Over the past couple of years I’ve had a number of comments and questions about active antennas, instigated by my ARRL book, Receiving Antennas for the Radio Amateur.

The “main ingredient” of an active antenna (in this discussion, we’ll center on the very short WHIP), is the preamplifier, which generally takes the form of an FET source follower.

A true source follower (or ideal cathode follower) is theoretically capable of INFINITE power gain). In practice, modern FET input op-amps have an input resistance on the order of a teraohm or so, and an input capacitance of about a picofarad.

Although we can’t QUITE get to infinite power gain with a real FET (or FET input op amp), we can get EXTREMELY high power gains. Assuming an output (source) resistance of 1Kohm and an input resistance of 1 teraohm, a voltage follower will have a power gain of 10^21:1…..not too shabby. (This is assuming essentially a DC signal, where the input parallel capacitance can be ignored).

At this point in time, there has been no mention of noise… but it is key to the problem.

Let’s consider the case of a low noise preamplifier (assume Noise Figure 1dB) used ahead of a receiver with Noise Figure 6dB. We can calculate the system Noise Figure, ie the cascade of the two elements, vs preamplifier gain.

It is easier to solve this problem by working in equivalent Noise Temperatures, \(T_s=T_1+T_2/G_1\). So converting the two Noise Figures to Noise Temperatures, we can calculate system Noise Temperature vs preamplifier Gain and plot it.

Above is a plot of system Noise Temperature vs preamplifier Gain, also shown on the right hand axis is the system Noise Figure.

For this scenario, there is a significant improvement in Ts and NFs as Gain increases… up to about 20dB, after which further increase in Gain makes very little difference.

Above, zooming in on the results, the behavior is very clear. Note that it is dependent on the scenario, and the plot will be different for different scenarios.

We do not need to consider the antenna gain to find this point of diminishing returns of Ts and NFs vs preamplifier gain, and to show that for this scenario, a Gain of 20dB or 100 is sufficient to have achieved most of the benefit of a preamplifier with NF=1dB.

The bigger question is whether that system Noise Figure or Noise Temperature is ‘sufficient’ for a specific application.

Again, it is easier to work in equivalent Noise Temperature.

Consider a scenario of an antenna system, with gain -30dB (or 1/1000) at 3.5MHz. ITU P.372 suggests the expected external or ambient noise is 47350000K, and with an antenna system gain of 1/1000, the noise power presented to the preamplifier is \({T_e}^{\prime}=\frac{47350000}{1000}=47350 \text{ K}\).

A useful metric in system design is the extent to which the external S/N is degraded by the receiver system, I will call it Signal to Noise Degradation (SND).

\(SND=10 log\frac{\frac{S_{ext}}{N_{ext}}}{\frac{S_{ext}}{N_{int}+N_{ext}}}\)Simplifying this by dividing top and bottom by \(S_{ext}\) we get

\(SND=10 log\frac{N_{int}+N_{ext}}{N_{ext}}\).

So, SND gives us a metric that simply depends on the external noise and the receiver internal noise, a quantitative measure of the system in an application context.

Let’s take the system Noise Temperature with the above scenario and 20dB of preamplifier gain to be 84K, we can calculate SND due to receiver system internal noise to be \(SND=10 log\frac{N_{int}+N_{ext}}{N_{ext}}=10 log\frac{84+47350}{47350}=0.08 \text{dB}\).

So, in this scenario, the receive system equivalent noise temperature is so low that there is only a 0.08dB degradation in the off-air S/N ratio.

You might see that a system Noise Temperature of 5000K (NFs=12dB) is not going to degrade S/N much (<0.5dB).

But here is the problem…

The methods presented here apply to linear systems, they do not capture the effects of non-linear behavior such as IMD noise.

It is easy to build a preamplifier with high gain, harder to build one with low noise, and even harder to build a broadband one with very load IMD noise.

This article explains with graphs the relationship between Signal / Noise degradation (see Signal to noise degradation (SND) concept) and LNA gain in the configurations discussed in the original article. See The oft asked question of how much an LNA improves a 70cm weak signal station for documentation of the scenario assumptions.

The critical value for SND is a personal choice, but for the purpose of this discussion, let’s choose 1dB. That is to say that the S/N at the receiver output is less than 1dB lower than the ultimate that could achieved with the antenna system given the external noise environment.

The total line loss in the example configurations was 2.6dB. The model assumes that LNA Noise Figure is independent of LNA Gain, though in the real world, there is typically some small dependence.

