KE7KUS
03-08-2012, 01:54 AM
INTRODUCTION
A few weeks ago I moved to a new QTH, which unlike my old QTH was in an HOA community. As with many HOA's, restrictions on antennas initially appeared to be a problem until I ran across a QST article on constructing a small magnetic loop antenna (SMLA) as a way to achieve fairly reasonable antenna performance while escaping the HOA Inquisition. After a little web browsing and planning I decided an SMLA was a good option for my purposes. This article documents my build and install of the SMLA at my new QTH.
BACKGROUND
The SMLA is a loop antenna constructed to be approximately 1/10 wavelength in circumference. It can be mounted horizontally or vertically. Horizontal mounting yields radiation patterns similar to a horizontal dipole mounted at an equivalent height. Vertical mounting yields a directional pattern biased in the plane of the loop, and with fairly uniform radiation at most elevation angles, to include robust performance in the 60-90 degree elevation regime.
Although my primary interest in building an SMLA was to escape the purview of my local HOA, I quickly realized that it would also meet my objective of putting an NVIS antenna on the air for EMCOM and local HF operations. A vertical SMLA's performance is not as robust as a resonant dipole up 1/2 wavelength, however, it's performance is comparable to a vertical antenna with less susceptibility to certain types of noise, making for a better SNR when compared to most HF verticals. Couple that with the extremely small size and the SMLA fit the bill on many fronts for my desired operations.
DESIGN
In its simplest form, the SMLA is simply a copper loop fed with a coupling loop and matched with coax stubs, which function as capacitors to tune the antenna to the desired operating frequency. More complex versions use a gamma match at the feed point in place of the coupling loop, and utilize a variable capacitor to tune the antenna across a broad range of frequencies. I wanted to start simple, as this was not only my first loop, but also my introduction to sweating copper. I elected to go with a combination of features. My loop is fed via a gamma match, but uses coax stubs for tuning. At a future point, I can upgrade the coax stubs to a variable capacitor with a little work. I planned to mount the antenna vertically a few feet off the ground, but low enough to keep the majority of the antenna below the fence line at my QTH.
Initial calculations using the Excel spreadsheet produced by Steve Yates, AA5TB, indicated my loop should be approximately 16 ft. in circumference to achieve reasonable performance on both 40m and 80m, both of which are essential to NVIS operations. I accepted the fact that particularly on 80m the antenna would have a very narrow bandwidth, essentially making it a fixed frequency antenna for any given coax stub. As I primarily planned to use digital modes for NVIS, this was not a terribly limiting factor, and fabricating coax stubs for SSB or CW ops would be a simple operation.
CONSTRUCTING THE LOOP
The loop was constructed by cutting 2 foot segments of copper pipe and joining them with the 45 degree elbows to eventually form an octagon approximately 60" in diameter. It is helpful to have a large workbench, or other elevated level surface that you can use to ensure the loop stays properly aligned during the soldering process. Also, thick leather gloves and a wet rag are useful to prevent burns and wipe the joint clean post-solder.
Start by cutting a 2-foot segment of copper, using emery cloth or fine sandpaper to rough up the end, and do the same for the inside of a copper elbow. Once the surfaces to be soldered are cleaned and sanded, coat them with flux. Once the parts are coated, join them, get them aligned and ready for soldering. This was my first experience sweating copper, but thanks to some tips from K7VZ and KE7NEA, the learning curve was quick. My first joint was a mess, with way too much solder, but after that it was fairly easy. I finally adopted the technique of heating the joint on the opposite side from where I was applying the solder. This ensured that the entire joint was at a temperature which would flow the solder when applied. Once I figured this out, the only other trick was to keep the wet rag handy for cleaning up the joint to make it look nice when complete.
I built my loop in three steps. I assembled three legs of the loop at a time on my bench, as I didn't have any place bigger to easily assemble the antenna:
1537
Once I got two of these built, I then put them together with the other two legs to make a complete loop. Once everything cooled, I made final adjustments to true the loop and then cut a 2" gap in the center of one side:
1538
The coax stub(s) will bridge the gap and the loop is fed opposite the stub. The loop is approximately 60" in diameter with a circumference of 16 feet.
The finished, mounted antenna fits in the yard quite nicely:
1566
The mount is constructed with 2x4 legs and a 1x4 section which holds the antenna at its base. The feed point sits about 8" off the ground and the top of the antenna is level with the 6 foot block wall behind my house. The feed point was constructed by soldering an SO-239 connector to some copper pipe strap and soldering the strap onto the loop opposite the capacitor mounting gap at the top:
1567
The gamma match I am using right now is pretty gimpy:
1568
I wasn't quite sure how long to make the gamma match tube and made it about 12" too short. I made up the difference with some 14 gauge stranded copper wire, but in hindsight, to prevent this problem next time, I would make the gamma match out of copper wire first, as the match point will be approximately the same regardless of what is used. Once I had an approximate length for the match in copper wire, I would then cut and form the soft copper tube into a more robust gamma match.
