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Thread: HF Mobile Magnetic Loop Antenna (Part 1)

  1. #1
    4x4 Ham Member Supporting Member KE7KUS's Avatar
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    HF Mobile Magnetic Loop Antenna (Part 1)

    I recently returned from Afghanistan, where I encountered a number of nifty radio toys. One of the neatest toys I saw was the Stealth Technologies ST-940B HF Mobile Magnetic Loop Antenna:

    http://www.stealth.ae/plugins/custom...categories=136

    After reviewing the specs on the antenna (and receiving a price quote of $5000 US from a distributor!) I became very interested in building an HF mobile magnetic loop of my own (and hopefully for much less $US). In addition to its compact size, the magnetic loop antenna offers several distinct advantages over a traditional HF mobile vertical antenna, which makes it a very attractive option for those desiring enhanced mobile HF performance. This article is the introductory article of a series I plan to post as I document the construction of an HF mobile loop antenna for my Jeep.

    Traditional HF mobile vertical antennas commonly use a whip and loading coil configuration. Losses in mobile HF setups can be enormous. The ARRL Antenna Handbook categorizes general HF vertical efficiencies at around 80% on 10m; however, efficiency can drop to around 0.2% on 80m. This means that a 100W radio will only radiate .2 watts on 80m using a traditional HF vertical. Even an incredibly efficient HF vertical with cap hats and a large diameter, high-quality loading coil will only achieve around 2% efficiency on 80m. The two major losses in an HF mobile vertical are coil losses and ground losses. Magnetic loop antennas mitigate both of these to provide increased performance.

    For those unfamiliar, a magnetic loop antenna is almost as simple as it sounds. A loop with a circumference of 1/8 to no more than 1/4 wavelength is formed out of copper or aluminum, fed on one side, and matched on the opposite side. The two main feed types are an inductive loop formed at the feed point, or a shunt feed using a gamma match. The loop can be matched with something as simple as a coax stub, resulting in a fixed frequency loop. More advanced loop designs use a variable capacitor to achieve a match on a range of frequencies. The loop does not necessarily have to be circular. Loops can also be octagonal, square, or rectangular. The more area encompassed by the loop, the better the performance, thus the most efficient loops are circular; however, square or rectangular loops exhibit minimal performance differences and can be more practical for some purposes (like a mobile HF loop installation.)

    Coil losses in an HF vertical antenna are a function of the coil Q-value. The higher the Q-value, the lower the losses in the coil; however, even the highest quality coils used in modern verticals are only capable of a Q of around 300. In contrast, a magnetic loop is often capable of Q-values well above 1000. An 18 foot circumference square magnetic loop made of 1" copper pipe will have a Q-value of around 1370 on 80m. The high Q-values of the magentic loop contribute to its superior performance in the mobile environment.

    The greatest loss that most HF verticals experience is ground loss. Again, magnetic loops perform very well in this environment. A vertically mounted magnetic loop sees very little ground loss, even when mounted very close to the ground, resulting in significantly better performance when compared to an HF vertical antenna. It is important to note that this applies only to vertical loops, as horizontal loops should be mounted at heights comparable to dipoles for a given band to achieve best performance. Another significant performance characteristic of vertically mounted loops is a strong radiation pattern from 60-90 degrees elevation. This characteristic makes vertical loops very well suited for NVIS operations.

    Again, for the unfamiliar, NVIS is an operating strategy which relies on high angle radiation below the MUF to generate signals reflected off the ionosphere which provide very uniform radio coverage for a 300-500 mile radius around the transmitting station. This type of operating eliminates the "skip zones" which are present with lower-angle radiation, and makes for very effective regional communications for emergency services, as well as in terrain which prohibits traditional line-of-sight (LOS) operations on VHF/UHF frequencies. In the amateur bands, most NVIS communications utilize the 40m band during daylight hours, and the 80m band after sunset; however, the anticipated changes in FCC regulations with respect to the 60m band have also made it more attractive to me for NVIS operations due to its good performance during nighttime operations, coupled with a dimensionally smaller antenna requirement. I wanted NVIS operation capability in my mobile HF setup, as I anticipate it being used primarily for contingency communications in my local/regional area, or for rugged terrain trail runs where traditional VHF/UHF comms are not suitable.

    In building my loop antenna, much of my pre-build planning was accomplished using the KI6GD loop calculator:

    http://www.standpipe.com/w2bri/software/loopcalc.exe

    This tool helped me quickly assess the performance factors I would be considering in the construction of my loop antenna. Although the KI6GD calculator does not model ground-plane "half-loops" such as the ST-940B (where the cargo rack is an integral part of the loop), I anticipated that conventional loop modeling would give a rough idea of expected "worst-case" performance, and actual performance would be slightly better than computed. The KI6GD software allows the user to quickly change loop circumference, conductor diameter, operating frequency and operating power to view calculated loop performance.

