Saturday, June 23, 2012

Looping the loop

Whenever I have taken a rig to operate in the Great Outdoors using a wire antenna I have often been disappointed. This is probably due to my use of inadequate antennas - Miracle Whips and the like. When I have tried wire I usually fail to get it up high enough. Usually when hurling a rock attached to a string into the trees, the rock rebounds off a branch and narrowly avoids hitting me on the head. If I do manage to get it up high the rock irremovably entwines itself with a branch just out of reach. There has to be a better way!

AlexLoop WalkHam carrying case

I have long been an enthusiast for magnetic loop antennas and have often wished I could use one as a portable antenna. My Wonder Loop was an attempt to make such an antenna, but it was less portable than transportable (by car) and has seen more use as a spare antenna from inside the shack.

I looked longingly at the WalkHam made by Alex PY1AHD and wished I could make a portable loop as neat and compact as that. If I had to make it myself it might never get done so I decided to bite the bullet and buy one of Alex's ready-made loops.

There are two versions of the AlexLoop. One is a kit using copper tube  for the radiating element and costs $199 US. The other version, called the WalkHam, uses stout coaxial cable for the main loop and comes ready built in its own custom made carrying case similar to a laptop case. The price of the AlexLoop WalkHam is $299 US. Shipping to the UK by express courier to the UK is a further $82 US. The total cost to me using PayPal was just over £250 at the present exchange rate.
AlexLoop WalkHam in its case
 The WalkHam is well made with gold plated connectors for the loop element. It is easy to assemble, though not so easy to pack away unless you have a photo to show how the parts go back! The mast is made of black plastic tubing and is in three push-together sections. Once assembled the antenna may be used whilst held aloft - hence the name. Most users will probably prefer some sort of mast.
The AlexLoop WalkHam ready for use.

The loop is 1 metre in diameter and tuned using an air spaced variable capacitor with a 3:1 reduction drive giving a 4 : 1 tuning range: 10m - 40m. Most magnetic loops including home-made ones only manage a tuning range of 3:1: 10m - 30m or 15m - 40m. My MFJ magnetic loop is the 40m - 15m version as it was bought during the last solar minimum when 12m and 10m were not much use!

The coupling loop has a diameter of about 20cm. The maximum power handling is 20 watts PEP, 10 watts continuous wave, making the WalkHam perfect for use with QRP radios like the FT-817 or Elecraft's new KX3!

Tuning as expected of a magnetic loop is extremely sharp but I noticed little or no hand-capacitance effect. With a little practice the loop can be tuned by peaking for maximum noise in the receiver. If the SWR isn't low enough then the tuning may be touched up using transmit and the rig's built-in SWR meter if it has one (both the FT-817 and KX3 do!) If not, a simple SWR indicator as I used in my Wonder Loop would be a big help.

Subjectively the AlexLoop seems to work as well as my MFJ magnetic loop in the attic, which itself is comparable to a full-size dipole. There are not many portable antennas that would beat the AlexLoop WalkHam for performance, unless you are able to erect a couple of 40-foot masts!

One thing that would improve the package would be a way of erecting the antenna so that it will stand on its own. I think my arm would soon get tired holding the WalkHam aloft! Possibly a short guyed mast made of sections of electrical conduit would do the job: magnetic loops don't need to be far above ground in order to work. Some users are reportedly using photographic tripods so I'll probably investigate that in due course.
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