To tune the gamma match, solder the feed point to the antenna and then cut about 3-4 feet of coax and form it into a stub across the antenna gap (center conductor to one side and braid to the other side...connect each side to the loop with pipe clamps.) Adjust where the wire coming from the center conductor of the SO-239 feed touches the loop. Start by setting the wire about 12" from the feed point and sweep the loop with an antenna analyzer. You will find a frequency where the SWR dips. It will probably be a fairly narrow, but sharp dip. I swept past the point several times before I noticed it. It was easier to see the dip with a slow frequency sweep. Once you find the frequency where the SWR dips set that frequency on the analyzer and adjust the point where the wire makes the gamma match to adjust the SWR down to as low as possible. You should be able to get a 1.1:1 or so match. Mark this point with a Sharpie marker...this is going to be where your gamma match attaches to the antenna. At this point I would cut the soft copper tube to length and attach at the match point with a pipe clamp. (Note, I did not do this in the photo above...this is how I would do it if I could do it all over again knowing what I know now.)
I bought about 30 feet of RG213 coax to fabricate the tuning stubs. Starting with the coax stub you probably still have attached to the antenna from the gamma match tuning described above, the antenna should have a 1:1 SWR and a resonant frequency. Now all you have to do is prune the coax to length to get it to resonate at your desired frequency. With RG213, I found that I could raise the resonant frequency about 200kHz for each inch of coax I pruned from 40-20m. What you want to do is stop pruning when you get about 200kHz below the desired resonant frequency. At this point, you want to take the coax shield from the end of the coax not attached to the loop and peel back the braid about 1/2". You'll have to play with how much braid is pulled back to get the stub to resonate the antenna at the right frequency. Once you've got it, tape it down and then cover the end back up, ensuring that there is plenty of tape between the center conductor of the stub and the peeled back braid. (This will prevent an arcing capacitor.) Mark the stub for it's resonant frequency and get ready to make the next one. In the picture below, the three long stubs are all around 3.5 feet long and tune to 7.035, 7.090, and 7.175 MHz respectively. The single shorter stub above that is tuned to 10.140 MHz. The two small stubs at the top are tuned to 14.070 and 14.215 MHz. Based on measured performance, I get about 60kHz of bandwidth from each 40m stub (<2:1 SWR), I get about 150kHz bandwidth on 20m. I cut a single stub each for 30m and 17m, both of which cover the whole band. I didn't have enough coax left over to cut an 80m or 60m stub, so that tuning remains untested at this time.
1569
Please see Part 2 (http://www.4x4ham.com/showthread.php?1899-Building-a-Small-Magnetic-Loop-Antenna-(Part-2)) for performance computation vs. on-air performance
A few weeks ago I moved to a new QTH, which unlike my old QTH was in an HOA community. As with many HOA's, restrictions on antennas initially appeared to be a problem until I ran across a QST article on constructing a small magnetic loop antenna (SMLA) as a way to achieve fairly reasonable antenna performance while escaping the HOA Inquisition. After a little web browsing and planning I decided an SMLA was a good option for my purposes. This article documents my build and install of the SMLA at my new QTH.
BACKGROUND
The SMLA is a loop antenna constructed to be approximately 1/10 wavelength in circumference. It can be mounted horizontally or vertically. Horizontal mounting yields radiation patterns similar to a horizontal dipole mounted at an equivalent height. Vertical mounting yields a directional pattern biased in the plane of the loop, and with fairly uniform radiation at most elevation angles, to include robust performance in the 60-90 degree elevation regime.
Although my primary interest in building an SMLA was to escape the purview of my local HOA, I quickly realized that it would also meet my objective of putting an NVIS antenna on the air for EMCOM and local HF operations. A vertical SMLA's performance is not as robust as a resonant dipole up 1/2 wavelength, however, it's performance is comparable to a vertical antenna with less susceptibility to certain types of noise, making for a better SNR when compared to most HF verticals. Couple that with the extremely small size and the SMLA fit the bill on many fronts for my desired operations.
DESIGN
In its simplest form, the SMLA is simply a copper loop fed with a coupling loop and matched with coax stubs, which function as capacitors to tune the antenna to the desired operating frequency. More complex versions use a gamma match at the feed point in place of the coupling loop, and utilize a variable capacitor to tune the antenna across a broad range of frequencies. I wanted to start simple, as this was not only my first loop, but also my introduction to sweating copper. I elected to go with a combination of features. My loop is fed via a gamma match, but uses coax stubs for tuning. At a future point, I can upgrade the coax stubs to a variable capacitor with a little work. I planned to mount the antenna vertically a few feet off the ground, but low enough to keep the majority of the antenna below the fence line at my QTH.
Initial calculations using the Excel spreadsheet produced by Steve Yates, AA5TB, indicated my loop should be approximately 16 ft. in circumference to achieve reasonable performance on both 40m and 80m, both of which are essential to NVIS operations. I accepted the fact that particularly on 80m the antenna would have a very narrow bandwidth, essentially making it a fixed frequency antenna for any given coax stub. As I primarily planned to use digital modes for NVIS, this was not a terribly limiting factor, and fabricating coax stubs for SSB or CW ops would be a simple operation.
