A ham recently posted a graph on QRZ to educate his fellow hams on the behaviour of transmission lines under mismatch.
Above is one of his graphs (the red arrow is my annotation). It plots Impedance variation along a mismatched 75Ω transmission line. The curves look graceful, but are they science or just pretty artwork? Continue reading Failure to treat impedance as a complex quantity leads to…
This article demonstrates use of a GR1606B RF impedance bridge for measurement of the feed point impedance of a MHz loaded mobile whip. The antenna is roof mounted on a vehicle and measurements are made looking into 4m of RG58C/U, then transformed to feed point impedance using three tools:
Key metrics are:
Continue reading Demonstration of the GR1606B for antenna Z measurement and calcs
This is a review of the BG7TBL noise source available on eBay for about $20 incl post. I have seen this recommended in various online forums and thought it worthy of review.
A quick mention of Excess Noise Ratio (ENR), it is a commonly used measure of the characteristic of noise sources. A noise source for testing low noise RF amplifiers needs to be less than 10dB, 5dB is common; for other receiver testing around 15dB is common, and for massive output for filter alignment etc the noise needs to be well above a spectrum analyser noise floor so an ENR of 50dB might be appropriate (but such high noise output makes it useless for LNA noise figure measurement),
Above is the device. The layout is pretty simple, it is a Zener noise source at the left followed by three MMIC amplifier stages. The circuitry at mid left is a DC-DC converter to supply 25V to the Zener.
There are a host of aspects so far that are concerning:
An online expert recently reported:
I tried to make an antenna loop for longwave with cat 5 and after it did no good I realized the twisted wires canceled each other out.
Or did they really cancel?
I constructed a loop of one Cat 5 pair and measured its inductance when both wires are bonded at the ends.
The conductors are 0.5mm diameter and spaced 0.9mm. To estimate the inductance we use the geometric mean radius (GMR) as the equivalent radius of the pair. GMR=(0.5*0.9)^0.5=0.67, diameter=1.34mm. So let’s calculate the inductance of a single turn circular loop of 0.8m perimeter and round conductor of 1.34mm diameter.
The estimate above is 850nH.
Above is the measurement, the screen is not readable, but it is 852nH, very close to the estimated 850nH. Continue reading Inductance of a loop of CAT5 pair
The Elecraft N-GEN is a low cost noise source which is quite suited to many applications, more so if the Excess Noise Ratio (ENR) is known.
ENR is a commonly used property to describe the noise power density of a source, it is calculated as ENR=10*log(Tne-T0)/T0 dB where Tne is the quivalent noise temperature and T0 is 290K.
This article describes a calibration procedure. Note that the calibration is specific to the device and cannot be applied to another N-GEN.
Above is a screenshot of the Spectrum Analyser scan. A text file of the frequency,power pairs is saved for input to a spreadsheet to calculate ENR vs frequency. Continue reading Calibrating the Elecraft N-GEN
At Equivalent noise bandwidth – IC-7300 SSB Rx Filter2 (2400Hz) the ENB of the receiver was measured at 2088Hz. This article goes on to calculate the power received from a Elecraft N-Gen noise source which has been measured to have Excess Noise Ratio (ENR) at 10.1MHz of 48.2dB.
Lets input the data to Field strength / receive power converter and find the received power.
The measurement is made is preamp off (so that the S meter is more realistic), and the supplied NoiseFigure is a guess… but the noise source is so strong (being some 30+dB above the receiver internal noise) that the result is barely sensitive to that assumption.
The calculator returns many results, we are interested in just the receive power in dBm. The results follow. Continue reading Calculation of received noise power given ENB and ENR
For a lot of experiments, knowledge of the Equivalent Noise Bandwidth (ENB) of a receiver is necessary. The ENB is the bandwidth of an ideal rectangular filter with the same gain as some reference frequency, 1kHz is usually specified for SSB telephony receiver sensitivity measurement.
Though filters are often specified in terms of bandwidth at x dB down, that metric is of relatively little value, the x is often 6dB but not always, the filters depart significantly from ideal or even common response.
In brief, a white noise source is connected to the receiver input, Filter2 (nominal 2400Hz bandwidth soft response) selected and set to standard PBT, and the audio output captured on a PC based audio spectrum analyser, Spectrogram 16 in this case.
Spectrogram is set to integrate over 30s to average the variations due to the noise excitation. The resulting graph and text spectrum log are saved.
The method is explained in detail at Measure IF Bandwidth.
Above is the spectrum plots, as receivers go this is relatively flat, lacking the usual tapering off above 1kHz (a technique to cheat on sensitivity specs).
Continue reading Equivalent noise bandwidth – IC-7300 SSB Rx Filter2 (2400Hz soft)
I bought an inexpensive GPS antenna on eBay, the requirement was for one that operated from 3-5V to suit both of my GPSDOs. The antenna is mounted in an electrical junction box on conduit above the roof for reliable coverage.
The question was whether the active antenna with 5m of RG174 coax and SMA male connector at A$6 posted was any good.
To map its behaviour, it was attached to a Trimble Thunderbolt GPSDO and Lady Heather used to plot rx signal level over about 30h.
Above is the plot. The pattern is not quite symmetric as there are obstructions in play, in fact some of the dips in performance are explained by specific trees and the roof profile. There is a gap to the south at low elevations, GPS satellites don’t fly there (MEO inclined 55° from the equator) .
Overall, it reaches similar strength at the zenith as other antennas tried.
Overall evaluation, it seems to work ok though the coax is a bit rough.
Messi & Paoloni Ultraflex 7 coax cable compared M&P UF7 with RG-213. This article does a similar comparison between M&P Ultraflex 10 and LMR400UF.
Both cables are of similar size, ~10mm overall, stranded centre conductor and foil+braid outer conductor. The shield stranding is different and the foil is copper in the UF10, aluminium in the LMR400UF.
Let’s take the loss factors calculated for TLLC and de-construct the conductor and dielectric loss for each line type.
Above is a comparison of the cables. Continue reading Messi & Paoloni Ultraflex 10 coax cable
A recent long running thread on QRZ entitled “True balanced auto-tuner” was sure to tease out some pretty woolly thinking… the word “true” was enough to signal the outcome.
There are only three words in the title, we can dismiss “true” as a harbinger of woolly thinking, and though people will argue the toss on the appropriateness of the term “auto-tuner’, most people share an understanding of the meaning. “Balanced” is another problem altogether.
After thirty odd posts, there has been no definition or discussion of the term balanced, or its advantages or disadvantages.
One of the recommendations by several posters is the old is new again solution, the once popular link coupled tuner and the work of WZ5Q featured in one of those recommendations.
WZ5Q describes a tuner inspired by WW8J. WZ5Q extended the design and provides a writeup on optimising balance.
Above is WZ5Q’s partial circuit. In the article he describes and shows:
Key to analysis of the topology is that the centre of the output inductor is grounded. This results in the circuit tending towards equal but opposite phase voltages on the output terminals. Continue reading True balanced tuner