Designing a Gamma Match – Simsmith design tool and confirmation of as-built antenna – comparison with Healey

A correspondent reading Designing a Gamma Match – Simsmith design tool and confirmation of as-built antenna referred me to (Healey 1969) and questioned my method.

I have tried several times to reconcile built and tuned antennas and NEC models with Healey and failed, leading me to think of the problem and devise a good approximation that did reconcile (for me).

This article attempts to reconcile the example given at Designing a Gamma Match – Simsmith design tool and confirmation of as-built antenna, an example where the two built antennas reconcile well with the ARRL published design article and NEC model.

Example for reconciliation

The example antennas are 4 element 144MHz Yagis built around 1970. They were originally designed with a 50Ω split dipole feed, or the option of a folded dipole with 4:1 half wave coax balun. Continue reading Designing a Gamma Match – Simsmith design tool and confirmation of as-built antenna – comparison with Healey

Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #7

Seventh part in the series documenting the design and build of a Guanella 1:1 (current) balun for use on HF with wire antennas and an ATU.

  • This article describes a measurment of common mode impedance Zcm of the packaged balun.

Packaging

The prototype fits in a range of standard electrical boxes. The one featured here has a gasket seal (a PTFE membrane vent was added later).

AtuBalun201

Above, the exterior of the package with M4 brass screw terminals each side for the open wire feed line, and an N(F) connector for the coax connection. N type is chosen as it is waterproof when mated. Continue reading Design / build project: Guanella 1:1 ‘tuner balun for HF’ – #7

Designing a Gamma Match – Simsmith design tool – how to

Designing a Gamma Match – Simsmith design tool and confirmation of as-built antenna posted a Simsmith design tool to assist in designing a Gamma Match.

Let’s walk through an example.

Above is an example for discussing the Gamma Match. In this case, the assumed feed point impedance of a simple split dipole feed is 17+j2Ω, and the challenge is to design a practical Gamma Match to match it to 50+j0Ω.

The design tool assumes that the connections to the open circuit stub are at the feed point, ie that the gap in the gamma arm outer is at the inboard end. There are other ways to build a gamma match and the model may not suit them without tweaking. Continue reading Designing a Gamma Match – Simsmith design tool – how to

Designing a Gamma Match – Simsmith design tool and confirmation of as-built antenna

Designing a Gamma Match – confirmation of as-built antennas was based on some online calculators to provide key values to a Simsmith model of a Gamma Match.

This article provides an updated Simsmith model that incorporates the necessary calculations (ie without depending on external calculators).

Much is written about the virtue of the Gamma Match, and near as much about how they work, and the difficulty in design and implementation.

Designing a Gamma match using a Smith chart showed a design method for a simple Gamma Match using a Smith chart as the design tool.

This article visits the implementation on a pair of antennas that I built 50 years ago, and are still in use today (albeit with some small preventative maintenance once during that interval). The basic antenna is a four element Yagi for 144MHz copied from an ARRL handbook of the time, probably based on NBS 688. It was designed to deliver a split dipole feed point impedance of 50+j0Ω.

I built them using a Gamma Match, partly to get some familiarity, but mostly to implement a Gamma Match that was reliable, weatherproof and lasting… features that are alien to most implementations I had seen at that point.

Both antennas were constructed and the Gamma Match adjusted for VSWR<1.1 using a Bird 43 directional wattmeter. The dimensions of each (including the key gamma dimensions) are the same, not surprising, but a confirmation of repeatability. See Novel Gamma Match Construction for more discussion.

Above is a dimensioned drawing of the construction. Continue reading Designing a Gamma Match – Simsmith design tool and confirmation of as-built antenna

Where is the best place to measure feed point VSWR – error in Zo

At Where is the best place to measure feed point VSWR I discussed location of the VSWR meter and projection of its reading to another point on a known transmission line.

One of the conclusions drawn in that article is:

Feed point VSWR can be estimated from measurements made at another place if the transmission line parameters are known. It, like all measurements, is subject to error but it may be a manageable error and indeed possibly better overall than direct measurement.

This article discusses some issues that may arise in referring measurements from one place to another (eg near transmitter to antenna feed point).

Characteristics of transmission line categories

Let’s consider two categories of transmission lines in terms of characteristic impedance Zo and propagation constant γ:

  • Lossless line; and
  • practical line.

A lot of theoretical analysis uses lossless line for simple explanations, and whilst for a lot of purposes, approximation of practical line as lossless line serves well, at other times the error may be significant.

Lossless Line

A Lossless Line has imaginary part of Zo equal to zero and the real part of γ equal to zero.

