Coming up in June this year I will heading off to the NSW central coast to participate in a DXpedition with three other radio enthusiasts. Although all involved are amateur radio operators we won't be transmitting on the amateur bands. Instead this DXpedition is aimed primarily at receiving medium wave broadcast band stations from the Pacific and the Americas.
In preparation for the trip I decided to build a receiving loop antenna which could be easily packed into a suitcase and assembled quickly at the receiving site. I have a Wellbrook receiving loop at my home which gives impressive performance on the MW bands; however, it is not suitable for portable operations.
There are a few options on the market and I decided to go for the Australian made Mini-kits EME 232 which comes just as an amplifier module which needs to be constructed. The advantage of this approach was that I was free to design the loop component of the antenna so as to maximise its portability. It also helped having the manufacturer based in Australia in case any problems arose.
The Mini-kits module is based on the proven LZ1AQ design, although it lacks the versatility of the original which can be used to switch in multiple loops. The trade off is that the Mini-kits version is a fraction of the cost of the LZ1AQ module.
Construction of the circuit board was fairly straightforward. Component wise the board comprises roughly 50% surface mount components. I had not attempted a surface mount construction before and was a little apprehensive as to how I would go, but as it turned out there was nothing to worry about. A fine point soldering iron, thinner spec solder and my magnifying LED lamp made the job easy.
Next was to flip the board over and install all the through hole components. This was straightforward enough.
The through hole component side of the board
Along with the main module I constructed the Mini-kits bias tee board which is used to power the module by sending 13.8 volts through the coaxial feed line.
Once the bias tee was completed it was time to tackle construction of the loop antenna itself. I dug about in the shed for some plumber's PVC pipe to construct the frame to give some mechanical strength to the loop. For the loop I had some LMR400 50 ohm coax in the shed which I cut to about three metres in length to give me a one metre diameter loop. On each end of the coax I soldered the inner conductor and the coax shield together and in turn soldered each of these ends on to two lengths of 3mm wire which plug into the module.
Next was the moment of truth. Would it work? I did a voltage check of the module as recommended by the manufacturer and found the voltage at the board was about 1.5 volts less than that specified. A quick email to Mini-kits who confirmed the voltage supplied would be sufficient.
Once satisfied with the voltage supply and the construction I plugged the antenna into a SDR receiver and gave it a test run. I was immediately impressed with the performance of the antenna. It has slightly more gain than the Wellbrook loop, but at the expense of a slightly higher signal to noise ratio. Given my noisy urban location I wasn't too worried about this as the DXpedition will be (hopefully) relatively quiet noise wise.
The physical construction of the loop is not suitable for a more permanent installation, although the antenna has been set up in my backyard for a couple of months now and faced up to all sorts of weather. I may look at building a LZ1AQ loop next time and experiment with some of the other antenna designs which can be utilised with this version.
A round up of the Dxpedition will follow in due course and I will let you know how the antenna performs in the field.
Until then, peace and love.
