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Re: CB power
I see that you are stating cut to length if not properly grounded.
tw twaldron wrote: > If the impedances are matched on the antenna, coax and connectors, the > length of the coax is insignificant. Cutting antenna coax to a specific > length to match wavelength in an otherwise matched system is an old > wives tale. Really old. > > tw > > > L. Ron Waddle wrote: > SNIP > >> The biggest cause of high SWR's is the fact that typically other parts >> of the transmission chain are radiating or are of a length that is >> chopping up your waves as they head for the tip of the antenna. Thus >> if your coax is not shielded with the shielding grounded properly at >> both ends, your coax length should be a half-wave or quarter-wave in >> length. For 11 meter band (CB radio), that's 8.25 feet if you want a >> quarter wave coax. sNIP |
Re: CB power
Mike Romain wrote:
> L. Ron Waddle wrote: > If the shielding on your coax is not grounded to a hard >> ground at both ends, your coax becomes an antenna, > > If this is the case, then why are antenna's electrically isolated from > the mounting brackets with a nylon bushing or rubber washer? Mag mounts Please examine a typical metal-bumper mounting bracket for a CB antenna, such as by Firestick. It consists of three things: 1. A metal screw-on coax connector, 2. A metal nut that screws onto the bottom of the coax connector, 3. A nylon bushing that is placed on *top* of the coax connector to isolate the antenna portion (the socket sticking up above the metal body of the bracket) from the metal bracket itself. The coax screws to the bottom. The metal shielding of the coax is attached to the metal screw-on portion, which in turn grounds via the metal nut to the bracket which in turn attaches to the bumper which (on a Jeep TJ) is a solid body ground. The center portion of the coax attaches to the socket that sticks up above the bracket, the portion of this apparatus that is electrically isolated from the bracket by that nylon bushing. I suggest that you go to the Firestick site and read their installation directions. They should help you figure this out. > even have the magnet slightly lower than the metal cup so it can't > ground out and it's covered with mylar. It's not mylar. it is a conductive plastic. Please read the description on the Firestick site of how their magnetic mounts work. > You have my curiosity up with your 'active shield' vs passive (one end > grounded) shield and mount setup I see. You don't need to look at me. Go to the Firestick site and read their installation directions and look at their photos of their CB radio mounts. Proper grounding of the coax shield at both ends is critical to reduce emitted noise in CB installations. A one-end-grounded shield can have a current induced into it by the signal passing through the center conductor, thus turning it into an antenna. A shield grounded on both ends can never have a current induced into it, since both ends are hard ground. Firestick sells a specially tuned antenna-coax apparatus which does not require two-end grounding, but this is because the coax is set up at the exact length needed to avoid adding distortion into the induced signal. -Elron |
Re: CB power
Mike Romain wrote:
> L. Ron Waddle wrote: > If the shielding on your coax is not grounded to a hard >> ground at both ends, your coax becomes an antenna, > > If this is the case, then why are antenna's electrically isolated from > the mounting brackets with a nylon bushing or rubber washer? Mag mounts Please examine a typical metal-bumper mounting bracket for a CB antenna, such as by Firestick. It consists of three things: 1. A metal screw-on coax connector, 2. A metal nut that screws onto the bottom of the coax connector, 3. A nylon bushing that is placed on *top* of the coax connector to isolate the antenna portion (the socket sticking up above the metal body of the bracket) from the metal bracket itself. The coax screws to the bottom. The metal shielding of the coax is attached to the metal screw-on portion, which in turn grounds via the metal nut to the bracket which in turn attaches to the bumper which (on a Jeep TJ) is a solid body ground. The center portion of the coax attaches to the socket that sticks up above the bracket, the portion of this apparatus that is electrically isolated from the bracket by that nylon bushing. I suggest that you go to the Firestick site and read their installation directions. They should help you figure this out. > even have the magnet slightly lower than the metal cup so it can't > ground out and it's covered with mylar. It's not mylar. it is a conductive plastic. Please read the description on the Firestick site of how their magnetic mounts work. > You have my curiosity