Steel wire CF dipole on 160m

A correspondent having seen recent discussion and models on eHam regarding steel dipoles, asked about the accuracy of my articles:

Galvanised steel wire CF dipole; and

Galvanised steel wire OCF dipole.

The eHam article gives the gain of a low half wave steel dipole on 160m as 0.5-1dB less than copper depending on steel composition. (The thread was entitled “galvanised steel wire”, but the model was clearly labelled steel. For discussion of the effect of galvanising, see Galvanised steel wire OCF dipole.)

The model used is not fully exposed, but the results are unlikely unless perhaps the permeability of the steel was ignored (NEC-2 does not natively model µr>1).

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Above are the gain plots from NEC-4.2 for three different material types, copper, steel, and steel resistivity with µr=1 (-wrong). Continue reading Steel wire CF dipole on 160m

Does VSWR damage HF ham transmitters

This Jan 2011 article has been copied from my VK1OD.net web site which is no longer online. It is for reference in further articles discussing the popular reflections explanations. The article may contain links to articles on that site and which are no longer available.

The statement is often made to the effect that:

VSWR will damage a HF ham transmitter, and the mechanism is that the ‘reflected power’ in a standing wave will be absorbed by the Power Amplifier (PA), increasing heat dissipation and damaging the PA.

There are two problems with this statement: Continue reading Does VSWR damage HF ham transmitters

Transmission line loss under mismatch explanations – the missing TWLLC model

At Transmission line loss under mismatch explanations I wrote that there is a lot of woolly thinking amongst hams about transmission line loss under mismatch and worked a simple example that could be done ‘by hand’ to show that formulas that some authors have produced as implementations of their explanations don’t stack up.

I also gave a solution to the Zo*3 scenario using TWLLC, but not the Zo/3 scenario which a few eagle hounds have pounced on as evidence that the solution would not support the article.

Not at all, the Zo/3 TWLLC solution was not given so as to keep the article short and within the attention span of modern hams though it was eventually a quite long article, and for that reason I will address it separately, here. Continue reading Transmission line loss under mismatch explanations – the missing TWLLC model

Transmission line loss under mismatch explanations

There is a lot of woolly thinking amongst hams about transmission line loss under mismatch, perhaps exemplified by Walt Maxwell (Maxwell 2001):

The power lost in a given line is least when the line is terminated in a resistance equal to its characteristic impedance, and as stated previously, that is called the matched-line loss. There is however an additional loss that increases with an increase in the SWR.

This article probes the folk lore with an example scenario designed to expose the failure of such thinking. Continue reading Transmission line loss under mismatch explanations

Some thoughts on the popular Carolina Windom antenna

The Carolina Windom is very popular with modern hams, and at the same time is commonly the discussion of problems in online fora.

The question is whether it is its popularity that is the reason for cries for help, or whether there is something inherently high risk in the ‘design’.

The original Windom

The first type is the classic ‘original’ Windom with single wire feed which folk lore explains as a horizontal wire being tapped at a point where Z matches the vertical  ‘single wire’ feeder, that there is not a standing wave on the feed line, and that it does not radiate. Traditional characterisation as a single-feeder Hertz denies the existence of the vertical radiating element.

It is a folly to designate the vertical wire as a non-radiating feeder, it carries an RF current that contributes to radiation just like current on the horizontal wire does. Continue reading Some thoughts on the popular Carolina Windom antenna

Are copper clad VHF whips a marketing ploy

A correspondent asked about whether the proprietary copper clad VHF whips are some kind of marketing ploy.

Lets consider a quarter wave whip for the 2m band. Made from 2.4mm stainless steel, it should deliver lowest VSWR(50) at about 483mm in length.

The properties of stainless steel whips will vary, but lets consider the case of one made from 17-7 PH Stainless Steel, a quite tough material that is popular with manufacturers of mobile whips.

17-7 PH Stainless Steel has higher resistivity than copper, but worse, it is weakly ferromagnetic which affects RF resistance.

Screenshot - 30_09_2015 , 09_54_15

Above is a calculation of the effect of skin depth, Rrf/Rdc is very high at 172, mostly a consequence of the permeability.

