CB radio installation in a 1998 Volvo wagon

[jonwienke]

The inductance of the coil will depend on the square of the number of loops, the average diameter of the loops, and the presence of nearby ferrous metal. Making the loops slightly larger than the minimum bend radius of the coax maximizes the inductance of the coil. For small diameter coax like RG58 or RG6, a loop diameter of 4-6 inches works pretty well. But for LMR-400 and similarly stiff coax, a diameter of 8-10 inches is more appropriate.

I generally make the coil as close as possible to the antenna mount and zip tie it together to keep it from uncoiling.

 

[Sonicadventure]

I'm using RG58, so you think if I have an extra 2-3 feet using the 4-6" loops zip ties together under the base of the antenna will be ok? By the way I'm using a shielded nickel plated NMO mount if that makes any difference. Thanks

 

[jonwienke]

2-3 feet extra isn't going to make a significant difference loss-wise.

 

[Sonicadventure]

What is the shortest antenna I could theoretically use? Because I park in a parking garage.

 

[jonwienke]

Get a folding antenna then. Antenna performance drops in direct proportion to how much shorter it is physically than electrically. A 1/4-wave antenna is 108" long, so if you had a 10.8" antenna it would only give you 1/10 of the signal, or 10dB less signal than the 108" antenna.

 

[Rred]

There are a number of 2' tall CB antennas, base and coil loaded with wire whip for the top portion so that even if they strike something, they just flex with no damage. IIRC Firestick also makes a 2' tall stick, which is helical wound, and K40 or K30 has something decent as well.

Any antenna which will not be destroyed by your daily routine, beats the heck out of a better antenna that is broken in two weeks.

And of course with an NMO or magmount, you can just keep a "highway" antenna in the trunk for those times when you'll be on the open road and want something better.

"One size fits none", like the tube socks ought to say.

 

[mmckenna]

On a station wagon, a NMO-27 will likely hit a parking garage, but they are very flexible.
If it's occasional, I'd either not worry about it, or just take it off when entering the garage.

Considering all the work you've put into this so far, I wouldn't sabotage it all by using a short antenna.

 

[RFI-EMI-GUY]

I have used the Larsen NMO-27. Wonderful antenna. If I recall Larsen has a spring accessory that replaces the fitting on top of the coil. If you install this, you can easily bend the whip over without too much strain on the base. Then either buy one of those gutter clips used for a 108 inch whip, or fashion some sort of insulated hold down bracket. You will need to shorten the whip using the cutting chart that includes the spring length.

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[crazyboy]

Actually have a NMO-27B here that I have been trying to sell off. It is brand new, was for a customer that went a different route.

 

[Sonicadventure]

Actually have a NMO-27B here that I have been trying to sell off. It is brand new, was for a customer that went a different route.

PM me for more details on that Larsen antenna


In response to the others, my ideal set up would be to have a Larsen 27 in a case under the back seat for road trips and long highway drives on the weekends, but ideally I like one that a smaller antenna kind of like a cop-style antenna that would be no taller than 16-18" that I could just leave on all the time for local driving to pickup nearby stuff only. There will also be times when I will just have the rain cap on the NMO mount. That's why I chose this set up to begin with I wanted the versatility of the NMO mount because I just can't run a huge steel whip anywhere where I do my local driving. I know my signal will suffer, but as long as I know I can slap a good Larsen antenna on anytime thanks to the mount I'm choosing that's OK with me.

 

[mmckenna]

Antenna length is tied to the operating frequency. There are ways to cheat this, but it -greatly- impacts functionality.
Issue is that the NMO mount will require you to stay with proper antennas. No reputable antenna manufacturer makes a CB that's 16 to 18".
You could try one of the cheap Wal*Mart/Truck stop magnetic mount antennas, but that sort of blows the purpose of the NMO mount.


If you were not going to transmit, you could put any antenna you want on there. A simple VHF quarter wave antenna will be about the length you want. It'll receive stuff that's -really- local, and will work really well if your CB has a Weather Channel function. But it will not work for transmitting.

 

[RFI-EMI-GUY]

Personally, I would go with an NMO-27 with spring and use an insulated gutter clip hold down when not in use. How low can this garage be? It should clear a folded NMO-27.

