Shortening coax aerial cable?

[crewman]

I have replaced my Hawk winded and aerial and replaced the coax down the mast. The supplied cable is far too long, can I simply cut it to a sensible length? Previous excess was wound round the mast. Seem to remember that the cable is a specific length for impedence(?).

 

[lw395]

It's OK to cut it. So long as you do a good job of fitting the connectors.

 

[[70521]]

No problem correcting the length in fact it should give a tiny improvement.

 

[Skylark]

A purist would probably try to cut a tuned length. IIRC, either half or full wave length at the mid frequency of operation multiplied by 0.95 velocity factor for typical coax.

Can't recall the mid frequency of marine band, say 160 MHz. Velocity of propagation (3 x 10^^8) divided by 160 x 10^^6 equals 1.875 x 0.95 is 1.8m.

So cut it in multiples of 0,9m. Tune using SWR meter if you're a real anorak. Completely ignore this if you're not.

 

[crewman]

Thanks for all the replies. I will shorten the cable and fit the plug tomorrow. I am no anorak, so will cut to a convenient length.

 

[Chalker]

There is absolutely no point in cutting a coax cable, feeding an antenna, to any multiple of any fraction of a wavelength. The whole point of a coax cable feeding a matched load (antenna) is that length is irrelevant except for its affect on loss. The shorter the better for loss.
See Utube for how to solder a coax connector of the appropriate type. If it has a quick fit fitting that screws into the braid and has a grub screw for the inner, laugh and reject it! If it's a 'UHF' type, smile gently (instead of laughing) and give in and use it. Proper clamp and solder types are preferable. Crimp types also acceptable but you need to aquire a crimp tool which will cost more than a soldering iron. Solder quickly as the inner insulation melts very easily. Tiny iron bits are no good, they only heat the joint slowly. 25W OK or 50W if temperature controlled, are good.
Cutting a wire antenna to length is important. 1/4 1/2 5/8 etc of a wavelength are all used in antenna designs.

 

[john_morris_uk]

A purist would probably try to cut a tuned length. IIRC, either half or full wave length at the mid frequency of operation multiplied by 0.95 velocity factor for typical coax.

Can't recall the mid frequency of marine band, say 160 MHz. Velocity of propagation (3 x 10^^8) divided by 160 x 10^^6 equals 1.875 x 0.95 is 1.8m.

So cut it in multiples of 0,9m. Tune using SWR meter if you're a real anorak. Completely ignore this if you're not.

With respect: this is a fallacy that's got written into so many texts it's become 'accepted fact'.

See: https://k5acl.files.wordpress.com/2017/03/another-look-at-reflections.pdf

 

[Skylark]

I wondered how long it would take for me to be "Nostrodamused" (see his amusing post on the YM forum).

The referenced document says clearly that coax losses have 2 components, reflective wave and attenuation. It says that there is a case to aim for a resonant length for single frequency operation but not for an amateur set-up operating on several frequencies.

Given that it's an 80 page and 50 year old document, I have to admit that I became bored after page 5 so probably missed something verifiably factual within.

Thanks for correcting my prior understanding.

 

[Skylark]

With respect: this is a fallacy that's got written into so many texts it's become 'accepted fact'.

Indeed, this is taken from an RSGB publication