At Where is the best place to measure feed point VSWR I discussed location of the VSWR meter and projection of its reading to another point on a known transmission line.
A correspondent has taken me to task and citing Telepost’s LP-100A manual: Continue reading LP-100A manual advice on VSWR measurement
(Sevick 2001) discusses efficiency of transmission line transformers that use nickel-zinc ferrites in Chapter 11 “Materials and power ratings” applied to broad band baluns.
In Chapter 11 he reports a range of measurements of two different basic configurations, a 4:1 Ruthroff balun and a 4:1 autotransformer and uses nickel zinc ferrite cores of types that are no longer readily available (and none were the K and 52 mixes he is said to have recommended).
The types of transformers he built are ones where core flux (and so core loss) at low frequencies is approximately proportional to the quotient of voltage impressed across the input terminals and number of turns, so core losses can be decreased by reducing voltage and/or increasing turns. These are Voltage Baluns, see Definition: Current Balun, Voltage Balun.
By contrast, the flux (and so the core losses) in Current Baluns is proportional to the common mode current times turns, and in antenna systems, that cannot be simply calculated using back of the envelope ohms law (though pundits often do it), see Baluns – Rule 500.
So Seviks experiments and discussion are not directly applicable to Current Baluns, yet they are cited by manufacturers, sellers, and users as rationale for their designs using nickel-zinc ferrites for Current Baluns. Continue reading Sevick’s comments on selection of ferrite mix
Steve (G3TXQ) posted a graph comparing Cecil’s (W5DXP) measurements of two turns on FT240-52 and FT240-K.
It is interesting to reconcile the #52 curves with Fairrite’s datasheets. A simple reconciliation is to compare results at the frequency where µ’ and µ” curves cross over. Continue reading Attempting to reconcile W5DXP & G3TXQ’s comparison of K and 52 mix ferrites
Among forum experts, there are ready recommendations for the ideal ferrite material (or mix) for a balun, often without knowing any detail of the application.
The ‘magic’ mixes include K. Perhaps they are devotees of Sevick.
Over some years I have searched for manufacturer’s data on K mix, and found only two references:
- Amidon who give a very brief table summarising characteristics, inadequate for RF inductor design; and
- Ferronics who give characteristic curves, albeit in less common format.
Problem is that Ferronics µi is 125 against Amidon’s 290… so their K materials are different.
One has hoped that an interested competent person might have made measurements of some samples from Amidon to give full characteristic curves, it isn’t that hard. Continue reading Ferrite K mix
I used an AIMuhf for Measuring balun common mode impedance – #2 using the SOL calibration facility.
AIM also claims to have a means of backing out a known transmission line between reference plane and DUT. This article discusses use of AIM’s Refer to Antenna facility.
AIM’s developer recently said of AIM’s Refer to Antenna facility:
Version 882 does have a problem with the Refer to Antenna function. Version 865A can be used for this function.
This function does have it’s limits though. It should only be used for good quality coax. The impedance and velocity factor of coax is not constant over the whole length and this limits the accuracy. Also the impedance may not be equal to the “nominal” impedance in the catalog. The impedance of 50 ohm cable can vary quite a bit. AC6LA.com has some interesting data showing how coax parameters vary with frequency.
Custom cal is much better when it is possible to put the cal loads at the far end of the transmission line. This takes into account variations in impedance, velocity factor, and loss and it can be used when there is coax and ladder line in one transmission line system.
Continue reading AIM 865A Refer to Antenna facility
At Measuring balun common mode impedance – #2 I mentioned a glitch on the AIMuhf scan that appears to be a defect of the instrument / client software and that it undermines confidence in the system.
The article documents a test of a known load to attempt to prove the measurement system good. Continue reading AIM 865A produces inconsistent / incorrect results
NEC-2 and NEC-4 support a GM card to translate and rotate a structure in three dimensions.
At Exploitation of NEC’s GM card I gave a small example of using the GM card.
Antennas often have some degree of symmetry and it is the key to simplification through use of the GM card.
This article gives a simple but practical example of exploitation of this often overlooked facility to a Lazy H antenna.
Continue reading Exploitation of NEC’s GM card #2
At Baluns – show me the numbers I raised the value of quantitative measurement of the complex common mode impedance of current baluns as a figure of merit for certain antenna applications.
At Measuring balun common mode impedance – #1 I showed a technique that can be used with a common antenna analyser that properly measures impedance (including the sign of reactance).
This article demonstrates a simple method of measuring the complex common mode impedance of a Guanella 1:1 current balun using a one port VNA, though of course the technique is equally applicable to a two port VNA.
This article shows a simple method of measuring the complex common mode impedance of a Guanella 1:1 current balun using a modern Antenna Analyser.
The instrument used for this demonstration is a AIMuhf with AIM 865A PC client. This is back level software as I have demonstrated AIM 882 produces internally inconsistent results. Continue reading Measuring balun common mode impedance – #2
A forum expert recently opined:
Unless there’s a compelling reason NOT to, always use CURRENT sources instead of voltage sources.
Like most online advice, the writer did not give reasons for is advice, and given the strength of it, an explanation is warranted.
Of course, many people giving compelling advice online don’t actually understand the matter and cannot give a rational explanation.
Continue reading NEC – current or voltage source