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[Sonicadventure]

Thanks all. My car doesn't have a rain gutter, but I do have an aluminum luggage rack that secures to the roof of the car with 6 steel screws. I could probably bend the antenna towards one of the corners of the rack.

The garage clearance is very low; the door height is probably 7 feet, with many pipes and other things in the garage as low at 5'11" in some places. Plus lots of electrical boxes and electrical conduit. Not a place to drive around with a CB antenna. If it was just a concrete garage I wouldn't mind a whip dragging in some places but there are huge transformers and junction boxes all around the ceiling. It's an old building built in the 1960's.

 

[RFI-EMI-GUY]

Thanks all. My car doesn't have a rain gutter, but I do have an aluminum luggage rack that secures to the roof of the car with 6 steel screws. I could probably bend the antenna towards one of the corners of the rack.

The garage clearance is very low; the door height is probably 7 feet, with many pipes and other things in the garage as low at 5'11" in some places. Plus lots of electrical boxes and electrical conduit. Not a place to drive around with a CB antenna. If it was just a concrete garage I wouldn't mind a whip dragging in some places but there are huge transformers and junction boxes all around the ceiling. It's an old building built in the 1960's.

So you have 71 - 57.5 = 13.5 inches to work within.

Here is a typical gutter clip. It has an insulated section so you can still use the antenna to receive while down. You can probably fashion something similar using PVC tubing and drilling some holes into it. If you place it near drivers side you can reach out and release it.

Kalibur K101X - CB Antenna Steel Whip Tie Down

There are also some motorized NMO mounts that will fold down flat, but the probability of breakage is high and I would suspect the grounding would be poor.

 

[RFI-EMI-GUY]

I had my NMO-27 and NMO-54 (6m ham) mounted atop a Ford 15 passenger E350 van. So you can imagine how much abuse they survived from tree branches and parking garages . The Corona balls did wear off from rubbing on concrete. But you can replace those.

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[Rred]

Uniden.
Arguably not reputable (G) center loading and 23-25" tall including the magnetic base and 2" of heavy spring on top of that, but the top is still a very flexible wire whip.
If you can clear 18", this would clear with just the top wire flexing over.
Close enough.

 

[RFI-EMI-GUY]

The best options for performance are the long 1/4 wave bumper or fender mounted whip which has directivity forced in one direction only, or a center mounted NMO-27 (or similar NMO mounted) with directivity approximately circular. All other CB antennas are inefficient radiators. I suppose if all you want is to hear truckers nearby, one of those short helical whips will work, but if you are broken down on a desolate stretch, it will be frustrating to try and reach anyone.

The problem with magnetic mount bases is that they do not adequately couple the current into the roof panel, and the counterpoise ends up being the coaxial cable. As a result directivity suffers, tuning is sloppy, and any noise from the vehicle is conducted to the antenna.



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[Rred]

Bumper mounted: Which has a problem of 1/2 the antenna being mounted below the level of the ground lane (the roof) and totally obstructed in 180 degrees by the ground mass of the vehicle. Dunno, I'd like to see some real antenna open range tests that show those aren't wet blankets for the bumper mounts.

 

[Sonicadventure]

So, again my plan is to mount a shielded 3/4" NMO mount directly in the center of the roof of my station wagon. The roof is going to provide a pretty good ground plane as it has no sunroof. We're talking about ~4' x almost 8' of flat steel.

I'm leaning towards buying a Larsen or MaxRad base loaded whip antenna which most of the time will be kept in the car. When I'm driving my 2-3 mile commute in the morning through two small towns I don't expect to A) have any issues where I'll be stranded and need help and B) because of all the building and train tracks I drive under I don't think I'd hear much CB chatter anyway. But I do want to at least have the ability to pick up local stuff if I see an accident or something wierd, and just want to cut the radio on to see what I can hear. Transmitting wouldn't be that important. I bought a set of 4 relatively cheap used NMO antennas from eBay that I plan to experiment with for my "local" use options. The Larsen antenna in the mix seems to be maybe my best option? Check out a photo:

 

[jonwienke]

None of those are CB antennas. The one is an 800MHz antenna which means that reception will probably be 20-30dB less than an actual CB antenna. The others (which appear to be UHF and/or VHF) won't be much better.

DO NOT transmit with any of these antennas or you will probably fry your radio.