Ian White gives the following diagram to explain what goes on at the coax to dipole junction.

He labels five currents in his explanation.

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# Category: Antennas

## Currents at a coax to dipole junction

## Let’s simplify that

## Fact check: “For an antenna, if it doesn’t resonate, it really doesn’t radiate!”

## A low Insertion VSWR high Zcm Guanella 1:1 balun for HF – coax bend radius

## Jupyter: one for the toolbox – decompose common mode and differential mode current components

## Antenna system ReturnLoss minima are interesting

## CMRR and transmitting antennas

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

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

## MFJ-854

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

## The example

## NEC model of 600mm a side square loop for field strength measurement

Ian White gives the following diagram to explain what goes on at the coax to dipole junction.

He labels five currents in his explanation.

An example of the utter nonsense posted on social media.

My very first posting as a trainee was to Bringelly HF receiving station in 1970. It had Rhombic antennas every 30° of the compass, and a few other antennas, but the mainstay of operation was the set of Rhombics.

The nearby transmitting station at Doonside had a similar antenna arrangement of Rhombics fed with two or four wire open transmission lines to transmitters in a central building, for most operations, no coax involved between transmitters up to 30kW and antenna feed points. Continue reading Fact check: “For an antenna, if it doesn’t resonate, it really doesn’t radiate!”

I see online discussion of specification bending radius for coax cables, and their application to ferrite cored common mode chokes.

A low Insertion VSWR high Zcm Guanella 1:1 balun for HF and follow on articles described a balun with focus on InsertionLoss.

Let’s remind ourselves of the internal layout of the uncompensated balun.

The coax is quality RG58A/U with solid polythene dielectric. The coax is wound with a bending radius of about 10mm, way less than Belden’s specified minimum bending radius of 50mm.

So, the question is does this cause significant centre conductor migration that will ruin the characteristic impedance: Continue reading A low Insertion VSWR high Zcm Guanella 1:1 balun for HF – coax bend radius

This article is principally a short commendation for Jupyter or Interactive Python for ham radio related projects for the quantitative ham. Python is a cross platform programming language that has a very rich set of libraries to support scientific and engineering applications, and a good graph maker.

The exercise for this demonstration is to decompose three measurements of currents on a two wire transmission line at a point into the differential and common mode components at that point, and to plot a phasor diagram of a solution to the measurements. Remember that common mode current and differential current in an antenna system are usually standing waves.

Above is a diagram explaining the terms used, I1 and I2 are the magnitudes of currents in each conductor measured using a clamp on RF ammeter, and I12 is the magnitude of the current when both conductors are passed through the clamp on RF ammeter, i12 is the phasor sum of the underlying i1 and i2. Continue reading Jupyter: one for the toolbox – decompose common mode and differential mode current components

One sees analyser sweeps of EFHW measurements posted online quite frequently, and a trend is that posters are quite pleased with the results.

Above is an example, a ‘user’s’ MyAntennas.com EFHW-4010 antenna with 23m of unspecified coax. Unfortunately it is a bit narrow, ordered up by an online expert. Continue reading Antenna system ReturnLoss minima are interesting

Since the widespread takeup of the NanoVNA, a measure of performance proposed by (Skelton 2010) has become very popular.

His measure, Common Mode Rejection Ratio (CMRR), is an adaptation of a measure used in other fields, he states that he thinks the application of it in the context of antenna systems and baluns is novel and that “CMRR should be the key figure of merit”.

Skelton talks of different ways to measure CMRR, but essentially CMRR is a measure of the magnitude of gain (|s21|) from Port 1 to Port 2 in common mode, with the common mode choke (or balun) in series from the inner pin of Port 1 to the inner pin of Port 2.

Note that this is the same connection as used for series through impedance measurement, but calculation of impedance depends on the complex value s21.

Above is capture of a measurement of a Guanella 1:1 common mode choke or balun. The red curve is |s21|, the blue and green curves are R and X components of the choke impedance Zcm calculated from s21. Continue reading CMRR and transmitting antennas

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!

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.

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

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

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

This article Reconciliation of transmitter power, EIRP, received signal strength, antenna factor, ground wave propagation etc @ 576kHz used a 600mm a side square loop which was originally designed for field strength measurements on the 40m in an effort to understand and document BPL (PLC) emissions.

As part of validation of the antenna, a free space NEC model with external excitation was developed. This article publishes a graphic summary of the antenna characteristic. The model antenna is loaded with 50+j0Ω and includes 10m of RG58A/U which was used for the BPL related measurements with FSM. Continue reading NEC model of 600mm a side square loop for field strength measurement