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
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.
The coil inductance is too low, try a solenoid of 13 turns, 40mm diameter and 40mm length.
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 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
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
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 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.
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
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
A correspondent reading recent articles on active loops for receiving asks:
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?
After scouring eBay for a packaged esp8266 with 220V 10A relay, two products were identified:
As is usually the case, finding a schematic and specifications is very difficult and the sellers were of no help (no surprises).
The LC Technology device was offered with indistinct pics that hinted it had a 8Mb flash chip, ESP8266EX processor, and a STC 15F104 8 bit processor on board for some unidentified purpose.
A schematic was eventually located for the Yunshan board, and from pics it appeared to have a 12E module on it which hinted the flash size.
A Yunshan module was purchased for about $10 posted, and it was indeed a 12E with flash-id 4016, so 4MB of flash memory.
The board does not incorporate a USB-TTL adapter which is a nuisance not just requiring an external adapter for programming, but there is no integration of the RTS and DTR signals as in the NodeMCU devkit. Adding a quality USB adapter (eg CP2102) would not increase the price a lot, you can keep the CH340G etc). Continue reading ESP8266 relay module review – Yunshan WiFi relay
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.
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
A common scheme for Lua scripted NodeMCU modules with automatically start the script init.lua is to incorporate some logic to test the condition of a GPIO pin to determine whether to boot to the application or drop to the lua prompt for programming etc. In fact the scheme can be elaborated to provide a simple multi level selection based on the time the input condition is applied.
The obvious pin to use is the pin that commonly has a “BOOT” or “FLASH” button on it, GPIO0 or D3. It is used to activate the ESP8266 boot loader if it is low during boot, so it must be left high at boot to allow the lua interpreter to run, but it can be pulled low shortly after boot up and tested from init.lua.
An example init script follows.
print("\n\nHold Pin00 low for 1s t0 stop boot.")
print("\n\nHold Pin00 low for 3s for config mode.")
tmr.delay(1000000)
if gpio.read(3) == 0 then
print("Release to stop boot...")
tmr.delay(1000000)
if gpio.read(3) == 0 then
print("Release now (wifi cfg)...")
print("Starting wifi config mode...")
dofile("wifi_setup.lua")
return
else
print("...boot stopped")
return
end
end
print("Starting app.lua")
dofile("app.lua")
Above is a pic of the helper. The DIP switch allows selection of the BOOT pulse in 1s increments. It has four connections, ground, Vdd, BootOut, and Reset (optional). The button near the DIP switch resets the helper which in turn will apply a 10ms reset pulse to the Reset line. Continue reading Reset helper for NodeMCU ESP8266 modules