Transmission lines – Page 38

Does common mode current flow inside coax?

The term “common mode current” applied to coaxial transmission lines is bandied about with abandon these days in online fora, awareness of its existence has increased if not understanding.

A simplistic analysis is that in TEM mode, ONLY differential current is supported inside a coaxial line, ie that at any point the current on the outer surface of the inner conductor is exactly equal to a current in the opposite direction on the inner surface of the outer conductor.

But, lets look at the wider context of the meaning of common mode current when a uniform coaxial line is connected to an antenna system. Whilst an antenna might have an obvious two terminal connection to the feed line, in the presence of ground, the current in those two terminals are not necessarily equal and opposite. Continue reading Does common mode current flow inside coax?

Last update: 12th June, 2017, 1:24 AM

Exploiting your antenna analyser #26

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 SOL 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

Last update: 30th January, 2017, 7:41 AM

Exploiting your antenna analyser – contents

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 #30 Quality of termination used for calibration

Exploiting your antenna analyser #29 Resolving the sign of reactance – a method – Smith chart detail

Exploiting your antenna analyser #28 Resolving the sign of reactance – a method

Exploiting your antenna analyser #27 An Insertion VSWR test gone wrong

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 SOL calibration

Exploiting your antenna analyser #24 Find coax cable velocity factor using an antenna analyser with SOL 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

Last update: 28th May, 2019, 10:28 AM

Rigexpert Antscope v4.3.1 dowload link?

At Rigexpert Antscope v4.3.1 released I commented on a new release of Antscope.

Correspondents have asked where I obtained v4.3.1.

Well, it seems the Rigexpert website is broken again, the URL to list the Antscope downloads produces garbage. Nevertheless, you can get a directory listing at https://www.rigexpert.com/files/antscope/ and yes, you will note that v4.3.1 is not listed… so it seems to have been either pulled due to defects or it is just a consequence of the web site problems.

Little loss, I use v4.2.57 on Rigexpert’s advice as it has better scales for impedance plots… and v4.2.57 is still published (at the time or writing) https://www.rigexpert.com/files/antscope/antscope040257.zip .

Last update: 15th January, 2017, 9:16 AM

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.

and;

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

Last update: 10th January, 2018, 9:04 AM

Rigexpert Antscope v4.3.1 released

It seems yet another new version of Rigexpert Antscope has been released, and it maintains the scale limits available for R,X plots to +/-2000Ω, it still does not allow the range permitted by v4.2.57 (+/-5000Ω).

No change details provided by Rigexpert.

Back to v4.2.57, though it is very likely it has undisclosed defects fixed in later releases.

Bottom line is that if you want an analyser with direct graphing of impedances over 2000Ω (eg measuring common mode choke impedance), think of a different analyser.

Last update: 31st December, 2016, 6:11 PM

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

Last update: 14th December, 2016, 6:55 PM

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.

Above 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

Last update: 4th December, 2016, 7:53 AM

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.

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

Last update: 23rd November, 2018, 4:22 PM

Grebenkember’s original Tandem match

(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

Last update: 26th November, 2016, 7:39 AM