Loop Antenna is Portable

We don’t know if [OH8STN] has a military background, but we suspect he might since his recent post is about a “DIY Man Portable Magnetic Loop Antenna.” “Man-portable” is usually a military designation, and — we presume — he wouldn’t object to a woman transporting it either.

[OH8STN] started with a Chameleon antenna starter kit. This costs about $100 and is primarily a suitable variable capacitor with a 6:1 reduction drive premounted and soldered. Of course, you could source your own, but finding variable capacitors that can handle transmit duty (admittedly, these can apparently handle about 10 W continuous or 25 W on single sideband) can be tricky, especially these days. Although he started with a kit, he did modify the antenna to switch between two different sets of ham radio bands. You can see the antenna in the video below.

Loop antennas aren’t ideal–but neither is any other small antenna. Because the loop is tightly tuned to a particular frequency, it requires retuning for even relatively small frequency changes, even though it can operate on many different frequencies. If you want more technical details, you might enjoy this recent presentation from [W4RAX]. The links at the end are worth checking out, too.

Of course, in addition to the starter kit, you need some other components, and the video shows it all. You also need some tools, but we were amused to see that for bending the aluminum loop, [OH8STN] simply wrapped it around a tree trunk of suitable circumference.

Loop antennas are popular with apartment- and other city-dwellers. But if you are just looking for exotic radiators, perhaps try sea water. Or you could have a look at a very short dipole-like antenna called the Poynting vector antenna.

16 thoughts on “Loop Antenna is Portable

    1. There is another reason not stated in the article why appartment dwellers like me prefer the loop over a piece of random wire. The wire is fine if you can hang it outside. Which is often not really possible in an appartment (because various bylaws and regulations prohibiting antennas, etc.).

      If you hang the wire indoors, it will be picking up all sorts of EMI crap from all your (and your neighbors’) appliances around. Good luck doing anything with that. Magnetic loop is sensitive mostly to the magnetic component of the signal, not to the electric one – so it effectivelly reduces the EMI. It is not a panacea, but every bit helps.

      1. The EMI issue is true for those of us living in the suburbs too these days. A loop will often have less of this to deal with than a long wire even for RX only especially if one can null some of it out by rotating the antenna.

      2. The loop sounds great. I would like to try one for the noise reduction properties I would also like to comment on the use of wire antennas from restricted locations… I have done this from a condo where I lived for 3 years. I used #26 magnet wire with a rubber band insulator on the far end. It is effectively invisible from more than a few feet away. I also used it from a multi-story motel that I frequently stayed at for business. I would always request a room on the top (4th) floor on the side facing the parking lot. The magnet wire antenna went out the window and I stretched it out to hook the rubber band over the top or the wrought iron fence on the far side. I ran QRP and worked DX, Europe, S. America & S, Africa with it from the Midwest.

  1. The power limitation is the air gap between the plates of the capacitor. There are various ways to create capacitors for a high power loop, such as vacuum variables or made with an insulating dielectric. I like the coaxial method of placing a small tube inside a larger tube insulated with PVC pipe. Easy to make.

    When I lived in an apartment 20 years ago I used a 1 meter loop on my desk inside the apartment. Furthest contact was Europe to New Zealand with 1 watt at each end and both stations running 1 meter loop antennas. A scheduled 20 meter CW contact to see if we could do it. Magnetic loops can work very well if you are limited to indoors or small antennas.

    On receive, one reason for the reduced noise is the very narrow bandwidth. Measuring the bandwidth is an indication of how well the antenna will perform – the narrower the bandwidth, the better. You can achieve bandwidths of 1 kHz which is great for CW but actually starts to distort SSB. But you have to tune the antenna very precisely for each frequency making it it a difficult antenna to use practically in many situations.

      1. Not as well; the key to performance with these loops is keeping the resistance in the circuit as low as possible. Trombone capacitors are not low in series resistance, plus their physical length can cause coupling problems with the loop itself. Vacuum caps are the best.

        1. Yes, performance and Q are directly proportional to the series resistance. But I am not sure why trombone capacitors have to have a higher resistance. They could be part of the same tubing that makes the antenna, hence no
          DC resistance.

          My personal favorite configuration is to make a loop of tubing and insert one end into the other so the capacitor is actually made from the loop’s tubing. That way there is nothing to couple and no connections to add resistance. Adjustment is a matter of varying how far the ends are overlapped.

  2. In 1976 I lived in a trailer park (NO ANTENNAS) and built a 6M loop antenna from 5/8″ copper tubing, some porcelain stand offs, a variable capacitor made HV by ripping off plates from a higher value cap that was porcelain insulated, a very small gear motor to tune the variable cap and a 3 foot squre piece of aluminum. I used it for 6M FM and found I could pretune the antenna by listening to the FM hiss. Then I would transmit and tune for SWR. It had very high Q was absolutly flat for reflected power. I used about 10W and found it to be quite effective. I keep it wired up to piece RG58 and a piece of zip cord for the motor. When I wanted to use it (mostly at night), I would carry the antenna out the front door, step up on the railing around the porch and lay the antenna on the roof. It was hard to see the antenna on the roof from the ground because the trailer was block pretty high. I lived there about a year and no one ever complained :-)

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    Sent from my BlackBerry® PlayBook™www.blackberry.comFrom: “Hackaday” <comment-reply@wordpress.com>To: “davegsc@gmail.com” <davegsc@gmail.com>Sent: 25 February, 2017 10:00 PMSubject: [New post] Loop Antenna is Portable

    Al Williams posted: “We don’t know if [OH8STN] has a military background, but we suspect he might since his recent post is about a “DIY Man Portable Magnetic Loop Antenna.” “Man-portable” is usually a military designation, and — we presume — he wouldn’t object to a woman tr”

    1. I used to make passive loops like this to boost AM radio out in the sticks where i lived at the time. As I recall I used a wooden X-cross about 30 inches square with about 7 turns of wire wrapped around it and a small variable cap on one corner connected to the ends of the loop.

      To use it, you put an AM radio near the coil with the radio oriented so it’s internal ferrite stick antenna is parallel to the axis of the coil, tune the radio to the station you want and then tune the coil’s capacitor to resonance. The signal strength would jump dramatically when the frequencies aligned.

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