At Efficiency and gain of Small Transmitting Loops (STL) I explained an approach to assessing the gain the efficiency of STL, and provided a link to a calculator to perform the calcs.
This expands on application of the concepts and introduces an enhanced calculator to perform the calculations.
Firstly, this technique applies to antennas where the VSWR characteristic is consistent with a feed point or virtual feed point where around the frequency of minimum VSWR, X varies with frequency much more than R. The simplified analysis assumes that R is constant, and change in X is the reason for the VSWR characteristic. See VSWR curve of a simple series resonant antenna for more information. Continue reading Enhancement of Calculate small transmitting loop gain from bandwidth measurement
David, VK3IL, describes a small transmitting loop (STL) at Portable magnetic loop antenna.
As far as I can glean from the article, it is made from a 3m length of LDF4-50B Heliax, and uses a Patterson match to tune it.
David offered measurement of VSWR around centre frequency for the loop matched on 40m. He has measured the VSWR=2.6 (the half power) bandwidth shown between markers 2 and 3 to be 22kHz. Continue reading VK3IL’s 3m circumference LDF4-50B loop on 40m
I have an IC2200H mounted on my operating table with 25mm clearance above the radio and ample room for convection currents to assist in heat removal. It is concerning that the case temperature reaches temperatures that are not safe to touch, temperatures in excess of 75° (55° above ambient) have been measured and that has not triggered the internal temperature protection… so it could get hotter still!
Whilst it might take a while for the radio to reach high temperatures, in the long term, it must dissipate around 139W when transmitting on HIGH power setting and at ambient temperatures as high as 35° in the shack. (Rated input is 15A at 13.6V for 65W out, leaving 139W of heat to be dissipated.)
This is one of those high power mobile radios that advertises no fan as an advantage, but it is clearly not up to the task!
The objective of this change is to keep the external parts below 60°, the (ASTM standard C1055 1999) 5 second human skin burn threshold.
Continue reading Cooling an IC2200H – update
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
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
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
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
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
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
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!