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Coaxial Cable Length

The w5dxp method works well. The only thing I don't like about it is your stuck using his measurements. That's ok If your tower and shack are in just the right place. A balanced L is so cheap and easy to make.
 
Okay I'll mention the one exception . When using coax for an impedance transformer the length is measured and specific to to frequency. When using coax for phasing elements , length is critical.

As CK stated...
When Abraham Lincoln was asked, "How long do you think a man's legs should be?" he replied, "Long enough to reach the ground."

Peculiar anecdote, I ran the coax down one side of the vehicle and could not get the computer out of the radio or the radio out of the computer. Receive noise was near 10 over and 250W would either shut the engine off or put it in limp mode until restarted.
Ran it down the other side and used 75 ohm , dual shield , high end RG-11 as a last resort. Last report is a real kilo dead hammer and no issues.

Last exception (for today anyway) and has nothing to do with mobile ops and not coax,

http://www.w5dxp.com/notuner/notuner.htm

~again , just some useless information unless you need it :)
99.99% of the time someone asks about proper length they are NOT talking about impedance matching or phasing so my general statement holds. In your mobile case it had to do with poor shielding and signal ingress/egress and nothing to do with length
 
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Thanks everyone for the input. The cable length issue is solved. Now I am moving on to a better antenna. I will start a new thread for that.
 
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Let me start with a greetings to all. I am new to HAM and this forum. I am in the process of setting up a RS and an antenna system. Back in the day I was heavily into CB radio. To receive/transmit a good signal there was an optimum length of cable that connected the antenna to the radio. I remember setting up my CB system in my Jeep and I had coax routed all over the floor board under the carpet. Is there an optimal length of coax to run from the antenna to the transceiver? I am running LMR400 coax from a Yaesu FT-450D to a Tram 1481 vertical antenna. Thanks.

a resonant half wave or multiple of at the operating frequency, or center frequency of the band your operating on. this will represent the antennas actual feedpoint resistance at the station end of the transmission line. ie: it will not act as a transformer, but if thats what you need its a quarter wave or multiple of. "coax length does matter"
 
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I'm not really sure of your point.
Are you saying that using a 1/4 wl of the same coax will "transform" (change) the actual feed line impedance?

Actually ANY length of coax will act as an impedance transformer. It just happens that a half wave repeats the impedance and a quarter wave inverts it. The actual feedline impedance does not change but the resulting impedance at the transmitter end of the antenna SYSTEM does.
 
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Actually ANY length of coax will act as an impedance transformer. It just happens that a half wave repeats the impedance and a quarter wave inverts it. The actual feedline impedance does not change but the resulting impedance at the transmitter end of the antenna SYSTEM does.

Thats where I was going to next.... It just changes the current/voltage loop (node) (use whatever term suits you) 90 degrees, and the transmitter sees that phase shift.

In a purely resistive load (yeah, sure...) this is a moot point.
 
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