[time-nuts] Why a 10MHz sinewave output

Jim Lux jimlux at earthlink.net
Tue Feb 7 22:54:23 UTC 2012

On 2/7/12 1:30 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?

A dipole has a feed point impedance of about 65-75 ohms.  A folded 
dipole has an impedance of 4 times that, so 300 ohm twinlead was a nice 

75ohms is, in general, the lowest loss impedance for a given dielectric 
(based on the ratio of inner and outer diameters and skin depth, etc.)

   30 some odd ohms is the highest power handling impedance (lowest peak 

> And do you know why other RF stuff and lab equipment is 50R?

Compromise of one sort or another.

http://www.microwaves101.com has a nice discussion of this..

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