Help tuning CB radio antenna

[jazngab]

So I have a Uniden 520xl cb, 2' firestick, and teraflex antenna mount on my driver side tail light. I've dremeled the underside of the mount to bare metal where the coax connector meets it, I've also dremeled the powder coat off the back of the mount where the 3 bolts holes are as well as the paint of the 3 bolt holes on the jeep. I'm trying to tune the antenna using a swr meter but I keep getting readings over 3. I've ran out of ideas, searched google, so I'm asking for help trouble shooting it. Any ideas?

 

[Mr. Bills]

First, confirm that the antenna mount is installed correctly, i.e., with the Delrin insulator in the proper location.

Next, read about the common causes of high SWR readings found here: http://www.firestik.com/Tech_Docs/swr-causes.htm

Then follow the SWR troubleshooting procedures found on these pages:

http://www.firestik.com/Tech_Docs.htm
https://www.wearecb.com/troubleshoot-a-high-swr.html']https://www.wearecb.com/troubleshoot-a-high-swr.html
https://www.rightchannelradios.com/blogs/learning-center']https://www.rightchannelradios.com/blogs/learning-center

 

[jazngab]

I’ve run through all the troubleshooting. Is my antenna stud mount set up correctly?

 

[Mr. Bills]

Your stud mount appears to be assembled properly.

I note that you did not use the upper hole in the Teraflex bracket that requires drilling a hole in the tub. This may be contributing to your issues. I used the hole that you left unused, drilled through the body, then sanded a small ring around the hole on both the inside and outside of the tub and on both the inside and outside of the bracket to ensure a good metal-to-metal bond between bracket and tub.

If you are relying entirely upon the fasteners for the taillight housing not only is your bracket substantially weaker than if you used the upper hole, the electrical bond between bracket and body will be marginal at best.

Don't fear the drill. That hole is there for a reason.

 

[jazngab]

Your stud mount appears to be assembled properly.

I note that you did not use the upper hole in the Teraflex bracket that requires drilling a hole in the tub. This may be contributing to your issues. I used the hole that you left unused, drilled through the body, then sanded a small ring around the hole on both the inside and outside of the tub and on both the inside and outside of the bracket to ensure a good metal-to-metal bond between bracket and tub.

If you are relying entirely upon the fasteners for the taillight housing not only is your bracket substantially weaker than if you used the upper hole, the electrical bond between bracket and body will be marginal at best.

Don't fear the drill. That hole is there for a reason.

View attachment 84624

Yea I know. It was more out of laziness at first. I just don’t think that alone would cause a 3+ swr reading. To note, my power source for the time being is the cb radio connected to the car adapter power. I know it’s much better to connect direct to battery but it’s for the time being as I only use it for off road use. Would that contribute to it at all? I originally also had my excess coax coiled up but I undid that but didn’t change much.

 

[jazngab]

@Mr. Bills the nylon plastic fastener washer thing should only be on top correct? There shouldn’t be one on the bottom too right?

 

[Mr. Bills]

1. Poor grounding is the #1 cause of high SWR readings. The electrical bond between the antenna bracket and body is critical, as is the bond between the antenna bracket and antenna mount. Although not absolutely essential, I would also recommend that you use a piece of 1/2"-1" braided copper strap between body and frame and another from exhaust pipe to frame.

This article on bonding is quite helpful: http://www.k0bg.com/bonding.html

You can buy copper strap here: https://www.amazon.com/dp/B003HGHQYM/?tag=wranglerorg-20
and here: https://www.amazon.com/dp/B01BIBQ940/?tag=wranglerorg-20 .
Just cut it to length and crimp ring terminals on each end.

2. Your power source won't affect SWR.

3. Loosely zip tie excess coax in a figure 8 or skein, never a coil.

4. Your stud mount is assembled correctly. The nylon insulator goes on top of the mounting surface for a Firestik K4A stud mount that uses a SO-239 connector, as illustrated on the left in Post #2.

Don't be confused by the two Delrin isolators I use for my Breedlove stud mount which employs ring terminals rather than a SO-239 connector and is akin to the center illustration for the Firestik K4.

