On 2/7/12 1:30 PM, Attila Kinali wrote:
On Tue, 07 Feb 2012 16:15:44 -0500
Mike Naruta AA8K<[email protected]> 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
match.
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
E-field)
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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