Antenna Position

How long are those antennas? The only spec I can find on them is that they'll handle 15 KW

Ninety degrees difference in the orientation of the two antennas? Or pointing straight out in front of your vehicle, sorta like a knight's lance in a jousting match? Or pointing straight back, parallel to the ground (similar to the lances, but the other direction), or out to the sides (needing three lanes to drive in ;0 ) I'm assuming that 0 degrees is straight up to be conventional, but perhaps you have a different "reference zero"?

Mobile whip antennas always assume a plumb installation, so the signal will radiate (and receive) equally in all directions towards the horizon. Tipping them forward or back skews the pattern, making them waste nearly all the energy fed to them, by pointing said signal into the sky above you, the road under you, and putting signal off to the sides of your vehicle cross-polarized with all other mobile and many base stations. That last bit cuts your usable signal by 90%.

For skip with a vertical antenna, having it dead vertical is about the best way to put as much signal as possible towards the horizon, so that the first "skip" (bounce off the ionosphere) will be as far away from you as possible.

What kind of vehicle are these on? If the vehicle isn't wide enough to allow for about 8.5 ft between them, any potential benefit to having two of them (and assuming the correct 75-ohm harness is in place) is lost. For *most* vehicles, a single correctly-installed antenna, ideally in the center of the largest span of horizontal metal you've got (like a van's roof), will work best.

I use a single 102" stainless whip ($20 from Radio Shack) and a stainless potbelly spring mounted up high on the rear quarter panel of my van, unless I've got parking issues (like a roof). When I can't accommodate that because of things like garage roofs & such, I switch to one of a couple of different antennas to the NMO mounts that are mounted to the roof of my van.

Perhaps a little more detail from you, maybe a picture of it as it is now? Otherwise, we're kinda limited to just spouting general principles to you.

Hope that helps some, and
73

 

Easy answer straight up not leaning forward.

 

Straight up will usually change the SWR on some trucks like the Cascadia, if you are using the perch mount on the mirror. Too much reflected signal from the A-Pillar.

 

Back before there were cell-phones, texting, and the internet, there was radio...

So much information from back then has gotten lost, making me feel like a dinosaur in comparison. I try to stay out of the cb forums because I am not much for posting something, then having a slew of these 'New Guys', or perhaps 'Older Guys' that may have gotten their view of things twisted over the years argue and pick apart my antiquated skills of yesteryear. I will say this though,...I used to be one HECK of a contest box and radio builder for the channel-6 'Ten Gallon Hat' club. I have witnessed those 102' steel whips pop like fuses under such power, and have seen nearby giant oak trees burst into flames as if gasoline were poured onto them. It was funny to 'PoP The Mike' at a traffic light and watch it change, confusing the snot out of folks, or better yet, watch the smoke roll out from the guy's car stereo next to you, killing his 'Boom Boom', and his engine. Back then, a cobra-148 with a decent antenna, nominal was 10-15 miles barefoot, and 30 miles on an open stretch when it was quiet...

That being cleared up, perhaps a bedtime story is in order...

I remember well, why truckers leaned their antenna's forward instead of straight up, and myself, building contest boxes and radio's, spent many many hours fine tuning a contest setup just before a competiton. I guess I was reminded of all of it, when posting in another forum about alternators. It was mostly done because the stacks on each side of the truck would interfere with the impedance of the antenna, causing the SWR to bounce up and down as the antenna waved back and forth. Someone 10+ miles away would complain you were fading in and out rapidly. A fibreglass whip standing straight up would lean right back toward the stack at 70+MPH. Most drivers would lean them forward just enough so that they would stand back straight up while going their 'cruising speed'. Of coarse, most trucks back then were ungoverned, so if you saw a KW with the 'Whips' leaning pretty far forward, it was a sign of someone running 'triple digits' out west. It didn't take long before leaning your antennas further forward became a symbol of how fast your truck would go, and guys used to stick 'LED's' at the tip of their whips to prove they could make them stand straight up at night by going super fast. Well,...Like with anything else competitive, guys would lean them further and further forward, but as more trucks started being governed, the initial reason slowly got lost and gave way to just simply style and/or looks. Many would use the excuse of 'Oh, it gets out better', rather than admit their truck was a slow-poke. Personally, I get a giggle out of all the trucks with their antennas pointed at 45 degrees for no other functional purpose other than not wanting to feel inferior because that's what everyone else has done. The general excuse these days has become something like 'Must be Somthin' To it Cause Every one Else Does It'. Well,..the correct answer from an ex-master contest station builder is both Yes and No.

