An Insertion VSWR test gone wrong
We often learn more from failures than successes, this exercise is one of those opportunities.
An online poster tried to validate his newly purchased MFJ-918 by measuring Insertion VSWR.
That is done preferably by measuring a good termination (dummy load) to validate that it has a very low VSWR, then inserting the Device Under Test (DUT) and measuring the VSWR as a result of insertion of the DUT.
The poster did not mention measurement of the dummy load alone, and it is a type that warrants validation.
Above is the poster’s test setup, his Rigexpert AA-170 is connected to the balun’s input jack using a M-M adapter. The output wires on the balun form a rough circle of about 550mm perimeter by eye. Continue reading Exploiting your antenna analyser #27
I recently purchased a Surecom SW-102 VSWR meter. It looked a little like a supercharged RedDot copy.
Above the Surecom SW-102 VSWR meter with backlight and photographed under normal interior lighting. The display lacks contrast, and overall is difficult to read due to size of text, fonts used, and lack of contrast. (The pic is taken with a screen protector installed, but the evaluation is based on the bare meter with original protective film removed as it degraded readability.) Continue reading Surecom SW-102 VSWR meter review
Find coax cable velocity factor using a very basic analyser
A common task is to measure the velocity factor of a sample of coaxial transmission line using an instrument that lacks facility to backout cable sections or measure SOL calibration (as discussed in other articles in this series). The older models and newer budget models often fall into this category.
The manuals for such instruments often explain how to measure coaxial cable velocity factor, and the method assumes there is zero offset at the measurement terminals (whether they be the built-in terminals or some fixture / adapters). In fact even the connectors are a source of error, especially UHF series connectors.
It is the failure to read exactly Z=0+j0Ω with a S/C applied to the measurement terminals that adversely impacts efforts to measure resonant frequency of a test line section.
The method described here approximately nulls out offsets in the instrument, measurement fixture, and even in the connectors used and for that reason may sometimes be of use with more sophisticated analysers.
Continue reading Exploiting your antenna analyser #26
A convenient list of ‘Exploiting your antenna analyser’ and short subject sub-titles, a table of contents for the series as it grows.
Exploiting your antenna analyser #30 Quality of termination used for calibration
Exploiting your antenna analyser #29 Resolving the sign of reactance – a method – Smith chart detail
Exploiting your antenna analyser #28 Resolving the sign of reactance – a method
Exploiting your antenna analyser #27 An Insertion VSWR test gone wrong
Exploiting your antenna analyser #26 Find coax cable velocity factor using a very basic analyser
Exploiting your antenna analyser #25 Find coax cable velocity factor using an antenna analyser without using SOL calibration
Exploiting your antenna analyser #24 Find coax cable velocity factor using an antenna analyser with SOL calibration
Exploiting your antenna analyser #23 Seeing recent discussion by online experts insisting that power relays are not suitable to RF prompts an interesting and relevant application of a good antenna analyser Continue reading Exploiting your antenna analyser – contents
Remote speaker-microphones and DMR portables discussed RF ingress to Speaker Mics (RSM) used with DMR radios in digital mode.
I purchased a RSM branded Kenwood but obviously a Chinese fake for an MD-390 for about $5 posted, but it turned out to be lousy with RF interference in the form of the ‘motorboat noise’ on transmit audio.
Dismantling the RSM I found there is precious little RF filtering, just a single SMD cap near the electret capsule.
Above is the modified RSM. Continue reading Another speaker mic modification to reduce RF interference
A correspondent wrote seeking clarification of the Telepost LP-100A claims re impedance measurement in the context of some of my previous articles on the sign of reactance.
I could see several mentions in the LP-100A manual and the LP_100Plot documentation and they do seem a little inconsistent.
The LP-100A manual states very clearly:
Note: The LP-100A cannot determine the sign of X automatically.
If you QSY up from your current frequency, and the reactance goes up, then the reactance is inductive (sign is “+”), and conversely if it goes down, then the reactance is capacitive (sign is “-“). A suitable distance is QSY is about 100 kHz or more. The LP-Plot program has the ability to determine sign automatically, since it can control your transmitter’s frequency. When it plots a range of frequencies, it uses the slope of the reactance curve to determine sign, and plots the results accordingly.
The first part states clearly that the instrument cannot directly measure the sign of reactance, and presumably measures the magnitude of reactance |X|.
Lets explore the second part in light of the overarching statement of the first part.
Above is the calculated R and X looking into 7m of Belden RG58C/U with a load 25+j0Ω. Also shown is |X|(as would be measured by the LP-100A) and calculated magnitude of phase of R,X, |φ|. Continue reading LP-100A impedance measurement
I bought a remote speaker-microphone (RSM) for a MD-390 DMR portable from 409shop.com, a 41-80K.
They assured me it was compatible with the radio in digital mode, but it turned out to be lousy with ‘motorboat noise’ on tx audio due to RF ingress tot he electret capsule.
Since the RSM was otherwise a good rugged and economical product, it was worth trying to rectify the RF ingress problem.
Above is a pic of the electret. Two fine tracks can be seen bonding the metal can of the electret to the -ve pin, so that is good… the can showed low resistance to the -ve pin. The +ve line is bypassed to the -ve line about 12mm from the electret with an unknown capacitor, but it was clearly not effective at 440MHz. Continue reading Another RFI mod of a speaker mic (41-80K) for DMR use
In a recent long running thread on impedance matching on one of the online fora, one poster offered the Ten-tec 540 manual as a reference for clarity on the subject (which of course got murkier with every posting as contributors added their version to the discussion).
The Ten-tec 540 was made in the late 1970s, one of the early radios with a solid state PA, and their manual give the
Technical facts of life to guide new owners to successful exploitation of this new technology.
technical facts of life is this little gem:
The standing wave ratio is a direct measure of the ratio between two impedances, ie an SWR of 3 to 1 tells us that one impedance is three times the other. Therefore the unknown impedance can be three times as large or three times as small as the known one. If the desired impedance that the transceiver wants to see is 50 ohms, and SWR of 3 to 1 on the line may mean a load impedance of either 150 or 17 ohms. …
This says that the SWR wrt 50Ω implies just two possible impedances, he is very wrong… it implies an infinite set of possible impedances. Continue reading Ten-tec on the meaning of SWR
Remote speaker-microphones and DMR portables discussed RF ingress to Speaker Mics(RSM) used with DMR radios in digital mode.
I purchased a Retevis SM that was advertised as original equipment for the MD-380, but turned out to be lousy with RF interference in the form of the ‘motorboat noise’ on transmit audio.
Dismantling the SM (fighting the way through TORX with PIN screws, what are they trying to protect) I found there is precious little RF filtering, just a single SMD cap at the end of a long (wrt 500MHz) branch track.
Above is the modified SM. Continue reading Retevis MD-380 speaker mic modification to reduce RF interference
At Remote speaker-microphones and DMR portables I discussed the propensity for RF ingress to speaker microphones on DMR radios in digital mode.
This article looks at another speaker mic problem, mechanical compatibility of the plug / jack arrangement.
The MD-390 is IP67 rated, which means that it has a water-resistant gasket around the speaker mic jacks.
Above, the soft rubber gasket surrounds the speaker mic jacks, and if you look carefully, you will note that the metal part of the jack is recessed in the gasket. This is not an unsual arrangement. Continue reading TYT (Tytera) MD-390 speaker mic plug compatibility issues