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.
Amongst the 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. …
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.
I have an IC-7410 with R1 of the firmware installed.
I have attempted to use its PTT tuner start feature (triggers ATU tune on PTT if frequency changed a significant amount) with an MFJ-993B ATU, but it fails.
The symptoms are that the IC-7410 does not transmit its tune carrier, it remains in the mode active when PTT was pressed.
Tuner operations initiated from the IC-7410 TUNE switch appear to all work as expected.
Above is a logic trace of the ATU control wires on PTT tune. everything looks good, when the /START signal is recognised as valid, the ATU asserts KEY and the IC-7410 should put tune carrier out… but it doesn’t and the ATU aborts after about 0.5s without tuning. Continue reading Icom IC-7410 – PTT tuner start doesn’t work
(Grebenkemper 1987) describes a directional coupler that has become very popular, especially in commercial implementation.
The simplified circuit above from Grebenkemper’s article illustrates the key elements of the directional coupler.
An important detail of the design is that the primary of the right hand transformer appears in shunt with the antenna load, and the magnetising impedance of that transformer core compromises Insertion VSWR. It is important that the magnetising impedance is sufficiently high (or the admittance sufficiently low) to not cause significant Insertion VSWR. Continue reading Grebenkember’s original Tandem match
The project is to build a test a couple of QRP VSWR detectors by KitsAndParts.com (http://www.kitsandparts.com/bridge.php) rated at 10W.
Above are the completed kits.
Above is the schematic. The bridge 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 KitsAndParts.com QRP SWR bridge
Walt Maxwell (W2DU) made much of conjugate matching in antenna systems, he wrote of his volume in the preface to (Maxwell 2001 24.5):
It explains in great detail how the antenna tuner at the input terminals of the feed line provides a conjugate match at the antenna terminals, and tunes a non-resonant antenna to resonance while also providing an impedance match for the output of the transceiver.
Walt Maxwell made much of conjugate matching, and wrote often of it as though at some optimal adjustment of an ATU there was a system wide state of conjugate match conferred, that at each and every point in an antenna system the impedance looking towards the source was the conjugate of the impedance looking towards the load.
Find coax cable velocity factor using an antenna analyser without using SOL calibration
A common task is to measure the velocity factor of a sample of coaxial transmission line using an instrument without using SOL calibration.
Whilst this seems a trivial task with a modern antenna analyser, it seems to challenge many hams.
We will use a little test fixture that I made for measuring small components, and for which I have made test loads for SOL calibration. We will find the frequency where reactance passes through zero at the first parallel resonance of an O/C stub section, this is at a length of approximately λ/2 (a good approximation for low loss coaxial cables above about 10MHz).
We will use a little test fixture that I made for measuring small components, and for which I have made test loads for SOL calibration.
The text fixture used for this demonstration is constructed on a SMA(M) PCB connector using some machined pin connector strip and N(M)-SMA(F) adapters to connect to the instrument.