This article was revised 03/01/2021 to correct an error: the NEC-5.0 model reported was actually the KK5JY LoG, fixed, apologies.
The Loop in Ground project is about a receive only antenna for low HF, but usable from MF to HF. The objective is an antenna of that is small, low profile, and can be located outside the zone where evanescent modes dominate around noise current carrying conductors, like house wiring to minimise noise pickup.
The antenna comprises a square loop of 3m sides of 2mm bare copper wire, buried 20mm in the soil.
This article reports measurement of feed point impedance and a ‘calibrated' NEC-5.0 model.
Feed point impedance measurement
Australia is experiencing a La Nina weather pattern this spring / summer, and it has rained and rained and rained. The ground has varied from saturated to nearly saturated, so measurements are a little atypical. Several measurements have been made, and the ones reported here are at a less saturated time, but the same broad pattern has been observed with all measurements.
Whilst the measurements to not calibrate exactly with the NEC model, they are quite close, and the model used here is adjusted for better reconciliation in the range 1-5MHz. The soil parameters used are σ=0.01, εr=20, which are suggestive of very ‘good' soil.
Calibrated NEC-5.0 model
The calibrated NEC model is probably the best predictor of behavior that other constructors might experience.
Key to the performance is system gain which depends greatly on MismatchLoss, which is quite dependent on the load impedance presented to the lig. A spreadsheet model was constructed from the NEC feed point impedance and expected ambient noise (per ITU P.372-14) and Signal to Noise Degradation (SND) calculated (Signal to noise degradation (SND) concept).
Provision made for a transformer as part of the system model. So parameters to the spreadsheet model included:
- NEC-5.0 model of feed point impedance and average power gain of a 3m a side square loop of 2mm HDC buried 20mm in soil σ=0.01, εr=20;
- ITU P.372-14 ambient noise precinct ( Rural used here);
- transformer ratio (2:1 turns, 4:1 impedance, with 1dB loss used here);
- receiver Noise Figure (6dB used here); and
- transmission line (10m of Belden 8215 RG-6/U used here).
The 75Ω feed line is chosen as a low cost feed line even though it is not 50Ω, in a real implementation with a buried feed line, flooded RG-6/U or RG-11/U might be very practical choices.
Above is the graph for the Rural precinct as mentioned, SND is in blue. It can be seen that is less than 3dB from 1.0-13MHz.
If you live in a very noisy neighborhood, the Residential precinct may be more appropriate to your environment.
Above is the graph for the Residential precinct as mentioned, SND is in blue. It can be seen that is less than 3dB from 1.0-28MHz.
Most designs of small Loop-on-Ground antennas use higher transformer ratios, and they may or may not be appropriate, but for this LiG in the specified soil, it is clear from the spreadsheet model that choosing other integer ratio transformers gives poorer SND response.
An IC-R20 was used for a listening test as with near zero length feed line, system gain is not significantly affected by common mode feed line contribution. Tests were conducted at 19:00 local time on the MW BC band, 80m, 40m and 20m bands. In all cases, external noise was audibly greater than internal noise (assessed with a 50Ω termination, some very minor contribution from the termination), and the receiver performed pretty much in line with prediction. There was no evidence to question the predictive models for Rural in this location. It was interesting that even an hour before sunset, many MW broadcast stations were heard at very good strength even though this location is not the the formal service area of any MW broadcast signals, stations in Canberra some 200km distant were at very good strength and excellent quality (steady signal, no buzz or other significant intermodulation distortion.
A work in progress…