    I began my calculations focusing on the 80m, 60m, and 40m bands due to my interest in NVIS work with the mobile antenna. I planned on building the roof-rack antenna out of 1" copper pipe over a copper mesh floor, with square steel tube as the structure material. I initially planned a roof rack measuring 40"W x 60"L to fit the YJ, knowing I could increase the size if needed. This would yield a vertical loop 5 feet long and 20" high (assuming the loop bar is designed to fold flat along the edge of the roof rack.) Plugging in the relevant numbers into the KI6GD calculator yielded the following output for 80m (calculations are for a square loop; however, rectangular loop numbers should look very similar):

    80m_Loop.jpg

    From this output, we see that the loop's efficiency on 80m is calculated to be 3.5%, which rivals, if not slightly exceeds the performance of a high quality HF vertical antenna. Keep in mind the high angle of radiation, which makes this a much better NVIS antenna than a traditional HF vertical. Also keep in mind the dimensions of the loop are 60"L x 20"H...a very compact package for 80m.

    Although the antenna promises reasonable performance on 80m, as mentioned above, the new regulations which up the ERP on 60m to 100W and allow both CW and digital mode operation piqued my interest and provide great promise for NVIS operations. The 60m band is suitable for NVIS operations, and offers greater efficiency for night NVIS as evidenced in the following calculation (60m Channel 1 was used for this run):

    60m_Loop.jpg

    The efficiency at 60m jumps up to almost 10%...a significant increase over performance on 80m. Stepping up to 40m shows even better results:

    40m_Loop.jpg

    The 15m band is the highest this antenna will tune. Above that, the loop is too large to function as a small transmitting loop. At 15m, however, the loop's performance is excellent:

    15m_Loop.jpg

    It should be noted that although the loop radiates well at high angles, it's low angle performance is also acceptable and should yield some DX work on the higher bands (20m, 17m, and 15m). The following NEC antenna diagram and radiation pattern show the general coverage provided by the antenna:
    [Figures provided by www.smeter.net]

    In addition to loss reductions associated with the magnetic loop, the antenna is also less susceptible to capacitive noise (power line noise, for example) which creates a higher SNR and subsequently improved performance. The high Q of the antenna creates a very narrow bandwidth, which can also aid in filtering out adjacent noise to a given signal, thereby also enhancing SNR.

    If we increase the length of the loop bar (and roof rack) from 60" to 72", the performance of the NVIS bands is noticeably improved. Efficiency on 80m jumps from 3.5% to 5.2%. On 60m, efficiency jumps from 9.7% to 14.1%. On 40m the jump is from 23.6% to 32% efficiency. Increasing the loop conductor size to 2" provides an even higher jump...80m efficiencies approach 10% while 40m efficiencies approach 50%.

    The antenna can be matched across the band from 3.9 to 21.350 MHz at 100W with a variable capacitor capable of 10-500pf and a 5kV rating. An air- or vacuum-variable capacitor can be driven by a simple 12V stepper motor using a control switch mounted inside the vehicle. With this configuration, tuning the loop is as simple as setting the desired operating frequency and adjusting the stepper motor until received audio noise is the loudest. Fine-tuning can be accomplished with the TUNE function on many radios and an inline SWR meter.

    Overall, the mobile loop antenna provides excellent performance in a very compact package. In coming posts, I will document the construction and installation of the loop, as well as its performance once installed. Until then...

    73,
    Kurt
    KE7KUS

  2. #2
    4x4 Ham Member AC0VH's Avatar
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    Be aware of RF exposure, the magnetic fields generated by resonant loops are quite high (obviously).

    The rule of thumb is that a minimum safe clearance is two lengths of the loop's longest dimension or 1/10th of a wavelength, whichever is greater, but you really need to run the numbers per OET 65.

    Most people assume that being low power (under 50W) make them safe for intermittent exposure, which is often true for verticals and dipoles. This is not the case with magnetic loops, especially so when you go >50W. Even QRP levels can generate a near field that is potentially over the exposure limits, which is difficult with anything other than a loop.



    I see people using them on HF packs and that is very risky, particularly since it's your head that in the field. Just be careful, I'd personally not want one right over my head like that, although at least you're not inside the near field broadside like the manpack people do. They've essentially made a portable MRI machine for themselves. The difference is that MRI machines are pulsed and very discontinuous, so your total dose is very small despite the high power. I should say, though, that there's not much medical consensus on non-ionizing radiation like this, so it might be perfectly safe. But radiation exposure is cumulative, so I suppose I worry about things I can control.



    http://transition.fcc.gov/Bureaus/En...et65/oet65.pdf

    http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2705217/

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

    http://sidstation.loudet.org/antenna-theory-en.xhtml

  3. #3
    4x4 Ham Member Supporting Member WB7X's Avatar
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    Welcome back Kurt.

