Amateur Radio – Page 83

VK3YE’s 3m circumference copper tube loop with RG213 stub tuning

Peter, VK3YE, describes a small transmitting loop (STL) in his video at https://www.youtube.com/watch?v=Cv_RnLpZ9gw.

As far as I can glean from the video, it is made from a 3m length of copper tube 19mm diameter, and uses about 1.8m of RG213 to tune it, and appears to have its centre 0.7m above ‘ground’ .

Let us firstly look at a free space model of the antenna using Reg Edwards’ RJELoop1 tool.

This model has its limitations, but the calculated inductance is of interest. We can calculate the inductive reactance to be 118Ω. The capacitive stub of RG213 will need around 107Ω reactance, and solving for RG213, we find that 1.94m gives 0.19-j107Ω. The resistive component is important as it is ignored by the above model. The stub resistance is a loss resistance, and we need to recalculate the efficiency. Efficiency=Rrad/Rloss=0.005/(0.19+0.0351+0.005)=2.17% (-16.6dB). We can also calculate the Q as 107/(0.19+0.0351+0.005)=465 and half power bandwidth as 7100/497=15.3kHz. Continue reading VK3YE’s 3m circumference copper tube loop with RG213 stub tuning

Last update: 4th August, 2015, 7:34 AM

2.4:1 balun design failure

A lost soul searching for enlightenment on impedance transformation sought advice on a transformer at 2.4 : 1 BALUN.

Inevitably one of the forum experts counselled:

Assuming your quad is a single-band HF antenna, a conventional transformer using #2 powdered iron would be my choice for the balun function. The reactance of the secondary winding would need to be at least 600 ohms.

So, let’s put the forum expert’s advice to a practical test.

Fleshing out the proposed solution

I have at hand a T200-2 core, so lets calculate the secondary turns to satisfy the proposed solution.

Above is calculation from a popular online calculator. For 14MHz, the secondary should be at least 23.8t. We will use 24t. Continue reading 2.4:1 balun design failure

Last update: 26th December, 2015, 9:01 AM

Thermal observations on Neosid 28-053-31 ferrite toroid

The Neosid 28-053-31 ferrite toroid is used in my HF Balun Project.

This article reports some thermal measurements and analysis made in relation to the project some years ago, but possibly of interest.

Above is the Neosid 28-053-31 ferrite toroid in an implementation of my HF Balun Project using XLPE wire for the winding. The core is a NiZn ferrite toroid of 63x26x19mm (larger than FT240 size). Continue reading Thermal observations on Neosid 28-053-31 ferrite toroid

Last update: 27th May, 2015, 7:56 AM

Interpreting temperature rise in ferrite cored RF transformers and inductors

We often see statements by hams where they draw inference from observed temperature rise of a ferrite core at RF. Lets consider the following statement.

The FT-240-43 balun MUST be quite efficient as it barely increased in temperature over a 5 minute over at 100W on SSB.

For the purpose of this explanation, lets assume barely increased in temperature means 5° increase in temperature from cold. Under these conditions, we can reasonably assume that almost all of the heat input to the core is consumed in raising the core temperature. Continue reading Interpreting temperature rise in ferrite cored RF transformers and inductors

Last update: 24th May, 2015, 11:07 AM

A walk through of a practical application of AIMuhf/AIM900A #2

I mentioned at A walk through of a practical application of AIMuhf/AIM900 that I wasn’t all together happy with feed point R at resonance, at 40Ω it was perhaps a touch high for a 2m quarter wave ground plane on a largish vehicle roof.

Repeated measurement of the DC resistance from the coax plug sheild to car body yielded unstable resistance ranging from 1 to 10Ω. If stable low DC resistance is not achieved, this feed line won’t work properly for RF. Continue reading A walk through of a practical application of AIMuhf/AIM900A #2

Last update: 26th December, 2015, 9:01 AM

A walk through of a practical application of AIMuhf/AIM900

This article describes the use of the Array Solutions AIMuhf/AIM900 to test a mobile antenna installation, a quarter wave whip for 2m with about 4m of RG58 cable which has been previously installed and tuned.

The exercise is motivated by a perception that the antenna is not working as well as it should.

Above is a scan of the VSWR. It indicates problems, there should be a main VSWR dip around the high end of the 2m band (147MHz), but instead the minimum is nearer 160MHz. Clearly there is an antenna connected to the far end of the line in some form (ie the inner conductor is not simply broken), but there could be a high resistance in the inner conductor or shield connection (the latter is common issue with this type of antenna base). Continue reading A walk through of a practical application of AIMuhf/AIM900

Last update: 26th December, 2015, 9:01 AM

Coupled coils – a challenge for hams!

One frequently sees discussions of coupled coils in ham fora, and the advice of the forum experts is commonly sadly lacking.

An example is the thread Impedance matching transformer where the OP is encouraged to make a transformer for 2:1 impedance transformation ratio based simply on turns ratio and a Rule Of Thumb for minimum number of turns.

Lets review a design where two windings of say 10µH and 20µH are wound on a toroidal core. With no flux leakage, the turns ratio would be 1:1.414. The model is a simple one of coupled coils and ignores self capacitance.

100% flux coupling

If there was no flux leakage, the mutual inductance is (10*20)^0.5=14.14µH, and we can build a three component model of the coupled coils along with the intended 100+j0Ω load.

Above the model for 100% flux coupling.

And above, the response of the network. At 7MHz, the input impedance is 48.7+j8.7Ω, not perfect, but close (VSWR=1.2). Continue reading Coupled coils – a challenge for hams!

Last update: 11th June, 2021, 6:51 AM

AIM 885A produces internally inconsistent results

Further to AIM 885 produces internally inconsistent results…

A new release, AIM885A appeared recently.

In the common theme of one step forward, two steps backwards, this version produces error popups when started.

The above popup appears twice when starting AIM885A. Just another symptom to undermine confidence in the system. It doesn’t make sense to me, and the program appears to otherwise start and run. Continue reading AIM 885A produces internally inconsistent results

Last update: 17th May, 2015, 6:57 PM

AIM 885 produces internally inconsistent results

AIM 882 produces internally inconsistent results and AIM 882 produces internally inconsistent results – more tests raised some issues with AIM882 software.

AIM885 has been released. Continue reading AIM 885 produces internally inconsistent results

Last update: 17th May, 2015, 6:57 PM

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

Last update: 9th March, 2015, 4:47 AM