Category: Transmission lines

UHF series coaxial connector characteristic impedance

Measurements of Insertion VSWR of UHF series connectors consistently show increasing Insertion VSWR with frequency, an issue that often impacts measurement accuracy.

My own article Exploiting your antenna analyser #12 is but one of many.

Measurements consistently hint that the defect is that the characteristic impedance is typically somewhere between 30 and 40Ω.

Above is a dimensioned drawing from Amphenol (https://www.amphenolrf.com/connectors/uhf.html). Continue reading UHF series coaxial connector characteristic impedance

Last update: 29th October, 2017, 11:16 AM

Review of the Amidon AB_200_10 balun

The Amidon AB_200_10 2-30MHz, 1KW balun and knock-offs have been around for a very long time, I recall Dick Smith selling them in the early 1970s in Australia.

They were regarded as the epitome of the art… but it was not a very well understood art.

Lets analyse the common implementation as a Ruthroff 4:1 voltage balun in a 50:200Ω scenario.

Ruthroff 4:1 voltage balun

In this implementation, Amidon’s instructions show 16 bifilar turns on a T200-2 core.

A very simple model is to consider the device as an ideal transformer with a shunt magnetising impedance equal to the impedance of the 16t winding that appears across the 50Ω terminals. This has its greatest effect at low frequencies and although it is specified from 2-30MHz, lets analyse it at 3.5MHz.

The powdered iron core has very low loss at 3.5MHz, sufficiently so that we can ignore the imaginary component of µr for this analysis and take µr to be 10+j0.

Above is a calculation of the magnetising impedance and admittance under those assumptions. The magnetising admittance (0.00-j0.0134S) appears in shunt with the transformed load admittance (0.02S) so we can simply add them to find the admittance seen by the transmitter (0.02-j0.0134S). Continue reading Review of the Amidon AB_200_10 balun

Last update: 9th November, 2020, 6:32 AM

A low Insertion VSWR high Zcm Guanella 1:1 balun for HF – more detail

This article expands on the detail behind A low Insertion VSWR high Zcm Guanella 1:1 balun for HF.

Choice of core

Online experts all have a preferred core material, but there is a dearth of measurement data to show the difference in actual use. If someone recommends a particular core material and cannot provide measured Zcm data to support the recommendation, regard it as a weak recommendation.

Beware the magic of unobtainium… just because something is hard to get is not an indication that it is desirable.

Above is the complex permeability characteristic of the #43 material used. Inductance calculators that do not take that frequency dependent complex characterisitic into consideration produce invalid results. (Duffy 2015) gives a suitable approximation, and there are links to calculators that do work properly at the bottom of this article. Continue reading A low Insertion VSWR high Zcm Guanella 1:1 balun for HF – more detail

Last update: 17th January, 2019, 4:45 AM

A low Insertion VSWR high Zcm Guanella 1:1 balun for HF

This article describes a Guanella 1:1 current balun which has high common mode impedance (Zcm) and low Insertion VSWR. It is for application on antennas that have low VSWR(50) on at least some bands, especially if they would be used without an ATU on some bands.

The purpose of the balun is to minimise common mode feed line current which may contribute to EMC problems when transmitting, and contribute to increased ambient noise when receiving. Reduction of feed line common mode current also helps in achievement of expected load impedance characteristic, radiation pattern and gain. This article gives measured Zcm, but the definitive test of the effectiveness of such a balun is direct measurement of common mode current Icm… and it is so easy.

Example applications are half wave centre fed dipoles, fan dipoles, trapped dipoles, G5RV with hybrid feed, ZS6BKW, trapped verticals, monopoles, ground planes.

To obtain low Insertion VSWR, the choke will be wound with 50Ω coax, to demonstrate the practicality of the design budget (but good quality) regular (ie solid PE dielectric) RG58C/U will be used. Foam dielectric is NOT recommended. Solid PTFE coax could be used, but avoid coax with steel cored inner conductor, it may be lossier than you think at low frequencies with the silver cladding is relatively thin.

The candidate core is the readily available FT240-43 (Fair-rite 2643803802, 5943003801), it is a low cost NiZn ferrite with medium µr, and its µr and loss characteristic contributes to a broad high impedance choke well suited to this application.

Above is a model of the expected Zcm with 11 turns of RG58C/U coax and an equivalent shunt capacitance of 4.6pF. Continue reading A low Insertion VSWR high Zcm Guanella 1:1 balun for HF

Last update: 24th April, 2019, 10:18 AM

A low cost 50Ω termination for measurement purposes

This article shows just how easy it is to make an inexpensive low VSWR load for antenna analyser validation / measurements.

