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. …
The humble ‘F barrel’ as it is known, the F81 adapter, is specified by IEC 61169-24:2009. That specification includes an extended performance type good to 3GHz.
It seems that every seller on eBay has worked out that they cannot sell F81 adapters unless they state that they are 3GHz rated… and they almost all lie.
Above is the internals of an F81 purchased as 3GHz rated on eBay. The construction is simply the popular construction used since the 1970s and good to almost 1GHz, good enough for VHF/UHF TV. Continue reading Anatomy of an F barrel (F81)
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.
At TV upgrade I reported a change in TV antenna pointing to a different and distant transmitter, and gave a Spectrum Analyser plot at the main TV receiver.
At that time, I adjusted the antenna accurately (within 1°) based on compass heading, but antennas are not perfect and two significant path obstructions may have bearing on best signal.
I could have run up and down the ladder making small adjustments and observing amplitude or better, RF S/N on the Spectrum Analyser but that is tedious and suboptimal so I purchased a DVB signal analyser.
Importantly, a good DVB analyser gives measurement of not just signal strength, but carrier to noise (C/N) ratio (which is actually RF S/N), Bit Error Rate (BER) and Modulation Error Ratio (MER), the last two very important statistics in optimisation and validation and not available on an ordinary Spectrum Analyser.
MER is calculated as the sum of the squares of the magnitudes of the ideal symbol vectors is divided by the sum of the squares of the magnitudes of the symbol error vectors. The result, expressed as a power ratio in dB, is defined as the Modulation Error Ratio (MER).
MER is a good overall single statistic for quality, but BER is more sensitive to occasional errors, so they are both important.
Above, the DVB analyser (the red device) enables a view of measurements whilst adjusting the antenna. The analyser here is connected to the masthead amp output and of course powers the masthead amp. Continue reading TV upgrade – MER optimisation
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.
The restack of TV channels, and then the allocation of spectrum immediately adjacent to a 4G mobile site that is 1km away and directly in line with Knights Hill (30km) caused me to rethink our TV source and switched to Mt Gibraltar (5km) to escape the 4G interference.
For whatever reason, the signal from Mt Gibraltar has dropped in level and is intermittently very inconsistent.
So, it is back to Knight’s Hill with a LTE filter to try to alleviate the interference from the in-line 4G site.
Above is the received channels at the TV set with a 6dB 75/50 pad inline. After the last restack, the five desired are now at least 90MHz lower than the edge of the 700MHz LTE (4G) allocation, and with an LTE filter in the masthead amplifier, it seems interference is not noticeable. Signal quality reported by the TVs is consistently 100%.
The spectrum analyser plot underestimates RF S/N due to the system noise floor.
The channels used are 35 (ABC),36 (WIN),37 (CTC),38 (CBN),39 (SBS), all 250kW. Ch 35 and 39 are on the BA tower, the others on the WIN tower.
Ham lore has it that man made noise on lower HF is radiated predominantly vertically polarised, this is offered and accepted by hams without explanation.
It can be shown by simple observation that the ambient noise level on lower HF is quite different in business or commercial areas, residential areas, and rural areas (ITU-R P.372-12). Not only is there a significant difference, the change happens quite rapidly with distance which suggests there is a dominant component (man made noise) and that the propagation path is a very local one (ground wave).
If you look around a typical residential neighborhood where hams might establish stations, the most obvious conductors that might carry and radiate noise currents from noise generators like appliances, leaky insulators etc are aerial power lines… which are usually closer to horizontal orientation (with horizontal E field) than vertical which seems inconsistent with the common observation that vertically polarised receiving antennas tend to capture more man made noise power than horizontal ones.
This article proposes a mechanism that may explain the apparent inconsistency between noise radiators and noise receivers.
Though this explanation is based on experience, the quantitative analysis here depends on interpretation of Recommendation ITU-R P.368-9 (2/2007) Ground-wave propagation curves for frequencies between 10 kHz and 30 MHz.
Whilst P.368-9 publishes a set of graphs like the one above for a limited set of grounds, ITU-R also publishes the program (GRWAVE.EXE) which can be used to calculate values for the user’s choice of ground and that is what was used for this article. The graph above is for a vertical monopole over ground with 1000W radiated, the antenna has directivity of 3, and the dashed line (inverse distance curve) is the field strength for a lossless ground (PEC). This can be verified with a spot calculation at 1km. Continue reading Polarisation of man made noise