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Author Topic: Magnetic Loop TX antennas  (Read 4471 times)
N4NUI
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Posts: 3




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« on: March 22, 2011, 12:01:25 PM »

After months of dealing with having to operate on HF digital modes from the parked mobile /portable with a HamStick dipole due to condo antenna restrictions, I read everything I could find in publications and on the web concerning magnetic loops.  I wanted 20m, 17m, 15m coverage. I used the calculator for octagonal construction at http://www.66pacific.com/calculators/small_tx_loop_calc.aspx . That calculator indicated that a circumference of 11.1 feet (3.3' diameter) would be within the dimensional limits and provide efficiency of 77% on 20m, 89% on 17m, and 93% on 15m if I used 1" copper (1.125 OD) plumbing pipe.

I purchased a used ($95) 10pF to 60 pf / 10KV vacuum variable capacitor (Comet) from Max Gain Systems to tune the loop.  I chose this cap because it nicely bracketed the minimum and maximum tuning capacitance 47pF to 21pF that the calculator indicated I needed for the three bands without being "over range" and difficult to achieve resolution at each setting. 

I first fabricated Faraday shield style feed loops from Belden RG-8U coax.  It appears that using the general design of the feed loop being 1/5th the diameter of the main loop results in the feed loop being a bit too large for the effective circumference of a octagonal loop.  1/5th of 3.3' = 7.92".  After four experiments. I got the match down to 1.0:1 at resonance of the main loop with an outside diameter of the feed loop being 6".  I then made a permanent shielded feed loop at that dimension by stripping the outer jacket and braid from RG-8U and inserting the rest into 3/8" soft copper plumbing tube.  As a note for others wanting to build a loop: buy, borrow, beg, a stable antenna analyzer to initially find the capacitor resonate point of the main loop and then to optimise the matching of the feed loop to the main loop. My MFJ 269 is a bit unstable and lacks resolution for this antenna Q. Wait until you get things matched and determine the tuning versus the capacitor shaft rotation degrees before you cast in stone the capacitor motor drive.

By going for minimum resistance and maximum efficiency and "Q" by using large diameter tubing and jeweler's silver solder for all the joints for the octagonal sections of the loop (8-pieces, 8-45 degree elbows = 16 loop joints) and the connection of the loop to the capacitor, I painted myself into a corner on the criticality of the tuning.  I believed the information in the literature and web about this, but I really underestimated the reality with the efficiency and Q of my construction.

I didn't want to invest in a motor drive for the capacitor until I had completed the experimental stage and I am glad I didn't.  The first thing I discovered is that the used capacitor I bought either doesn't make it up to 47pf or something in the loop or calculator is "off."  The loop would tune down to 14.150, but not below that before I ran out of shaft rotation.  I constructed a coaxial (RG-8U / 26pF/Ft) capacitor to pad the vacuum variable and it would then tune down (max cap) to 13.5MHz on the MFJ-269 analyzer. Here is another note: the real-world dielectric strength of the standard Belden 8214 is NOT up to the voltage potentials of these antennas beyond QRP levels. (Baby, light my fire!)

Take home info:
At QRP levels and at 3' off the ground and in the clear, the loop antenna very greatly out performs the HamStick dipole on a 10' mast in the same physical location for both RX & TX.  It is very noise-quiet compared to the dipole and this provides a very good SN ratio at the receiver. It is noticeably directional in the plane of the loop. If I keep the vacuum variable capactor rather than building a new "trombone" cap, I will need to find an 8 KV shunt cap or fabricate one from brass and dielectric.  Also, the drive motor input to the vacuum variable capacitor shaft will need a resolution of about 1/2 of 1-degree and sufficient torque (high resolution stepper) to tune the main loop to resonance across the range of each of the three bands.   It is easy to "hide" in a condo situation on a patio, deck, balcony, attic, etc. BUT, at 100 watts input, the field around the antenna is very strong and the voltage across the cap in my configuration is about 4,281V on 20m and 3,166V and this is a major hazard.  The 1" (1.125" OD) copper pipe, 45-degree elbows, copper plate for the capacitor mounting plates, and jeweler's silver solder are bloody expensive these days.  So, there is considerable effort expended to build and optimize the antenna, and the cost of materials is very high compared to a highly loaded stealth antennas of other configurations.

