Cutting Coax

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duncanville1
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Joined: Mon Aug 22, 2011 8:24 pm

Cutting Coax

Post by duncanville1 »

I have heard for years that NMO mounts are "tuned" with the correct amount of coax and to not trim much of it off. Now I have a remote mount 900mhz rig mounted less than three foot from the antenna. At this point I have about 15ft of extra coax wire tied out of the way. I would like to cut the excess away and install a new mini-U on the end. If I do that will I loose any tuning in the feed line? The wound up coax looks unprofessional and I am sure its a source of loss in the feed line.
Jim202
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Joined: Sun Sep 09, 2001 4:00 pm

Re: Cutting Coax

Post by Jim202 »

duncanville1 wrote:I have heard for years that NMO mounts are "tuned" with the correct amount of coax and to not trim much of it off. Now I have a remote mount 900mhz rig mounted less than three foot from the antenna. At this point I have about 15ft of extra coax wire tied out of the way. I would like to cut the excess away and install a new mini-U on the end. If I do that will I loose any tuning in the feed line? The wound up coax looks unprofessional and I am sure its a source of loss in the feed line.


Been in this field for over 45 years now and cut the cables to fit the mounting location all the time. Do not cut the cable too short so as not to be able to replace the connector if needed. leave some slack. Not sure where your hearing this old wife's tale, but I don't put much faith in it.

I would have some concern that the antenna is only 3 feet away. Is this on a trunk of a vehicle or what? I would also be concerned with people being in the near field of the antenna for RF radiation.

Jim
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FireCpt809
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Joined: Sat Jan 19, 2002 4:00 pm
What radios do you own?: Alot..

Re: Cutting Coax

Post by FireCpt809 »

I have always cut off extra coax as not to have excessive loops that can result in more loss. I do leave about a 1 foot at the end for service if the radio has to be taken out of the vehicle.
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Bill_G
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Re: Cutting Coax

Post by Bill_G »

NMO mobile coax kits are not tuned lengths per se. In fact, they are as long as they are to prevent them from becoming critical lengths at typical land mobile freqs simply by applying a connector. I'll say it another way - they are 15ft long so that they are not tuned lengths. They are 15ft long so you don't have to worry about the impedance of the transmission system.

I am horrible at explaining the math. Suffice it to say at 900mhz, the cable would have to be well under 3ft long before you would have to worry about quarter inch increments causing vswr problems. Cut it to length plus some service loop, and enjoy the radio.

That said, I have seen some very strange problems in other bands, especially at the lower freqs, caused by accidental lucky lengths cut attempting to neaten the install. I have also seen some perfect cable bundles turn into inductors that ate every bit of energy at the freq of interest. These things come into play more quickly when the antenna is mounted on a less than optimal ground plane, or has reflecting metallic surface adjacent to the whip. ie: a UHF gain antenna on a TMB through rusty screws, a VHF 5/8ths wave on a ladder rack, or maritime VHF half wave on fiberglass. Crazy things can happen.
duncanville1
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Re: Cutting Coax

Post by duncanville1 »

Thanks all, that was my feeling also, but wanted to get others feedback.

Jim, yes the antenna is mounted on the trunk and the radio (remote MSC 2000) is mounted under the package tray.
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