EMRFD Message Archive 7980
Message Date From Subject 7980 2012-12-24 19:42:21 Tim Directional Coupler Construction I want to build (I know, getting away from QRP!) a QRO-type directional coupler as part of a SWR meter.
1.5 kW ballpark ability, 160M to 10M, would be great.
ARRL handbooks have had these (and corresponding electronics for computing/displaying SWR) for decades. But I want to actually understand the tradeoffs that get me to:
1) Power handling ability
2) Flatness across desired spectrum
Some handbooks show FT82-67 toroids in the directional coupler. Looking in the Fair-Rite tables, material 67 seems to be remarkably flat across the HF spectrum. Maybe that's why this is chosen for the toroids. See e.g. http://www.fair-rite.com/newfair/materials67.htm
Can anyone remind me of the math, that would show that an FT82-67 is appropriate at the kW level? Do I look at "loss factor" in spec sheet? What math gets me from input power, to the parameter in the spec sheet that I shouldn't exceed? Maybe something like... find max RF current at 50 ohms, turn that into flux density, turn that into field strength? I took graduate level E&M but lord knows I don't own a flux density meter or even really understand the units!!!
7981 2012-12-24 22:39:26 davidpnewkirk Re: Directional Coupler Construction 7982 2012-12-25 05:43:49 Tim Re: Directional Coupler Construction 7983 2012-12-25 11:38:26 davidpnewkirk Re: Directional Coupler Construction 7986 2012-12-25 21:47:36 NeilDouglas Re: Directional Coupler Construction Tim,
There is some useful information on transformer frequency flatness to be
7990 2012-12-26 17:52:56 davidpnewkirk Re: Directional Coupler Construction > Both of the above articles are "monimatch" circuits which involve some neutralization to reduce coupling between line and transformer (and other strays).Hmm. I saw the DeMaw article being about inline meters that use transformers rather than coupled lines, and Bruene's article being a technical survey of the math and practice of multiple approaches, including the Micromatch and Monimatch, in addition to the transformer-based sampling approach we now treat as standard because it is (within limits) practically frequency-flat.
I only recently graduated to a (homemade) transformer-based matching indicator myself, having at last tired of the frequency-dependent wackiness of indications obtained with Monimatch coupled-line methods when I'm fiddling with harmonic-ridden vintage, vacuum-tube-based ham circuits--a wackiness heightened the "50-W" test load I was using for awhile: a string of five seriesed 10-ohm wirewound power resistors with their reactance tuned out by means of a series capacitor. When neither your matching indicator nor your dummy antenna are response-flat over even an octave, there's metrology fun a-brewin'.
amateur radio W9VES
7992 2012-12-26 22:04:02 Kerry Re: Directional Coupler Construction This is a subject that's interested me for some time so, having little to do today, I began a post on it.
The post turned into something more extensive than I intended so I have put it as a pdf in the folder "VK2TIL (kerrypwr) ".
Those who are interested can access it; I stress that I am no "expert" on this subject (or any other).
8000 2012-12-27 12:54:37 Tim Re: Directional Coupler Construction As long as we are referencing useful QST articles on directional couplers... I think the best I've found is KL7IBA's "High Power Operati