    I'm wondering if flying combat sorties in AFG is more or less dangerous than RF exposure?

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    4x4 Ham Member AC0VH's Avatar
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    Quote Originally Posted by WB7X View Post
    Welcome back Kurt.

    I'm wondering if flying combat sorties in AFG is more or less dangerous than RF exposure?
    We all gotta die of something, eh?

    BTW, did a quick calculation and looks like the tuning cap could see very high voltage. I have it around 3kV at 100W on 75m and that considered loop would peak at around 5kV on the cap on 20m. If you did use a 18' circumference loop, it would present a Q of about 1200 and a radiation resistance of 0.0025 ohms. That seems like it would be very narrow banded, although very sensitive.

    Hard to see the diagrams, they're pretty small, but that pattern looks more like a half loop instead of a true resonant loop. That would be a good model for a loop like you've shown over a vehicle body. That should help with bandwidth and efficiency, too. A loop over good ground is better than one passively coupled to ground. So the vehicle acting as an artificial ground is good.

    The proximity to the magnetic field not withstanding. :-)

    Keeping in mind also that magnetic fields are not shielded per say by ferrous materials, like sheet steel... You can redirect high frequency magnetic fields but not shield them quite like E-fields. We call that magnetic flux shunting. I'd be interested if when transmitting a magnetic loop would play havoc with steel-shielded electronics inside the car, particularly those bolted to the outer skin of the body.

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    4x4 Ham Member Supporting Member KE7KUS's Avatar
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    Quote Originally Posted by WB7X View Post
    I'm wondering if flying combat sorties in AFG is more or less dangerous than RF exposure?
    Well, considering I was flying with 2000W of 9.8GHz radiation 18" in front of my feet for three months, I'd say 100W 18" above my head periodically is a dramatic improvement. And here in Phoenix, they only use small caliber weapons when they shoot at you.

    Thanks, Marty...good to be back...for sure.

  6. #6
    4x4 Ham Member Supporting Member W5LMM's Avatar
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    I like the setup, however, with theirs, it seems you have to buy their roofrack, and I'm happy with mine. I would like a loop though!

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    I ran across your post while searching for ideas for a HF antenna for my Jeep Patriot. Have you made any progress in constucting one? I have built several HF mobile antennas over the years and thought I would like to give a loop a try.

    73
    Hank
    W5HJ

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    I am glad this thread got "bumped" back into circulation. I have been looking for a NVIS antenna to put on my Lance camper. Lots of space and a nice sheet of aluminum. As I stated in a recent post, I am concerned about a solar panel that is also on the roof. 60m is my band of choice as well.

    I will follow this project with interest. Thanks for the calculator and good description. Lots for me to learn here.

    Ron, N1AHH

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    4x4 Ham Member Supporting Member KE7KUS's Avatar
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    I'm in the process of acquiring the radios for the vehicles right now. In the meantime, I'm working on securing two decent variable capacitors to use in the mobile antennas. Once the parts are all accrued I'll get to putting it together and document everything here. Stay tuned!

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    We will be looking forward to it.

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    Check this home built version of the costly military mobile mag loops.

    http://pa3ect.eu/magloop.html - bottom of page
    http://pa3ect.eu/magloopeng.html
    http://pa3ect.eu/indexeng.html


    YouTube
    http://www.youtube.com/watch?v=XC0JXW9hBbY

    George

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    I recently returned from Afghanistan, where I encountered a number of nifty radio toys. One of the neatest toys I saw was the Stealth Technologies ST-940B HF Mobile Magnetic Loop Antenna:
    Hello Kurt,

    Really happy that you've liked ST-940B. Meanwhile, it has been discontinued on 2010 and replaced with newer Type 9400 Super SkyRider Autotune Magloop.
    9400resized.jpg
    9400leafletF.jpg9400leafletR.jpg

    Also, we are currently launching our new 93-series of Autotune Whips comprising 4 models:
    9300 - 1.5-30Mc (MilSpec)
    9310 - 1.8-30Mc (HAM)
    9320 - 1.6-30Mx (Marine)
    9360 - 1.6-30Mc (commercial)

    The new series already dubbed in Middle East as "USB-Stealth Antenna" for their inbuilt software aided configuration and diagnostics features.





