I am no expert when it comes to baluns, but here are several comments that are based upon some research that I have recently done on the subject:
1) Now, unlike in the past, the most commonly used balun is the current balun. Most current baluns use some type of transmission line (if they do, they are called transmission line transformers). If the load on a balun’s output does not represent a 1:1 SWR for the balun, the impedance measured at the input of the balun will be close to the balun load impedance only when the electrical length of the balun’s line is a multiple of an electrical half wavelength. I suggest that you try putting 25 ohm and 100 ohm resistive loads on your baluns and see what you measure at the input. The BL-2 uses a relatively short length of coax (~12 in I believe), however, 12 in of RG58 will show 80+j33 at its input at 30 MHz when terminated with 100 ohms (using TLW). My high power DXE 1:1 current choke (balun) has a much longer length of coax than the BL-2. It reads 43 +j32 ohms at 30MHz with a 25 ohm load (measured with a VNA 2180 network analyzer). If you can determine the length and type of coax in the balun that you are using, you can use ARRL’s TLW software to estimate what the actual balun load impedance is by inputting the line parameters and the impedance you measure at the balun input. 2) What happens to balun performance as the SWR increases is a question that I have yet to find an answer to. I am not aware of any balun manufacturer that attempts to quantify this behavior with any spec. There seems to be general agreement that baluns do not like highly reactive loads. 3) Balun heating is a complex issue that many “experts” don’t agree on. Looking at it simplistically, there are two sources of heating in current baluns (voltage baluns are very different than current baluns when it comes to heating). The first source of heating is the loss in the transmission line used in the balun. If a short length of high quality coax is used, the loss (ie, heating) should be relatively small. However, the confined space and poor heat transfer in the balun housing could mean that even a small amount of loss can be problematic in a poorly designed, high power balun. What many “experts” don’t realize is that the normal (differential mode) signal flowing down the coax does not cause any heating in the ferrite core in a current balun (this is not the case with a voltage balun). This is because this signal is completely contained between the outer surface of the center conductor and inner surface of the shield in the coax. What does cause heating in a current balun is the common mode current flowing on the outer surface of the shield on the coax used in the balun. This is the unwanted current that the balun is intended to reduce. Also, a number of articles claim that ferrite core temperatures can reach critical values (where the core fails) well before the core saturates and thermal runaway occurs. 4) Heating from common mode current is another area where the “experts” don’t agree. Heating due to common mode current is a function of transmitter power, design of the balun (ie, type of core, core size/mix, style of balun, heat transfer of housing, balun load impedance, etc) and the “system” issues that affect the balun’s performance. Depending on the values of the important system parameters, adding a balun can either increase, decrease, or have no effect upon the common mode current flowing on a transmission line. The common mode current and the resistive component of the balun’s common mode impedance are critical factors that determine the amount of heating in the balun’s ferrite core. A surprising number of “experts” make claims for power handling capabilities of various types of baluns without even mentioning the importance of these “system” issues. Tom Rauch (W8JI) and Tom Thompson (W0IVJ) have written articles that use EZNEC to quantitatively show the importance of these “system” issues when it comes to estimating common mode currents on transmission lines. However, as enlightening as these articles are, I am doubtful that EZNEC can be used to accurately predict the common mode current in any given application. At this point, given the difficulty of either measuring, or predicting, what the common mode current will be in a given application, I have concluded that the best way to evaluate core heating in a balun is to install it and test it at the intended power level. Hope this helps, Bill N0CU P.S.: an “expert” is someone who may be wrong, but is never in doubt! -- View this message in context: http://elecraft.365791.n2.nabble.com/BL2-Antenna-Balun-Usage-tp7630889p7631020.html Sent from the Elecraft mailing list archive at Nabble.com. ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
