This article explores the design / analysis of a passive receive only antenna 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.
The example antenna is a K6SE 14’x29′ Pennant optimised for 160m.
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. I will use the Residential precinct as a lot of users will be deploying this type of antenna on a relatively small residential block in the suburbs.
So, design median ambient noise figure Fam is 65.4dB.
Pennant antenna
An NEC model of the Pennant was built and run at 1.8MHz.
Note the feed point Z=240-j33Ω, this is the source impedance to be loaded by the receiver input impedance transformed by the feed line. Receiver input impedance is not necessarily a tightly controlled parameter, so this example with use a value a little off nominal to be realistic.
Modelled antenna gain is -34.7dB, approximate reciprocity is assumed and this gain figure will be used for receiving. Directivity is 6.8dB, average gain is -34.7-6.8=-41.5dBdB.
SimNEC model of mixed load system
A SimNEC model was built of:
- the Pennant antenna as source (from an NEC-4.2 model);
- 100m of RG6 feed line; and
- receiver with input impedance of 60+j20Ω.
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.
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 -2.11dBW, so MismatchLoss is 2.11dB.
Above, the MismatchLoss is verified using Calculate Kurokawa’s power reflection coefficient from Zload (or Yload or S11) and Zsource.
Next, the reduction in power from -2.11dBW to -3.617dBW tells use that the RG6 under these mismatch conditions has a loss of 1.507dB.
The total feed system loss is 3.617dB.
Receive SND calculation
The receive antenna system (including feed system) has average gain of -41.5 + -3.617 = -45.1dB, lets round it down to -46dB.
Let’s assume the receiver noise figure is 20dB, receivers often need some front end attenuation when used on low bands with broadband antennas… so 20dB is not unrealistic.
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 | 65.400 dB |
| Field Strength Noise Temperature | 1.006e+9 K |
| Field Strength Excess Noise Ratio | 65.400 dB |
| Field Strength Excess Noise Temperature | 1.006e+9 K |
| Field Strength (2000.0 Hz) | 2.177 μV/m 6.76 dBμV/m 0.005778 μA/m -44.76 dBμA/m 1.26e-14 W/m^2 -139.00 dBW/m^2 |
| Normalised Field Strength (1 Hz) | -26.25 dBμV/m -77.77 dBμA/m 6.29e-18 W/m^2 6.29e+4 SFU 6.29e+8 Jy |
| Antenna system factor | 21.33 dB/m |
| Antenna system gain | -46.00 dBi |
| Receiver input resistance | 50.0 Ω |
| Receiver Noise Figure | 20.00 dB |
| Receiver Noise Temperature | 28710 K |
| Receiver Bandwidth | 2000.0 Hz |
| Receiver distance to source | 1.000 m |
| Receiver Voltage (external) | 0.1867 μV -14.58 dBμV |
| Receiver Power (external) | 6.974e-16 W -121.56 dBm 2.526e+4 K |
| Receiver S/N | -0.556 dB |
| Receiver (S+N)/N | 2.74 dB |
| S/N degradation | 3.30 dB |
The important metric here is S/N degradation, 3.3dB. This means that the system S/N is 3.3dB worse than ultimate (ie for a noiseless receive system).
If the external noise was 5dB higher, SND would be higher (but not by the same amount). So, a solution that might be acceptable in a relatively noisy residential precinct might not suit a user on a very quiet rural block.
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.
SimNEC model of 50Ω load system
So, would this be better with 50Ω feed line and a 50Ω receiver.
Above, the SimNEC model of the changed scenario.
Mismatch at the antenna terminals verifies.
So, this scenario is just a little worse that the RG6 scenario.
Conclusions
- The Pennant antenna on 1.8MHz has low gain, but sufficient to be used without a preamp, particularly in noisy environments.
- The Pennant may require a preamp for best effect in low noise situations.
- There is likely to be some MismatchLoss at the Pennant terminals, 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.