up with your 'active shield' vs passive (one end > grounded) shield and mount setup I see. You don't need to look at me. Go to the Firestick site and read their installation directions and look at their photos of their CB radio mounts. Proper grounding of the coax shield at both ends is critical to reduce emitted noise in CB installations. A one-end-grounded shield can have a current induced into it by the signal passing through the center conductor, thus turning it into an antenna. A shield grounded on both ends can never have a current induced into it, since both ends are hard ground. Firestick sells a specially tuned antenna-coax apparatus which does not require two-end grounding, but this is because the coax is set up at the exact length needed to avoid adding distortion into the induced signal. -Elron |
Re: CB power
Mike Romain wrote:
> L. Ron Waddle wrote: > If the shielding on your coax is not grounded to a hard >> ground at both ends, your coax becomes an antenna, > > If this is the case, then why are antenna's electrically isolated from > the mounting brackets with a nylon bushing or rubber washer? Mag mounts Please examine a typical metal-bumper mounting bracket for a CB antenna, such as by Firestick. It consists of three things: 1. A metal screw-on coax connector, 2. A metal nut that screws onto the bottom of the coax connector, 3. A nylon bushing that is placed on *top* of the coax connector to isolate the antenna portion (the socket sticking up above the metal body of the bracket) from the metal bracket itself. The coax screws to the bottom. The metal shielding of the coax is attached to the metal screw-on portion, which in turn grounds via the metal nut to the bracket which in turn attaches to the bumper which (on a Jeep TJ) is a solid body ground. The center portion of the coax attaches to the socket that sticks up above the bracket, the portion of this apparatus that is electrically isolated from the bracket by that nylon bushing. I suggest that you go to the Firestick site and read their installation directions. They should help you figure this out. > even have the magnet slightly lower than the metal cup so it can't > ground out and it's covered with mylar. It's not mylar. it is a conductive plastic. Please read the description on the Firestick site of how their magnetic mounts work. > You have my curiosity up with your 'active shield' vs passive (one end > grounded) shield and mount setup I see. You don't need to look at me. Go to the Firestick site and read their installation directions and look at their photos of their CB radio mounts. Proper grounding of the coax shield at both ends is critical to reduce emitted noise in CB installations. A one-end-grounded shield can have a current induced into it by the signal passing through the center conductor, thus turning it into an antenna. A shield grounded on both ends can never have a current induced into it, since both ends are hard ground. Firestick sells a specially tuned antenna-coax apparatus which does not require two-end grounding, but this is because the coax is set up at the exact length needed to avoid adding distortion into the induced signal. -Elron |
Re: CB power
Mike Romain wrote:
> L. Ron Waddle wrote: > If the shielding on your coax is not grounded to a hard >> ground at both ends, your coax becomes an antenna, > > If this is the case, then why are antenna's electrically isolated from > the mounting brackets with a nylon bushing or rubber washer? Mag mounts Please examine a typical metal-bumper mounting bracket for a CB antenna, such as by Firestick. It consists of three things: 1. A metal screw-on coax connector, 2. A metal nut that screws onto the bottom of the coax connector, 3. A nylon bushing that is placed on *top* of the coax connector to isolate the antenna portion (the socket sticking up above the metal body of the bracket) from the metal bracket itself. The coax screws to the bottom. The metal shielding of the coax is attached to the metal screw-on portion, which in turn grounds via the metal nut to the bracket which in turn attaches to the bumper which (on a Jeep TJ) is a solid body ground. The center portion of the coax attaches to the socket that sticks up above the bracket, the portion of this apparatus that is electrically isolated from the bracket by that nylon bushing. I suggest that you go to the Firestick site and read their installation directions. They should help you figure this out. > even have the magnet slightly lower than the metal cup so it can't > ground out and it's covered with mylar. It's not mylar. it is a conductive plastic. Please read the description on the Firestick site of how their magnetic mounts work. > You have my curiosity up with your 'active shield' vs passive (one end > grounded) shield and mount