Often the choice of LNA Gain drives the choice of a single stage or two stage LNA, which has cost implications.

Above is a chart showing SND vs LNA Gain. It can be seen that as LNA gain is increased, SND improves rapidly with a knee around 15dB LNA gain above which SND improvement is slower.

For SND lower than 1dB, one would choose LNA gain of more than 12dB for this scenario.

Above is a chart showing SND vs LNA Gain. It can be seen that as LNA gain is increased, SND improves rapidly with a knee around 20dB LNA gain above which SND improvement is slower.

For SND lower than 1dB, one would choose LNA gain of more than 18dB for this scenario.

Above is a chart showing SND vs LNA Gain. It can be seen that as LNA gain is increased, SND improves rapidly with a knee around 5dB LNA gain above which SND improvement is slower.

For SND lower than 1dB, one would choose LNA gain of more than 2dB for this scenario.

The bigger question for this scenario is whether there is worthwhile benefit in using an LNA at all. If very low LNA Gain is sufficient, no LNA is probably nearly as good… it is just not needed and likely to have significant downsides with negligible benefit.

Even for this simple practical scenario where all else is held constant, the SND response to changing LNA Gain depends on the external noise level, and the response is not a simple linear one, but one with a distinct knee.

It is not safe to simply use as much gain as possible / available as high gain amplifiers are more prone to IMD, and more likely to overload the following receiver creating IMD.

Excessive gain is not a safe solution.

In high noise environments it takes little effort to achieve low SND, in fact an LNA is probably not warranted.

At the other extreme, the very external low noise level of a satellite path means that a suitable LNA is vital to low SND, and it probably needs high gain to achieve low SND, even for an LNA with very very low Noise Figure.

It is complicated, and it is a multi-dimensional problem… and that is where the G/T spreadsheet may assist.

Rules of Thumb may not be reliable.

Be wary of over simplified rules, this is a complicated problem for linear systems, then candidate solutions need to be tested carefully to see whether they are unduly affected by IMD etc.

Since the kind of improvements made to achieve a high performance weak signal receive system tend to be expensive, a little time to make measurements and on desk studies to understand the problem and design a solution may be a good investment.

]]>This article focusses on just one question in a quite similar configuration, what is the advantage given by the LNA?

The scenario will be evaluated for both terrestrial and satellite paths.

Above is the assumed ambient noise environment, it has great bearing on the results. More on that later.

The system G/T statistic has been used to quantify comprehensively the performance of a receive station over many decades. It has widest application in weak signal / low noise receive systems, eg satellite paths. G/T is the ratio of system antenna gain to equivalent noise temperature, most often given in dB as dB 1/K or simply dB/K.

The article Effective use of a Low Noise Amplifier on VHF/UHF gives some insight into G/T and its application.

Let’s use my G/T spreadsheet to model the scenario.

Assumptions:

- feed line types and lengths as listed in the spreadsheet;
- ambient noise as given in table above;
- LNA option is a MVV-432-VOX;
- system is linear, ie no non-linearity, zero IMD.

Two key metrics are calculated in the spreadsheet, G/T and Signal to Noise Degradation (SND).

- G/T can be used to calculate the S/N ratio given a receive field strength; and
- SND is the degradation of the external S/N ratio by system internal noise (see Signal to noise degradation (SND) concept).

Either metric can be used to calculate the S/N improvement due to adding the LNA.

S/N is a measurable quantity, and an S-meter does not give S/N with any accuracy. Higher S-meter deflection is not synonymous with higher S/N ratio.

The following assumes an equivalent external noise temperature that is of the order of what might be expected to illustrate the analysis technique, but measured values should be used for a specific scenario.

Above is the baseline terrestrial model with provision for the LNA, but without it.

A perfect receive system would have SND=0dB, so at 6.67dB, this degrades ‘off-air’ S/N substantially.

Above is the terrestrial model with LNA.

A perfect receive system would have SND=0dB, so at 0.73dB, this degrades ‘off-air’ S/N by a quite small amount, and some 6dB improvement over the baseline terrestrial model with no LNA.

The following assumes an equivalent external noise temperature that is of the order of what might be expected to illustrate the analysis technique, but measured values should be used for a specific scenario.

Above is the baseline satellite model with provision for the LNA, but without it.

A perfect receive system would have SND=0dB, so at 16.64dB, this degrades ‘off-air’ S/N hugely.

Above is the satellite model with LNA.

A perfect receive system would have SND=0dB, so at 5.14dB, this degrades ‘off-air’ S/N substantially, but 12dB improvement over the baseline satellite model with no LNA.

