All right, here we go: by popular demand here is some info about my loop.
Living in New York City is the worst possible nightmare for an Amateur Radio enthusiast: no antennas on top of the roof of your tiny little apartment and for the most also no car at all. The only chance you have to transmit something is being portable. Yep... that's right! Over here you will have to get used to that "/P" suffix after your callsign.
All my equipment - battery included - must be light enough to be carried on a bicycle since I often operate from the City Parks. Imagine the scene: curious people giving you funny looks, bums asking for money, kids asking questions, sometimes even cops asking what the heck is that all about.
Since I moved here I always used a MP-1 Super Antenna and I was quite satisfied with it. Recently I wanted to experiment with an exotic type of aerial that I have never tried before: the magnetic loop.
Since I don't have a garage nor an impressive toolset, my portable loop had to be as easy as possible to build.
After some googling I came out with a fairly good solution that I believe is simple enough for anybody to replicate (If I made it here, you can make it anywhere!)
So, here is what you need if you want to build your own:
- Various Schedule 40 1/4" PVC pipes and junctions used for the support structure. I used two X junctions, six T junctions and about seven feet of pipe.
- Around 10 to 13 feet of RG-213 coaxial cable needed to build the loop itself. Mine is 11 feet long, but the exact length doesn't really matter.
- Three SO-239 panel connectors
- Two PL-259 connectors for the RG-213
- Some small electrical wire to connect the capacitor
- Some ticker electrical wire to make the coupling loop (you will need a fifth of the length of the main loop)
- A few electrical jumper connectors to make removable connections on the air variable capacitor
- Some extra coax cable like RG-6, RG-58 or RG-59 to cover the bands that your variable capacitor cannot eventually reach by itself
- A high voltage (no less than 2KV) air variable capacitor in the range of 20-250 pF (a 500 pF capacitor would be too big. This is a portable antenna! see below)
- Some screws to secure the SO-239 to the PVC X junctions, some electric tape
What about the tools? Here is what I had:
- Soldering iron (mine is 30W but a more powerful is definitely better)
- Electrician scissors (I had to buy those...)
- Small saw to cut the PVC pipes
- a drill would be gold if you have it (I didnt't, so I had to drill the holes using one tip of my scissors. Not fun at all...)
I believe you could buy the whole above material (if you don't have it already) for about $50. The capacitor is the most difficult thing to procure but once every while you can find good deals on ebay. I got mine (a dual gang 4500V 15-250 pF) for about 6 bucks.
The net is full of excellent resources about this antenna so I won't duplicate other people's already excellent work. I just want to show how I built mine so that you get inspired to build your own.
The key concept is that you will have to sustain a loop made with the RG-213. Any way to reach this is acceptable as long as the loop will stand at about 2 feet from the ground.
The loop will have to be left open and at the two extremes of it you will connect your variable capacitor. To make the loop portable I have used two SO-239 connectors. Their purpose is to sustain mechanically the loop and to provide an attach point for the electrical connections to the capacitor.
At the opposite side of the loop you will put another smaller loop made with tick electrical wire. It will be 1/5 of the length of the main RG-213 loop. The smaller loop will then be connected to the feedline that goes to your transceiver. The two loops will be physically disconnected from each other with their distance being up to around two inches. You only have to make sure that they are on the same plane.
That's it. The antenna is done and ready to be used. My 15 to 250 pF capacitor allows to tune from 10.2 to 20.7 MHz. I mean, such a small loop has a very poor performance on 7 MHz (about 9%) but still you might want to use it there. That's the reason why I had to use the RG-6 cable to extend my tuning range. I have a smaller piece that I put in series to reach 21 MHz, and a bigger one that I use in paraller to go down to 7.
Does it work? It does... I mean, don't imagine that it will make miracles but I believe its a good antenna: I work DX stations without much trouble on phone or PSK31 using just about 30 Watt. The only problem is that being an high Q device the tuning is as sharp as a razor blade you can't really move around the band without having to re-tune the capacitor (which becomes annoying after a few times you do it).
Here is all you need to know if you want to proceed:
- Magnetic Loop Calculator
- The best article on Magnetic Loops I could ever find (by N4SPP)
- W2BRI's site explains how to properly build a loop complete of coax capacitors
- A loop made by KR1ST (no less!)
Good Luck and good DX!
73's de WW3WW