Here are some simple methods I have used for transforming impedance to match
75 ohm cable with 50 ohms:
Method #1: alternating sections of 75 ohm and 50 ohm cable - each cut to
0.082 wavelength. Example 50 ohm antenna (or down-lead) > .082 L 75 ohm
coax > .082 L 50 ohm coax > 75 ohm coax feed-line (any length). Simply
reverse the order for the opposite transformation.
Method #2: 50 ohm antenna (or down-lead) > 0.141 wavelength of 50 ohm coax
> 0.163 wavelength 75 coax shorted stub > 75 ohm coax feed-line (any
length) from the stub "T" connection. This method is useful for
grounding a down-lead at the tower base by connecting the shorted end of the
stub to the tower ground.
Method #3: same as method #2 except substitute 0.188 wavelength 50 coax for
the shorting stub.
All of the above methods are low enough Q to have flat reflection across
any single ham band. With proper selection can achieve several of the
upper bands with small increase in SWR.
Matt
KM5VI
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|