Category: Transmission lines

A quick and simple, but effective test of coax matched line loss

Background

From time to time, ham radio operators may question whether a section of installed and used coax is still good or significantly below spec and needs replacement.

A very common defect in coax installed outside is ingress of water. The earliest symptoms of water ingress are the result of corrosion of braid and possibly centre conductor, increasing conductor loss and therefore matched line loss (MLL). Any test for this must expose increased MLL to be effective. Continue reading A quick and simple, but effective test of coax matched line loss

Last update: 24th August, 2024, 8:24 PM

nanoVNA-H – Deepelec test jig #2

I have found you can never have enough of these things. It is very convenient to leave some measurement projects set up while work continues on some parallel projects.

Above is the kit as supplied (~$8 on Aliexpress). Note that it does not contain any male turned pin header… more on that later. Continue reading nanoVNA-H – Deepelec test jig #2

Last update: 18th July, 2024, 9:20 AM

Dave Casler’s “why so little loss?”… a fact check!

Dave Casler sets out in his Youtube video to answer why two wire transmission line has so little loss . With more than 10,000 views, 705 likes, it is popular, it must be correct… or is it?

He sets a bunch of limits to his analysis, excluding frequency and using lossless impedance transformation so that the system loss is entirely transmission line conductor loss.

He specified 300Ω characteristic impedance using 1.3mm copper and calculates the loop resistance, the only loss element he considers, to be 0.8Ω.

Above is Dave’s calculation. Using his figures, calculated \(Loss=\frac{P_{in}}{P_{out}}=\frac{100}{100-0.27}=1.0027\) or 0.012dB. Continue reading Dave Casler’s “why so little loss?”… a fact check!

Last update: 9th July, 2024, 6:27 AM

Effective measurement of common mode current on a two wire line – a user experience

This article reports and analyses a user experiment measuring current in a problem antenna system two wire transmission line.

A common objective with two wire RF transmission lines is current balance, which means at any point along the transmission line, the current in one wire is exactly equal in magnitude and opposite in phase of that in the other wire.

Note that common mode current on feed lines is almost always a standing wave, and differential mode current on two wire feed lines is often a standing wave. Measurements at a single point might not give a complete picture, especially if taken near a minimum for either component.

MFJ-854

The correspondent had measured feed line currents using a MFJ-854.

Above is the MFJ-854. It is a calibrated clamp RF ammeter. The manual does not describe or even mention its application for measuring common mode current. Continue reading Effective measurement of common mode current on a two wire line – a user experience

Last update: 24th June, 2024, 9:00 AM

VNWA-3E – a ferrite cored test inductor impedance measurement – s11 reflection vs s21 series vs s21 pi

This article is a remeasure of NanoVNA-H4 – a ferrite cored test inductor impedance measurement – s11 reflection vs s21 series vs s21 pi using a VNWA-3E of both a good and sub-optimal test fixture estimating common mode choke impedance by three different measurement techniques:

  • s11 shunt (or reflection);
  • s21 series through
  • s21 series pi;

Citing numerous HP (and successor) references, hams tend to favor the more complicated s21 series techniques even though the instruments they are using may be subject to uncorrected Port 1 and Port 2 mismatch errors. “If it is more complicated, it just has to be better!”

s21 series pi is popularly know as the “y21 method” (The Y21 Method of Measuring Common-Mode Impedance), but series pi better describes the assumed DUT topology.

What is an inductor?

Above is the test inductor, enamelled wire on an acrylic tube, an air cored solenoid. Continue reading VNWA-3E – a ferrite cored test inductor impedance measurement – s11 reflection vs s21 series vs s21 pi

Last update: 17th June, 2024, 1:20 AM

NE6F’s common mode current tester – Part 1

A correspondent asked my thoughts on a Youtube video featuring…

NE6F’s common mode current tester

Above is the schematic of NE6F’s common mode current tester.

The concept is that current probes A and B are placed either side of a current mode choke, and by calibrating and switching between them, a relative reading of current on one side compared to the other may be found. Continue reading NE6F’s common mode current tester – Part 1

Last update: 16th August, 2024, 12:51 PM

The obsession with measuring extreme spot frequency impedances of broadband common mode chokes

The fashion for measuring HF broadband common mode chokes for antenna systems is to use the s21 series through measurement technique, the basis for which is specious as discussed elsewhere on this blog.

Let’s look at an example common mode choke, this time for suppression of ham transmitter ingress to a VDSL2 line.

The spectrum of interest is 1.8 to 10.2MHz, this is the overlap between VDSL2 spectrum and ham bands above 1MHz.

Somewhat arbitrarily, a design specification was drawn up for a prototype choke which would be tested for effectiveness. The draft specification was: Continue reading The obsession with measuring extreme spot frequency impedances of broadband common mode chokes

Last update: 29th May, 2024, 5:39 PM

Study of suitability of UHF bulkhead adapter to a Diamond x50-A antenna system

There must a a thousand articles on the ‘net on why UHF series connectors are good or bad, this is another.

The example

The example for discussion is a Diamond X-50A 2m/70cm vertical antenna on about 11m of LDF4-50A feed line, N type connectors are used throughout.

At commissioning, a sweep looking into the feed line was made using an Rigexpert AA600 analyser and the results saved. The file used for this study is a sweep from 143-151MHz.

Above is the UHF series bulkhead adapter studied in the simulation. It is 50mm end to end, the simulation uses 60mm to account for the impedance discontinuity in the mating plugs. The adapter is modelled as 60mm of lossless 35Ω line with VF=0.7 (typical of UHF series adapters). Continue reading Study of suitability of UHF bulkhead adapter to a Diamond x50-A antenna system

Last update: 8th May, 2024, 9:12 AM