LP-100A impedance measurement

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

Another RFI mod of a speaker mic (41-80K) for DMR use

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

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

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.


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

Walter Maxwell’s teachings on system wide conjugate matching

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.

This is popularly held to be some nirvana, a heavenly state where transmitters are “happy” and all is good. Happiness of transmitters is often given in online discussion by hams as the raison d’être for ATUs . Continue reading Walter Maxwell’s teachings on system wide conjugate matching