Devices inserted in transmission lines often characterised by one or more of:
- Insertion VSWR (the input VSWR when terminated with a matched load);
- Return Loss (RL) in dB (20 times the log of the magnitude of the complex reflection coefficient); and
- Insertion Loss.
Practitioners often find Insertion VSWR (1) of most use as it indicates whether the device is worse than other system devices, the weak link in the chain if you like. You might see a coax antenna switch specified to have InsertionVSWR<1.2 to 60MHz.
Return Loss (2) is a function of VSWR and vice versa, so it appeals when the designer thinks in terms of Return Loss rather than VSWR (and it is a better metric for VSWR<1.2). You might see a coaxial relay specified to have ReturnLoss>30dB to 500MHz.
Insertion Loss (3) is not so readily compared to the other two which are measures of input reflection with a matched termination. It often yields some numbers that appear very acceptable, but might be deceptively so. You might see a coaxial relay specified to have ReturnLoss>30dB to 500MHz. You might see a coax antenna switch specified to have InsertionLoss<0.2dB to 100MHz. Continue reading InsertionLoss implies InsertionVSWR in lossless devices
Accurate GPS positioning has been of interest in automating data collection in field strength surveys of antennas on HF. To be useful, the positions need to be accurate to 100mm.
RTKLIB is an open source program package for GNSS positioning.
A number of experiments were conducted using a U-Blox LEA-6T assembly with integral small patch antenna (cost ~A$40) and RTKLIB v2.4.3(b8) using a base station feed from Geoscience Australia (with thanks). The test location is about 200km from the reference station at Symonston ACT.
Above, the LEA-6T and a RS-232 to USB adapter are strapped to a roof rack bar with stretch film. The high mounting position gives good view of the sky.
Above is a position plot of about an hour with the vehicle stationary (though in Kinematic mode) beside the house. Sky view was slightly restricted by the house and a line of tall trees to the west.
For the most part the track is smooth, but there is one section where the track in a more heavily treed section is markedly jaggy.
The Atten APS3005S is a 0-30V 5A linear DC power supply.
This later model includes a thermostatically controlled fan which at moderately light currents short cycles (10s on 20s off) and is very annoying… especially since it sits above my desk.
This project describes application of the generic heating / cooling controller (hcctl) to control the fan, reducing the short cycling nuisance.
Continue reading Atten APS3005S – a better thermostatic fan control
On a transmission line with standing waves, the voltage varies cyclically along the line, and is dependent also on power.
This article explains a method to use an analyser to predict the peak voltage level at a point for a given frequency and power based on measurement or estimation of complex Z or Y at that point using a suitable antenna analyser.
Lets say you have some critical voltage breakdown limit and want to use your analyser to find any non-compliance at the proposed power level.
Let us assume that the not-to-exceed voltage at that point is 1000Vpk. Let’s allow a little margin for variation due to factors not fixed, let’s actually use 800Vpk as the limit. We will use the maximum permitted power in Australia, 400W.
Continue reading Exploiting your antenna analyser #22
The popular End Fed Half Wave is all things to all men, but this article compares an End Fed Half Wave, Inverted L, and Half Wave Dipole with some common parameters:
- frequency: 7.1MHz;
- flat top length: 20m;
- Height above ‘average’ ground (0.005,13): 10m;
- lossless balun / matching device.
- ground connection: Inverted L = 2Ω, End Fed Half Wave = 100Ω; and
- effective common mode choke used on the dipole.
Above is the modelled gain for all three. Continue reading End Fed Half Wave / Inverted L / Half Wave Dipole
I have noted recently the increasing popularity of the so-called End Fed Half Wave antenna, though the term often includes harmonic operation of the antenna.
It seems that at the heart of common ham understanding of this antenna system is that some kind of two terminal feed device creates a scenario with current on the nominal radiator, and zero common mode current on the feed line. If that feed device is small, its contents bears little influence on the current distribution on the feed line and radiator (the device behaviour approaches that of a simple circuit node).
