Need a CB antenna wizard

[K8BBL]

Gonna get a bit long winded ... apologies in advance.
We're installing in a Ram truck (steel body, not aluminum). Using front stakehole mount. Grounding is not an issue, flatstrap copper and drills and taps solved all that. Our 5' Firestick works perfectly. SWR ~1.1, no issues & it works well even with the antenna being that close to the cab (mount is about 8" from the vertical wall of the cab). The tip of the 5' Firestick is ~39" above the cab. We're trying to swap the 5' Firestick for a base loaded whip (55") which will end up being about 34" above the cab. The only reason we want to swap it is appearance, the whip will "hide" better. We cannot get the SWR below 3. 18' of RG58 gets us to it and we need every bit of it, no way to shorten the coax. Here are my thoughts and this is where I need a guru ...
The Firestick has it's loading coils at the top where they are well above the cab. The base loaded whip has the loading coils at the bottom where they sit about 8" from the vertical steel wall of the cab. I'm thinking that regardless of what we do to the length of the whip the SWR will not be changeable unless we get the loading coil away from the wall of the cab (we have not tried to cut the whip shorter yet, didn't want to ruin it if there's no chance of it working). Opinions or ideas anyone (aside from using a different mount, which we don't want to do)?

 

[slowmover]

NMO w/Laird on roof’ll whup that. Base-load. Stick yours up there.

6’ Skipshooter a better choice than others for topload. Bad location at cab. Rear of bed, better. Roof, best.

A quarter wave antenna for 27-Mhz is 9’.
All else is compromised.

Whatever the antenna you have to give it what IT wants.

.

 

[ForestRunner98]

OP...although I am not close to an antenna expert I think you answered your own question. That loading coil so close to the cab is what’s causing the issue. I don’t think there is anyway around it. I wouldn’t cut the antenna.

 

[FPR1981]

The Laird only handles 200 watts max? Such a disappointingly low figure for such a reputable name.

 

[slowmover]

The Laird only handles 200 watts max? Such a disappointingly low figure for such a reputable name.

(In a friendly manner):

49” antenna. And, given what CB offers, good luck hearing as far as that 200W (hypothetical) can TX.

I regularly try to speak with guys running BIG RADIOS who don’t have ears for ****.

A Laird/Larsen atop a private vehicle with an analyzer indicating sufficient bonding, etc, will very likely out-talk the bigger amp rigs.

A President Lincoln II+ is — so far — the mobile with the best ears I’ve run (DSP). A KL-7405 behind it does as much as what most antenna systems are capable RX/TX.

50-150W isn’t surpassed by my pushing past 200W with a different amp in a big truck (pickups and cars always better) except on Sideband or skip with base stations both of which haven’t anything to do with mobile operations (timely & accurate road information).

Sure, anchor a 108” whip and do the rest.
How many will?

The problem is always the poor state of the average radio rig (mobile). The other guy.

50-150W + DSP you expect to Hear & Be Heard.

There are occasions I have an extended conversation with a well-sorted base. It’s always a little funny some guy near me in traffic (same direction) asks who the heck I’m talking with.

There does come a point where wattage offers nothing extra to the operator and becomes an irritant to everyone around.

A Laird C27 on an NMO to 3/8 adaptor rooftop until you hit the big road and install the PRESIDENT Texas. (Change nothing else).

My pickup that’s the spread from about 10’ to 13’; or the range from E-Z city maneuvering to real highway performance.

Not the wattage, per se.

A 108” in the truck bed rear stake hole is not taller. and is directionally lopsided. You think it’d be better with 350W vs the Texas on the roof with about 180W out on the highway?

Cleaning up the audio receive is the thing in my experience.

.

 

[mmckenna]

The Laird only handles 200 watts max? Such a disappointingly low figure for such a reputable name.

These types/brands of antennas are designed for LMR use. In that industry, it's very rare to see any mobile radios running more than 100 watts. That's because it's more about designing an entire system, end to end. When done correctly, excessively high power outputs are not required.