CONSTRUCTING THE LOOP
The loop was constructed by cutting 2 foot segments of copper pipe and joining them with the 45 degree elbows to eventually form an octagon approximately 60" in diameter. It is helpful to have a large workbench, or other elevated level surface that you can use to ensure the loop stays properly aligned during the soldering process. Also, thick leather gloves and a wet rag are useful to prevent burns and wipe the joint clean post-solder.
Start by cutting a 2-foot segment of copper, using emery cloth or fine sandpaper to rough up the end, and do the same for the inside of a copper elbow. Once the surfaces to be soldered are cleaned and sanded, coat them with flux. Once the parts are coated, join them, get them aligned and ready for soldering. This was my first experience sweating copper, but thanks to some tips from K7VZ and KE7NEA, the learning curve was quick. My first joint was a mess, with way too much solder, but after that it was fairly easy. I finally adopted the technique of heating the joint on the opposite side from where I was applying the solder. This ensured that the entire joint was at a temperature which would flow the solder when applied. Once I figured this out, the only other trick was to keep the wet rag handy for cleaning up the joint to make it look nice when complete.
I built my loop in three steps. I assembled three legs of the loop at a time on my bench, as I didn't have any place bigger to easily assemble the antenna:
1537
Once I got two of these built, I then put them together with the other two legs to make a complete loop. Once everything cooled, I made final adjustments to true the loop and then cut a 2" gap in the center of one side:
1538
The coax stub(s) will bridge the gap and the loop is fed opposite the stub. The loop is approximately 60" in diameter with a circumference of 16 feet.
The finished, mounted antenna fits in the yard quite nicely:
1566
The mount is constructed with 2x4 legs and a 1x4 section which holds the antenna at its base. The feed point sits about 8" off the ground and the top of the antenna is level with the 6 foot block wall behind my house. The feed point was constructed by soldering an SO-239 connector to some copper pipe strap and soldering the strap onto the loop opposite the capacitor mounting gap at the top:
1567
The gamma match I am using right now is pretty gimpy:
1568
I wasn't quite sure how long to make the gamma match tube and made it about 12" too short. I made up the difference with some 14 gauge stranded copper wire, but in hindsight, to prevent this problem next time, I would make the gamma match out of copper wire first, as the match point will be approximately the same regardless of what is used. Once I had an approximate length for the match in copper wire, I would then cut and form the soft copper tube into a more robust gamma match.
To tune the gamma match, solder the feed point to the antenna and then cut about 3-4 feet of coax and form it into a stub across the antenna gap (center conductor to one side and braid to the other side...connect each side to the loop with pipe clamps.) Adjust where the wire coming from the center conductor of the SO-239 feed touches the loop. Start by setting the wire about 12" from the feed point and sweep the loop with an antenna analyzer. You will find a frequency where the SWR dips. It will probably be a fairly narrow, but sharp dip. I swept past the point several times before I noticed it. It was easier to see the dip with a slow frequency sweep. Once you find the frequency where the SWR dips set that frequency on the analyzer and adjust the point where the wire makes the gamma match to adjust the SWR down to as low as possible. You should be able to get a 1.1:1 or so match. Mark this point with a Sharpie marker...this is going to be where your gamma match attaches to the antenna. At this point I would cut the soft copper tube to length and attach at the match point with a pipe clamp. (Note, I did not do this in the photo above...this is how I would do it if I could do it all over again knowing what I know now.)
I bought about 30 feet of RG213 coax to fabricate the tuning stubs. Starting with the coax stub you probably still have attached to the antenna from the gamma match tuning described above, the antenna should have a 1:1 SWR and a resonant frequency. Now all you have to do is prune the coax to length to get it to resonate at your desired frequency. With RG213, I found that I could raise the resonant frequency about 200kHz for each inch of coax I pruned from 40-20m. What you want to do is stop pruning when you get about 200kHz below the desired resonant frequency. At this point, you want to take the coax shield from the end of the coax not attached to the loop and peel back the braid about 1/2". You'll have to play with how much braid is pulled back to get the stub to resonate the antenna at the right frequency. Once you've got it, tape it down and then cover the end back up, ensuring that there is plenty of tape between the center conductor of the stub and the peeled back braid. (This will prevent an arcing capacitor.) Mark the stub for it's resonant frequency and get ready to make the next one. In the picture below, the three long stubs are all around 3.5 feet long and tune to 7.035, 7.090, and 7.175 MHz respectively. The single shorter stub above that is tuned to 10.140 MHz. The two small stubs at the top are tuned to 14.070 and 14.215 MHz. Based on measured performance, I get about 60kHz of bandwidth from each 40m stub (<2:1 SWR), I get about 150kHz bandwidth on 20m. I cut a single stub each for 30m and 17m, both of which cover the whole band. I didn't have enough coax left over to cut an 80m or 60m stub, so that tuning remains untested at this time.
1569
Please see Part 2 (http://www.4x4ham.com/showthread.php?1899-Building-a-Small-Magnetic-Loop-Antenna-(Part-2)) for performance computation vs. on-air performance