The results above depend on an assumption that the system is linear, in particular this means insignificant IMD.

Note that many ham LNA designs and products lack front end selectivity which predisposes them to worse IMD due to the higher aggregate signal levels reaching the active device.

High gain antenna systems tend to be frequency selective and provide some measure of front end selectivity. It is worth evaluating S/N ratio with a low value precision attenuator inserted before the LNA, if S/N improves there is significant IMD… and addressing that improves internal noise.

As mentioned, external noise is scenario dependent, antenna pattern dependent, antenna azimuth and elevation dependent, and should be evaluated (see link below).

The model has many variables, and more system components could be added if needed. Any of these can be varied to answer questions like:

- what is the impact of less LNA gain;
- is optimal gain just sufficient to offset the coax loss;
- is a NF=0.8dB Gain=10dB LNA better;
- should I choose an LNA with more gain, or lower noise figure;
- what is the benefit of LDF4-50A feed line;

etc.

Whilst discussions on social media about with opinions and hand waving, it is quantitative analysis of your own scenario that gives the most valid answers.

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To some extent, the project was inspired by KK5JY’s Loop on Ground (LoG).

This article presents a comparison of Signal to Noise Degradation metric (see Signal to noise degradation (SND) concept) for both antennas, the common elements being:

- based on NEC-5.0 models (as detailed in earlier LiG articles);
- soil parameters used are σ=0.01, εr=20 (calibrated to measurements at the LiG test site);

The LoG models are for 4.6m sides, 2mm wire at 10mm height above ground, and an approximately optimal 450Ω:50Ω transformer.

The LiG models are for 3.0m sides, 2mm wire at 20mm below ground, and an approximately optimal 200Ω:50Ω transformer.

Above is a plot of SND for both antennas over the range 0.5-15MHz.

They are quite different responses.

It can be seen that from about 3-11MHz there is not a lot of difference between the antennas, but the LiG degrades slowly above 11MHz whereas the LoG degrades quickly below 3MHz.

A work in progress…

]]>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.

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.

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…

]]>The antenna comprises a square loop of 3m sides of 2mm bare copper wire, buried 20mm in the soil.

Above is the site marked out for earthworks, but excavation of a narrow slot 25mm deep. On the far side of the loop is an already installed plastic irrigation valve box for the transformer.

Above, the excavation implement… an attachment for the 62cc weed wacker which is designed for cutting neat vertical edges in the grass along paths etc. It was not very expensive, so seemed worth a trial. It worked very well.

Above, wire laid and slot backfilled. This will be watered and rolled with the mower over coming weeks to grow grass roots and settle the soil.

Above, ABS tent pegs were used to secure the corners of the wire loop.

In a couple of weeks, the feed point impedance will be measured and compared to models.

After one week of settling, including lots of rain, a quick preview with a hand held receiver is promising.

A work in progress…

]]>This article explains a little of the detail behind the graph.

The graph is based on a series of NEC-5.0 models of the loop in ground antenna. Key model parameters are:

- 3m a side;
- ‘average’ soil (σ=0.005, εr=13);
- depth=0.02m; and
- frequency 0.5 to 10MHz in 0.1MHz increments.

The models were scripted by a PERL script, and the output parsed with a Python script to extract feed point Z, structure efficiency, and average power gain (corrected to 4πsr).

The summarised NEC data was imported into a spreadsheet and an approximate model of the system built, comprising:

- Receiver input impedance 50+j0Ω;
- a length of transmission line (10m of Belden 8215 RG6/U);
- an ideal transformer (4:1);
- source impedance derived from the NEC data.

Calculation includes:

- transmission line loss and impedance transformation;
- transformer assumed ideal plus an allowance for transformer loss (1dB);
- mismatch loss; and
- average antenna gain.

Above is an extract of the spreadsheet.

Mismatch loss is an important element of the system behavior. A convenient place at which to calculate mismatch loss is the feed point of the loop in ground.

Above is a plot of the loop feed point impedance, the source impedance in the receive scenario.

Above is a plot of the loop load impedance, the receiver impedance transformed by transmission line and transformer. The varying impedance is a result of using 75Ω line.

The combination of these allows us to calculate mismatch loss.

Above is a plot of the calculated mismatch loss which must be added in to the system gain model.

From the system model, and an estimate of ambient noise from ITU-R P.372-14, we can calculate SND.

Above is a plot of SND.

Note that P.372-14 is based on a survey with short vertical monopole antennas, so it is likely to overestimate noise received by a horizontally polarised antenna (and therefore the SND estimate will be low).

