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Thread: Coax in-feed question

  1. #1

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    Default Coax in-feed question

    Howdy all, I got a problem. I want to put a mag mount antenna on my Subaru Outback (up on top between the luggage racks) but am having trouble finding one with a long enough cable to run out the back. I need 17' and all I see (worthwhile ones) are 10', 12', 5 m, etc. Now I can piece one together but that costs a fortune so I've been wondering, what about getting a shorter one and then extending the cable a little? So my question is: Is this is a viable option or will I lose too much signal in the process? Anybody ever done this and how did it work for you?

    Thanks for your help.

  2. #2

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    The question is why would you want to? It has been my experience that if you pinch the cable between the door and door jamb that the cable will fail.
    Same is true if you pinch it between the door and window.
    The right way of doing it would be to take your car to a Two Way Radio shop, pay them to punch a hole in the roof, put the antenna on the roof with a permanent mount.
    With all the antennas on these new vehicles today, XM Radio, On Star, GPS, Keyless Entry, what is one more antenna in the grand scheme of things?
    The only reason not to just extend it would be - because the length of the coax - when the antenna is not 50 ohms J-0, the coax would become inductive or reactive and a true SWR reading could not be obtained, unless the coax was some sort of 1/2 wave length multiple. 70 cm is not a harmonic of 2 meters... The manufacturers usually finds some compromise length that is acceptable to both and that is why the cable is that length to begin with.
    If the length doesn't suit you, the next logical step is to visit your local CB radio shop and pick up a piece of RG8 coax in the length you believe you need to reach from the radio to the antenna. Take apart the magnetic mount, unsolder the old coax, route the new coax where you need it to go. Cut it to length, solder it to the magnet mount, crimp the PL terminal on the other end. That should get you into your local repeaters, don't worry about the loss unless you are trying to access distant repeaters while mobile.
    If accessing distant repeaters becomes a problem, you could always carry a J-Pole style antenna with you that you could deploy roadside when you wanted to operate.

    I would go to a large field, far away from trees, buildings, anything metal - before trying to tune the antenna, else you might just end up with a stub - trying to cut it to resonance.
    I would even try adding antenna length while checking SWR before cutting your only antenna.

    PL connectors have an inherit impedance bump around 300 Mhz, adding more connectors just exacerbates the problem, and most PL connectors will work their way loose in time in a mobile set up, causing more problems then they solve.. Using one good cable is always better than using two marginal cables and a barrel connector.

  3. #3

    Join Date
    Mar 2018
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    Quote Originally Posted by Sixmeters View Post
    The question is why would you want to? It has been my experience that if you pinch the cable between the door and door jamb that the cable will fail.
    Same is true if you pinch it between the door and window.
    The right way of doing it would be to take your car to a Two Way Radio shop, pay them to punch a hole in the roof, put the antenna on the roof with a permanent mount.
    With all the antennas on these new vehicles today, XM Radio, On Star, GPS, Keyless Entry, what is one more antenna in the grand scheme of things?
    The only reason not to just extend it would be - because the length of the coax - when the antenna is not 50 ohms J-0, the coax would become inductive or reactive and a true SWR reading could not be obtained, unless the coax was some sort of 1/2 wave length multiple. 70 cm is not a harmonic of 2 meters... The manufacturers usually finds some compromise length that is acceptable to both and that is why the cable is that length to begin with.
    If the length doesn't suit you, the next logical step is to visit your local CB radio shop and pick up a piece of RG8 coax in the length you believe you need to reach from the radio to the antenna. Take apart the magnetic mount, unsolder the old coax, route the new coax where you need it to go. Cut it to length, solder it to the magnet mount, crimp the PL terminal on the other end. That should get you into your local repeaters, don't worry about the loss unless you are trying to access distant repeaters while mobile.
    If accessing distant repeaters becomes a problem, you could always carry a J-Pole style antenna with you that you could deploy roadside when you wanted to operate.

    I would go to a large field, far away from trees, buildings, anything metal - before trying to tune the antenna, else you might just end up with a stub - trying to cut it to resonance.
    I would even try adding antenna length while checking SWR before cutting your only antenna.

