Improved cooling for the MFJ-949E

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At A look at internal losses in a typical ATU I demonstrated that it is quite easy to raise the temperature of the coil in the MFJ-949E to an unsafe level, even with quite modest power.

The most heat sensitive component in this ATU is the coil, specifically the coil supports which are probably polystyrene, and the melting temperature of polystyrene is around 100°.

This article documents modification of my MFJ-949E to reduce the risk of damage under some operating conditions. Continue reading Improved cooling for the MFJ-949E

Last update: 25th April, 2020, 6:55 AM

CH340, CH341 USB to RS-232 chip compatibility

One of the several sources of chips for USB to RS-232 converters is WCH, a Chinese firm offering their CH340 and CH341 chipsets.

CH340T

Information is scarce as their website is mostly in Chinese language.

CH34x chips have become more common, perhaps as consumers have fled the Chinese Prolific product line (for good reason).

In true Chinese style, downgrade knock offs of Arduino boards (eg the Arduino nano) have appeared, the downgrade is achieved by use of CH340 USB to RS-232 converters instead of the FTDI chips on the genuine nano. Continue reading CH340, CH341 USB to RS-232 chip compatibility

Last update: 31st March, 2016, 9:17 AM

Some thoughts on a two turn small transmitting loop

Small transmitting loops (STL) are very popular with hams, and a fashion is developing for N turn loops. This article lays out some thoughts on a 2 turn STL.

Firstly, to the meaning of “small transmitting loop’. There are a range of definitions used, and they mostly centre around the concept of a size sufficiently small that current is approximately uniform. The issue is about the meaning of sufficiently. Accuracy of estimation of radiation resistance of small transmitting loops sets out a rationale for a single turn loop for criteria that perimeter<λ/10.

This article will compare NEC-4.2 models of loops with the following key parameters:

  • 1m diameter (the loop perimeter is 0.07λ);
  • 20mm copper conductor;
  • frequency is nominally 7.1MHz;
  • 16 segments per turn
  • when not specified as in free space, the loop centre is 1m above ‘average’ ground (σ=0.005, εr=13);
  • the loop is directly fed in the middle, opposite to the tuning capacitor position, cap down;
  • pitch is 0.15m.

The model is sensitive to all these parameters. Continue reading Some thoughts on a two turn small transmitting loop

Last update: 7th March, 2017, 8:26 AM

Ultrafire XML-T6 LED torch – a fix for the dysfunctional mode memory ‘feature’

On review of the Ultrafire XML-T6 torch, I found the mode switching / mode memory so dysfunctional that it rendered the torch useless in my evaluation.

XML-T6This article describes a work around  that makes the thing usable (IMHO). Continue reading Ultrafire XML-T6 LED torch – a fix for the dysfunctional mode memory ‘feature’

Last update: 9th August, 2022, 2:21 PM

End fed Zepp

The so-called End fed Zepp (EFZ) is often cited as the basis for many more recent antenna designs, and is leveraged to provide and explanation… though few hams understand how the EFZ actually works.

End fed Zepp

Screenshot - 13_03_16 , 08_38_13

Above is a diagram from the ARRL Antenna Handbook  (Silver 2011).

Though a short conductor is shown to the right of the right hand feed line wire, the length is not specified or discussed in the accompanying text. It is popularly held that this is a “counterpoise” that provides a path for current equal to that flowing left into the main horizontal wire.

Let us explore the EFZ using NEC. The models are a reflection left to right of the above diagram, ie the feed is on the left hand end. Continue reading End fed Zepp

Last update: 14th March, 2016, 7:08 PM

Fridge / freezer setup

The operating temperatures of refrigerators and freezers used for food storage is important to safe storage of food and to minimisation of energy costs.

The US FDA recommends the refrigerator should be set to 40F (4.4°) and the freezer to 0F (-17.8°).

