Some tools for designing a Guanella 1:1 balun using ferrite toroids

In designing a Guanella 1:1 balun, selecting a ferrite core that has been characterised by the manufacturer simplifies the design process greatly.

The manufacturer’s full characterisation includes curves for complex permeability vs frequency and from these the magnetising impedance of the core can be calculated. Note though that tolerances on magnetics are usually fairly wide and they can be quite temperature dependent.

The inductor will usually exhibit a self resonance that is not revealed by the above calculation, but can be reasonably well modelled by adding a small equivalent shunt capacitance, see (Knight 2008). This equivalent capacitance is usually very important and not so easy to estimate, and is often best estimated by careful measurement of the self resonant frequency of the inductor (taking care to back out fixture effects). With experience, one can make a fairly good first guess so that the process is not too iterative.

Some writers say that Cs increases as turns are increased, but (Knight 2008) shows quite the opposite.

Controlling inductor self resonance is a lot about controlling added stray capacitance, eg connecting wires, encapsulation in conductive boxes etc.

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Above is a plot of common mode impedance of a FT240-43 ferrite toroid with 11t wound in Reisert cross over style and Cs=3pF. Different scenarios will give different results, but the form will tend to be similar to above. Continue reading Some tools for designing a Guanella 1:1 balun using ferrite toroids

Designing a Guanella 1:1 balun using the ‘unknown’ ferrite toroid

At Characterising an unknown ferrite toroid an ‘unknown’ ferrite toroid was characterised. This article uses that information for design of a Guanella 1:1 current balun.

The proposed design uses 11t of small coax wound in the Reisert ‘cross-over’ style.

The impedance of a single turn vs freq was used to predict the impedance of an 11t choke. Such a choke exhibits a self resonance that can be represented as due to an equivalent shunt capacitance. This equivalent capacitance is not easily estimated, and can best be determined by calibrating an analytical model of the choke for the same self resonance as exhibited by a real choke.

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Above is common mode impedance from an analytical model of the choke, assuming an equivalent self capacitance of 11pF.
Continue reading Designing a Guanella 1:1 balun using the ‘unknown’ ferrite toroid

Characterising an unknown ferrite toroid

The ‘unknown’ toroid is wound with a single turn and measured with a VNA, an AIMuhf in this case.

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Of interest in the first instance is the apparent inductance of the single turn winding at low frequencies where typically permeability µ is fairly constant and core loss is fairly low. Continue reading Characterising an unknown ferrite toroid

Mini60 antenna analyser

There seems a never ending stream of low end antenna analysers appearing.

The Mini60 antenna analyser is one in that vein, and is sure to prove popular because of its low price. As is common, there does not appear to be an English language user manual and the specifications in eBay ads are not very reliable (eg weight: 200kg).

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Above is a screenshot from an online demo of the Mini60 on a 7MHz antenna. Continue reading Mini60 antenna analyser

Transmit performance of 2m hand held transceivers – absolute gain estimates

Transmit performance of 2m hand held transceivers reported relative field strength measurements for some transceiver / antenna combinations.

This article documents a more careful measurement of the absolute field strength of one combination, and application of that knowledge to the other results.

Measurements of field strength were done with Lou Destefano’s (VK3AQZ) VK3AQZ RF power meter (RFPM1) and a small loop antenna.

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Above, the RFPM1 RF power meter.

VhfUhfLoopAbove is the small loop used for field strength measurement. It is 2mm hard drawn round copper wire formed into a circle 185mm in circumference, and a common mode choke is used to connect the loop to the RFPM1 power sensor. The common mode choke is 0.6m of RG58C/U with 0.5m of ferrite sleeves over it and its loss is accounted for in the “Other Loss” item.
Continue reading Transmit performance of 2m hand held transceivers – absolute gain estimates

Accuracy of AIMuhf system – AIM910A vs several recent versions on a ferrite cored inductor

AIMuhf

Yet another release of AIM software is available, 910A at the time of writing. I have downloaded and tested 8 versions this year, most have been wanting. Again, there is very little detail on what has changed and likely impact on historical measurments.

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A quick set of measurements was made on my test inductor pictured above. Continue reading Accuracy of AIMuhf system – AIM910A vs several recent versions on a ferrite cored inductor

Transmit performance of 2m hand held transceivers

This article documents measurements of transmit performance of three hand held 2m radio with several antennas.

Measurements of field strength were done with Lou Destefano’s (VK3AQZ) VK3AQZ RF power meter (RFPM1) and a small loop antenna.

RFPM1-VhfLoop

Above, the field strength meter, a RFPM1 with small loop antenna oriented for max gain in the direction of the DUT.  The instrument reads -73.5dBm with no signal, -69.5dBm with the strongest transmitter with the loop removed, and around -30dBm for the various transmitters with the loop in place… so the meter reading is predominantly due to the loop mode pickup.

All three transmitters have different power. The table below reports power into a 50Ω load and does not take account of mismatch with the various antennas.

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Above a comparison of the configurations on a field strength test at 1λ. The relative column factors the different transmitter power and FS to obtain  a comparative figure independent of power. Mismatch is almost certainly a significant part of the explanation of different performance, but it is quite difficult to measure in this sort of application without disrupting the DUT.

BoafengINF641Antenna

It is interesting that there is little difference observed with the Baofeng on two different antennas, when the Boafeng antenna is clearly inefficient, see the thermograph above.

 

Using the AIM to measure matched line loss

A correspondent wrote seeking explanation of difficulty he was having measuring line loss using the advice given in the AIM manual using a scan with either O/C or S/C termination:

Note the one-way cable loss is numerically equal to one-half of the return loss. The return loss is the loss that the signal experiences in two passes, down and back along the open cable.

Because my correspondent was using one of the versions of AIM that I know to be unreliable, I have repeated the measurements on a cable at hand using AIM_900B to demonstrate the situation.

The test cable I have used is 10m of RG58C/U which I expect should have matched line loss (MLL) of 0.26dB, but I expect this to be a little worse as it is a budget grade cable that I have measured worse in the past. Continue reading Using the AIM to measure matched line loss

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.

AIMuhfRepeated 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

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.AIMuhf

 

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

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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