I'm this far [] from giving up on this CB...

I bought a brand new Cobra 29 CB this weekend. I hooked it up to my setup of whips on the toolbox (only one plugged in, if you are going to bitch to me about two antennas, do it somewhere else, not here). The antenna warning light on my radio lit up immediately when I tried to transmit. I ran a ground wire from the mount directly to the frame of the pickup, that damn antenna warning light still came on. With this setup (if I remember right) the SWR's were between 1 and 2... so acceptable. This setup also worked well with the old Midland radio I had... but I still can't figure out why the light keeps coming on. Hell, I even took a multimeter to check the ground, and I scraped some paint off of a door mounting nut and the thing beeped when I touched the toolbox and the antenna stud.

Tonight I made a home made 100% steel hood mount bracket, and I ran a ground wire from the coax to the mounting bolt. Once again, the antenna warning light came on and my SWR's were off the charts at about 3. This is what throws me off the most, why the hell are my SWR's so high with this setup? The only thing else I could do to ground it is to wire the SOB to the negative battery terminal.

Anyone have any clue on what the hell is wrong with this thing? I've tried numerous combo's of coax, mounts and antennas and nothing seems to make that damn light shut off.

The only time I was ever able to get the light to shut off was when we took the mount off of the toolbox, and placed it directly onto the negative terminal of the battery. This was the only time that light shut off. What the hell is wrong?

ANY help would be appreciated, otherwise this thing is going back to the store.

just get the antenna tuned and truck beds are not good grounds

407Link said:

just get the antenna tuned and truck beds are not good grounds

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+1 If you have firestik's they are a breeze to tune with a friend. Start in the same parking lot and then get farther apart (maybe a mile or 2) and see what works best.

407Link said:

just get the antenna tuned and truck beds are not good grounds

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The antenna is tuned, and that helps in no way. I used my home made all steel hood mount, put it on with a firestik tuneable tip antenna, and my SWR's were just a bit lower than 2 on 19... however the antenna warning light STILL came on. If I ran a short (<6") ground wire from the mounting bracket itself to the negative of the battery my SWR's dropped down to about 1.1 and the antenna warning light would turn off.

It shouldn't make a damn difference if the truck bed is a ground when I ran a ground wire straight to the frame. The only time I've been able to get that f*cking light to shut off is with a magnetic antenna or when there's a wire going to the negative battery terminal.

You should understand the differences between a DC ground and a RF (radio frequency) ground. The two are not always the same, a piece of wire more than a tiny fraction of a wavelength at 27MHz (even a few inches) typically will not conduct well enough to work as an RF ground between the antenna and the frame or body due to having too much inductance (this fully affects the "tuning" of your antenna, and is why the light keeps coming on).

On the other hand, you could have three or four 9-foot long pieces of wire (or metal tubing) attached directly to your antenna's mount splayed out in all directions connected to absolutely nothing at all and have a perfect SWR. These wires would then be acting as what's called a "Ground Plane" for the antenna (though this would be more practical for fixed-base station use than with a mobile unit in a vehicle).


What you need to do is attach the antenna mount directly to a solid sheet of metal, such as the center of a metal toolbox (which may require several bonding straps if where you're mounting it is not a welded part of the main part of the box), or even better yet, directly in the center of your truck's roof (the antenna requires a substantial mass of metal directly underneath if it's to work well as an RF ground or ground plane).
The bedrail can also work decent, but only with a full length (96-102") whip if it's anywhere near the cab.

4x4junkie said:

You should understand the differences between a DC ground and a RF (radio frequency) ground. The two are not always the same, a piece of wire more than a tiny fraction of a wavelength at 27MHz (even a few inches) typically will not conduct well enough to work as an RF ground between the antenna and the frame or body due to having too much inductance (this fully affects the "tuning" of your antenna, and is why the light keeps coming on).

On the other hand, you could have three or four 9-foot long pieces of wire (or metal tubing) attached directly to your antenna's mount splayed out in all directions connected to absolutely nothing at all and have a perfect SWR. These wires would then be acting as what's called a "Ground Plane" for the antenna (though this would be more practical for fixed-base station use than with a mobile unit in a vehicle).

What you need to do is attach the antenna mount directly to a solid sheet of metal, such as the center of a metal toolbox (which may require several bonding straps if where you're mounting it is not a welded part of the main part of the box), or even better yet, directly in the center of your truck's roof (the antenna requires a substantial mass of metal directly underneath if it's to work well as an RF ground or ground plane).
The bedrail can also work decent, but only with a full length (96-102") whip if it's anywhere near the cab.

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Thank you for the information. So I understand what you are saying about the wires not effecting the RF ground that much, however, I'm wondering why my SWR becomes better when I ground the antenna MOUNT straight to the battery (probably a 8" long wire)? Because once the mount is grounded to the battery, then the light also turns off.

Here's the problem I guess, my toolbox is aluminum... What I'm thinking now is going back to a hood mounted antenna with a 4ft stainless steel whip on it, so what I'm wondering now is the best way to ground that mount. I never had a problem with ground on my old radios, however this one seems to pick up on anything that isn't grounded to the battery.

Though I'd like to put an antenna on my roof, its just not an option at this point to put a hole in my roof. So therefore some other option has to be found.

A good magnetic mount works great for up on the roof, no hole needed (magnet mounts attain RF ground through capacitive coupling to the roof's metal, another case where RF ground differs from DC ground).

