Coax length

[Tennmogger]

As BigJimCruising said, just use what you need. Tune the antenna for least reflected power at the radio end of the coax. The radio will be happy and output full rated power.

So, why all the recommendations for an exact length of coax? Because if you want to be able to actually know the forward/reflected power (and thusly SWR) AT THE BASE OF THE ANTENNA, then the coax length has to be an exact 1/4 or odd multiple of 1/4 wavelength in the coax at that frequency. Then the SWR will be the same at the radio end as it is at the base of the antenna.

That means you must know the velocity factor of the coax to compensate for the slower signal speed in the coax. Obviously that length is exact frequency related even across a band, and completely different for another band.

Just use random coax lengths that fit, and tune each antenna that you might put on a mount, for that mount.

Bob

 

[4x4mike]

Interesting.

Just use random coax lengths that fit, and tune each antenna that you might put on a mount, for that mount.

How does this work with non tunable antennas? Or is it more of a you get what you get for a given antenna?

 

[ScoutII]

Here's a good write up on coax basics.

http://www.signalengineering.com/ultimate/coax_basics.html

Its a good read and not very long.

Cut and paste from the question and answer section

Coax Length Issues Simplified

Question: What is the "correct" length of coax?
Answer: The shortest length that makes it from the radio to the antenna.

Question: Are there any exceptions to the above rule?
Answer: 75 Ohm harnesses for Co-phasing is the only exception.

Question: Why do most mobile antenna makers recommend 18 feet of coax?
Answer: You got me, they claim you should use 1/2 wavelength multiples of coax. 18 feet isn't even close to being a 1/2 wavelength in any 50 Ohm coax you will find. Check some commonly used coax using the above formulas. RG-58, the most commonly used mobile antenna coax length would have to be 12 feet to be a 1/2 wavelength. RG-8X would need to be 14 feet.

 

[Chief_919]

As others have said, the old 18ft thing is just CB myth. Use as little as you need, leaving enough extra in case you ever want to move the radio a little.

 

[Tennmogger]

CB is 27 MHz more or less. quarter wave is 246/27 = 9.1 ft, or 109 inches, about what we use for full quarter wave CB whip. But, in coax we must multiply by velocity factor which is 0.66 in RG-58. That makes quarterwave in coax of 6 ft. Need ODD multiple of quarter wavelength in coax to recreate same SWR as base of antenna (not halfwave). Three quarter-waves is 6 ft x 3 = 18 ft. Or you could use 6 ft (one quarterwave) or 30 ft (five-quarters), etc. 18 ft is rather practical length for vehicle installation. Of course I rounded off all those numbers. If you want to aim for a proper coaxial length, 18 ft isn't a bad start as used by lots of manufacturers who have no idea what channel to cut the coax for.

Ideally, one would then measure at what exact frequency the installed "18 ft" of coax was three quarterwaves long, and make antenna measurements at that frequency, at the radio.

How to tune a non-tunable antenna? You can't, it's non-tunable. Don't buy a non-tunable antenna. They build those for people who don't really care how their radio works

Bob WB4ETT

 

[paddlenbike]

Thread bump. Has anyone ordered coax from pasternack.com? It's the only place I can find custom length cable (I need about 19') with FME on one end and a through-hole NMO mount on the other.

http://www.pasternack.com/fme-jack-nmo-mount-connector-rg58cu-cable-assembly-pe37843-p.aspx

 

[4x4mike]

You don't need/want fme for this application. As it is you're drilling a huge hole in your roof so you don't need the small end liKe I used for my mag mount. I used the small end becuase I was feeding the coax through small holes not a 3/4 inch one. Just get a PL-259 end and call it good.

 

[paddlenbike]

You don't need/want fme for this application. As it is you're drilling a huge hole in your roof so you don't need the small end liKe I used for my mag mount. I used the small end becuase I was feeding the coax through small holes not a 3/4 inch one. Just get a PL-259 end and call it good.

You're right, I forgot I was running an FME to PL-259 adapter on the radio, DOH!
If anyone has cabling recommendations, I'm open to suggestions. I need roughly 19' of PL-259 to NMO for a 2M setup with a 1/4 wave antenna.

 

[4x4mike]

Just hit up ebay if you need a custom length. HRO, Universal, Cheap Ham, etc all have NMO's but if you need a custom length someone will have to make it if you don't want to make it yourself. They are somewhat simple with few options so its an easy purchase.