Above is an AA-600 sweep of the prototype from 10kHz to 100MHz. VSWR reads 1.02 in ‘All’ mode at 100MHz… better than the inherent accuracy of the instrument.

How expensive?

It is made from two 100Ω 1% 1206 SM resistors purchased on eBay for about $2/100, so about $0.04 for the resistors, and 40mm of bare copper wire (0.5mm phone / data wire in this case).

In use, it is held in contact with the coax socket (in this case an N type) with a pair of disposable plastic first aid tweezers (yep, you can buy them on ebay for about $0.20/pair).

While you are at it, make a good short circuit termination by scrunching up a bit of (clean) kitchen aluminium foil and press that against the coax socket conductors.

Try both of these on your antenna analyser and see how it stacks up.

Last update: 27th September, 2017, 4:46 PM

“No short antenna is more efficient than a resonant-length antenna”

A recent online posting asserted that an antenna is optimal when itself resonant, and fed with a resonant feed line length so delivering a purely resistive load to a source, and further that implementors needed to be careful that a shorter dipole could be offset to some extent by a longer feed line but it would be inferior because:

no short antenna is more efficient than a resonant-length antenna

… but does that stand scrutiny?

An NEC experiment

Lets walk though an experiment using NEC-4.2 models of a dipole of 2mm copper wire at 10m height at 7.1MHz over average ground (σ=0.005, εr=13).

Key assumptions:

  1. source has a Thevenin equivalent source impedance of 50+j0Ω;
  2. feed line is lossless.

The results are sensitive to the model assumptions.

We will calculate the ratio of radiated power to the power delivered by the transmitter to a matched load, let us call it TransmitEfficiency for the purposes of this article. Continue reading “No short antenna is more efficient than a resonant-length antenna”

Last update: 22nd September, 2017, 6:21 AM

Failure to treat impedance as a complex quantity leads to…

A ham recently posted a graph on QRZ to educate his fellow hams on the behaviour of transmission lines under mismatch.

Above is one of his graphs (the red arrow is my annotation). It plots Impedance variation along a mismatched 75Ω transmission line. The curves look graceful, but are they science or just pretty artwork? Continue reading Failure to treat impedance as a complex quantity leads to…

Last update: 22nd September, 2017, 6:08 AM

Inductance of a loop of CAT5 pair

An online expert recently reported:

I tried to make an antenna loop for longwave with cat 5 and after it did no good I realized the twisted wires canceled each other out.

Or did they really cancel?

Parallel connection

I constructed a loop of one Cat 5 pair and measured its inductance when both wires are bonded at the ends.

The conductors are 0.5mm diameter and spaced 0.9mm. To estimate the inductance we use the geometric mean radius (GMR) as the equivalent radius of the pair. GMR=(0.5*0.9)^0.5=0.67, diameter=1.34mm. So let’s calculate the inductance of a single turn circular loop of 0.8m perimeter and round conductor of 1.34mm diameter.

The estimate above is 850nH.

Above is the measurement, the screen is not readable, but it is 852nH, very close to the estimated 850nH. Continue reading Inductance of a loop of CAT5 pair

Last update: 2nd September, 2017, 8:52 PM

Messi & Paoloni Ultraflex 10 coax cable

Messi & Paoloni Ultraflex 7 coax cable compared M&P UF7 with RG-213. This article does a similar comparison between M&P Ultraflex 10 and LMR400UF.

Both cables are of similar size, ~10mm overall, stranded centre conductor and foil+braid outer conductor. The shield stranding is different and the foil is copper in the UF10, aluminium in the LMR400UF.

Let’s take the loss factors calculated for TLLC and de-construct the conductor and dielectric loss for each line type.

Above is a comparison of the cables. Continue reading Messi & Paoloni Ultraflex 10 coax cable

Last update: 18th August, 2017, 7:14 PM

EME Calc v11.11 reconciliation issues

A correspondent wrote about trying to reconcile by G/T worksheet with EME Calc.

Many times I have tried to validate it and run into problems. At one time I reported them to the author, but they were never acknowledged, much less fixed.

The specific problem on this occasion relates to the receiver performance tab.

Above is a screenshot (with my annotations) where I have basically stripped the configuration down to a receiver attached to a noiseless antenna with lossless line. Continue reading EME Calc v11.11 reconciliation issues

Last update: 22nd July, 2017, 8:13 AM