 

[MaloStapalo]

1. Poor grounding is the #1 cause of high SWR readings. The electrical bond between the antenna bracket and body is critical, as is the bond between the antenna bracket and antenna mount. Although not absolutely essential, I would also recommend that you use a piece of 1/2"-1" braided copper strap between body and frame and another from exhaust pipe to frame.

This article on bonding is quite helpful: http://www.k0bg.com/bonding.html

You can buy copper strap here: https://www.amazon.com/dp/B003HGHQYM/?tag=wranglerorg-20
and here: https://www.amazon.com/dp/B01BIBQ940/?tag=wranglerorg-20 .
Just cut it to length and crimp ring terminals on each end.

2. Your power source won't affect SWR.

3. Loosely zip tie excess coax in a figure 8 or skein, never a coil.

4. Your stud mount is assembled correctly. The nylon insulator goes on top of the mounting surface for a Firestik K4A stud mount that uses a SO-239 connector, as illustrated on the left in Post #2.

Don't be confused by the two Delrin isolators I use for my Breedlove stud mount which employs ring terminals rather than a SO-239 connector and is akin to the center illustration for the Firestik K4.

Thanks for that! Why would you want to go from exhaust pipe to frame for a ground?

 

[jazngab]

1. Poor grounding is the #1 cause of high SWR readings. The electrical bond between the antenna bracket and body is critical, as is the bond between the antenna bracket and antenna mount. Although not absolutely essential, I would also recommend that you use a piece of 1/2"-1" braided copper strap between body and frame and another from exhaust pipe to frame.

This article on bonding is quite helpful: http://www.k0bg.com/bonding.html

You can buy copper strap here: https://www.amazon.com/dp/B003HGHQYM/?tag=wranglerorg-20
and here: https://www.amazon.com/dp/B01BIBQ940/?tag=wranglerorg-20 .
Just cut it to length and crimp ring terminals on each end.

2. Your power source won't affect SWR.

3. Loosely zip tie excess coax in a figure 8 or skein, never a coil.

4. Your stud mount is assembled correctly. The nylon insulator goes on top of the mounting surface for a Firestik K4A stud mount that uses a SO-239 connector, as illustrated on the left in Post #2.

Don't be confused by the two Delrin isolators I use for my Breedlove stud mount which employs ring terminals rather than a SO-239 connector and is akin to the center illustration for the Firestik K4.

Thank you. That’s pretty helpful. I was convinced it had to be a grounding issue. So I see you have the same antenna mount. How much of the powercoating on the mount and tj body paint did you expose bare metal? I sanded about a 1 cm circumference around each bolt hole as well as in the hole for both the mount and on the tj. Do I need more metal to metal contact? I don’t want to go nuts sanding paint off the tj exposing it to potential rust spots.

 

[JMT]

@jazngab.

Readings over 3 for which channel, 40 or 1? Or both?

Where are you located when you are using your SWR meter?

Where is your Jeep located when you are using your SWR meter?

 

[Mr. Bills]

. . . Why would you want to go from exhaust pipe to frame for a ground?

Grounding the exhaust pipe to the frame is frequently recommended, but the reasons why are seldom stated.

Just for fun I googled "Why would you want to go from exhaust pipe to frame for a ground?" and the very first search result was http://forums.qrz.com/index.php?threads/grounding-the-exhaust-pipe-to-chassis.43993/ which contains the following exchange on a ham radio forum:


How/Why does the exhaust create electrical noise?

The exhaust pipe can generate [or radiate] electrical noise . . . .

An exhaust pipe is bolted to the engine at the exhaust manifold. Many vehicles hold the long exhaust pipe up, in several places, with insulated/flexible straps to reduce mechanical vibration and audible noise. Lets call these straps "insulators." The length of the exhaust pipe becomes an antenna, collecting and re-radiating your ignition spark and other noise to your transceiver antenna like a "passive repeater." The longer the length of insulated exhaust pipe, the better radiator. By attaching shorting straps from the exhaust pipe to ground (frame) in several places along the length and end, you lower the impedance and thus reduce the ability of the pipe (antenna) to radiate.

