I learned early, length needed plus a foot or so. Cable just "that much" too short is a pain and costly.
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Coaxial Cable Length
- Thread starter Cackalacky Kid
- Start date
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.
The best length calculator to determine coax length is a simple tape measure. Measure the length it takes to get from the radio to the antenna. That's 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 lengthOkay 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
So sad. I have had some good mobiles. If I have to use heliax the fun is done. Will give everybody a look at what a Earthdreams Honda 1.5 T looks like on the analyzer.
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"
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.
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.
For single band use a half wave or multiples of is not a bad idea if you wish to connect an analyzer etc.
That's useless for a multi-band situation.
He'd be best off using the shortest cable to read from point A to point B.
Those are like unicorns in a Mobile Installation, although it can be faked with a tuner.