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Author Topic: 4:1 Balun with T200-2 core  (Read 6896 times)

Posts: 849

« Reply #15 on: June 16, 2014, 11:30:11 AM »

picking a nit... teflon pipe tape is soft and easily deforms.  has to for value as pipe joint packing.  if a toroid edge can nick and short enamelled wire, teflon tape is not going to buy you any protection.  looks pretty, conforms smoothly, won't do a darn thing for protection.  I suspect putting an ohmmeter probe on the toroid and another on a conductor of the balun will be informative here.  Scotch #27 is the gold standard... first, it's thick, and second, the fiberglass filaments provide extra spacing from the coil if the tape base melts under heat.

Posts: 382

« Reply #16 on: July 11, 2014, 12:35:51 PM »

The inductance of 17 (or 2 × 17?)turns on a T200-2 core is about 3.5µH or about 77 ohms reactance at 80 m. So 77 omhs is parallelling 200 ohm at the secundairy of the balun. Most of the transmit power is lost in the balun.
Frits, PAoFRI,

This is an iron core with high Q. Assuming Q of 200, the reactance is 77 Ohms and resistance of 0.35 Ohms. The reactance part is easily matched and not much is lost in 0.35 Ohms.

I think that T200-2 is good for a KW. One way to find out how good it is, transmit and see whether it heats. IMHO, it will stay cool at 100W.

If you have doubts, look at sizes of iron toroids in automatic antenna tuners. In 100W tuners they seem to be size 50. One reason is that they work at 200 C and beyond. 

Ignacy, NO9E

Posts: 709

« Reply #17 on: July 13, 2014, 04:37:11 PM »

so many things...

First if your doing a transformer style balun there are single core designs to follow though they are often
not as good in bandwidth as a two core balun.  See Sevik et al.

A t200-2 x2 using #14 enameled will work if  wound and connected as a Guanella style and offers good
working bandwidth. 

A good single core design can be found here
but you have to use the recommended core.  Note the teflon tubing around the wire is not to protect them.
its there to create a parallel pair transmission line so the teflon is actually spacer.

Baluns are as much art as science and building them right is important.


Posts: 42

« Reply #18 on: July 20, 2014, 11:57:35 PM »

Hi Scott,

How are you feeding your fan dipole?  I guess with the resonant antennas of your fan dipole you could feed them with coax now?  The 300ohm twinlead won't match too well up at the feed point, but it wouldn't be a big deal since its loss is low enough and you are running a tuner.

I hadn't before seen NB6Z's page with the antenna design and instructions.  I think it's a nice, well thought out, multband design that should work great with little trouble.  He thought carefully about the feedline length, system loss, and lowering stress on the tuner for the different bands (40M-10M).  Not paying attention to these things can cause a lot of trouble to hams trying to use one antenna for multiple bands.

Alison gave a great link for a 4:1 current type balun (thanks Alison!)  I would rewind yours following those instructions.  Probably a 1:1 current balun would also work fine at your shack coax to balanced line transition and would be easier to wind.

Too true about teflon plumber tape -- very thin and weak stuff (even with multiple wraps), and the 3M fiberglass tape is great stuff (like all 3M tape!)

73 and let us know how your new setup works,


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