Mullard QY4-400 (4-400) x 2 Class C AM plate and screen

Valve datasheets often give one to few suggested sets of operating parameters for a valve in one several modes, and designers are left to sort of interpolate or extrapolate to their own set of parameters, eg loaded power supply voltage.

This article is a walk through and comparison of RF Power Amplifier Tube Performance Computer applied to the subject scenario.

Let’s extract from the Mullard datasheet.

We will use the Va=3kV scenario for this comparison. Continue reading Mullard QY4-400 (4-400) x 2 Class C AM plate and screen

A desk study of a matching scheme for a short base loaded Marconi on 137kHz

A correspondent recommended a simple configuration of a base loaded shortened Marconi for 137kHz, referencing an online posting by another ham.

I was  assured that this configuration is simple, very effective and very popular. It has been used for a very long time, so it must be good.

Well, let’s do an analysis.

The recommended antenna

The online poster’s equivalent circuit of his 137kHz base loaded vertical. The resonant frequency of this circuit is actually 136.979kHz, let’s assume the inductance is correct and that C is C=99.969pf and the circuit is resonant at exactly 137kHz. Continue reading A desk study of a matching scheme for a short base loaded Marconi on 137kHz

A desk study of a matching scheme for a cap hat loaded Marconi on 137kHz

Reworked for average ground type (σ=0.005, εr=3) …

A common scheme for narrow band match of an end fed high Z antenna discussed discussed the kind of matching network in the following figure.

A common variant shows no capacitor… but for most loads, the capacitance is essential to its operation, even if it is incidental to the inductor or as often the case, supplied by the mounting arrangement of a vertical radiator tube to the mast. In any event, and adjustable capacitor may be a practical addition to help with matching under varying environmental factors.

This article is an expose on technique rather that a recommended antenna design. Continue reading A desk study of a matching scheme for a cap hat loaded Marconi on 137kHz

VNA fixture for measuring Zcm of a common mode choke – twisted pair wound

VNA fixture for measuring Zcm of a common mode choke – coax wound discussed issues with common ham practice for measuring coax wound common mode chokes.

The article left readers with some homework:

  • Does the same thing occur if the core is wound with twisted pair that is well represented as a uniform two wire transmission line?
  • Are the resistors beneficial?
  • Do they degrade fixture behavior?
  • Then, why are the used so often?

This article addresses those questions.

Does the same thing occur if the core is wound with twisted pair that is well represented as a uniform two wire transmission line?

Let’s treat the common mode choke as a black box with two input terminals at left and two output terminals at right with voltages as annotated above. Continue reading VNA fixture for measuring Zcm of a common mode choke – twisted pair wound

VNA fixture for measuring Zcm of a common mode choke – coax wound

A common online question is what sort of fixture is appropriate to measure the common mode impedance of a common mode choke.

Above is a screenshot from a Youtube video by Trx Lab, probably about 2016 vintage. I see many problems with the fixture, lets start with the resistors. Continue reading VNA fixture for measuring Zcm of a common mode choke – coax wound

Does RBN give a reliable metric for comparing antennas – more detail

Does RBN give a reliable metric for comparing antennas? gave an example of signal strength measurement and the effect of fading over time.

This article goes into a little more depth on the subject using a further data capture of 600 measurements 10s apart.

Above is a plot of signal strength of an 80m A1 Morse (CW) beacon measured in 20Hz bandwidth over 100min (a terrestrial path of length 105km). Continue reading Does RBN give a reliable metric for comparing antennas – more detail

Reconciliation of transmitter power, EIRP, received signal strength, antenna factor, ground wave propagation etc @ 576kHz

This article reconciles measurements with path predictions for a MW AM transmitter on 576kHz. The techniques used could be used to validate / assess the performance of a transmitter.

Source

The source is a MF AM transmitter on 576kHz located about 74km distant.

Above is the station data from the ACMA licence register. Conveniently it gives the EIRP as 132kW, we would expect something a little less than 150kW from the nominal 50kW transmitter, system efficiency calculates to 80%.

The EIRP would have been calculated from a set of field strength measurements at the time of commissioning. Continue reading Reconciliation of transmitter power, EIRP, received signal strength, antenna factor, ground wave propagation etc @ 576kHz

Does RBN give a reliable metric for comparing antennas?

I see that lots of hams depend on HF RBN to compare to antennas, or to compare before and after a change.

Experience says that A/B comparisons on HF are subject to variation in Ionospheric propagation paths, and that variation can be wide in range and rapid.

An example

Above is a plot of signal strength of an 80m A1 Morse (CW) beacon measured in 20Hz bandwidth over 15min snapshot (a terrestrial path of length 105km). Continue reading Does RBN give a reliable metric for comparing antennas?

Origins of the PIK (PIC Iambic Keyer)

I rode my motorcycle over to visit Ray, VK2COX, and we rode our bikes across to Canowindra  (pronounced /kəˈnndrə/ kə-NOWN-drə) for a burger at the famous Garden of Roses Cafe.

Over lunch, Ray described his new cigarette packet sized CW rig build that he would take up onto the local hill on his block for a little play, and told me he was going to jam his favorite 3xNE555 CW keyer into it.

I offered to design him an accurate keyer based on an 8 legged DIP chip and less than a dozen parts overall. I designed the logic in my head on the way home to Canberra, and started programming it that night in May 2001.

Screenshot - 18_04_16 , 19_47_22

The PIK is described at PIK – PIC Iambic Keyer. Above is the generic circuit diagram of the PIK. Continue reading Origins of the PIK (PIC Iambic Keyer)