WB7X View Post
    Maybe you can reach me in Peoria.
    You laugh, but I can't work Paul, KE7NEA, on 2m simplex and he's only 6 miles from me over off Grand Ave. Apparently there is a large school complex with tall metal buildings between us that pretty much kills whatever LOS we have. (At least according to Google Earth.)

    Several of us have talked over the past year or two and expressed some frustration with being able to work places like Honduras, but not being able to work each other across the Valley, or stay in touch with the Valley on HF from places like Barlett Lake, Payson, Ash Fork, etc. I think it goes to show that antenna planning and NVIS operating techniques are two critical skills for hams who are interested in either consistent regional ops, or emergency services. I know it's been a great learning experience from me...both for operating technique, and also for antenna construction. It's funny...I hear a lot of hams on the radio these days lamenting how HOA restrictions have totally crushed their radio ops, but I decided early on to make it a technical challenge and learning experience, rather than something to complain about. I probably won't make DX Honor Roll any time soon with this setup, but it's definitely made me a better radio operator and helped me understand antennas a lot more than I did prior to starting the project. More than anything, it's made me want to drop the dough on an MFJ analyzer of my own so I can continue experimenting with antennas. I'm excited to take the loop concept mobile and see what kind of performance I can get in a lightweight, low-profile mobile setup.

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    4x4 Ham Member WB7X's Avatar
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    Kurt,
    Interesting about the school in the way on VHF. I talk to a guy in Buckeye that is behind the White Tanks from me and he does well on VHF simplex.
    That one is a puzzler.

    I know guys in Globe and Payson if you want to try something on 40 one evening.

    Enjoy your experimentation.

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    Quote Originally Posted by WB7X View Post
    Kurt,
    Interesting about the school in the way on VHF. I talk to a guy in Buckeye that is behind the White Tanks from me and he does well on VHF simplex.
    That one is a puzzler.

    I know guys in Globe and Payson if you want to try something on 40 one evening.
    I think part of my and Paul's problem is that I'm using an H-pol 2m loop and he's using a V-pol stacked 5/8 wave vertical. That coupled with the school probably doesn't help things...to the point that there's no signal path.

    I'd love to work with you all on 40m one night. I'm finishing up studying for my Flight Engineer written exam this weekend. Once I have that knocked out, I should be back on the air in the evenings more frequently. I'll give you a shout next week and see if we can set something up.

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    Quote Originally Posted by KE7KUS View Post
    I think part of my and Paul's problem is that I'm using an H-pol 2m loop and he's using a V-pol stacked 5/8 wave vertical. That coupled with the school probably doesn't help things...to the point that there's no signal path.

    I'd love to work with you all on 40m one night. I'm finishing up studying for my Flight Engineer written exam this weekend. Once I have that knocked out, I should be back on the air in the evenings more frequently. I'll give you a shout next week and see if we can set something up.
    FE?

    Are you giving up lawn darts for "heavies"?

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    How does this work for NVIS on 40m? Let's say in the range of 100 miles.

    Kurt sounded great at 50w on 7.195 even with QRM. He turned the power up, but I couldn't really notice too much difference. Then this LID contester started calling CQ on top of us. We were able to reconnect later around 7.167, but I was having antenna tuner problems myself, so we cut the conversation short.
    Virgil - K7VZ
    Offroad and on the air.

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    I'm two for two now with 4x4 Ham HF QSO's outside the Valley with this antenna. Earlier in the week I made a 40m contact with Chris, K6OZY, who was working QRP from the eBay HQ parking lot in San Jose, CA. Total contact distance was 592 miles and the signal was 599 with 5W on both ends, despite a moderate level of noise on the band. We worked for over an hour, despite Chris' antenna being taken out by a wayward security guard. I did align the antenna so the plane of the loop was along the 300 degree heading from my QTH to San Jose. I would say 600 miles is outside what is considered the normal radius of NVIS communications, which I've seen generally touted as a 300-400 mile radius around a point. That being said, I'm not surprised at the antenna's performance, as radiation in the plane of the loop is fairly uniform (in theory) from 15 degrees elevation up to 90 degrees elevation. In practice, I have experienced fairly consistent contacts in the 40/30m bands out to about 1000 miles. Higher band performance has been seen out to 2000 miles, but the loop stops acting as a small magnetic loop above the 30m band, so that's not surprising.

    Working Virgil, I also aligned the antenna so the plane of the loop was in the direction of Virgil's QTH. RX was also good on this QSO, despite heavy QRM on either side of the 2.5k hole we found in the band. Virgil peaked around S8, but his ERP was unknown due to the fact that he was running 100W into a G5RV on 40m without a tuner, and had high SWR.

    Overall, I have been very happy with the performance of this loop when compared with its size, profile, and cost. It was definitely worth building and I can't wait to make my planned improvements.

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    I have recently become interested in Magnetic Loop antennas. The first one that I built is based on a May 1994 QST article "You Can Build: A Compact Loop for 30 through 12 Meters", using an 8 foot length 5/8" flexible copper tubing. I have an air variable capacitor to tune it. I have been happy with my results so far. I can tune 80 through 20 meters.
    IMG_1643.jpg

    My next loop is is based on a design of Rich, K8NDS. His qrz.com page has a nice description of his antenna and links to youtube videos of his antenna in use. My version of his antenna is made of 1.5" PVC pipe, wrapped with 3" wide 8 mil copper strap. He calls this a helically loaded magnetic loop. Rich runs a Kilowatt into his. I am awaiting a new vacuum variable capacitor to arrive from Ukraine. I expect to be able to tune 40 through 10 meters with my design.
    IMG_1660.jpg

    Another interesting variation of the magnetic loop antenna was in the November 2011 QST by WV6N. Maybe I'll build one of these some day.

    There is a yahoo group for loop antennas that has lots of pictures to give you design ideas.

    73 de KD7KEF, Jack

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