If you're going to put the antenna on the hood, attach the mount to the hood directly, and then use a couple bonding straps at each corner of the hood to the firewall or fender.
Or you could mount it directly on the inside lip of the fender itself too, but make sure your bracket is as short as possible (I'd still bond the hood to the firewall too). A 5' or better antenna will also work better, as it'll still be fairly close to the cab (unless you're thinking of putting it out at the front corner, then a 4-footer will probably be fine).


My guess as to why your match was better with the wire attached to your battery post... that wire and the battery (and other wires connected to it) was somehow acting as a ground radial for your antenna (the 3 or 4 wires of the ground plane example I gave above are each called "radials"). This phenomenon usually happens as a matter of chance given the randomness of the wire lengths involved.

Hopefully that helps

4x4junkie said:

A good magnetic mount works great for up on the roof, no hole needed (magnet mounts attain RF ground through capacitive coupling to the roof's metal, another case where RF ground differs from DC ground).

If you're going to put the antenna on the hood, attach the mount to the hood directly, and then use a couple bonding straps at each corner of the hood to the firewall or fender.
Or you could mount it directly on the inside lip of the fender itself too, but make sure your bracket is as short as possible (I'd still bond the hood to the firewall too). A 5' or better antenna will also work better, as it'll still be fairly close to the cab (unless you're thinking of putting it out at the front corner, then a 4-footer will probably be fine).


My guess as to why your match was better with the wire attached to your battery post... that wire and the battery (and other wires connected to it) was somehow acting as a ground radial for your antenna (the 3 or 4 wires of the ground plane example I gave above are each called "radials"). This phenomenon usually happens as a matter of chance given the randomness of the wire lengths involved.

Hopefully that helps

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Tried the magnetic mount thing too, scratches my roof, so that's a no go.

When I referred to hood mount, I was talking about one that mounts to the fender and comes out between the hood and the fender, like this one:

SWR can also be affected by proximity to other metal objects. Although RF behaves a little differently at 29MHz (CB freqs) than at 146MHz (Ham Radio 2m band), it's still a concern.

I had a 2m mag mount on the cowl next to the FM antenna, and when I measured it, it was nearly 3:1 SWR. Once I moved the antenna to the center of the hood, it improved to about 1.5:1, which is MUCH better. I'm gonna mount a permanent NMO to the center of the roof, which is probably the best place, but I haven't gotten around to it yet.

So, try the antenna in different places. Some could be better than others. Maybe the back bumper, or the back corner of the bed rail, or the center of the toolbox, or anything. I really like MFJ's little frequency analyzer, but it might be a bit much for CB work, like the MFJ-207 or the (much better) MFJ-249B. Makes measurements really handy.

And also, make sure you have a good heavy physical ground between the radio and the vehicle chassis, too. A length of outer copper braid from a piece of coax makes a decent grounding strap. This is a good idea in addition to the existing DC ground wire.

Spott

I got that same radio you got. I also run a 5' K40 fiberglass whip and 9' mini 8 coax. I was getting the antenna warning and I replaced the antenna stud. the little ground wire on my antenna is not hooked to anything and my SWR is 1.5:1 with the antenna mounted on my mirror.

02RangerXLT said:

Tried the magnetic mount thing too, scratches my roof, so that's a no go.

When I referred to hood mount, I was talking about one that mounts to the fender and comes out between the hood and the fender, like this one:

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Gotcha.
A fender mount like that should work fine. Just try to keep it away from the cab as much as you can if you're using a fairly short antenna.

BTW, 29 MHz is ham band also lol (10-meters)
(CB is 27 MHz )

Wow... all these issues, and the jeep guys I wheel with have problems too!

I must have got damn lucky with my cobra 75 wx st and 4' steel whip mounted on my roll bar. I could recieve/transmit roughly 5 miles on hwy, flat ground and never lost anyone in the bush on UA.

Gonna have to throw an SWR meter on it one day and check it out.

wahlstrom1 said:

Wow... all these issues, and the jeep guys I wheel with have problems too!

I must have got damn lucky with my cobra 75 wx st and 4' steel whip mounted on my roll bar. I could recieve/transmit roughly 5 miles on hwy, flat ground and never lost anyone in the bush on UA.

Gonna have to throw an SWR meter on it one day and check it out.

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I had great luck with my Midland 1001Z and a 4ft firestik in a hood mount, same with my dad's 26 year old Midland... however this stupid little light has got me searching to find some problem, even if my SWR's are good.

Thanks for the help, gentlemen.

The light shouldn't come on if the SWR is good.

Anything above a 1.5:1 SWR is worth messing with to get it lower (IIRC, those lights are calibrated to come on at around 2:1 SWR or higher).

4x4junkie said:

The light shouldn't come on if the SWR is good.

Anything above a 1.5:1 SWR is worth messing with to get it lower (IIRC, those lights are calibrated to come on at around 2:1 SWR or higher).

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F*cking damnit!!!!!!!!!!!!!!!!!!!!! I have this stupid mount grounded in 3 places (4 if you include the bolt its actually hooked to) and it still turns that damn light on. SWR is about 1.2:1, even less maybe, but that stupid ass light keeps coming on... see the pictures for yourselves:






Any suggestions now? If I don't get something by tomorrow night I'm going to run a ground wire to the battery, and make the stupid POS light shut off.