    PL connectors have an inherit impedance bump around 300 Mhz, adding more connectors just exacerbates the problem, and most PL connectors will work their way loose in time in a mobile set up, causing more problems then they solve.. Using one good cable is always better than using two marginal cables and a barrel connector.
    Thanks Sixmeters, you've pretty much covered all the things that I wanted to know about. I thought about pinching the cable but figured it would be running out the back door and I don't open it much. Still... Windows, bad idea I thought because of rain and stuff. And I thought about punching a hole in the roof but don't want to do that to the car. "because the length of the coax", connectors coming apart, and everything else is exactly waht I was afraid of so that's out. Not too worried about distant repeaters at the moment, just want a beter antenna than I currently have. It's an OLD L bracket mount between the hood and fender. Works lousy and kinda ugly. Well, looks like I'm gonna have to stick with the same mount and just get a new antenna. Thanks again.

  4. #4

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    Nov 2017
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    Quote Originally Posted by Sixmeters View Post
    The question is why would you want to? It has been my experience that if you pinch the cable between the door and door jamb that the cable will fail.
    Same is true if you pinch it between the door and window.
    The right way of doing it would be to take your car to a Two Way Radio shop, pay them to punch a hole in the roof, put the antenna on the roof with a permanent mount.
    With all the antennas on these new vehicles today, XM Radio, On Star, GPS, Keyless Entry, what is one more antenna in the grand scheme of things?
    The only reason not to just extend it would be - because the length of the coax - when the antenna is not 50 ohms J-0, the coax would become inductive or reactive and a true SWR reading could not be obtained, unless the coax was some sort of 1/2 wave length multiple. 70 cm is not a harmonic of 2 meters... The manufacturers usually finds some compromise length that is acceptable to both and that is why the cable is that length to begin with.
    If the length doesn't suit you, the next logical step is to visit your local CB radio shop and pick up a piece of RG8 coax in the length you believe you need to reach from the radio to the antenna. Take apart the magnetic mount, unsolder the old coax, route the new coax where you need it to go. Cut it to length, solder it to the magnet mount, crimp the PL terminal on the other end. That should get you into your local repeaters, don't worry about the loss unless you are trying to access distant repeaters while mobile.
    If accessing distant repeaters becomes a problem, you could always carry a J-Pole style antenna with you that you could deploy roadside when you wanted to operate.

    I would go to a large field, far away from trees, buildings, anything metal - before trying to tune the antenna, else you might just end up with a stub - trying to cut it to resonance.
    I would even try adding antenna length while checking SWR before cutting your only antenna.

    PL connectors have an inherit impedance bump around 300 Mhz, adding more connectors just exacerbates the problem, and most PL connectors will work their way loose in time in a mobile set up, causing more problems then they solve.. Using one good cable is always better than using two marginal cables and a barrel connector.
    Are you serious? You believe that about half wave lengths of coax and the rest? I'm shocked.

  5. #5

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    Quote Originally Posted by KI7UJM View Post
    Thanks Sixmeters, you've pretty much covered all the things that I wanted to know about. I thought about pinching the cable but figured it would be running out the back door and I don't open it much. Still... Windows, bad idea I thought because of rain and stuff. And I thought about punching a hole in the roof but don't want to do that to the car. "because the length of the coax", connectors coming apart, and everything else is exactly waht I was afraid of so that's out. Not too worried about distant repeaters at the moment, just want a better antenna than I currently have. It's an OLD L bracket mount between the hood and fender. Works lousy and kinda ugly. Well, looks like I'm gonna have to stick with the same mount and just get a new antenna. Thanks again.
    You just solved your own problem and didn't even know it...

    There is 100 reasons why you shouldn't mount the antenna next to the hood, at the edge of the fender.

    Most everything from the transmission shift to the fuel pump to the fuel injection uses some type of pulse width modulation....

    Not to mention the fact that you will pick up ignition / electrical motor noise and the fact that you can't bond the antenna to the body of the vehicle with a simple fender mount.

    Yes WZ7U - the laws of physic's hasn't changed in the past 100 years, nor do I think they are going to change anytime soon to accommodate someone that doesn't want to install an antenna properly..
    The reason why we use a Smith Chart is to be able to predict what happens when we change coax length. The only time coax length isn't important is when you have a dedicated antenna, installed properly, that is tuned to 1 frequency.

