[TowerTalk] FW: RG-149: 50 ohm/70 ohm - does it matter?
maflukey at gmail.com
Sat Dec 14 02:00:55 EST 2013
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. A Smith Chart really helps for
planning in this case. Smith v3.1 is available in a free version...
Or if you are working out in the field try iSmith for the iphone.
BTW, trying to feed a non-resonant 50 ohm load with a random length of 75
ohm cable without transformation can produce a very poor match at the
Good luck with your cable!
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