In another long running discussion on QRZ about End Fed Antennas, WA7ARK offered a contribution:
(1) Back in post #30 I showed that with a halfwave wire fed close to its end works just like the same wire fed in the center; the only difference being the feed point impedance. I let EzNec figure this out; I didn’t have to explain it with any mysterious “displacement” currents. Shown as (1) in the attached.
Since, in the model, the source is a constant current source, that forces the current on either side of the source to be equal, and the radiation pattern predicted by EzNec reflects that, because the patterns for the end-fed and center-fed match… (go back and look at post #30)
His post #30 is of a 67′ dipole at 66′ above poor ground @ 7.18MHz, fed at one end.
Above is the current distribution of my approximate re-creation of his model in NEC-4.2. It reconciles with his published graphs. Continue reading Discussion of WA7ARK’s contribution to a QRZ thread on an End Fed Dipole
An online expert somewhat exasperated that the audience hasn’t absorbed his wisdom elaborated in apparently many previous posts said:
We’ve been round and round on this discussion but in a current mode balun aka a common mode choke the losses due to the windings and core are common mode not differential mode losses. You DO NOT dissipate your transmitted and received signals, which are carried as differential mode signals, as choke losses.
I know you’ve been reminded of this many times and don’t expect you to accept it now but that’s how common mode chokes work.
Now there is a sense in ham radio forums that repetition transforms assertions to fact, but setting that aside, let’s look at the assertion from an energy conservation point of view. Continue reading woolly thinking on the nature of feed line common mode current
This article is documentation of a capacity test of 5 x Hobbyking 2500mAh 18650 LiIon cells (9210000181-0).
The cells were purchased on 26/02/2018 (~$7 + shipping) and received at about 30% charge. They were each charged in a XTAR VC2 Plus charger at 0.5A until charged.
The cells are 65mm long, and do not claim to contain protection modules which are prudent in some applications.
Each cell was then discharged at 1A (0.4C) to 2.8V, the discharge was captured.
Continue reading Hobbyking 2500mAh 18650 LiIon cells (9210000181-0) initial capacity test
Calibration of the 4-20mA input
This article is a tutorial on calibrating the 4-20mA input which is designed for flexibility that is achieved through exploitation of the calibration.
The input device for this tutorial is a Pt100 RTD temperature sensor and inexpensive Chinese Pt100 – 4-20mA converter (loosely) calibrated for -50-150°. The Pt100, the converter, the load resistor, the divider resistors on the MCU board, and the MCu voltage reference all introduce error which is compensated in this end to end calibration procedure.
For this demonstration, two scenarios are measured:
- probe in still air whose temperature is captured with an accurate thermometer; and
- probe in boiling water whose temperature is calculated from known altitude and barometric pressure.
Another option would be to use a container of water filled with ice to obtain close to 0° for scenario 1… you don’t need a triple point cell for the end system stability and accuracy.
Temperature of boiling water
Using Calculate cooking time for soft boiled egg :
No, we are not boiling an egg, but the results include the temperature of the boiling water under current altitude and pressure. Continue reading IoT water tank telemetry project – part 2
I have been asked a few times about my article Implementation of G5RV inverted V using high strength aluminium MIG wire, and conversations ran to the suitability of the wire to a radial system on Marconi type antennas.
Firstly, a progress report on the antenna, no news to report and that is good news, there have been no issues so far. Inspection of connections without disassembly has not shown signs of corrosion or fatigue. Continue reading Aluminium ground system suitability for ham radio station
An online expert recently expounded on detailed design of a balun, this is an excerpt about wire sizing.
The wire gauge used limits the current handling capacity of the wire, run too thin a wire and it will heat up. Run much too thin of a wire for the power in use and it will fuse open. Current carrying capacity of wire is typically rated for either power transmission applications or chassis wiring applications. The latter, and higher, current capacity for a wire is relevant to designing a balun. How much current your 50 watt signal generates depends on the impedance its looking into. If you’re talking about a 50 ohm system, with a perfect match you’ll deliver one amp through your balun wires when driving 50 watts into it. Allowing for say a 4:1 SWR the worst case current(@12.5 ohms) is 2 amps. If you’re using this as a tuner balun, perhaps to drive a multi-band doublet then the impedance can vary widely so over sizing the wires is easy insurance. Here’s a table of wire current carrying capability: https://www.powerstream.com/Wire_Size.htm
For convenience, the relevant part of the table linked above is quoted for discussion.
So, the poster recommends wire with chassis wiring rating of 2A for 50W with reserve capacity for worst case VSWR=4. Continue reading Baluns – wire size insanity
This is the first in a series of articles describing a simple maker / DIY IoT water tank telemetry system.
- capture water depth, temperature and relative humidity;
- IoT connectivity;
- solar / battery powered;
- wireless connection;
- use existing inexpensive electronic modules.
Design choices made initially:
- 4-20mA water pressure sensor for depth measurement;
- ESP8266 Wemos D1 mini pro for the MCU and wireless elements;
- NodeMCU / Lua software environment;
- external antenna for improved WiFi range;
- 6V 100mA PV array;
- module with TP4056 batter charger and cell protection chip;
- 2500mAh 18650 cell;
- AM2320 temperature and humidity sensor;
- bipolar transistor switch for boost converter;
- Thingspeak RESTful interface for data accumulation and presentation.
Above is a block diagram showing the major system components. Almost all of the electronics is on easily obtained low cost electronic modules source from eBay, assembled on a Veroboard backplane. Continue reading IoT water tank telemetry project – part 1
The ARRL handbook for radio communications (Ward 2011) gives guidance on designing with ferrite cored inductors:
Ferrite cores are often unpainted, unlike powdered-iron toroids. Ferrite toroids and rods often have sharp edges, while powdered-iron toroids usually have rounded edges.
Because of their higher permeabilities, the formulas for calculating inductance and turns require slight modification. Manufacturers list ferrite AL values in mH per 1000 turnssquared. Thus, to calculate inductance, the formula is
L = the inductance in mH
AL = the inductance index in mH per 1000 turns-squared, and
N = the number of turns.
Example: What is the inductance of a 60-turn inductor on a core with an AL of 523? (See the chapter Component Data and References for more detailed data on the range of available cores.)
Lets follow the example through. Continue reading ARRL guidance on design of ferrite cored inductors
Many antennas can be represented near their series resonance as a series RLC circuit, and in many cases R changes very slowly with frequency compared to X. This provides a convenient and good approximation for the behaviour of the antenna impedance in terms of a simple linear circuit.
Series resonant circuit
The response of a simple series resonant RLC circuit is well established, when driven by a constant voltage source the current is maximum where Xl=Xc (known as resonance) and falls away above and below that frequency. In fact the normalised shape of that response was known as the Universal Resonance Curve and used widely before more modern computational tools made it redundant.
Above is a chart of the Universal Resonance Curve from (Terman 1955). The chart refers to “cycles”, the unit for frequency before Hertz was adopted, and yes, these fundamental concepts are very old. Continue reading Antenna half power bandwidth and Q, concept and experimental validation
Given that most versions of the AIM software that I have tried have had serious defects, I approach the latest release, AIM912, with caution.
An interesting opportunity presented when a correspondent sent me some .scn files captured from an AIM4170B using AIM912.
Above is the correspondents .scn file opened in AIM912. Continue reading AIM system – AIM912 initial checkout