[time-nuts] Why a 10MHz sinewave output
John Ackermann N8UR
jra at febo.com
Tue Feb 7 22:11:55 UTC 2012
On 2/7/2012 4:30 PM, Attila Kinali 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.
> 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?
In coax cables, it turns out that about 30 ohm impedance offers the
greatest power handling capacity for a given diameter, and 70 ohms is
nearly optimum for minimum loss. 50 ohms was a compromise for both
factors, and IIRC had some other desirable physical characteristics.
Twin-lead cable is in theory balanced (neither conductor grounded) while
coax is unbalanced (shield is grounded). The balun is a "balanced to
unbalanced" converter, and can also be designed to transform impedances,
so a 4:1 ratio matches 300 to 75 ohm line.
More information about the time-nuts