    Let's use a Ringo Ranger as an example. A ringo ranger is essentially a dummy load. It is a poor radiator with a gain factor of 1 - (Unity).
    The match on the Ringo Ranger is a tuning ring that is adjusted to minimum VSWR, along with the length of the radiator.
    The laws of physics tells us that if you have an antenna that is 50 ohms J -0, that all of the power applied, minus the loss factor of the coax will be absorbed by the load and radiated.
    When the VSWR is not 1:1 - then anything reflected is radiated back towards the Generator, applied to the next available radio wave + the voltage and it will travel up and down the coax until finally it is either radiated by the antenna or lost in the coax - heat.

    By putting the antenna in the middle of the roof, the OP is at least trying to provide a decent ground plane for the antenna.. The RF will capacitive couple with the body of the car, even with a magnet mount at that frequency.
    When you move the antenna down to the front of the car and next to the fender, the antenna can no longer be resonant because you take the ground plane away from the antenna..
    Think reflector behind your TV Sattelite dish.
    The physic's part tells us that the RF will follow the path in the direction with the most mass ( metal), not that there is much metal in a Scooby Doo, but anyways, his antenna would work best - but not very well in the opposite direction - towards the rear of the vehicle. This makes the antenna highly directional, turn the car in the other direction and the radio don't talk so good....
    That is why the op is having problems..

    It makes no difference how many times the door is opened or closed, eventually the coax will fail - just from it being pinched. Not to mention the fact that the purpose of the dielectric is to maintain a certain distance between the braid and the center conductor - which changes when you pinch it - creating a hot spot in the coax when you transmit, especially at any half wavelength point in the coax.

  6. #6

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    My guess is physics hasn't changed in a million plus years but that still doesn't make the whole half wave of coax argument any truer than it was yesterday; unless the outer shield of the coax is a radiator of RF. So, if your antenna systems are coax length dependent, I would be looking for why you have common mode current on the line. None of this even touches quarter wave tuning stubs either...

    Which has nothing to do with where on the car the antenna is mounted. Now, directivity of the signal due to ground plane abnormality - yes.

  7. #7

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    I have no idea where six metres gets some of his information.
    70 cm is not a harmonic of 2 meters
    It is - and is the third harmonic in the series. 145MHz being the fundamental, the 2nd harmonic is 290MHz and then the 3rd is 435MHz. A 2m wave matches really well as a wave at 70cm as the impedance is the same.

    We're talking about sticking a mag mount on a vehicle roof. All this messing around just makes me smile. Sure, the cable will eventually crush, but at the other end water will get in, and the sun, the heat, the cold can make the cable split where you don't see it and the water gets in. Mag mounts are consumables. When they start to mis-perform, you either slap on a new cable, or thrown them away and buy a new one. You don't mess around with adaptors. Buy one, slap it on. Forget cable RF radiation, forget the radiation pattern. Mobile is all about compromise and luck. If you put the mag mount in the middle of the roof, the extra cable drag at high speeds can pull the damn thing off. The distorted radiation pattern from off centre mounting means a non-symmetrical polar plot, that can help and hinder. Trunk (boot) mounting can also work pretty well in practice - it's everyday, practical antenna placement for many business radio systems. Look at a typical Police vehicle. They have numerous antennas on the roof nowadays, all fitted by the workshops with a hole, often no anti-rust treatment of any kind, and they last pretty well. They live with the variety of polar patterns. There is simply NO point trying to squeeze ultimate performance from a mobile install, because with the noise of the vehicle inside, working at the limit of reception is painful. Where you want good comms, you don't work at the fringes.

    All this stuff about reactive components is just sidetracking. A proper VHF/UHF mobile antenna does NOT need precise lengths of cable to ensure that the meter reads correctly. It really doesn't work that way at all. In commercial radio workshops all over the world, people cut a wave to a bit longer than they need, and then snip away with a VSWR meter - usually dead centre of the roof. At minimum VSWR, they put the meter away and the antenna is tuned. You then slap it on the vehicle in the best place you can manage for convenience. My van has a number of antennas on it, and only one is in the middle. When putting some new radios in, I lost the labelling. Six months later I discover the one I've used as the best central one was actually the one right at the back, and I had not noticed. Things that are important at HF really don't matter that much at VHF.

    A half wave dipole with NO balun works fine in practice, even though the feeder should be radiating (and possibly even is) - but in practice, a dipole like this can give a perfectly OK VSWR and perform with no issues. The only real issue with them, when used vertically is that the lower driven element interacts with the feeder cable screen, unless you mount it on a stand off horizontal tube greater than a wave. Less than that, the resonant frequency shifts away from where it should be by measurement. If you don't believe me, build one and try it.

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