Precise RF small transmitting loop

Precise RF have announced two small transmitting loops for amateur radio, this article looks at the Precise High Gain Loop.

Description

The antenna is described at (Precise RF 2017).

Above is an extract from a table in the brochure comparing the subject antenna to some others.

On a quick scan, the standout figure is gain of 2.8dBd presumably at a loop height of 4.57m (15′), and without qualification of frequency. Elsewhere in the brochure there is a note that 80m requires an optional ‘resonator’… presumably a larger loop.

Lets review the meaning of dBd

The ITU Radio Regulations (ITU 2012) gives us a definition for antenna gain that captures the meaning of dBd that is accepted by most regulators and industry world wide.  Continue reading Precise RF small transmitting loop

The sign of reactance – SM6WHY’s take

As the popularity of low cost, low end antenna analysers increases, client software appears to enhance the capability of the analyser.

The SARC-100 is one of these low end analysers, it and its many close derivatives are marketed under various model names.

The sign of reactance discusses a major weakness of these and many other low end instruments in that they do not ‘measure’ the sign of reactance, displaying the magnitude of reactance and leaving it to the user to solve the sign problem.

SM6WHY is one of the many who have produced software for the SARC-100 that purports to solve the sign of reactance problem. He gives this graphic on his website to demonstrate the capability of his software used with a SARC-100 (which does not sense the sign of reactance).

Above is part of the graphic he offers. Though the image is poor quality, the VSWR plot appears smooth and quite typical of that which might be obtained by measuring an antenna system near its VSWR minimum.

However the accompanying Smith chart plot which has points plotted with both negative and positive reactance is inconsistent with the VSWR plot and appears flawed.  Continue reading The sign of reactance – SM6WHY’s take

A search for some mid power white wide angle LEDs

I have a project which needs some mid power (~3W) white wide angle (120+°) LEDs.

The obvious source ie eBay which means running the gamut of Chinese sellers, sellers who rarely understand the product they sell and probably expect the same of buyers.

Buying electronic components on eBay

Component sales tend to fall into categories:

  1. those with headline descriptions that have very brief description of characteristics; and
  2. those whose descriptive content claims well known part numbers for which datasheets can separately be found;
  3. those with detailed specifications offered.

In the case of category 1, it is very hard to have confidence that the components will deliver required performance, and headline descriptions on eBay are often used as competitive search keywords and do not apply to the goods on offer. These are probably best skipped unless they are the only option.

Category 2 provides a better option, and the question then on delivery is whether the goods are compliant with the part number offered. There is a considerable risk of counterfeit or fake parts that are not equivalent to the claimed part number, even where brand names are cited.

The third category can provide suitable product, but it takes some leg work, more than ‘due diligence’ to check the description for consistency and form an idea about its reliability, fit to the requirements and then value for money, seller reputation etc. This can be a lot of work for a few dollars worth of parts, but is a better option than category 1.  Continue reading A search for some mid power white wide angle LEDs

Shunt matching a loaded HF whip – discussion

Shunt matching a loaded HF whip with just a VSWR meter gave a direct answer and supporting explanation to an online poster’s question about optimising an 80m loaded mobile vertical with shunt matching, specifically the inductor needed and an adjustment procedure.

The original poster clearly had the impression that this improvement of the original VSWR=1.3 would make a large difference.

The only other option for me is to remove the shunt and set my swr back to 1.3:1 and not be able to communicate.

Continue reading Shunt matching a loaded HF whip – discussion

Power in a mismatched transmission line

This is a republication of an article posted on VK1OD.net Jun 2012.

This article presents a derivation of the power at a point in a transmission line in terms of ρ (the magnitude of the complex reflection coefficient Γ) and Forward Power and Reflected Power as might be indicated by a Directional Wattmeter. Mismatch Loss is also explained. Continue reading Power in a mismatched transmission line

Shunt matching a loaded HF whip with just a VSWR meter

A question was asked on one of the popular online forums:

How to get the most out of an 80 mobile antenna?…I am using a hustler antenna and I had the swr down to 1.3:1. I started researching how to make the antenna better and it seems that maybe an inductive shunt at the base of the antenna to ground would help. I don’t have the equipment to analyze the antenna and the shunt reactance. I made a 9 turn coil 1″ in diameter and 1″ long using n0. 12 awg thnn wire. I installed the coil at the base of the antenna and now the best swr that I can get is 1.8:1. So is there a way that I can set up the coil and antenna using only an swr meter?…

After 50 responses, none of the online experts have offered a direct answer or explanation.