You might ask, isn't the exhaust pipe at DC ground? Yes it is, but so are both sides of many antenna's like a Folded Dipole, Quad, Triangle, Halo, most Screwdrivers and so on. So it appears that DC ground and RF ground are different animals and must be analyzed accordingly. "Do not rely upon ohm meter reading to determine RF grounds!"

Every large chunk of conductive material on your vehicle needs to be tied electrically to the other metallic parts (in more than one place), with strap or braid. Example: Hood to fender, fender to frame, frame to engine, frame to exhaust, frame to body, trunk to body, A/C lines to frame, etc. Strap is better in that it has a lower RF impedance than braid and should be used where the flexibility of braid is not required. Some vehicle manufacture's do a good job of tying their parts together electrically, others do not. . . .

I hope this helps.

73,
Terry, K7FE

 

[MaloStapalo]

Grounding the exhaust pipe to the frame is frequently recommended, but the reasons why are seldom stated.

Just for fun I googled "Why would you want to go from exhaust pipe to frame for a ground?" and the very first search result was http://forums.qrz.com/index.php?threads/grounding-the-exhaust-pipe-to-chassis.43993/ which contains the following exchange on a ham radio forum:


How/Why does the exhaust create electrical noise?

The exhaust pipe can generate [or radiate] electrical noise . . . .

An exhaust pipe is bolted to the engine at the exhaust manifold. Many vehicles hold the long exhaust pipe up, in several places, with insulated/flexible straps to reduce mechanical vibration and audible noise. Lets call these straps "insulators." The length of the exhaust pipe becomes an antenna, collecting and re-radiating your ignition spark and other noise to your transceiver antenna like a "passive repeater." The longer the length of insulated exhaust pipe, the better radiator. By attaching shorting straps from the exhaust pipe to ground (frame) in several places along the length and end, you lower the impedance and thus reduce the ability of the pipe (antenna) to radiate.

You might ask, isn't the exhaust pipe at DC ground? Yes it is, but so are both sides of many antenna's like a Folded Dipole, Quad, Triangle, Halo, most Screwdrivers and so on. So it appears that DC ground and RF ground are different animals and must be analyzed accordingly. "Do not rely upon ohm meter reading to determine RF grounds!"

Every large chunk of conductive material on your vehicle needs to be tied electrically to the other metallic parts (in more than one place), with strap or braid. Example: Hood to fender, fender to frame, frame to engine, frame to exhaust, frame to body, trunk to body, A/C lines to frame, etc. Strap is better in that it has a lower RF impedance than braid and should be used where the flexibility of braid is not required. Some vehicle manufacture's do a good job of tying their parts together electrically, others do not. . . .

I hope this helps.

73,
Terry, K7FE

Thank you for that information!

 

[Mud dauber]

What is the best rated coaxial cable to run from the Antenna?

 

[jazngab]

@jazngab.

Readings over 3 for which channel, 40 or 1? Or both?

Where are you located when you are using your SWR meter?

Where is your Jeep located when you are using your SWR meter?

1. Both
2. In the Jeep. Doors closed.
3. In a parking lot

 

[JMT]

1. Both
2. In the Jeep. Doors closed.
3. In a parking lot

Try a different location. Won’t hurt. No trees or obstructions around. There could be some interference in that parking lot your not aware of.

I tuned mine last week to 1.5 on both channels.

 

[jazngab]

@Mr. Bills i was thinking is it possible my antenna is too short? I have a 2’ firestik and it’s about the same height or maybe slightly shorter than my hardtop. Could the hardtop be interfering since it’s mounted so close to it without being taller than the hardtop?

 

[Mr. Bills]

It is often more difficult to get a good SWR reading with a shorter antenna, but you should be able to get it down to 2.0-2.2 mounted in that location, which is in the "tolerable" range (1.5-1.9 being "good enough" and 1.0-1.5 being "good").

