This article explores the design / analysis of a passive receive only antenna, a K9AY loop, for the 160m band (1.8MHz).
The example and calculations assume linear systems, if there is significant nonlinearity that gives rise to significant IMD, IMD noise is not captured by the analysis.
Results are for the scenarios calculated and may not be extensible to different scenarios.
Another caveat: I have reservations about transmission line modelling in SimNEC, especially for composite conductors, but for the purposes of the discussion, assume that it is reasonably correct.
Above is the K9AY loop from a model by W7EL.
Design objective
The objective here is to design a receive only antenna system that can be relatively remote from local noise sources (like house wiring), and captures enough external signal and noise that the receiver internal noise does not degrade S/N too much.
I will use the metric Signal to Noise Degradation (SND), see Signal to noise degradation (SND) concept.
External noise
Let’s estimate expected external noise using ITU-R P.372. For this workup, I will use the Quiet Rural precinct which is fairly demanding because of the lower expected noise environment.
So, design median ambient noise figure Fam is 46.3dB.
K9AY antenna
An NEC model of the K9AY antenna was built and run at 1.8MHz.
Modelled antenna gain is -25.6dB, approximate reciprocity is assumed and this gain figure will be used for receiving. Directivity is 8.07dB, Average gain is -25.6-8.07=-33.67dB.
SimNEC model of mixed load system
A SimNEC model was built of:
- the K9AY antenna as source (from a NEC-4.2 model);
- 100m of RG6 feed line; and
- measured input impedance of an example IC-7300 receiver.
Above is the SimNEC model of the receiver feed. A quirk of SimNEC means that the source impedance must be specified as the complex conjugate.
Note that there are standing waves on the RG6, so impedance transformation details depend on the feed line length.
The model includes an ideal transformer at the antenna terminals, the optimum turns ratio here is 3 (impedance ratio 9).
If loaded with its complex conjugate, the source type specified here would deliver 0dBW to the load. Because of the mismatch, the power delivered to T1 is -0.018dBW, so MismatchLoss is 0.018dB.
Next, the reduction in power from -0.018dBW to -1.527dBW tells use that the RG6 under these mismatch conditions has a loss of 1.5dB.
The total feed system loss is 1.5dB.
Receive SND calculation
The receive antenna system (including feed system) has average gain of -33.67 + -1.5 = -35.17dB, lets round it to -35dB.
Lets assume a receiver at highish sensitivity, receiver noise figure is 10dB. That might lead to significant IMD noise due to the broadband nature of the antenna.
We will use Field strength / receive power converter.
… and the results are:
| Frequency | 1.8000 MHz |
| Field Strength Bandwidth | 2000.0 Hz |
| Field Strength distance to source | 1.000 m |
| Field Strength Noise Figure | 46.300 dB |
| Field Strength Noise Temperature | 1.237e+7 K |
| Field Strength Excess Noise Ratio | 46.300 dB |
| Field Strength Excess Noise Temperature | 1.237e+7 K |
| Field Strength (2000.0 Hz) | 0.2415 μV/m -12.34 dBμV/m 0.0006409 μA/m -63.86 dBμA/m 1.55e-16 W/m^2 -158.10 dBW/m^2 |
| Normalised Field Strength (1 Hz) | -45.35 dBμV/m -96.87 dBμA/m 7.74e-20 W/m^2 774 SFU 7.74e+6 Jy |
| Antenna system factor | 10.33 dB/m |
| Antenna system gain | -35.00 dBi |
| Receiver input resistance | 50.0 Ω |
| Receiver Noise Figure | 10.00 dB |
| Receiver Noise Temperature | 2610.0 K |
| Receiver Bandwidth | 2000.0 Hz |
| Receiver distance to source | 1.000 m |
| Receiver Voltage (external) | 0.07349 μV -22.68 dBμV |
| Receiver Power (external) | 1.080e-16 W -129.66 dBm 3912 K |
| Receiver S/N | 1.76 dB |
| Receiver (S+N)/N | 3.98 dB |
| S/N degradation | 2.22 dB |
The important metric here is S/N degradation, 2.2dB. This means that the system S/N is less than 2.2dB worse than ultimate (ie for a noiseless receive system). This might not be acceptable to some users, the solution is a bigger antenna or a suitable preamplifier.
Note that a preamp may well degrade things, it is not uncommon for preamps to have significant IMD. Likewise for running a receiver at maximum sensitivity.
Conclusions
- The K9AY antenna on 1.8MHz has low gain.
- There is likely to be some MismatchLoss at the K9AY terminals, but with a suitable transformer it is not likely to be huge.
- That said, the combination of low antenna gain and possibly needing to set the receiver to higher Noise Figure to reduce IMD noise means that small losses may be important.
- The article sets out to demonstrate a design process rather than to design an antenna system to be copied.