[time-nuts] Why a 10MHz sinewave output
albertson.chris at gmail.com
Tue Feb 7 22:05:07 UTC 2012
On Tue, Feb 7, 2012 at 1:30 PM, Attila Kinali <attila at kinali.ch> wrote:
> On Tue, 07 Feb 2012 16:15:44 -0500
> Mike Naruta AA8K <aa8k at comcast.net> wrote:
>> On 02/07/2012 03:59 PM, Attila Kinali wrote:
>> > while TV and radio uses 75R. (there was once a reason
>> > for this, but i don't know it).
>> A 4:1 balun takes old 300 ohm twinlead to 75 ohms.
The required balun would need a 4:1 _impedence_ ratio. The square
root of four would be the required turns ratio. So it is a 2:1 balun
that you need. It's an easy integer number.
If you want you balun to have an integer turns ratio like 1:1, 2:1,
3:1 and so on then the impedence rations are going to be 1:1, 4:1 or
9:1. which means you'd need coax to be either 300, 75 or 33 ohm.
Those are the only choises you have if you start with 300 ohm and
restrict your balun to have a simple integer turns ratio.
A balun that matches 300 to 50 cable would have a 5:2 turns ratio but
5:2 is not exact. You can't to 300 --> 50 exactly. 49:20 is very
close but there is not exact turns ratio that works
> Thanks! This explains half it :-)
> Do you know why a 4:1 balun was used?
> And do you know why other RF stuff and lab equipment is 50R?
> Attila Kinali
> Why does it take years to find the answers to
> the questions one should have asked long ago?
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