On Tue, 25 Aug 2009 15:35:48 -0400, Dan Zimmerman N3OX wrote:
>>
>> On Tue, 25 Aug 2009 14:59:27 -0400, Tom Haavisto wrote:
>>
>> >http://www.k1ttt.net/technote/matching.html
>>
>> >Shows how to match 50 to 75 ohm cable
>>
>> Not only is this complicated and frequency sensitive (it only covers
>> a single band), it is un-necessary.
>With a 50 ohm load, the mismatch on a 75 ohm line can be up to 2.25:1,
By what math? A 50 ohm (purely resistive) antenna is a 1.5:1 mismatch to
75 ohm coax (and an antenna that is 50 ohms but not resistive would be a
mismatch to 50 ohm line). The SWR falls along the length of any real
transmission line due to line loss, so an SWR bridge (and transmitter
output) would see a somewhat lower VSWR. For example, 150 ft of Belden foam
RG11 (8213) would look like 1.4:1 to the transmitter with a 50 ohm
resistive load.
>and that is sufficient to cause my barefoot, auto-tuner-less radio to fold
>back.
Perhaps at 2.25:1, but hardly at 1.4:1, which is typical of a 1.5:1 at the
antenna.
>The problem will get worse if the load is 35 ohms or 40 ohms, where
>you'd just casually use 50 ohm cable, but end up with pretty bad SWR at
>the radio with 75 ohm cable.
Agreed. But that's a different problem. Like you, I'd use 50 ohm cable on
that antenna.
A study of impedance curves for dipoles of various heights (in the ON4UN
book and ARRL Antenna Book) has convinced me that 75 ohm coax is the ideal
cable for high dipoles and 50 ohm cable is the best choice for low dipoles.
Through my own studies with NEC, I've learned that 50 ohm cable is the
ideal cable for a 20/15/10 fan dipole, but that a high 80/40 fan still
wants 75 ohm coax. BUT -- given a choice between much lower loss and no
50/75 ohm mismatch, I'll take the much low loss 99 times out of 100.
Another point -- I wouldn't think of setting up a station without an
antenna tuner, if for no other reason than making the rig happy over the
entire width of 160M, 80/75M, and 10M. And you WILL need it to use antennas
on more than one band.
73,
Jim K9YC
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