## W3LPL’s paired WSPRlite test – test 1

Frank, W3LPL conducted two interesting experiments with WSPRlites on 20m from the US to Europe essentially. This article discusses the first test.

The first experiment was a calibration run if you like to explore the nature of simultaneous WSRP SNR reports for two transmitters using different call signs on slightly different frequencies (19Hz in this case) feeding approximately the same power to the same antenna.

The first test uses two WSPRlites feeding the same antenna through a magic-T combiner producing a data set consisting of 900 pairs of SNR reports from Europe with only about 70 milliwatts from each WSPRlite at the antenna feed.

The data for the test interval was extracted from DXplorer, and the statistic of main interest is the paired SNR differences, these are the differences in a report from the same station of the two signals in the same measurement WSPR interval.

There is an immediate temptation of compare the average difference, it is simple and quick. But, it is my experience that WSPR SNR data are not normally distributed and applying parametric statistics (ie statistical methods that depend on knowledge of the underlying distribution) is seriously flawed.

We might expect that whilst the observed SNR varies up and down with fading etc, that the SNR measured due to one transmitter is approximately equal to that of the other, ie that the simultaneous difference observations should be close to zero in this scenario.

What of the distribution of the difference data?

Above is a frequency histogram of the distribution about the mean (0). Interpretation is frustrated by the discrete nature of the SNR statistic (1dB steps), it is asymmetric and a Shapiro-Wik test for normality gives a probability that it is normal p=1.4e-43.

So lets forget about parametric statistics based on normal distribution, means, standard deviation, Student’s t-test etc are unsound for making inferences because they depend on normality.

Nevertheless, we might expect that there is a relationship between the SNR reports for both transmitters, We might expect that SNR_W3GRF=SNR_W3LPL.

So, lets look at the data in a way that might expose such a relationship.

Above is a 3D plot of the observations which shows the count of spots for each combination of SNR due to the two transmitters. The chart shows us that whilst there were more spots at low SNR, the SNRs from both are almost always almost the same.

A small departure can be seen where a little ridge exists in front of the main data.

Lets look at in 2D. Continue reading W3LPL’s paired WSPRlite test – test 1

## Small untuned loop for receiving – is an amplifier necessary?

I have a 30″ square loop of #12 wire that I use for receiving, and when I attach it to the receiver on 40m, the audio output voltage goes up three times or more. Do I need an amplifiers, or will it worsen things?

It is possible to determine the ambient noise temperature from the true noise power change over that of a matched termination.

The equivalent noise temperature of the receiver is implied by its Noise Figure when it is terminated with a matched termination. Noise due to an open circuit or short circuit input is not defined.

The correspondent re-measured with a termination, and as it turned out, the results were much the same, so lets work the case of voltage increasing by a factor of three.

Without going any further, we can calculate the degradation in External S/N by the receiver, total noise power is proportional to (3^2) times internal noise, so S/N degradation is 10*log(9/(9-1))=0.51dB… very little.

It is true that an amplifier is unlikely to improve things and will be likely to degrade things because of intermodulation distortion that is inherent in them, more so if it overloads on broadband signal input.

But let’s go on to estimate the ambient noise figure Fa.

It is really important for this process that the AGC does not change the receiver gain, and there is no overload or clipping. The latter means DO NOT SWITCH THE AGC OFF, the S meter deflects, you need extra input attenuation to keep things linear.

Now lets assume the receiver has a Noise Figure of 6dB (most modern HF transceivers are in that ball park).

We need to estimate the gain of the antenna, we will use Calculate small loop Antenna Factor.

Ok, terminated in 50Ω, the untuned small loop has a gain of -43.4dBi. So, it captures only a very small portion of the external noise, but even so it delivers sufficient to the receiver to increase the output voltage by a factor of 3. Continue reading Small untuned loop for receiving – is an amplifier necessary?

## Small untuned loop for receiving – optimal loop load resistance

Small untuned loop for receiving set out a model for calculating the S/N degradation of an active untuned small loop antenna system.

The calculations in Small untuned loop for receiving – Trask noise and gain analysis might prompt the question of what is the optimal resistive load for an untuned small loop.

This article explores the topic for a simple model where the equivalent noise temperature of the amplifier is independent of source impedance.

## A simple model for a small loop

We can construct a simple model where the loop behaves as a fixed  pure inductance, and its load is a fixed pure resistance.

This is a reasonably good model for a small loop, perimeter < wl/10, not too bad for perimeter up to wl/3.

The source impedance becomes the loop’s inductive reactance Xl which is proportional to frequency, and the load is Rl.

Above is a plot of the relative power developed in the load vs the ratio of Rl/Xl.

There is a maximum where Rl=Xl, and the power captured falls away either side. Continue reading Small untuned loop for receiving – optimal loop load resistance

## Small untuned loop for receiving – Trask noise and gain analysis

The article Small untuned loop for receiving mentioned Trask’s active loop amplifier.

(Trask 2010) published a two stage design using passive augmentation, arguing certain benefits of the approach.

• Zin=2.25Ω
• NF 2.42dB
• Voltage gain 36dB
• OIP2 80dBm
• OIP3 40dBm

This article presents a noise gain analysis for the 8m perimeter loop used in the article Small untuned loop for receiving to achieve a S/N degradation of no worse than 1dB at 7MHz.

The analysis assumes linear components, that there is no significant intermodulation distortion in the preamplifier. That is a significant challenge on which success of the system depends.

## External noise

From the above chart (ITU-R P.372-12 (7/2015)), we can take the external or ambient noise figure Fa to be about 45dB at 7MHz, Ta=290*10^(45/10)=9.17e6K. Continue reading Small untuned loop for receiving – Trask noise and gain analysis

## Small untuned loop for receiving

This article walks through a case study for a small single turn untuned loop with attached 50Ω balanced preamplifier and 50Ω coaxial output to a high grade communications receiver. The objective is to achieve system S/N ration not poorer than 1dB below the external S/N (ie ExternalS/ ExternalN).

