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

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.

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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

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)

Helical loading and Calculate small transmitting loop gain from bandwidth measurement

Several correspondents have asked about the application of Calculate small transmitting loop gain from bandwidth measurement to the helically loaded small transmitting loop.

The helically loaded small transmitting loop appears to be the invention of K8NDS and is described at Stealth Antennas for the Radio Amateur and (K8NDS nd). It may not be a novel idea as it was analysed at (Maclean 1978).

Without getting too much involved in the inventor’s specious arguments which attribute magic properties to his antenna, this article focusses on whether / why the calculator will or will not provide valid results for the antenna.

At Stealth Antennas for the Radio Amateur he makes the statement

A solid copper tube “Magnetic Loop” exhibits a certain inductance per foot of the total circumference of the antenna.

The statement seems to belie a basic understanding of inductance, the inductance of a given conductor formed into a single turn loop is not simply perimeter multiplied by some constant “inductance per foot”. Continue reading Helical loading and Calculate small transmitting loop gain from bandwidth measurement

End fed matching – PA3HHO design review

 

A correspondent having read End fed matching – design review raised a similar design by PA3HHO which uses a#43 ferrite toroid as part of an end-fed matcher, see Multi band end-fed (English).

The text and diagram  are inconsistent, but to allow him the benefit of doubt, lets consider the FT240-43 with a 3t primary… this is his lowest loss configuration.

Continue reading End fed matching – PA3HHO design review

A method for initial ground loss estimates for an STL

Over recent weeks, I have run literally hundreds of thousands of NEC models of small transmitting loops (STL) over real ground. The objective was to try to discover some simple methods for initial design of a STL, particularly an estimate of ground loss of STL mounted near natural ground. Continue reading A method for initial ground loss estimates for an STL