Find coax cable velocity factor using a very basic analyser
A common task is to measure the velocity factor of a sample of coaxial transmission line using an instrument that lacks facility to backout cable sections or measure OSL calibration (as discussed in other articles in this series). The older models and newer budget models often fall into this category.
The manuals for such instruments often explain how to measure coaxial cable velocity factor, and the method assumes there is zero offset at the measurement terminals (whether they be the built-in terminals or some fixture / adapters). In fact even the connectors are a source of error, especially UHF series connectors.
It is the failure to read exactly Z=0+j0Ω with a S/C applied to the measurement terminals that adversely impacts efforts to measure resonant frequency of a test line section.
The method described here approximately nulls out offsets in the instrument, measurement fixture, and even in the connectors used and for that reason may sometimes be of use with more sophisticated analysers.
Continue reading Exploiting your antenna analyser #26
A convenient list of ‘Exploiting your antenna analyser’ and short subject sub-titles, a table of contents for the series as it grows.
Exploiting your antenna analyser #26 Find coax cable velocity factor using a very basic analyser
Exploiting your antenna analyser #25 Find coax cable velocity factor using an antenna analyser without using OSL calibration
Exploiting your antenna analyser #24 Find coax cable velocity factor using an antenna analyser with OSL calibration
Exploiting your antenna analyser #23 Seeing recent discussion by online experts insisting that power relays are not suitable to RF prompts an interesting and relevant application of a good antenna analyser Continue reading Exploiting your antenna analyser – contents
A correspondent asked about the use of a Jaycar LO1238 ferrite core in VK3IL’s EFHW matching unit for 40m and up. The LO1238 implementation would use 3t primary and 24t secondary on the core.
If the transformer is simply used without an ATU between it and the radio, and we assume that the antenna system is adjusted to present low VSWR(50) to the radio, a simple approximation involves calculating the magnetising admittance of the 3t 50Ω winding, and calculating the portion of total input power that is dissipated in that admittance.
Using the calculator at Calculate ferrite cored inductor, the admittance (G+jB) of the 3t winding is 0.00177-j0.00204S. (The impedance of a sample wind could be measured with a suitable analyser and converted to admittance.) Continue reading End fed matching – VK3IL design on LO1238
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
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.
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
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)
This article describes my build of a Radio-Kits SWR meter (v1.1) and post implementation review.
- 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.
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.
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
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.
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