Such an antenna has utility in that it can be rotated to null out a strong noise source from a direction other than the desired signal.

The analysis assumes linear components, that there is no significant intermodulation distortion in the preamplifier. That is a significant challenge on which success of the system depends.

This is a rework of an earlier article which presented a ‘back of the envelope’ noise and gain analysis now presented as a more accurate model embodied in a spreadsheet to allow convenient exploration of variations to the scenario.

## External noise

From the above chart (ITU-R P.372-12 (7/2015)), we can take the external or ambient noise figure Fa to be about 45dB at 7MHz, Ta=290*10^(45/10)=9.17e6K. Continue reading Small untuned loop for receiving

## The Mobius strip loop – ham benefits

(Baum 1964) describes his “Moibus strip loop” (sic).

In fact it is not made from a strip conductor but rather a circle of round tube  with a gap at the top, and containing a transmission line which is cross connected to the outer tube at the gap.

Two main features are claimed for this antenna:

1. cancellation of induced Compton currents in the centre conductor due to incident gamma radiation; and
2. transformation of the feed point voltage V to 2V at the transmission line at the loop feed T joint.

Feature 1 is claimed to improve S/N when irradiated by gamma radiation, the effect would be of most benefit in the event of a nearby nuclear bomb. Given that most ham stations are not EMP hardened, this is unlikely to be of material benefit to those ham stations. Continue reading The Mobius strip loop – ham benefits

## Quiet HF antennas and E and H fields in the near field zone

Hams often postulate that certain HF antennas are “low noise’ antennas.

There are many possible explanations for why an antenna captures less noise power than another, this article discusses the distribution of electric and magnetic fields (E and H) very near to a radiator, and the power captured by antennas that respond more to E or H fields.

Electromagnetic radiation consists of both and E field and a H field, and they are in the ratio of η0=µ0*c0Ω, the so-called impedance of free space, often approximated to 120πΩ or 377Ω. Close to a radiator there are components of E and H additional to the radiation components, the ratio of E/H is not simply 377Ω.

Fig 1 shows the magnitude of the ratio E/H near a quarter wave vertical over average ground at 3.6MHz. |E/H| depends on location near the antenna, and with increasing distance it converges on 377Ω.
Continue reading Quiet HF antennas and E and H fields in the near field zone

## LNR Precision small transmitting loop

LNR Precision have announced a small transmitting loop for amateur radio.

## Description

The antenna is described at (LNR Precision 2016).

The loop itself appears to be 3/8 Heliax or similar (nominally 9.5mm outer conductor diameter) in a rough circle of 45″ (1.143m) diameter.

Little information is given of the internals, but the promotional material gives a VSWR curve for a matched antenna at 7.065MHz. To their credit, they give the height above ground and ground type for their tests.

The VSWR=3 bandwidth scaled from the graph is 18kHz.

If we assume for a moment that the VSWR measurement was captured at a substantial height above ground, its behavior approaches that of the antenna in free space. Taking the assumption that the published curve is similar to the antenna in free space, we can estimate efficiency based on earlier assumptions. Such antennas very close to ground have a directivity of about 6dB (dependent on ground parameters), and that can be used with efficiency to estimate gain in proximity to ground.

The assumed values and published VSWR curve indicate an antenna system half power bandwidth of 15.6kHz and Q of 453 which implies efficiency of 2.8%.

The actual value for radiation resistance is likely to be with -50-+100% of the free space value used, and that rolls up as an uncertainty of +/-3dB in the calculated efficiency and gain. Continue reading LNR Precision small transmitting loop

## Current and voltage implications of a small transmitting loop power ratings

This article gives a simple method for calculating the key voltage and current in a small transmitting loop using observed or expected behaviour and Calculate small transmitting loop gain from bandwidth measurement.

## Method

Above is a model hypothetical 1m diameter loop of 10mm conductor on 40m with 1% radiation efficiency.

Lets say it is rated for input power being the lesser of 10W continuous, or 30W PEP SSB. Continue reading Current and voltage implications of a small transmitting loop power ratings

## CHA P-Loop 2.0 small transmitting loop

Chameleon have released their CHA-P-Loop 2.0 small transmitting loop. This article considers the likely efficiency on 40m based on their published measurements and Efficiency and gain of Small Transmitting Loops (STL).

## Description

The antenna is described at http://chameleonantenna.com/CHA%20P-LOOP%202.0/CHA%20P-LOOP%202.0.html.

This analysis does not consider the proprietary Power Compensator option for lack of sufficient information.

The loop itself appears to be LMR400 coax or similar (nominally 8.0mm outer conductor diameter) in a rough circle of 34″ (0.863m) diameter.

Little information is given of the internals, but the promotional material gives a VSWR curve for a matched antenna at 7.15MHz. To their credit, they give the height above ground and ground type for their tests, though elevation above ground was between 1/2 diameter to a full diameter of the P-LOOP 2.0 is a little vague.

## Basic loop (34″)

The VSWR=3 bandwidth scaled from the graph is 27.0kHz. The shape of the curve near minimum suggests that were the scan points sufficiently close, the minimum VSWR would be very close to 1.0 and it is taken as 1.0.

If we assume for a moment that the VSWR measurement was captured at a substantial height above ground, its behaviour approaches that of the antenna in free space. Taking the assumption that the published curve is similar to the antenna in free space, we can estimate the gain and efficiency based on earlier assumptions.
Continue reading CHA P-Loop 2.0 small transmitting loop