Calculation of equivalent self capacitance of Small Transmitting Loop – ARRL

In Calculation of equivalent self capacitance of Small Transmitting Loop I mentioned that (Straw 2007), The ARRL Antenna Book 21, gave an expression for equivalent self capacitance of a Small Transmitting Loop of one turn:

C=HD where C is in pF, D in cm, and H comes from a given table of length/diameter ratios from 0.1 to 1.0. ARRL cites (Medhurst 1947) for this expression. Medhurst’s work was for solenoids.

Values of the Constant H for Distributed Capacitance

Length to
Ratio       H
=====       =====
0.10        0.96
0.15        0.79
0.20        0.78
0.25        0.64
0.30        0.60
0.35        0.57
0.40        0.54
0.50        0.50
1.00        0.46

A 1m diameter loop of 10mm diameter conductor has l/d=0.01, so it is not covered by the table, and you might form the view from the table that H tends to 1.0 or thereabouts as l/d approaches 0, but that is an extrapolation and dangerous. Continue reading Calculation of equivalent self capacitance of Small Transmitting Loop – ARRL

Calculation of equivalent self capacitance of Small Transmitting Loop

Small Transmitting Loops behave fairly much like an ideal inductance in series with some small resistance. They do however exhibit a self resonance at a frequency where the perimeter is approximately a half wavelength. This can be expected to slightly alter the Xl vs frequency characteristic below the self resonant frequency (SRF), more so as the SRF is approached.

This departure can be compensated for to some extent by addition of a small equivalent shunt capacitance. Continue reading Calculation of equivalent self capacitance of Small Transmitting Loop

An NEC-4.2 model of VK5BR’s 1m square loop for 20m

(Butler 1991) gives a design for a Small Transmitting Loop (STL) for 14MHz and some other bands.

He gives key design data:

Tube Diameter d   0.75 inch
 Circumference S  12.7 feet
 Area A =   10 square feet
 Frequency f =  14.2MHz
 Power P   100 watts
 Radiation Resistance Rr =   0.137 ohm
 Loss Resistance RL =   0.064 ohm
 Efficiency n =  68%
 Inductance L =   3.27 micro-henry
 Q factor =   723
 Inductive reactance XL =   291 ohms
 Bandwidth B =   19.6kHz
 Distributed capacity Cd =   10.4pF
 Capacitor potential Vc =   4587V
 Tuning capacitor Ct =   28pF

The data above appear to ignore some important factors, and estimate some others based on an assumption of uniform current. Continue reading An NEC-4.2 model of VK5BR’s 1m square loop for 20m

Underhill on Small Transmitting Loop efficiency

The meaning of the terms efficiency and radiation resistance are often critical to understanding written work on antennas, yet different authors use them differently, often without declaring their intended meaning.

Mike Underhill (G3LHZ) is an enthusiastic proponent of Small Transmitting Loops and in his slide presentation (Underhill 2006) challenges the proposition that their efficiency is low.

The line taken broadly is to introduce his own interpretation of efficiency and to challenge by experimental evidence other views on expected efficiency. Continue reading Underhill on Small Transmitting Loop efficiency

DK7ZB’s balun

(Steyer nd) describes the DK7ZB balun / match for VHF and UHF Yagis.


To understand how the “DK7ZB-Match” works look at the left picture. Inside the coax cable we have two currents I1 and I2 with the same amount but with a phase shift of 180°.

No. At any point along the coaxial line, a current I on the outer surface of the inner conductor causes an equal current in the opposite direction on the inner surface of the outer conductor.

As the currents are shown with the designated directions, I2=I1, not I2=I1<180.

A consequent simplification is that I4=I2-I3=I1-I3.

There is an issue with the current arrow I3 in the lower right of the diagram. It might imply that the only current in the conductors is I3, but the current between the nearby node and lower end of the shield is I3-I1.

If the structure was much much shorter than the wavelength, there would be negligible phase change in currents along the structure, so I1 would be uniform along the centre conductor, I2 uniform along the inside surface of the outer conductor, and I3 uniform along the outer surface of the outer conductor.

The diagram notation does show that I3 (which is equal to the dipole drive imbalance) is uniform along the structure, and that I3 flows to ground.

It seems that the diagram appears in (Straw 2003).

DK7ZB goes on:

If we connect a dipole or the radiator of a Yagi direct to the coax, a part of I2 is not running to the arm 2 but down the outer part of the coax shield. Therefore I1 and I4 are not in balance and the dipole is fed asymmetric.

But how can we suppress the common-mode current I3? A simple solution is to ground the outer shield in a distance of lambda/4 at the peak of the current.

So, the length of the structure is in fact a quarter wavelength electrically, or close to it to achieve the choking effect. I3 will be in the form of a standing wave with current maximum at the lower (‘grounded’) end, and current minimum at the upper end.

It happens also that his usual configuration of this balun is that there is a standing wave on the inside of the coax, and so I1 and I2 are not uniform along the conductor, and whilst it is relevant to the designed impedance transformation, it is inconsequential to reduction of dipole current imbalance.

DK7ZB continues with the development of his variation of a Pawsey balun:

But now we get a new interesting problem: For the transformation 28/50 Ohm we need a quarterwave piece of coax with an impedance of 37,5 Ohm (2×75 Ohm parallel). The velocity of the wave inside the coax is lower than outside (VF = 0,667 for PE).

The outside of the shield has air (and a litle bit of insulation) in the surrounding and VF = 0,97. For grounding the common mode currents this piece should have a length of 50 cm, with a VF = 0,667 and a length of 34,5 cm this piece of coax is to short. By making a loop of this two cables as shown in the picture down we get an additional inductivity and we come closer to an electrical length of lambda/4. Ideal is coax cable with foam-PE and a VF = 0,82

schleifeAbove is DK7ZB’s implementation of his balun with the loop and additional inductivity.

I copied the above implementation and measured the common mode impedance Zcm.


Above is the Zcm measurement. There is a quite narrow self resonance where Zcm is quite high for about 10MHz bandwidth centred on 125MHz, but at 144MHz Zcm=83-j260Ω which is too low to qualify as a good common mode choke.

Like all narrowband / tuned common mode chokes, tuning to the desired frequency band is essential to their effective operation.

Like most published balun designs, this one is published without measurements to demonstrate its operation or effectiveness.


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).

Clip 045

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 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 VSWR50 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.

Screenshot - 30_09_2015 , 09_54_20

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 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


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