setup I see. You don't need to look at me. Go to the Firestick site and read their installation directions and look at their photos of their CB radio mounts. Proper grounding of the coax shield at both ends is critical to reduce emitted noise in CB installations. A one-end-grounded shield can have a current induced into it by the signal passing through the center conductor, thus turning it into an antenna. A shield grounded on both ends can never have a current induced into it, since both ends are hard ground. Firestick sells a specially tuned antenna-coax apparatus which does not require two-end grounding, but this is because the coax is set up at the exact length needed to avoid adding distortion into the induced signal. -Elron |
Re: CB power
Hi Mike
As I said before a mag mount is capacitive coupled to ground A plate (on the base of the mag mount) separated from the vehicle metal acts like (actually is) a capacitor (You know what makes a cap, 2 plates or more seperated by a dielectric) In DC it's isolated, but RF = AC, AC flows through a cap. I don't pretend to be an expert in antenna theory but You both (L Ron Waddle) have certain assumptions wrong. Once again: This site is very accurate in describing the theory of 11 meter (CB) antennas and such. I suggest everyone read it and try to understand it all. The author really knows what he's talking about. It is not BS. http://www.signalengineering.com/ultimate/index.html Cheers Mike Romain wrote: > L. Ron Waddle wrote: > If the shielding on your coax is not grounded to a hard > >> ground at both ends, your coax becomes an antenna, > > > If this is the case, then why are antenna's electrically isolated from > the mounting brackets with a nylon bushing or rubber washer? Mag mounts > even have the magnet slightly lower than the metal cup so it can't > ground out and it's covered with mylar. > > You have my curiosity up with your 'active shield' vs passive (one end > grounded) shield and mount setup I see. > > Mike > 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 > 88 Cherokee 235 BFG AT's - Gone to the rust pile... > Canadian Off Road Trips Photos: Non members can still view! > Jan/06 http://www.imagestation.com/album/pi...?id=2115147590 > (More Off Road album links at bottom of the view page) |
Re: CB power
Hi Mike
As I said before a mag mount is capacitive coupled to ground A plate (on the base of the mag mount) separated from the vehicle metal acts like (actually is) a capacitor (You know what makes a cap, 2 plates or more seperated by a dielectric) In DC it's isolated, but RF = AC, AC flows through a cap. I don't pretend to be an expert in antenna theory but You both (L Ron Waddle) have certain assumptions wrong. Once again: This site is very accurate in describing the theory of 11 meter (CB) antennas and such. I suggest everyone read it and try to understand it all. The author really knows what he's talking about. It is not BS. http://www.signalengineering.com/ultimate/index.html Cheers Mike Romain wrote: > L. Ron Waddle wrote: > If the shielding on your coax is not grounded to a hard > >> ground at both ends, your coax becomes an antenna, > > > If this is the case, then why are antenna's electrically isolated from > the mounting brackets with a nylon bushing or rubber washer? Mag mounts > even have the magnet slightly lower than the metal cup so it can't > ground out and it's covered with mylar. > > You have my curiosity up with your 'active shield' vs passive (one end > grounded) shield and mount setup I see. > > Mike > 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 > 88 Cherokee 235 BFG AT's - Gone to the rust pile... > Canadian Off Road Trips Photos: Non members can still view! > Jan/06 http://www.imagestation.com/album/pi...?id=2115147590 > (More Off Road album links at bottom of the view page) |
Re: CB power
Hi Mike
As I said before a mag mount is capacitive coupled to ground A plate (on the base of the mag mount) separated from the vehicle metal acts like (actually is) a capacitor (You know what makes a cap, 2 plates or more seperated by a dielectric) In DC it's isolated, but RF = AC, AC flows through a cap. I don't pretend to be an expert in antenna theory but You both (L Ron Waddle) have certain assumptions wrong. Once again: This site is very accurate in describing the theory of 11 meter (CB) antennas and such. I suggest everyone read it and try to understand it all. The author really knows what he's talking about. It is not BS. http://www.signalengineering.com/ultimate/index.html Cheers Mike Romain wrote: > L. Ron Waddle wrote: > If the shielding on your coax is not grounded to a hard > >> ground at both ends, your coax becomes an antenna, > > > If this is the case, then why are antenna's electrically isolated from > the mounting brackets with a nylon bushing or rubber washer? Mag mounts > even have the magnet slightly lower than the metal cup so it can't > ground out and it's covered with mylar. > > You have my curiosity up with your 'active shield' vs passive (one end > grounded) shield and mount setup I see. > > Mike > 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 > 88 Cherokee 235 BFG AT's - Gone to the rust pile... > Canadian Off Road Trips Photos: Non members can still view! > Jan/06 http://www.imagestation.com/album/pi...?id=2115147590 > (More Off Road album links at bottom of the view page) |