Its very difficult to see the subtle differences in antenna position on a big-truck (this does NOT apply to 4-wheelers or small vans) at 5, 10, 50, or even 500 watts of power, but push a carrier of 15 kilowatts with a PeP of more than 32,000 and the slightest nuance of imbalance can send that 3.5 million volts of electricity somewhere you don't want it to be. It used to be rather hilarious (trying not to laugh.LOL) back then to have a driver with a 10+ kilowatt box he set up himself come in to my shop and proclaim every time he keyed his mic, his poo-hole would burn as if there were hot sauce on it.....:ROFL: .....Yup this actually used to happen.

Anyways, After many many trucks and hours of tweaking this kind of power, I figured out that on a big truck, stacks or not, standing still, the absolute 'Q' (sweet spot of perfection) was almost always exactly 23 degrees forward of straight up within 1 degree off the mirors. The actual reason for all the tech-savvy geeks reading this, revealed by all that power, is that the antennas are not exactly centered within the 'ground plane' effect of the truck. The baloon of inductive reactance vs capacitive reactance is offset toward the direction of greatest metal surface in reference to the antenna (IE. the back), so leaning the antenna forward slightly brings what metal there is in the front closer to the tip, where the effect is greatest. It is not a complete balance from front to back by any means, but helps quite a bit. As handlebar kindly put it, the further from straight up, the more the polarization of the signal is skewed. 23 degrees is the 'Q' because anything past that point and polarization loss starts to overcome the gain from balancing the 'Ground Plane'.

Conclusion,...Whatever angle forward that is required to achieve 23 degrees while going down the road.


Maybe my bedtime story was entertaining,...Rawze

 

It’s hard to tell how you have them mounted by what you posted but if they are horizontal to the ground instead of vertical they you will have a huge loss (can be over 20dB) due to cross polarization. Almost all CB’s use vertical polarized antennas so if you go directly opposite that you will have high loss of signal. Mounting them at 45 degrees will give you a 3dB loss but 90 degrees off is theoretically infant loss and over 20dB in real life.

http://www.tscm.com/polariza.pdf

 

Is that the new math I keep hearing about where 3X4=11?

P=E^2/R, so E=SQRT(PR). Ppeak=32,000, R(antenna)=50 ohms, E= 1,264.91 volts peak. Wondering where the extra 3,498,735.089 volts came from.

Just mount the antenna a couple degrees forward of vertical at 70 MPH. Otherwise cross polarization loss occurs. At 90 degrees (jousting angle) that equals a loss of 20 dB. As mike stated on the Cascadia you must go further ahead sacrificing some loss due to cross polarization or you will seldom see even a reasonably low SWR.

 

thats exactly why I do not post in these forums, too many know-it-alls that have no idea how to fathom such things, so this will be my last post in here. I am not your teacher, and it is obvious that most would kill themselves or render their private parts inert from trying.

it is obvious that you have no clue as to what went on back then, so I will clarify a bit so you can go off yourself trying it out. How about 6 250-amp leese-neville alternators gutted. They provide raw 3-phase a/c. All of them connected in parallel fed into a custom built 3-phase xfmr (southern trabsformer built them for us), upping it to 875KV. push that into a single 1608 through a quadrupler = 3.5 million on the plate, and only 150-watts drive will push 25-32KW out of the box on a bird wattmeter.

BTW, your kindergarden math has a serious flaw as well. The feedpoint of any decent antenna is almost all current, but as the energy travels upward along the element, it is converted back into almost all pure voltage by the time it reaches the tip. If the antenna is designed well, then that voltage can be infinitely high. So YES, try having a couple three million volts dance through the truck at 27 MHZ, and you will feel like someone stuffed you into a microwave oven, it on high.

There is no reason to pick apart an old-mans words to prove yourself or to gain attention to yourself. After-all it WAS just a bedtime story.

(I suppose next, some joker in here will proclaim that 39 kilowatts DC @ 1.3-1.5MV is not possible from only 6 alternators, witch is of coarse easily done as well)

 

It would depend what you have them mounted on..if you mount them straight up are they going to be too close to the side of the truck...I personally don't see a need to run dual antenna I would just run with one.