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
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
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.
Above is a calculation of the effect of skin depth, Rrf/Rdc is very high at 172, mostly a consequence of the permeability.
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
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.
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
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.
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
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
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
(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.
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)
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
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
At Characterising an unknown ferrite toroid an ‘unknown’ ferrite toroid was characterised. This article uses that information for design of a Guanella 1:1 current balun.
The proposed design uses 11t of small coax wound in the Reisert ‘cross-over’ style.
The impedance of a single turn vs freq was used to predict the impedance of an 11t choke. Such a choke exhibits a self resonance that can be represented as due to an equivalent shunt capacitance. This equivalent capacitance is not easily estimated, and can best be determined by calibrating an analytical model of the choke for the same self resonance as exhibited by a real choke.
Above is common mode impedance from an analytical model of the choke, assuming an equivalent self capacitance of 11pF.
Continue reading Designing a Guanella 1:1 balun using the ‘unknown’ ferrite toroid