How it works:
A loop antenna is effectively a tuned RLC circuit. What does this mean? It means the primary coupling loop inductively couples to the secondary. The secondary loop has a variable capacitor to electrically adjust its resonance. Alternatively, Some people use a toroidal transformer, and still others use a gamma match.
The variable capacitor is the biggest decision on the limitations of your antenna. Some folks use a variable capacitor that is smaller and handles low wattage (before arc-over). This would be for QRP (less than a watt) or Shortwave/Ham Radio listening. Others pick up a costly vacuum variable capacitor (Great for high power since they handle large wattage) and go all out. Keep in mind that an air gap of 1mm equates to a breakdown voltage of about 1KV, which can be encountered when as little as 20 watts is used. A Magnetic loop antenna for 1-5 watts can be made fairly inexpensive.
The circumference of a true magnetic loop should be less than 1/10th wavelength and the coupling loop at 1/5 ratio to the main loop. Here is a great calculator for sorting out lengths, values, etc. You could use a very specific capacitor and not have it variable if you were only staying on a specific frequency, but what fun would that be?
What can it do:
This thing is an amazing easy homebrew project. I can tune by simply listening to noise (for highest volume when tuning) but can also use an SWR meter and can cover multiple bands.
In my testing with my antenna analyzer I found I get 1.5:1 SWR. This is ranging from 5.6Mhz to 8.8Mhz. Now with the Primary Coupling Lop at 4 feet exactly and the Secondary at 18.3 feet I am getting a single ham band. I could actually get 7Mhz – 14Mhz by doing the Primary Coupling Loop at 3.4 feet and the Secondary at 17 feet. I will do this but am enjoying getting some of those ShortWave signals I have never received before.
I live in a townhouse and have “Attic Antennas” exclusively so I need a good multi-band antenna with low SWR.
18.3 feet of copper pipe and a 2kw variable capacitor….. A nice start to the project and a great excuse to start my magnetic loop build. The Variable capacitor was a gift from another ham in my radio club.
I bend the larger copper pipe to form a circle. Then I laid it out to get an idea of what it would look like and start to think about how I wanted to mount it.
I laid out the ends onto a board and hammered the ends flat. Then drilled holes into the flat portion for a place to mount the pipe.
The variable capacitor has screws on either side so this is a good mounting spot for the secondary ends. I bent the freshly drilled holes up and mounted them directly to the variable capacitor. If this is not possible on your variable capacitor then be sure to keep the connection leads as short as possible.
I ran to the store and got a quick 4 feet more of copper pipe so I didn’t have to cut down the 18 feet. I was afraid of losing the 20 meter (7Mhz) band. At first I tried a 20 watt soldering iron with copper tip…. no luck. I ended up having to use a small torch to heat the copper pipe hot enough to take solder.
What does this look like all put together?
Yeah, OK.. I get it. At 8 feet tall and 4 feet wide, this is not going with to all the hams in the park or meet ups. This will however make it to field day and was a very fun project.
So how would we draw this out:
Notes on magnetic loop antennas:
For HF the size is very good compared to a dipole antenna or some others. It is however very directional, which can be good or bad, depending on use.
Also, The magloop does have an Electric field. It is just not found until you get a bit away from the antenna, my old field strength meter does show an electric field on its little vertical antenna when I get about a half wavelength or more away from the loop. Inside that, it shows nothing. But don’t lose track of the fact that a radio wave must have both E and h fields perpendicular to each other in order for the wave to propagate. This type of antenna just doesn’t seem to have much of the E up close to it.
AA5TB gives us a great view of the antenna radiation pattern in the following diagram.
I have heard there are reports of mag loops at 100 watts blistering paint on wood a foot or two away. That said, this article on antenna radiation and fields seems to suggest that that would be not possible.
Warning!!! This does not mean the antenna does not have significant dangers associated. On transmit, mag loops develop very high voltages across the tuning capacitor. At 100 watts, they are in the thousands of volts. Even at QRP power, they may be enough to be dangerous. Exercise care, especially if there are kids or pets around.
Main problems with magnetic loops:
They are usually low power (or receive only) since the rating on the variable capacitor must be so high if transmitting above 10 watts.
If you are using this for higher frequencies then the wavelength is shorter. This means the coupling and secondary loop are significantly smaller for something like 2 meters and therefor are more portable and are thus lighter.
Next question would be “What is a reasonable parts list?”
- 1 x Variable Capacitor
- 1 x 8 foot long board
- 1 x 18 feet of copper refrigerator piping (craigslist for free)
- 1 x 4 foot copper piping (hardware store for $12)
- Screws to mount to board
- Old RG-8 cable with PL-259 end (fun fact – SO259 means socket for a 259 end. PL259 means the plug side of it)
- Something to mount the copper pipes and cap to the board. (Alternatively you could use PVC piping and zip-ties instead of wood, screws and cable runners)
G4ilo says this is a good parts list for a more permanent DIY Mag loop build:
- 250pF single gang variable capacitor with reduction drive
- Plastic case
- Self adhesive feet
- 2 x 4mm gold plated binding posts for the main loop connections
- 2 x 4mm gold plated spade terminals for the main loop connections
- 250cm length RG-213 coaxial cable for main loop element
- RCA (phono) plug and socket for coupling loop input connection
- BNC socket for coupling loop output connection
- 50cm insulated wire for coupling loop
- Length of RG-174 mini coaxial cable for coupling loop
- N7VE SWR Indicator Kit or similar (see below)
- 2m white uPVC electrical conduit, for fabricating loop support mast
- White plastic cup hooks
- Self adhesive Velcro (hook and eye fabric fastener, from XYL’s sewing box)
- Large plastic tuning knob
- White car number plate fixing bolt covers (to cover tuning capacitor fixing screws)
- Bolt for photographic tripod mount (optional)
Here is yet another build of a mag-loop antenna.
Have fun and 73’s,
Let me know what questions you want me to address or if anything is unclear. My email is firstname.lastname@example.org