My CB antenna mount is not in the optimum location to transmit or receive radio signals, compounded by the fact that the antenna is only a few inches from a steel jerry can. It is an example of balancing competing priorities and making the best compromise for one's particular needs. I knew at the outset that a 2' antenna in that location would not perform as well as the same antenna in an unobstructed spot. However, I wanted a short antenna for use around town because in that situation garage door clearance is as or more important to me than Tx range. I use quick disconnects and switch to a 4' Firefly antenna for road trips and offroad runs when Tx range is as or more important to me than overhead clearance.


My notes show that the best SWR I could get with my 2' Firestik II sandwiched between my hard top and a steel jerry can was 2.1. The SWR of my 4' Firefly antenna mounted in the same location is 1.3.

 

[Imapepper]

No, do you have an ohmmeter?

 

[TurboTJ]

The shorter the antenna the worse the VSWR will be at the band edges. This is because greater inductance is needed to resonate out the greater capacitance of the short antenna. This reduces the matched bandwidth and incidentally reduces the efficiency of the antenna. The inductance in a Firestik comes from the spiral winding.

With all other factors remaining the same, if you want better VSWR at the band edges get a longer antenna.

BTW, the longer your cable, the better the VSWR will be at the radio end just because of the loss of the cable. Loss is not good per se, but to a certain extent it will help conceal a poor match at the antenna end.

One more thing. I think it has been stated here or on some other forum that you shouldn't coil coax because this will create inductance which will degrade impedance match. While it is true that it will create inductance to the current flowing on the outside of the shield, it has no affect on the currents flowing within the coax and therefore will not affect the match. That is of course unless there is no counterpoise or ground plane in which case the inductance might actually help because it chokes off the current on the outside of the shield.

 

[jazngab]

It is often more difficult to get a good SWR reading with a shorter antenna, but you should be able to get it down to 2.0-2.2 mounted in that location, which is in the "tolerable" range (1.5-1.9 being "good enough" and 1.0-1.5 being "good").

My CB antenna mount is not in the optimum location to transmit or receive radio signals, compounded by the fact that the antenna is only a few inches from a steel jerry can. It is an example of balancing competing priorities and making the best compromise for one's particular needs. I knew at the outset that a 2' antenna in that location would not perform as well as the same antenna in an unobstructed spot. However, I wanted a short antenna for use around town because in that situation garage door clearance is as or more important to me than Tx range. I use quick disconnects and switch to a 4' Firefly antenna for road trips and offroad runs when Tx range is as or more important to me than overhead clearance.


My notes show that the best SWR I could get with my 2' Firestik II sandwiched between my hard top and a steel jerry can was 2.1. The SWR of my 4' Firefly antenna mounted in the same location is 1.3.

I exposed more bare metal at the contact points of the mount and the tub to improve ground connection. It dropped the swr slightly but still above 3.

No, do you have an ohmmeter?

Yes I do. How would I use it to test the ground? I’ve read touch the exposed mount and the negative battery terminal hit obviously that’s on the other end of the vehicle.

The shorter the antenna the worse the VSWR will be at the band edges. This is because greater inductance is needed to resonate out the greater capacitance of the short antenna. This reduces the matched bandwidth and incidentally reduces the efficiency of the antenna. The inductance in a Firestik comes from the spiral winding.

With all other factors remaining the same, if you want better VSWR at the band edges get a longer antenna.

BTW, the longer your cable, the better the VSWR will be at the radio end just because of the loss of the cable. Loss is not good per se, but to a certain extent it will help conceal a poor match at the antenna end.

One more thing. I think it has been stated here or on some other forum that you shouldn't coil coax because this will create inductance which will degrade impedance match. While it is true that it will create inductance to the current flowing on the outside of the shield, it has no affect on the currents flowing within the coax and therefore will not affect the match. That is of course unless there is no counterpoise or ground plane in which case the inductance might actually help because it chokes off the current on the outside of the shield.

So that’s a lot of fancy cb talk for a noob like myself. What do you suggest I try to do to remedy my situation? I’ve tried understanding this ground plane concept but I’m not getting it.