On 7/19/2010 10:17 PM, Don Tucker wrote:
> Here, here. Too much time splitting hairs when it comes to SWR. The ham on
> the other end probably doesn't even hear the diff between a low and
> relatively low or moderately high SWR.
>
> Don W7WLL
> ----- Original Message -----
> From: "Jim Brown"<jim@audiosystemsgroup.com>
> To:<Towertalk@contesting.com>
> Sent: Monday, July 19, 2010 6:14 PM
> Subject: Re: [TowerTalk] 75ohm cable
>
>
>
>
>> You can have up to a 2:1 mismatch (SWR) at the transmitter using 75 ohm
>> line
>> if the antenna is a flat 50 ohms. If your 75 ohm line happens to be a
>> multiple of a quarter wave length that will transform the 50 ohm load to
>> 100
>> ohms seen at the transmitter end.
>>
> So what? Any rig with an antenna tuner, including tube power amps, will
> drive
> that just fine. Loss in the line due to mismatch depends ONLY on the
> mismatch
> between the antenna and the line.
True, but the additional loss in the coax depends on the matched loss
and the additional loss (in the line) due to SWR. With a low loss line,
it's miniscule, but if using RG8 on 440 it could be substantial. I
used 3/4" 75 ohm Aluminum hard line for years feeding a staked pair of
14L 2-meter KLM's at 50' on a hill, 350 feet from the house. There was
very little loss in that system. I am not a fan of 1/2" Aluminum hard
line as I found it to be fragile. Of course it didn't hurt that I had
nearly a full reel of 3/4" in the garage. <:-))
73
Roger (K8RI)
> The worst that can get with a 2:1 mismatch
> is a few tenths of a dB.
>
> There is NO loss created by a mismatch between the TX and the line -- all we
> need to do is make the output stage happy, and an antenna tuner does that
> just fine. Remember that ALL of the matching methods for use at the antenna
> being suggested are very frequency sensitive -- they will work only on one
> band, and not necessarily over a relatively wide band. It is FAR more
> flexible to use a tuner in the shack than to worry about the small mismatch
> at the antenna.
>
> FAR too much time is spent in the wringing of hands over a relatively small
> mismatch. If the output stage is happy, be happy.
>
> 73, Jim Brown K9YC
>
>
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