    I have many self explanatory PDFs but don't know how to upload. Some more info can be traced on our facebook page being updated by Sergey Lomadze who combines A65BU with crazy dune jumping.
    http://www.facebook.com/StealthTelecom

    73! George
    Attached Images Attached Images

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    You still wokring this?

    Quote Originally Posted by KE7KUS View Post
    I recently returned from Afghanistan, where I encountered a number of nifty radio toys. One of the neatest toys I saw
    Overall, the mobile loop antenna provides excellent performance in a very compact package. In coming posts, I will document the construction and installation of the loop, as well as its performance once installed. Until then...

    73,
    Kurt
    KE7KUS

    Kurt..

    any movement on this? I am going to work something for my LJ. I have a warrior rack on it, I need to find some copper and a cap.

    AL KA1FFO

  14. #14
    4x4 Ham Member AC0VH's Avatar
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    Came across this discussion about RF safe distance on QRZ.

    http://forums.qrz.com/showthread.php...-magnetic-loop

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    Kurt-
    I'm curious if you have made any progress on this antenna. I have a 99 TJ that I have been thinking about this exact project for... and with the price quote from the US distributor of the 9400 skyrider running nearly $15k.. I'd prefer to build one.

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    Quote Originally Posted by KE7KUS View Post


    [/CENTER]

    In addition to loss reductions associated with the magnetic loop, the antenna is also less susceptible to capacitive noise (power line noise, for example) which creates a higher SNR and subsequently improved performance. The high Q of the antenna creates a very narrow bandwidth, which can also aid in filtering out adjacent noise to a given signal, thereby also enhancing SNR.

    If we increase the length of the loop bar (and roof rack) from 60" to 72", the performance of the NVIS bands is noticeably improved. Efficiency on 80m jumps from 3.5% to 5.2%. On 60m, efficiency jumps from 9.7% to 14.1%. On 40m the jump is from 23.6% to 32% efficiency. Increasing the loop conductor size to 2" provides an even higher jump...80m efficiencies approach 10% while 40m efficiencies approach 50%.

    The antenna can be matched across the band from 3.9 to 21.350 MHz at 100W with a variable capacitor capable of 10-500pf and a 5kV rating. An air- or vacuum-variable capacitor can be driven by a simple 12V stepper motor using a control switch mounted inside the vehicle. With this configuration, tuning the loop is as simple as setting the desired operating frequency and adjusting the stepper motor until received audio noise is the loudest. Fine-tuning can be accomplished with the TUNE function on many radios and an inline SWR meter.

    Overall, the mobile loop antenna provides excellent performance in a very compact package. In coming posts, I will document the construction and installation of the loop, as well as its performance once installed. Until then...

    73,
    Kurt
    KE7KUS
    The calculator you used is completely unable to evaluate real world efficiencies simply because it does not 'ask' for the data that alloows efficiency to be computed. It assumes zero connection losses, zero ground losses and a tuning capacitor of infinite Q. All are unachievable in the real world. I have no idea why they even included that field.
    Try Owen Duffy's calculator: http://owenduffy.net/calc/SmallTrans...oopBw2Gain.htm
    I think the numbers will be sobering.
    My next question is why would the small loop yield a higher S/N on power line noise?

    Dale W4OP

  17. #17
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    I played with NVIS a little bit. It's a neat idea but I don't think it works that well with the frequencies hams have to work with. I did not see any difference in my NVIS setup vs a screwdriver type antenna. I might work on it in the future though. Looked at a mag loop as well. I don't like the RF exposure issues a mag-loop has. DON'T IGNORE RF EXPOSURE! Doing so is at the very least poor amateur practice. HOWEVER do post some pics of your mag-loop project! I might end up trying to build one just for giggles.
    Just googled StealthTelecom's stuff. Wholly cow their stuff is expensive! Quality but too rich for normal folks. I think they are going after the government market. I need to win the lottery so I can try some of their gear out!

    R

  18. #18
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    Quote Originally Posted by K7POC View Post
    I played with NVIS a little bit. It's a neat idea but I don't think it works that well with the frequencies hams have to work with. I did not see any difference in my NVIS setup vs a screwdriver type antenna. I might work on it in the future though. Looked at a mag loop as well. I don't like the RF exposure issues a mag-loop has. DON'T IGNORE RF EXPOSURE! Doing so is at the very least poor amateur practice. HOWEVER do post some pics of your mag-loop project! I might end up trying to build one just for giggles.
    Just googled StealthTelecom's stuff. Wholly cow their stuff is expensive! Quality but too rich for normal folks. I think they are going after the government market. I need to win the lottery so I can try some of their gear out!

    R
    If one is inside of a steel vehicle and the mag loop is on the roof, is RF exposure still a problem?

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