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
Displays VSWR, forward power, reverse power and supply voltage
Peak reading power meter
Bar graph or numerical format
Reverse power alarm with adjustable threshold
Auto turn on in presence of RF – sensitivity about 1 watt
Optional turn off after preset time – 10-240 seconds
Backlit LCD display with variable brightness
Reverse polarity protection
I purchased the kit some years ago, and on receiving it and reviewing the circuit I formed the view that it was likely to have unacceptable Insertion VSWR on 1.8Mhz, and probably 3.5MHz bands… so I lost interest in assembling the kit. However, I have belatedly constructed the kit, calibrated and tested it.
The kit is supplied as a PCB and parts, no casework is supplied.
The board was difficult to solder, the strain relieved ground plane connections of components have very little donut to contact for heat transfer and are much harder to solder than the other pads. The strain relief is a dubious feature that makes soldering difficult.
Above, the kit assembled in a die-cast aluminium box. An opening for the LCD was milled into the box, and holes drilled for the rest of the fit up. The kit does not lend itself to this boxing as the buttons out the top and display out the front are a problem to fit up. A poor mechanical design.
(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