 

[W0JOG]

You're running into an impedance miss-match with all that tinkering. Like someone said above, put a Larssen on the roof and you'll have a better system than anything you can patch together anywhere else.

 

[prcguy]

I don't think the problem has anything to do with the coil being 8" from the cab, the ground plane is just not sufficient for the antenna. You can test this by attaching a wire or two to the grounded base of the antenna and stretch that across the truck bed to simulate a ground radial and preferably several. I'll bet that matches up just fine thus leaving you with a problem that is not easy to solve with a base loaded antenna.

If you can get to the coax connection on the back of the NMO mount you could experiment by adding a 25pf disc ceramic or dipped silver mica capacitor with a voltage rating over 200 volts. That will fool the antenna into having more capacitance between the coil, whip and ground plane. If 25pf makes it better but not quite perfect try 50pf.

200 watts into 50 ohms is 100 volts, so that's why I recommend a capacitor voltage rating at least 200 volts to match the power rating of the base loaded antenna.

 

[jonwienke]

A bed stake hole mount next to the cab is a crap location, probably one of the worst possible, except underneath the vehicle. Beside the poor ground plane issue, the cab is going to reflect a lot of signal back into the antenna and back to the radio no matter how you tune it, and you're going to have a crappy asymmetric radiation pattern as a result. Nothing you put there is going to work worth crap compared to a decent antenna mounted near the center of the cab roof.

Adding extra power to try to compensate for a crap antenna setup is just putting lipstick on a pig.

 

[prcguy]

I've had good results with long CB and HF amateur antennas mounted about 8" back from the cab on the upper bed rail. The cab only sticks up about 2ft above the bed rails and that's a small fraction of a wavelength at 27MHz and will not reflect that much. Its the size in wavelength of something that is a problem and not the size compared to the antenna. A 1ft long CB antenna next to something 2ft tall doesn't matter that much because the radiated wave off the antenna is much bigger than the antenna and a truck cab is only a little speed bump in the big picture.

I think a linearly loaded antenna like a Firestick or a 9ft whip will match up just fine on the bed rail but a base loaded antenna reacts a little different to its surroundings. I think it might have something to do with the capacitance of the whip to surrounding metal being skewed a bit and maybe reduced with a base load where its more consistent without most of the loading right at the base.

When you shorten an antenna its impedance goes lower. Many base loaded antennas have a tap in the loading coil to raise the feedpoint impedance and they are usually designed to match well on a typical roof or trunk lid. A linearly loaded whip like a Firestick is lower impedance than a full 1/4 wave whip (under the same conditions) and when you have a lot of wire wound as a whip that might provide more capacitance from the whip to the body and might help with matching when the ground plane is poor.

Maybe, all just speculation on my part.

A bed stake hole mount next to the cab is a crap location, probably one of the worst possible, except underneath the vehicle. Beside the poor ground plane issue, the cab is going to reflect a lot of signal back into the antenna and back to the radio no matter how you tune it, and you're going to have a crappy asymmetric radiation pattern as a result. Nothing you put there is going to work worth crap compared to a decent antenna mounted near the center of the cab roof.

Adding extra power to try to compensate for a crap antenna setup is just putting lipstick on a pig.

 

[jonwienke]

Having 2/3 of an antenna blocked by any metal structure seems like a really bad idea under the best of circumstances. It's not as bad if the antenna is larger and proportionately less of it is blocked, but that doesn't make it a good idea.

 

[prcguy]

But the physical size of the antenna doesn't matter, its the frequency and wavelength it operates at. A 2ft tall cab is not a big obstruction at 27MHz.

Having 2/3 of an antenna blocked by any metal structure seems like a really bad idea under the best of circumstances. It's not as bad if the antenna is larger and proportionately less of it is blocked, but that doesn't make it a good idea.

 

[slowmover]

But the physical size of the antenna doesn't matter, its the frequency and wavelength it operates at. A 2ft tall cab is not a big obstruction at 27MHz.

