Ten-tec on the meaning of SWR

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. …

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

Retevis MD-380 speaker mic modification to reduce RF interference

Remote speaker-microphones and DMR portables discussed RF ingress to Speaker Mics(RSM) used with DMR radios in digital mode.

The problem

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.

A solution

Above is the modified SM. Continue reading Retevis MD-380 speaker mic modification to reduce RF interference

TYT (Tytera) MD-390 speaker mic plug compatibility issues

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

Icom IC-7410 – PTT tuner start doesn’t work

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

Speaker-microphones and DMR portables

Speaker-mics (RSM) are popular with portables (hand-helds), and it turns out that a lot of the implementations which appeared to work properly with conventional FM radios have issues on DMR portables.

The frustration in buying these things online is that sellers typically have no idea of what they are selling.

This article deals with degraded audio in a form that is often described as motor boating. Continue reading Speaker-microphones and DMR portables

Reflected power alarm for the MFJ-993B

This article describes an add-on to a MFJ-993B auto ATU to provide an audible alarm when reflected power exceeds a set threshold. A deficiency of the original design IMHO.

The solution uses the generic heating / cooling controller (hcctl) configured for its alarm function only, including a function to silence the alarm.

screenshot-03_12_16-18_34_50Above is the directional coupler part of the MFJ-993B. The REF test point is designed to present voltages within the range 0-5V when used within the stated power ratings. Continue reading Reflected power alarm for the MFJ-993B

Radio-Kits SWR meter – build and review

This article describes my build of a Radio-Kits SWR meter (v1.1) and post implementation review.

Advertised features:

  • HF coverage – 1.8-30MHz
  • 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.

Implementation

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.

rkswr01

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.

rkswr02

Above is the interior of the box showing the LCD display and the external BNC connectors fitted (substituted for the ubiquitous UHF connectors supplied with the kit). Continue reading Radio-Kits SWR meter – build and review

Grebenkember’s original Tandem match

(Grebenkemper 1987) describes a directional coupler that has become very popular, especially in commercial implementation.

screenshot-22_11_16-07_23_25The 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

KitsAndParts.com QRP SWR bridge

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.

kitsandparts002

Above are the completed kits.

bridge_sch

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

DMR distance limit

I have heard ‘traditional wisdom’ that there is a timing limit imposed by the DMR protocol limit that limits radio path lengths to less than 70km.

There is indeed a limit and it is due to the 2ms guard interval between time slots, and half of that time is available for accomodation of round trip radio propagation delay (RTT), so 1ms with c0=3e8m/s gives a one way distance limit of 150km.

To put the ‘traditional wisdom’ to the test, I took a UHF DMR handheld up onto a nearby hill and successfully had a medium length QSO via the VK2RAG repeater netoworked to VK1BGT using the VK1RBM repeater.
Continue reading DMR distance limit