In message , alchazz
writes
On Mon, 18 May 2009 16:01:04 +0100, Ian Jackson wrote:

In message , Bill M
writes
alchazz wrote:
I am looking for a 6 to 1 balun, 300 ohms balanced to 50 ohms coax. I
don't need one that handles transmitter power as I am only using it for
a receiving antenna.
I have on hand several hundred feet of 50 ohm coax, RG58 modified
(IEEE 802.3). I want to use it to connect my 300 ohm TV dipole antenna
in my attic to my HDTV in the basement. And, of course, I would need a
50 ohm to 75 ohm match at the HDTV.
We only watch over-the-air TV. Why? Because I live within 2 miles of
three 1000 ft TV towers. In fact, the dipole in the basement works
quite well. But I do get dropouts during storms; go figure. I get 17
channels for free, so why go cable or FIOS. Nothing worthwhile there

So can someone point me to a good site for balun design. I've done
the Google searches, but all that I find seem to be for high power
applications.
Any help would be appreciated.
Al


Have you tried the 50 ohm cable directly (with a 4:1 xfmr at the dipole)
to see how it works? The 300 ohms for the dipole, etc only apply at one
single frequency and with signals that strong there really shouldn't be
an issue with additional loss. Reflections will not show up visibly with
digital tv.

Part of the issue here is winding the balun and set transformer at UHF
frequencies. Not impossible to do with inexpensive parts (multicore
ferrites) but I wonder if its really necessary?

GL,

Yes, just go ahead and use a normal 4:1 off-the-shelf TV/FM balun. They
are only a couple of dollars. The effects of the incorrect impedance
transformation will be negligible, and there's little to be gained by
trying for 'perfection'. If it doesn't work, you've lost virtually
nothing. [Note that, if it doesn't work, it won't be because the ratio
is wrong, or that the coax isn't 75 ohms.

Yes, I thought I might try that. You guys are right. It should not be an
issue at these signal levels. I have a bunch of the 4:1 baluns that I
have gotten from our town's recycling station.

But it is still odd that I get dropouts when I am so close to the towers.

That might well be due to multiple off-air reflections. Digital TV isn't
totally immune to the problems these cause (but, of course, you don't
see the reflections like you do on analog TV pictures).

Even if you live in an area of high signal strength, it's often
necessary first to get a clean signal from the antenna, which may need
to be a 'high-gain' design in order to reduce reflections. If you've got
too much signal at the TV set, you can always fit in-line attenuators at
the TV input (fixed barrels or variable 'twiddlers').
--
Ian