Feeding at a current maximum, and three other options

Feeding at a current maximum visited the common practice of designing to feed a multi band dipole with open wire feed at or very near to a current maximum.

I explained that feeding at the current maximum may provide sub-optimal performance on the popular T-match ATU as its losses tend to be worst with low R loads, aggravated by the use of 4:1 baluns for even lower R.

On the other hand, feeding at a voltage maximum might exceed the ATU’s voltage capacity, or perhaps be outside of the matching range of the ATU.

Well if neither of these is optimal in all cases, what about half way between. It has been done, the odd eighths wave feed line on an 80m half wave is another of the recipes you will hear.

Lets explore the options of a half wave dipole at 3.6MHz with four different feed line lengths (Wireman 551). Continue reading Feeding at a current maximum, and three other options

Low power Guanella 1:1 tuner balun using a pair of Fair-rite 2643625102 suppression sleeves

A reader of Low power Guanella 1:1 tuner balun using a pair of Jaycar LF1260 suppression sleeves asked whether there is an equivalent Fair-rite core for the project.

Fair-rite part 2643625102 (~$4 each from Element14) is of similar size and also medium µ, but slightly different characteristic to the LF1260. Continue reading Low power Guanella 1:1 tuner balun using a pair of Fair-rite 2643625102 suppression sleeves

The Army Loop (Patterson match)

The ARRL and other publications refer to the Army Loop or Patterson match.

Patterson described his antenna system at (Patterson 1967). Hams seem to call any configuration that uses only capacitors in the matching circuit a Patterson or Army loop, though they are incorrect.

The ARRL Antenna Book 21 has a nonsense circuit that cannot work.

Another ARRL example, one that does work

Screenshot - 01_12_2014 , 13_51_37Above is a diagram from a much earlier ARRL and as far as I can ascertain, this is McCoy’s version the so-called ARMY Loop. (McCoy 1968) gives the middle capacitor as 500pF variable which would reduce the matching range. Continue reading The Army Loop (Patterson match)

Spoiling balun action with ‘shielded twin’

In a QST column in 2008, a correspondent asked the question

… I have the ladder line terminated to double coaxes that run about 12′ (4m) inside the house to an antenna tuner. Should this pair of coaxes be grounded at one end or both ends?

ShieldedTwin

The Doctor gave a detailed diagram (above) and his advice was… Continue reading Spoiling balun action with ‘shielded twin’

OCF dipoles

Off centre fed (OCF) dipoles are very popular with Hams on HF.

Many claims are made for variations of the theme, but the essential common feature is that the offset feed point provides a feed point impedance that is not too unsuited to 50Ω coaxial feed line at the fundamental resonance of the dipole and half wave harmonics. Continue reading OCF dipoles

What does an Antenna Tuning Unit (ATU) do?

An Antenna Tuning Unit (ATU) performs a simple but important function in many transmitting systems.

Almost all things called an ATU are simply impedance transformers, and almost always, narrow band impedance transformers (meaning that when adjusted, they achieved the desired transformation over only a narrow frequency range).

Screenshot - 03_05_2014 , 15_06_33

ATUs come in a range of configurations, each designed for a specific set of characteristics. Above is the heart of a Palstar AT2K T Tuner, just three real passive components that are fully explained by conventional linear circuit theory. Continue reading What does an Antenna Tuning Unit (ATU) do?

Feed point voltage – W5WSS 7′ dipole

Background

W5WSS describes his antenna at (eHam 2014). It is essentially a shortened dipole with capacity hats for 20m.

The configuration appears from several postings to be this shortened dipole with a Balun Designs 1115du balun at the center and an adjacent LDG Pro 200 automatic ATU.

Continue reading Feed point voltage – W5WSS 7′ dipole