Antenna performance is sensitive to soil parameters, especially those close to the surface and subject to variation with recent rainfall etc.

This is after all a feasibility study, and within acceptable uncertainty, the antenna system would seem to be feasible for low HF and even 160m receive.

]]>Let’s take ambient noise as Rural precinct in ITU-P.372-14.

An NEC-5.0 model of the 3m a side LiG gives average gain -37.18dBi. An allowance of 2.7dB of feed loss covers actual feed line loss and mismatch loss.

Above, calculated SND is 0.9dB. For this scenario (ambient noise and antenna system), the receiver S/N is 0.6dB worse than the off-air or intrinsic S/N ration. For Residential precinct ambient noise, SND is less at 0.3dB.

The above graph shows the system behavior over 0.5-10MHz, it is a combination of the effects of noise distribution; antenna gain; mismatch; transformer and feedline losses; and receiver internal noise.

A key measure of the ability to decode a radio signal is its Signal to Noise ratio (S/N) at the demodulator (or referred to some common point).

We can speak of think of an external S/N figure as \(S/N_{ext}=10 log\frac{S_{ext}}{N_{ext}}\) in dB.

Receiver systems are not perfect, and one of the imperfections is that they contribute undesired noise.

So, the S/N becomes \(S/N=10 log\frac{S_{ext}}{N_{int}+N_{ext}}\).

A useful metric in system design is the extent to which the external S/N is degraded by the receiver system, I will call it Signal to Noise Degradation (SND).

\(SND=10 log\frac{\frac{S_{ext}}{N_{ext}}}{\frac{S_{ext}}{N_{int}+N_{ext}}}\)Simplifying this by dividing top and bottom by \(S_{ext}\) we get

\(SND=10 log\frac{N_{int}+N_{ext}}{N_{ext}}\).

So, SND gives us a metric that simply depends on the external noise and the receiver internal noise, a quantitative measure of the system in an application context.

You might think that receiver Noise Figure does just that, but it does not. Receiver Noise Figure assumes the external equivalent noise temperature is 290K, a laboratory metric.

The methods presented here apply to linear systems, they do not capture the effects of non-linear behavior such as IMD noise.

Though the calculation is not difficult, a convenient online calculator is at Signal to Noise ratio degradation by receiver internal noise.

ITU P.372 ambient noise might also be useful.

Let’s work an example using Simsmith to do some of the calculations.

Scenario:

- 20m ground mounted vertical base fed against a 2.4m driven earth electrode @ 0.5MHz;
- 10m RG58A/U coax; and
- Receiver with 500+j0Ω ohms input impedance and Noise Figure 20dB.

An NEC-4.2 model of the antenna gives a feed point impedance of 146-j4714Ω and radiation efficiency of 0.043%, so radiation resistance \(Rr=146 \cdot 0.00043=0.0063\).

Above, the NEC antenna model summary.

Above is a Simsmith model of the system scenario.

R1 and G model the antenna, G uses Rr for Zo, and R1 contains the balance of the feed point impedance.

With the useZo source type, the source would deliver 1W or 0dBW to a conjugate matched load.

The next important figure is the power into the 500Ω load L. it is -58.3dBW. Simsmith has calculated the solution to the antenna loss elements, mismatches and coax loss under standing waves. Effectively, the average gain of the antenna system (everything to the right of L) is -58.3dB. Such an antenna is likely to have a Directivity of around 4dB, in fact the NEC model calculates 4.8dB. So the maximum gain is -58+4.8=-53.2dB.

The burning question is whether it is sufficiently good to hear most signals. Well, a better question is how much does it degrade off-air signal to noise ratio (S/N). All receivers degrade S/N, but how much degradation occurs in this scenario.

We need to think about the ambient noise. Lets use ITU-R P.372 for guidance on the expected median noise in a rural precint.

Above, ambient noise figure @ 0.5MHz is 75.54dB.

Now lets calculate the Signal to Noise Degradation (SND).

At 4.58 dB it is not wonderful, the weakest signals (ie those with low S/N) we be degraded significantly, stronger signals (those with high S/N) will be degraded by the SAME amount, but for instance reducing S/N from 20 to 15dB is not so significant.

Applying this to your own scenario

The information fed into the calculations included:

- Rr;
- feed point impedance;
- transmission line details;
- Rx input impedance and NF; and
- Ambient noise expectation.

To calculate your own scenario, you need to find these quantities with some accuracy.

Tools:

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