Elecraft produces a directional coupler that may interest QRP aficionados. It comes with instructions for 20dB and 30dB coupling factors rated at 25 and 250W respectively from 1 to 30MHz.
This article reviews the magnetics design of the -20dB / 25W coupler.
The coupler uses a type of Sontheimer coupler (Sontheimer 1966) and these are commonly poorly designed. The first question is whether the magnetising impedance of T2 which appears in shunt with the load is sufficiently high to not give rise to poor insertion VSWR. Continue reading Elecraft CP-1 directional coupler – magnetics review
Should you trust your VSWR meter? asked an interesting question, and Should you trust your VSWR meter – detector linearity discussed a problem apparent in may VSWR meters.
This article illustrates one method of linearisation of the detector response of a practical VSWR meter.
Radio-kits SWR meter
This article contains an analysis of the analogue circuitry of the Radio-kits SWR meter.
The directional coupler at top left contains half wave peak detectors for forward and reflected waves. They are wired to the two compensated op amps at lower right (the connections are not shown on the circuit as the coupler may be remote, follow the terminal designations). Continue reading Should you trust your VSWR meter – linearisation
Should you trust your VSWR meter? asked an interesting question, and based on experience, including a relevant example, concluded:
The answer is no, like any measurement instrument, prove that it is trustworthy in the intended application.
It went on to ask:
If the VSWR meter is designed to fail, why does it fail?
This article contains an analysis of the analogue circuitry of the IC-7300 directional coupler to explain the likely cause of its poor behaviour.
IC-7300 directional coupler schematic
Above is an extract of the IC-7300 circuit in the area of the directional power coupler used for VSWR measurement. The circuit is a quite conventional Bruene coupler, and its response is similar to several types of directional couplers that produce a DC output voltage from a half wave detector. Continue reading Should you trust your VSWR meter – detector linearity
One often sees newbies ask about their VSWR meter readings, and a common observation is that the measured VSWR is better at low power and as power is increased, VSWR increases.
With the evolution of the ‘shack in a box’, and knowledge and experience to match, the problem is often reported observed with the transceiver’s internal VSWR meter.
Some of these ‘shack in a box’ have some pretty nifty features, for example the very popular Icom IC-7300 not only has an internal VSWR meter for the HF bands, but it can perform an assisted sweep and display the results graphically.
Isn’t that a great idea, so convenient, all good!
Or is it? Continue reading Should you trust your VSWR meter?
From time to time one sees discussion online about consistency of ‘measured’ VSWR at different power levels (on the same instrument).
A question often asked is:
I tune up at 10W and achieve VSWR=1.5, and when I increase power to 100W, the VSWR increases. Which should I believe?
The first thing to note is that good antenna systems SHOULD be linear, VSWR should be independent of power, it is if the system IS linear.
For the most part they are linear, even though many antenna systems contain elements such as ferrite cored inductors that may exhibit some small level of non-linearity in ‘normal’ operation.
Non-linearity caused by for instance saturation of magnetic materials, loss of permeability where the temperature of ferrite cores reaches Curie point, arcing of capacitors or other insulating materials is NOT normal linear operation of a GOOD antenna system. If high indicated VSWR at high power is caused by any of these effects, it is flagging a problem that requires attention.
That said, a significant non-linear element may be the VSWR meter itself.
A common, if not most common way to make these meters is to use a half wave detector to convert the direction coupler RF outputs into DC to drive an ordinary moving coil meter. These meters commonly assume that the detector DC output voltage is exactly proportional to the RF input voltage.
Lets look at the accuracy of that process.
Above is a plot of the detector output vs RF input voltage for a commercial 200W VSWR meter. The measurements cover input power from 10 to 100W.
Continue reading VSWR meter trap for the unwary
This article reports initial impressions of an Anytone AT-D868UV hand held VHF/UHF dual mode (DMR/FM) radio.
Above, the AT-D868UV, purchased for about A$225 incl post from Hong Kong. This model had a GPS though that is unusable on ham DMR networks, so it is wasted money if you like. They may be more expensive through online shops that collect GST, and of course in countries where tariffs are applied to make them great again, prices may be higher.
Continue reading Anytone AT-D868UV: initial impressions
The WSPRlite flex requires external low pass filters for each band of operation.
Since my experiments will be conducted on the 40m band, the following low pass filter meets the requirement. The inductors and capacitors make a seven element Chebyshev filter as designed by G3CWI.
Above, the filter is assembled on a piece of matrix board with two BNC connectors. The inductors are fixed with hot melt adhesive, and the whole thing served over with heatshrink tube. It is not waterproof. Continue reading 40m filter for WSPRlite flex
I bought a remote speaker-microphone (RSM) for a DMR portable from eBay (~$12 posted). Experience says that these suffer RF ingress which is seriously bad in DMR due to the amplitude modulation of the transmitted signal.
This RSM had somewhat improved filtering around the electret compared to others I have purchased. Continue reading Another RFI mod of a speaker mic for DMR use
A correspondent wrote asking about the design of a matching network for a Half Square antenna for 80m, voltage fed at one end.
Above is the current distribution on the half square voltage fed. It is essentially two in-phase vertical quarter waves separated a half wavelength, a broadside array.
Feed point impedance at resonance is very high 5700Ω, and being a high Q antenna, they are very sensitive to dimensions, nearby clutter etc. Note that this is calculated for an antenna in the clear, it will be different where trees or conductive mast exist nearby. Continue reading 80m voltage fed Half Square matching workup
At Surecom SW-102 VSWR meter review I wrote a review of a meter which I had purchased a little over a year ago, it was at v4.5.
One of the many problems identified was inconsistency of displayed values.
Surecom’s versions are confusing, the highest number is not necessarily the latest version. It appears a partial version history from their current page advertising the SW-102 is:
OLD VERSION : V3.3 ,V3.8 ,V4.5,V4.9 ,V5.0,V5.1
2017-8 NEW VERSION : V2.02 ,V2.03
The following image is from Surecom’s current page advertising the SW-102, and I assume that the version shown here (v2.6) is the latest at time of writing.
The image captures the outputs of two tests with poor and good dummy loads.
Let’s check the displayed values for internal consistency. Continue reading Surecom SW-102 VSWR meter review – v2.6