All well and good, but one might search in vain for the recommendation in general.

Anyone is welcome to prove the assertion wrong.

I agree that a better location will result in better performance.

One of the very best pickup truck radio rigs I’ve ever heard was a set-up by the best installer (Robert) at Clays Radio (San Antonio) for a guy did deliveries all thru the Eagle Ford Basin.

Nine-foot (108”) whip a little forward of the 8’ bed center point. May have been on a riser. I gave him reports as I went east on IH-10 that evening as he sat at the Ackerman Road parking lot.

Couldnt NOT hear that guy 10+ miles. Heard him a few other times, as well. Every detail was superb for audio strength & clarity.

.

 

[prcguy]

Of course there are much better mounting locations than the bed rail, but life is full of compromises driven by restrictions. My Toyota Tundra, which I just sold last month had a Tarheel 100HP screwdriver antenna mounted about 8" behind the cab with the feedpoint maybe 6" off the bed and the factory mount on the vertical bed wall. On CB and 10m it worked very well. On HF bands 80m and up it worked extremely well and I would often be the strongest station in a group of others on 40m running 100 watts on base stations.

The match on CB/10m was very good with no external matching components.

All well and good, but one might search in vain for the recommendation in general.

Anyone is welcome to prove the assertion wrong.

I agree that a better location will result in better performance.

One of the very best pickup truck radio rigs I’ve ever heard was a set-up by the best installer (Robert) at Clays Radio (San Antonio) for a guy did deliveries all thru the Eagle Ford Basin.

Nine-foot (108”) whip a little forward of the 8’ bed center point. May have been on a riser. I gave him reports as I went east on IH-10 that evening as he sat at the Ackerman Road parking lot.

Couldnt NOT hear that guy 10+ miles. Heard him a few other times, as well. Every detail was superb for audio strength & clarity.

.

 

[jonwienke]

But the physical size of the antenna doesn't matter, its the frequency and wavelength it operates at. A 2ft tall cab is not a big obstruction at 27MHz.

It is if your antenna is basically the same height as the cab, or even shorter. Only the portion of the antenna clear of the obstruction can effectively radiate a signal past it.

 

[prcguy]

Doesn't work like that. If the antenna was 1 inch long at 27MHz the truck cab will still only distort the pattern a little. There will be more effect from the rest of the vehicle body pulling the radiation pattern down towards the horizon in the direction of the most mass (like the cab) and a higher angle of radiation and less signal at the horizon on the side of the truck where the antenna is mounted. At VHF or UHF it will be a different story.

I'll say again, its has nothing to do with the physical size of the antenna and everything to do with the frequency and wavelength. Do you really think a 1ft tall antenna emits a little 1ft tall wave at a frequency where a wavelength is 36ft tall? The radiated pattern is bigger than the antenna in this case.

It is if your antenna is basically the same height as the cab, or even shorter. Only the portion of the antenna clear of the obstruction can effectively radiate a signal past it.

 

[jonwienke]

If the antenna was an inch tall behind a 24-inch metal obstacle, it's basically not going to radiate anything.

 

[prcguy]

At 27MHz no, but my point is a 24" high cab next to it is not a big deal due to the wavelength. In fact a CB antenna mounted 8" behind a truck cab on a top bed rail should have more radiation in the direction of the cab towards the opposite side front of the truck. The ground plane provided by the cab and hood is more than the bed rail area and the part of a truck with the most continuous sheet metal will pull the pattern down towards the horizon more than an elevated bed rail.

Do the same thing with a VHF or UHF antenna and the cab will then reflect more signal away from the cab and you should have less signal in the direction of the cab. Not counting the size of the rear window and signals at UHF being able to pass freely through the rear window opening out the front window because the window openings may be taller than 1/2 wavelength.

If the antenna was an inch tall behind a 24-inch metal obstacle, it's basically not going to radiate anything.