Direct answer

The coil inductance is too low, try a solenoid of 13 turns, 40mm diameter and 40mm length.

Explanation

An antenna of this type at minimum VSWR will have a feed point impedance of near zero reactance and resistance equal to 50 divided by the measured VSWR, so in this case 39Ω. Continue reading Shunt matching a loaded HF whip with just a VSWR meter

G4JNT’s observation of bandwidth effects on WSPR SNR

G4JNT reported some measurements of WSPR reported SNR vs input signal at (Talbot 2010).

His experiment connected a WSPR modulated RF source directly to an SDR receiver, and he recorded WSPR’s receive SNR reports vs input attenuation and configured SDR receiver bandwidth. The direct connection means the test is not subject to normal radio path effects like fading.

The table above is derived from Talbot’s, his information about the RF source (-30dBm) and attenuator settings are converted to receiver input power (dBm).

Above is the same data charted. A linear line fit to the 300Hz data is also included, it is a very good fit. The issue that Talbot raised is that the reported SNR is quite dependent on receiver bandwidth. Continue reading G4JNT’s observation of bandwidth effects on WSPR SNR

W3LPL’s paired WSPRlite test – test 2

Frank, W3LPL conducted two interesting experiments with WSPRlites on 20m from the US to Europe essentially.

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 simultaneously feeding approximately the same power to the same antenna.

This article is about the second test which he describes:

The second test uses a WSPRlite directly feeding the same stacked Yagis, and the second WSPRlite feeding nearly identical stacked Yagis that point directly through the other stack located four wavelengths directly in front. Power at each antenna was about 140 milliwatts for each WSPRlite.

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 antenna relative to the other depends on their gain in the direction of the observer. Even though the two identical antennas point in the same direction for this test, the proximity of one antenna to the other is likely to affect their relative gain in different directions.

What of the distribution of the difference data?

Above is a frequency histogram of the distribution about the mean (4.2). Each of the middle bars (0.675σ) should contain 25% of the 815 observations (204). It is clearly grossly asymmetric and is most unlikely to be normally distributed. A Shapiro-Wik test for normality gives a probability that it is normal p=4.3e-39.

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. Continue reading W3LPL’s paired WSPRlite test – test 2

Small untuned loop for receiving – NEC model

A correspondent wrote suggesting that he had seen online NEC patterns showing a 30″ square small untuned loop to have a gain of around 10dBi, more than 30dB better than given by Calculate small loop Antenna Factor.

Firstly, lets describe a loop for study, a square diamond with sides of 760mm (30″) of 2mm diameter copper fed in one corner at 7.1MHz.

Calculate small loop Antenna Factor

Calculate small loop Antenna Factor models a small loop in free space (therefore does not include ground losses).

Above is the calculator result, the key figures are Antenna Factor 31.75dB and Gain -44.5dBi.

NEC-4.2 model

An NEC-4.2 model was constructed with external excitation (1V/m) incident on the loop which has a 50+j0Ω load inserted at the feed point to represent the receiver load.

Here is the model source.

CM Small square untuned loop
CM NEC-4.2
CM 
CM 1. Plane wave excitation
CM 
CM Owen Duffy
CM Note: rotations might not work properly in various NEC-2 versions, beware of segment size issues in NEC-2.
CE
GW    1    5    -0.38    0    -0.38    0.38    0    -0.38    0.001
GM    1    3    0    90    0    0    0    0    1
GM    0    0    0    90    0    0    0    2    1
GE    0
LD    5    0    0    0    58000000
LD    4    1    1    1    50    0
GN    -1
EK
EX    1    1    1    0    45    0    0    0    0    0
FR    0    0    0    0    7.1    0
EN

The key result to be extracted from the model run is the current in the 50Ω resistor in segment 1 of wire 1. The magnitude of the current is 5.1204E-04, so the voltage developed in the resistor V=5.1074-04*50=0.02554V. Antenna Factor is the ratio of the E field excitation to the terminal voltage of the receiver, so in dB it is 20*log(1/0.02554) =31.83 dB/m.

The NEC model’s 31.83 dB/m is close to the calculator prediction of 31.75dB/m, but includes the benefit of the lossy ground reflection . Continue reading Small untuned loop for receiving – NEC model

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