Above is the kind of current distribution envisaged by many. The equivalent source is shown at the end fed feed point The red curve is the magnitude of current, the horizontal line represents the nominal radiator, and the vertical line represents the common mode conductor formed by the feed line. The feed line is often of arbitrary length, arbitrary route, and it may connect to real ground via an arbitrary impedance. Pretty much everything about this antenna system is random save the length of the nominal radiator. Continue reading The magic of End Fed Half Waves (EFHW)
A correspondent wrote about the apparent conflict between Exploiting your antenna analyser #11 and Alan, K0BG’s discussion of The SWR vs. Resonance Myth. Essentially the correspondent was concerned that Alan’s VSWR curve was difficult to understand.
For convenience, here is the relevant explanation.
By definition, an antenna’s resonant point will be when the reactive component (j) is equal to zero (X=Ø, or +jØ). At that point in our example shown at left, the R value reads 23 ohms, and the SWR readout will be 2.1:1 (actually 2.17:1). If we raise the analyzer’s frequency slightly, the reactive component will increase (inductively) along with an increase in the resistive component, hence the VSWR will decrease, perhaps to 1.4:1. In this case, the MFJ-259B is connected to an unmatched, screwdriver antenna mounted on the left quarter panel, and measured through a 12 inch long piece of coax. This fact is shown graphically in the image at right (below).
Note that the graph is unscaled, and that frustrates interpretation. The text is also not very clear, a further frustration. It is easy to draw a graph… but is the graph inspired by a proposition or is it supporting evidence. Continue reading Exploiting your antenna analyser #21
Desk study of opportunity to improve linearity.
At Chinese AD8307 power measurement module #2 I showed measurement of the linearity of an AD8307 based RF power meter.
The specification linearity is +/-1dB, which is poorer than one might like in a power measuring instrument.
The diagram above from the AD8307 datasheet shows the internal architecture, including 9 stages of cascaded log detector cells that attempt to give a log response over around 100dB range. The issue is that in the transition region between detector cells, error is worse than well inside an individual detector cell’s range.
Above is a sweep from -65 to -6dBm at 10MHz after calibration of slope and offset. The linear fit to the blue curve shows slope is 20mV/dB and intercept 1.8015 for 0dBm means the offset is -1.8015/0.02=-90.08dBm. Log conformance is 0.2dB (well within spec at this frequency, temperature etc).
Continue reading Chinese AD8307 power measurement module #4
A very long time ago, I purchased a W2AU 1:1 balun on the maker’s claims that it was good from 1.8 to 30MHz.
These were very popular at the time, but as voltage baluns they achieve good current balance ONLY on very symmetric loads and so are not well suited to most wire antennas.
Above is W2AU’s illustration of the internals.
Mine barely saw service before it became obvious that it had an intermittent connection to the inner pin of the coax connector. That turned out to be a poor soldered joint, a problem that is apparently quite common and perhaps the result of not properly removing the wire enamel before soldering.
Having cut the enclosure to get at the innards and fix it (they were not intended to be repaired), I rebuilt it in a similar enclosure made from plumbing PVC pipe and caps, and took the opportunity to fit some different output terminals and an N type coax connector.
Above is the rebuilt balun which since that day has been reserved for test kit for evaluating the performance of a voltage balun in some scenario or another. Continue reading W2AU 1:1 voltage balun
This posts shows a measurement of ambient noise and comparison with the data given at Expected ambient noise and its more detailed references.
The test scenario is my 40m station, a G5RV inverted V dipole with tuned feeders, a balun and ATR-30 ATU. Antenna system losses are less than 1dB.
The chart above gives a range for expected ambient noise at 40m.
Above is a screen shot from a spectrum analyser measuring power in 1kHz bandwidth from 7.0 to 7.1MHz. The band is mostly unoccupied, and the mean noise power is about -99dBm, it would be 3dB higher in 2KHz bandwidth (ie -96dBm). Continue reading Expected ambient noise – in practice