Re: CB power
Hi Mike
As I said before a mag mount is capacitive coupled to ground A plate (on the base of the mag mount) separated from the vehicle metal acts like (actually is) a capacitor (You know what makes a cap, 2 plates or more seperated by a dielectric) In DC it's isolated, but RF = AC, AC flows through a cap. I don't pretend to be an expert in antenna theory but You both (L Ron Waddle) have certain assumptions wrong. Once again: This site is very accurate in describing the theory of 11 meter (CB) antennas and such. I suggest everyone read it and try to understand it all. The author really knows what he's talking about. It is not BS. http://www.signalengineering.com/ultimate/index.html Cheers Mike Romain wrote: > L. Ron Waddle wrote: > If the shielding on your coax is not grounded to a hard > >> ground at both ends, your coax becomes an antenna, > > > If this is the case, then why are antenna's electrically isolated from > the mounting brackets with a nylon bushing or rubber washer? Mag mounts > even have the magnet slightly lower than the metal cup so it can't > ground out and it's covered with mylar. > > You have my curiosity up with your 'active shield' vs passive (one end > grounded) shield and mount setup I see. > > Mike > 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 > 88 Cherokee 235 BFG AT's - Gone to the rust pile... > Canadian Off Road Trips Photos: Non members can still view! > Jan/06 http://www.imagestation.com/album/pi...?id=2115147590 > (More Off Road album links at bottom of the view page) |
Re: CB power
> If the coax is not properly grounded at both ends (again, I am talking
> about the SHIELDING part of the coax, not the CENTER part -- a coax has > *TWO* things in it that carry electricity, doh!), your likely result is to > be high SWR's. And the ground on both ends has to be to a ground that's at > the exact same potential on both ends and said ground has to be the same > ground as the transmitter because the transmitter is bouncing its waves > courtesy of said ground. The coax cable, has a 50ohms impedance, IF the RF circuit closes within the coax. That is the RF must travel through the center electrode, through the antenna and then return through the shield of the cable. In this case the RF currents in the core of the coax cable and in the shield have opposite directions, and they cancel each other's electromagnetic field, so that the cable doesn't radiate. That's why a coaxial cable is used, so that the electromagnetic field is contained within the shield of the cable (between the core and the shield). If you create an alternative ground path, by hard grounding both ends of the coax on the chassis, part of the RF will leak and will not return through the coax shield, but through the car's body shell. This WILL change the coax cable's impedance increasing the SWR, and WILL make it radiate part of the signal causing interference to other cables parallel to it's route. You could tune your RF amplifier output or your cable length to match the impedances to reduce SWR, but you won't be able to reduce interference to the car's circuits. For a proper installation, you have to ensure that the RF circuit ground path closes through the coax cable shield only. A good way to achieve this, is by not grounding the CB end of the coax cable, i.e. don't ground the chassis of the transmitter unit directly on the car body. The GND power supply wire of the CB must be used, which might have an RF choke inline (otherwise one must be added) that prevents the RF circuit to close through it. And a relatively low frequency 27MHz RF is much more difficult to choke than a higher 100MHz or 1 GHz RF signal. A choke must be added inline to the positive power supply wire too, the RF can travel from the car's chassis through the battery to the positive terminal easily. If the antenna end of the coax needs to be grounded on the car's body depends on the antenna used. But if you have grounded the CB unit properly ONLY from it's GND power supply wire, and have installed RF Chokes in both +12v and GND wires, then you will not have problems for either case. Regards, Bill Spiliotopoulos. '96 XJ, '06 TJ. |
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