LZ1AQ Wideband Active Loop Project
I began this project last winter. It is a very good design that uses a low input impedance, which is important because a small 3 foot diameter loop looks like a dead short at the feed point (electronically), thus it has a very low impedance. A sincere thank you to Chavdar Levkov LZ1AQ for this circuit design, as it is the best preamplifier that I've used so far.
In my design, I substituted 2SC5551 transistors for the PN2222A transistors that Chavdar uses. Any 12v bias-T circuit can be used to send power from your receiving position down the feedline to the active loop. I use a bias-T circuit that is built in to my Misek / Lankford phasing unit to send the voltage to the preamplifier. L4,5 and 6 are wound on a binocular core (BN73-202). Also, Chavdar's circuit uses CAT5 cable. I use 75ohm coax, so a .1 uF capacitor and a 1uH inductor is utilized at the feed point of the loop preamplifier to stop the power from shorting out at the feed point transformer. These components stop the DC current and filter it to the voltage input section that you see in the above diagram.
The above photo illustrates my first prototype LZ1AQ loop. I was so impressed with it, that I made two more and combine their signals with the Misek Lankford phaser at my main listening position. I use the above loop for listening to MW and SW out in the yard with my dog. The design calls for a low inductance loop. To achieve this, I used a wider conductor for the loop. The loop is made from 1.5 inch aluminum flat bar, which is 1/8th inches thick. The enclosure that I use for all of my loops is a 4 x 4 x 2.5 inch electrical junction box made of sturdy PVC. At the lowest point of the enclosures, I drill three small holes for drainage of any moisture that may collect inside. The moisture fallows the gravity and keeps my circuits nice and dry. My two loop array loops are mounted right to the box, and I use a short length of angle iron to mount the box to a mast.
I have no specific measurements or numbers to offer as proof, but from my experience with loop antennas, I can say that this wider loop is very directional, and the preamplifier circuit offers the best signal to noise ratio I have seen in the active loops I have experimented with so far. I do most of my listening on the Medium Wave Broadcast Band, but this antenna works very well from 20 KHz up to 30 MHz. By turning the loops, I can null or peak signals. I may post another blog entry with information on the Misek / Lankford phasing unit, which is another great tool to have for nulling or peaking signals (including noise sources).
If you are interested in building this project, a Google search for LZ1AQ loop antenna will bring up all kinds of information with regard to construction and the performance of this loop antenna.