Temperatures vary inside the cabinets, and they vary over time with opening and closing doors, and introduction of warmer goods for storage.

Many spot temperature checks are helpful but they don’t provide a very complete picture, and opening the door to make measurements disturbs the very thing being measured. Continue reading Fridge / freezer setup

Last update: 14th March, 2016, 9:51 AM

A tutorial on initial design of operating conditions for a valve amplifier

A correspondent asked for a walk through of use of a couple of my online design tools for a 6m 350W single ended valve PA using three QQE06-40 valves. The request was perhaps inspired by a design he had seen, but I sound a caution about a large number of parallel valves (6 sections in this case).

Key design parameters:

  • HV power supply fully loaded: 1200V;
  • Power output: 350W;
  • Class: AB1;
  • Vak min: 180V (from datasheet anode curves);
  • Pi output network, Q at least 12 (for reduction of harmonics on the FM broadcast band), select 15.

The datasheet gives max supply voltage at 450 for plate and screen AM, which implies max ‘instantaneous’ DC supply voltage in AB1 SSB telephony of 900V… so 1200V goes beyond the guaranteed ratings. Of more concern, it is probably close to 1400V lightly loaded, 56% greater than the maximum instantaneous supply implied by the AM specifications.

Assumptions:

  • output network efficiency 90%;
  • valves load share equally.

An advantage of a high Q design is that it requires higher input C which makes accommodating the self capacitance of the 6 valve sections somewhat easier. A disadvantage is lower efficiency.

Screenshot - 12_03_16 , 16_21_24

Above is a calculation using Calculate initial load line of valve RF amplifier. Note the anode dissipation, a total of over 190W is quite high for 120W total valve rating (though these are pretty robust valves). Total DC anode current will be almost 0.5A, so the HV power supply must deliver 1200V @ 0.5A. The 0.5A figure is within the absolute maximum of 720mA (for 6 sections).
Continue reading A tutorial on initial design of operating conditions for a valve amplifier

Last update: 13th March, 2016, 7:49 AM

Thinking about SOTA, EFHW and EMR safety

There seems to have been a revival in use of the so-called End Fed Half Wave antenna.

The prospect that a small radio such as the FT817, a magic match box and 10m of wire makes a good 20m field station appeals to many a SOTA enthusiast.

Let us model a scenario with a FT817 powered by internal battery and sitting on an insulating platform (eg a pack) 0.3m above natural ground, a 10m wire strung up into a tree at a 45° angle, and a 1m long mic cord stretched up at 45° in the other direction. The is the popular so-called ‘no counterpoise’ configuration.

A simplified model of just the current paths without regard to the bulk of the radio, or the effect of the helix of the mic cord illustrates an approximate current distribution. The model uses 1W RF input to the antenna over ‘average ground’ (σ=0.005, εr=13).

 

Clip 142

Above is a plot of the current distribution. Current is a minimum at the open ends, a boundary condition for the problem, and maximum in the middle of the half wave. We expect H field to be greatest near the current maximum, and E field to be greatest near the current minima. Continue reading Thinking about SOTA, EFHW and EMR safety

Last update: 14th April, 2024, 1:36 PM

SPICE model of Zcm of a Guanella 9:1 current balun

At Why the preference for Guanella 1:1 current baluns for HF wire antennas I compared a 1:1 Guanella balun with a 4:1 using the same component baluns.

Broadly, the findings were that the common mode impedance of the 4:1 balun was around a quarter of that of the component baluns, and mediocre at that.

This article extends the reference, documenting the SPICE model extended to a 9:1 balun.

Screenshot - 07_03_16 , 11_17_27

Above, the same component baluns were interconnected to make a 9:1 balun. Note that Zo of the TL section has been increased to 150Ω to suit the notional nominal 50 to 450Ω broadband transformer. Continue reading SPICE model of Zcm of a Guanella 9:1 current balun

Last update: 9th March, 2016, 6:48 AM