If I can get a second mortgage, I am going to build a 7.5' diameter for 40m that should have 75% efficiency (-1.2 db below 100%).
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N3OX
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« Reply #1 on: March 22, 2011, 08:51:42 PM »

I didn't want to invest in a motor drive for the capacitor until I had completed the experimental stage and I am glad I didn't.  The first thing I discovered is that the used capacitor I bought either doesn't make it up to 47pf or something in the loop or calculator is "off."  The loop would tune down to 14.150, but not below that before I ran out of shaft rotation.  I constructed a coaxial (RG-8U / 26pF/Ft) capacitor to pad the vacuum variable and it would then tune down (max cap) to 13.5MHz on the MFJ-269 analyzer. Here is another note: the real-world dielectric strength of the standard Belden 8214 is NOT up to the voltage potentials of these antennas beyond QRP levels. (Baby, light my fire!)

Did it flame out INSTANTLY or did it take a second?  It might be the losses and subsequent heating in the coax stub that get you, not the ultimate voltage standoff of the cable.  A foot-long stub of RG-8/U used as a capacitor only has a Q of about 1500 (nearly 0.25 ohms resistance and 380 ohms reactance, abt 29pF equivalent).  While that Q is maybe comparable to a cheap sliding-connection air variable, it's a far cry from the Q of the vac. variable.

Quote
If I keep the vacuum variable capactor rather than building a new "trombone" cap, I will need to find an 8 KV shunt cap or fabricate one from brass and dielectric.


You could also think about doing a loop-lengthening transplant of longer legs in two sides, make it into a somewhat oblong shape.  You'd want to keep symmetry about the feedpoint and capacitor but you could either make it wider or taller depending on where you need to hide it.  This might give fewer headaches than trying to fabricate a HV capacitor.

Quote
Also, the drive motor input to the vacuum variable capacitor shaft will need a resolution of about 1/2 of 1-degree and sufficient torque (high resolution stepper)

Geared down stepper might be nice.

Quote
The 1" (1.125" OD) copper pipe, 45-degree elbows, copper plate for the capacitor mounting plates, and jeweler's silver solder are bloody expensive these days.  So, there is considerable effort expended to build and optimize the antenna, and the cost of materials is very high compared to a highly loaded stealth antennas of other configurations.

Yeah, it should be straightforward to do better for efficiency with considerably less cost if you don't have such a low radiation resistance.  A loaded dipole that fit in the same bounding box will have considerably higher radiation resistance and not so much attention needs to be paid to losses.  The downside is that it's hard to tune.

I'm working on a remotely tunable loaded dipole but... well, "working on it" is kind of a euphemism for "some pretty complete bits are sitting in my shed waiting for me to build the second capacitance hat."

I do like my magloop though.  Mine isn't such fat tubing as yours and it's still an eminently usable antenna.  I happened to have a 300pF variable and built a four foot octagon: I can get all the way down to 40m and have had several very satisfying QSOs with good reports vs. a much bigger antenna.   Not BETTER of course, but roughly aligning with model predictions (good agreement would require tons of measurements) and certainly very usable.

I actually have had more fun using it than some of my big antennas just because it adds an extra flavor to trying to work a DX station on 30 or 40 or whatever.  I get a real kick out of using a four foot octagon.  A couple years ago, I didn't have time to actually go OUT to field day, so I set up a chair, FT-857, boat battery, and the magloop up in my own backyard and operated that way.  I carefully rolled up my nearby 40m half-wave vertical to detune it sufficiently.  I didn't want to be a "parasitic cheater" like I was on one of my early test QSOs with the loop Grin 

I do think the price of doing it right is quite a bit steeper than it needs to be for the performance and I think it's probably worth thinking about alternatives, but I'm still glad I built it.  I'd wanted to try one ever since I first read about them.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WV6N
Member

Posts: 1




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« Reply #2 on: March 22, 2011, 11:05:36 PM »

Please take a look at:

http://www3.qrz.com/db/wv6n

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N3LCW
Member

Posts: 152




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« Reply #3 on: March 26, 2011, 10:25:51 AM »

Take a look at http://g0cwt.co.uk/magloops/  inparticular, the mobile loop.

and then my QRZ web page.


Using Ben's idea I built a loop for 80/160M with most impressive results.  The first attempt took less than 2 hours and I was on the air that night on 80M.

Andy
N3LCW


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