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Above is a calculation of the end to end RF resistance of the whip, 15.3Ω. Continue reading Are copper clad VHF whips a marketing ploy

Some tools for designing a Guanella 1:1 balun using ferrite toroids

In designing a Guanella 1:1 balun, selecting a ferrite core that has been characterised by the manufacturer simplifies the design process greatly.

The manufacturer’s full characterisation includes curves for complex permeability vs frequency and from these the magnetising impedance of the core can be calculated. Note though that tolerances on magnetics are usually fairly wide and they can be quite temperature dependent.

The inductor will usually exhibit a self resonance that is not revealed by the above calculation, but can be reasonably well modelled by adding a small equivalent shunt capacitance, see (Knight 2008). This equivalent capacitance is usually very important and not so easy to estimate, and is often best estimated by careful measurement of the self resonant frequency of the inductor (taking care to back out fixture effects). With experience, one can make a fairly good first guess so that the process is not too iterative.

Some writers say that Cs increases as turns are increased, but (Knight 2008) shows quite the opposite.

Controlling inductor self resonance is a lot about controlling added stray capacitance, eg connecting wires, encapsulation in conductive boxes etc.

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Above is a plot of common mode impedance of a FT240-43 ferrite toroid with 11t wound in Reisert cross over style and Cs=3pF. Different scenarios will give different results, but the form will tend to be similar to above. Continue reading Some tools for designing a Guanella 1:1 balun using ferrite toroids

Differential flux leakage in a Guanella 1:1 balun – an experiment

The article reports a simple experiment on the balun described at Low power Guanella 1:1 tuner balun using a pair of Jaycar LF1260 suppression sleeves to assess the loss with near zero common mode current.

This test would not subject dielectrics to high electric field strength.

LF1260x2-01

The balun above had the two wires at one end connected together, and a current of 1.41A at 7MHz passed between the terminals of the device at the other end.

The device so configured looks like a s/c transmission line stub and we would expect that the input impedance would be a very small resistance and small inductive reactance. Continue reading Differential flux leakage in a Guanella 1:1 balun – an experiment

Differential flux leakage in a Guanella 1:1 balun

This article has been copied as reference for a new article from my VK1OD.net web site which is no longer online. The article may contain links to articles on that site and which are no longer available.

I have been asked by a correspondent to comment in the context of my model of a Guanella 1:1 balun wound on a ferrite toroid (Duffy 2008a) on the impact of differential flux leakage as discussed in the ARRL 2011 Handbook on the predicted losses in a Guanella 1:1 balun using a ferrite toroidal core

ARRL

The ARRL 2011 Handbook (Silver 2011  20.23) states [i]f the line is made up of parallel wires (a bifilar winding), a significant fraction of the flux associated with differential current will leak outside the line to the ferrite core. Leakage flux can exceed 30% of the total flux for even the most tightly-spaced bifilar winding.

This might suggest that differential current will contribute significantly to balun core losses and consequently transmission loss. The claim is made without explanation or substantiation, or without making conclusions about any resultant loss. This is the makings of fear, uncertainty and doubt (FUD), and hardly the enlightenment that readers might expect. Continue reading Differential flux leakage in a Guanella 1:1 balun

Review of G3LDO STL (Radcom Sep 2010)

(Dodd 2010) describe a small transmitting loop (STL) and gave some meaningful performance measurements. It is rare to see such measurements and he is to be congratulated.

The loop is an octagon of perimeter 4.7m which at 14.2MHz is 0.224λ so although many will consider it meets the requirements of an STL, the common formula for radiation resistance Rr of a STL fail for perimeter above about 0.1λ (see Accuracy of estimation of radiation resistance of small transmitting loops).

Dodd gives calculations of one of the many simple loop calculators which gives Rr as 0.422Ω, it is probably closer to 160% of that value. This is an important quantity as it has direct bearing on calculated efficiency.

Dodd’s NEC model should have used a better figure for Rr, but it seems unlikely that the structural losses were fully included and its bandwidth prediction will be impaired.

G3LDO01

Above is Dodd’s measurement of antenna VSWR at 20m. This is most useful as it allows estimation of the half power bandwidth of the antenna. In this case, the antenna is not perfectly matched at its centre frequency, the residual VSWR is 1.07. The graph allows scaling off the VSWR=2 bandwidth as approximately 42kHz.
Continue reading Review of G3LDO STL (Radcom Sep 2010)