Practical line

A practical line has non-zero imaginary part of Zo and non-zero real part of γ, and these are frequency dependent.

Under standing waves, attenuation along a practical line is not uniform, in most practical applications conductor loss/m is higher than dielectric loss so loss is higher near current maxima than near current minima.

For the purpose of this article, it is the frequency dependence of Zo, particularly the non-zero imaginary part that is significant.

A model

A model of a load similar to a 7MHz half wave dipole fed with 10m of RG58A/U was created in Simsmith to provide a basis for discussion. Whilst the model is subject to some errors computation, it is much less than comparing two field measurements at both ends of a transmission line.

VSWR at each end of the transmission line

Let’s look at the ACTUAL VSWR. Actual means that if you were to observe the standing waves on the line (eg with a voltage probe), this is the VSWR you would expect to observe.

Firstly, observe that the source end VSWR (orange) is a little lower than the load end VSWR. This is by virtue of the attenuation on the line. The difference between the two can be calculated, but it is moderately complicated. Continue reading Where is the best place to measure feed point VSWR – error in Zo

Implementation of G5RV inverted V using high strength aluminium MIG wire – 7 year review

This article continues on from Implementation of G5RV inverted V using high strength aluminium MIG wire documenting review after 7 years operation under a wide range of temperature, humidity and wind conditions. It adds some contemporaneous pics of some parts of the system.

Support structure

Above is a view of the steel mast (2021) with the Inverted V G5RV rigged from the top of the 11m mast using a halyard through a purchase on a small gibbet to offset the antenna and feed line from the mast. There are lateral guys at 7m height, and the left hand one is non-conductive synthetic fibre rope. Atop the mast is a 2m/70cm vertical.

Above is a view of the feed point (2023) of the G5RV at 1mm AGL. The galvanised telescopic TV mast is 55 years old and starting to show signs of rust, likewise for the gibbet which was painted with cold gal back then. Continue reading Implementation of G5RV inverted V using high strength aluminium MIG wire – 7 year review

Relationship between radiation efficiency and minimum VSWR for common short helically loaded verticals

This article explores the relationship between radiation efficiency and minimum VSWR for common short helically loaded verticals.

For clarity, \({RadiationEfficiency}=\frac{FarFieldPower}{InputPower}\).

Such antennas are often advertised with a “minimum VSWR” or “VSWR at resonance” figure, but rarely show gain figures. One might wryly make the observation that that is how one might sell dummy loads rather than antennas.

Well, these things do radiate, so they are not very good dummy loads. Lets explore a theoretical example on the 40m band to inform  thinking.

Unloaded vertical

Above is a NEC5.2 model of a vertical on a wagon roof. Continue reading Relationship between radiation efficiency and minimum VSWR for common short helically loaded verticals

Refurbishing aluminium antenna parts affected by weather / corrosion

Weather is not kind to aluminium antenna parts, often giving rise to corrosion that may result in high resistance joints that then reduce radiation efficiency.

It is good practice to document antenna behavior at installation, and through life, measurements can be compared to that benchmark to possibly reveal changes resulting from corrosion. See Diagnosing a possible antenna problem by comparison with a baseline for more discussion.

WARNING: wear eye protection when using any of the appliances mentioned in this article.

Corrosion preventative compounds

Above is the kind of structure that often develops corrosion in the overlapping / clamped tube sections. They can be recovered by cleaning the corrosion products out, and reassembling them with purpose specific conductive waterproof grease containing irregularly shaped shavings of zinc. The way in which it works is that the sharp particles of zinc penetrate the insulating oxide layer on clean aluminium providing a low R conducting path, and the grease prevents ingress of water and oxygen, so preventing corrosion.

Some forms use a silicon grease base for applications where mineral grease is incompatible with wire insulation, but they are more expensive. Continue reading Refurbishing aluminium antenna parts affected by weather / corrosion

Balanced ATUs – the Holy Grail?

It seems that the Holy Grail of many ham HF enthusiasts is a “true balanced ATU.”

The word “true” in there bodes poorly!

It seems that while there are plenty of online experts who have very strong opinions on common mode current, baluns and ATUs, it is very rare that we see quantitative evidence of their assertions, measurements even.

Less commonly does a “true balanced ATU” description include valid measurement of common mode current as evidence of operation.

A “true balanced ATU” project by LY1O in unusual in that it contains a probe purported to measure and display current balance.

Above is a schematic of LY1O’s measurement system, it has a pair of current transformers each with half wave diode detectors in each leg of ATU output. It is important to note that the detectors convert the RF AC wave into a DC value close to the peak value of the AC wave… so they respond to the magnitude only of the current in each leg. Continue reading Balanced ATUs – the Holy Grail?