Hi Folks,Recent articles in "The LowDown" and discussions on the LowFER net have covered some interesting approaches to receiving antennas for LF. Prompted by a comment on the net, and with some recent work to tidy up my antenna connections offering a chance to take pictures and measurements, I thought it may be of interest to share some technical details with LWCA members.
For LF listening, I have a "Random Wire" spanning 80 ft between trees, suspended at around 20 ft above ground. A down lead meets a ground rod, and from there, to a run of 75 ohm coax back to the house. A transformer is used to improve matching between the wire and feeder. This has always featured separate primary and secondary windings, and there is no connection made between the antenna's ground rod and the coax shield - Common practice in "low noise" antennas. The setup has been in place for several years, and while it was effective from MF through HF, reception at lower frequencies was troubled by increasing noise and reduced signal strength, becoming progressively severe as frequency was reduced.
Theory suggests there are two reasons for this 1). In the primary circuit, low inductance means much of the signal from the antenna is shunted to ground, rather than being coupled into the transformer’s magnetic circuit. 2) The low inductive reactance at the antenna end of the feedline presents a short between the braid and center conductor, and a path for noise on the coax shield outer surface to reach the receiver’s input.
To test this and in search of a better solution for LF reception, I explored my “junk box” for different ferrite cores. A few different examples were found, and these were tested by measuring the inductance achieved with a 10 turn winding. An inexpensive L-C-R meter was used to make measurements. The objective of this test was to identify the core which presented the highest inductance for the set number of turns. The best candidate was a sleeve of the type designed to be placed over cables to reduce common mode current. Its dimensions are 1.125" long, inner diameter 0.25", outside diameter 0.615". It yielded around 500 uH for the ten turn coil, and was chosen for further testing.
An initial 9:1 impedance transformer was wound with 12:4 turns of #32 a.w.g. enameled wire and tested in the receive circuit. Reception of WWV (60 kHz) was improved, but there was still noise, identified as conducted noise, rather than QRM picked up by the antenna. This is easily distinguished by leaving the coax connected to the transformer, and disconnecting the antenna. The winding count was increased to 24:8, and a significant improvement noted. The completed transformer, and the terminal box setup are shown in the attached pictures. A second 1:1 transformer can be inserted in the feedline near the receiver, providing further defence against conducted or common mode noise. For dedicated VLF reception, a capacitor ~200 pF connected across the secondary of this second transformer is an effective way to suppress broadcast band signals and the receiver overload they can cause.
With this configuration, reception of data signals in the 20 kHz range is achieved, and tuning lower one hears atmospheric, rather than man-made noise. It is hoped that this will enable reception of SAQ during some future transmitting event, as well as reception of 2200m activity.
Further up in frequency, the antenna system is quite usable at 22m for HiFER reception, but tails off in response above the 20 m amateur band. This seems partly to be due to the new transformer, since HF reception was better with the previous transformer. This compromise is quite acceptable, especially as another receiving antenna is available for HF duties.
*Ferrite sleeves of similar dimensions are available from Palomar Engineers, Digikey etc, look for type 75, 73 or 31 material