What Every CB'er Shoul Know

Well it doesn't take long for the "real" experts to get over my head to where I wouldn't know if what they were saying was fact or not!:smt102:smt102

 

Mostly I'd have to agree with your ummm collection but
Number 3 is in error a 5/8ths mobile would need to be 21.5 ft.
Who told you that a rubber ducky antenna was a 1/4 wave?

Technically CB isn't "ground wave"

To answer your question regarding length an antenna can be made shorter (or longer) than the 1/4 wave (108 inches ) by use of a matching network .
physical length and performance aren't the same thing, but there's a direct correlation.
Up to the .64 wavelength increasing length increases gain.
Maybe we have a definition problem. Iif you think you can design a 5/8ths wave smaller than a 1/4 of a wave length (108 inches ) then, if not radiator length what is your definition of a 5/8ths wave antenna vs a quarter?
Calling a mobile antenna for `27 MHz a 5/8 is just marketing hype and snake oil
There is a reason we don't all use 12 inch antennas. Why bother with a `103 to 105 inch whip

19 is mostly irrelevant . Most of the "recommendations" don't give even the transparency of the half wave length gives.
unless you are playing with phasing harnesses there is no benefit from some "magic length" of coax, and not very much loss from having some extra.
Get one long enough to reach from the radio and the antenna.
Well get a little extra because :
20 and 21 A coil of ~8 inches is called a "big ugly balun" (poor choice of terminology) and only stops common mode currents (on the outside of the shielding ) that you don't want. That would make the counterpoise inside the cab with you not outside the vehicle.
If you think cable length causes/cures a problem you are fooling yourself just like the ATU fools the radio.
What kind of cheep coax can't take a 6" radius turn?
58 I won't bother to argue that even though it's more likely accidental altering the tuning while installing the tip not the presence of a small piece of vinyl.

 

"58 I won't bother to argue that even though it's more likely accidental altering the tuning while installing the tip not the presence of a small piece of vinyl."

Then take a wirewound fiberglass antenna where adding or removing the cap cannot alter the length of the tunable tip since there is none. Measure with an analyzer, or whatever type SWR meter you feel comfortable with. Example a Firestick 4', non tunable tip type. You can clearly see the frequency of minimum dip move as much as 20 channels tip on VS tip removed. The effect is greater at the tip because this is the point of highest resonant rise of voltage assuming the antenna is well tuned and resonant at the frequency being measured. This makes the tip the most sensitive portion of the antenna to adding a dielectric (no mystery here, dielectric stores charge energy Q=CE^2/2, if you increase E the voltage, you increase the stored energy in Joules/second, power not returned as reflection). Where the dielectric constant is greater the more the RF wave is slowed down, causing the resonant frequency of the antenna to be lower.

Since wavelength is related to frequency at a given velocity the antenna appears to be electrically longer with greater dielectric constant surrounding it. Given air K=1, and taking into account geometric effects (electrical wavelength affected by diameter of conductor) we get a velocity factor around 95 percent, lower if conductor is surrounded by a dielectric > 1 (air). The plastic surrounding the antenna raises K to a value greater than air = 1 thereby slowing down the wave. Since frequency is cycles per second and free space velocity = C (light speed), any given frequency will have a defined distance traveled by the wave during 360 electrical degrees. CB is called 11 meters for a reason. One cycle at 27.205 MHZ is about 11 meters long. If we slow down the wave the distance is lowered for one full cycle. Not hard to see if the wave is slower it travels less distance during 360 degrees. On channel 20 the time is 1/27205000 of a second for 360 degrees, conversely in one second the wave cycles 27,205,000 times. The point being adding more dielectric at the top makes the antenna 'appear' longer to the RF wave because the velocity factor is lower.

Have you never had a 102" stainless whip well tuned for CB and not wondered why the fiberglass version is 96", not due to winding compressing resonant length? Dielectric constant slowing down the wave requires less antenna length to be resonant on the same frequency. This is not rocket science nor is it hard to understand, i.e., removing the tip raises the frequency of the resonant point of the antenna whereas putting it on lowers it. By 10 to 20 channels depending upon the size, thickness, and dielectric constant of the plastic used to make the tip.

 

Outlaw CB
OK if you insist I'll argue the point.
While what you say about dielectric constants of plastics reducing the velocity factor and making coated antennas shorter than bare wire versions is true, the effect is miniscule when applied to only a small part of the antenna
Also the tip is the least sensitive to the plastics effect, the closer you put it to the feed point the more effect it has.
I built two identical models (dipoles) 12 Ga. cu wire optimized for lowest SWR (1.44) then coated the top 20 inches with 40 thousands inch thick "soft PVC" (dielectric constant = 4.5) and got a SWR of 1.45 . That 0.01 is well beyond the sensitivity of the average SWR meter. and completely insignificant regardless of the accuracy of the meter.

Just 10 inches of the same PVC just above the feed point throws the SWR up to 2.32.
Making the 20 inch "tip" a full 1 inch in diameter only raised the SWR to 1.47 still negligible. at 0.03 difference.

 

Not rocket science to know a dielectric stores charge and is not affected by a magnetic field. The feedpoint is a lower impedance than the tip. Therefore the voltage is higher at the tip than the feedpoint which one would hope is close to the coax impedance. Current is highest at the point of lowest impedance, voltage is lowest, at tip conditions are reversed, high school ohms law. Since the voltage is higher (since the impedance is higher) at the tip more energy is stored in the dielectric meaning greater effect from the dielectric. From :

http://www.cv.nrao.edu/course/astr534/AntennaTheory.html From this:

"The radiated electric field strength E is proportional to the integral of the current distribution along the antenna. The current at the center is just the driving current I=I0e−it and the current must drop to zero at the ends of the antenna, where the conductivity goes to zero."

Since this is a dipole the "current at the center" is the feedpoint when considering a monopole, i.e., a typical CB antenna used in mobile stations. If not too difficult I suggest you look at half the dipole to consider voltage VS length in a monopole. Since "current must drop to zero at the ends of the antenna, where the conductivity goes to zero", the impedance is therefore highest. Ohms law dictates at a given power level this point is where the voltage is highest. Not difficult to see since the driving source at the feedpoint accelerates electrons in the antenna. They are accelerating towards the tip. Again the point of highest voltage is the point most affected by a dielectric as the plastic (dielectric) is not affected at all by magnetic fields. Yes, even a plastic coated refrigerator magnet sticks to you guessed it, your refrigerator. Through the plastic. Sorry this theory is above you from reading your posts but you can argue away now I will not make another post here. No doubt the engineer Firestick consulted to make their point '58' has no time to argue it either. Too many important things to do and little time to continue this 'argument'.

 

See post #11 .........................................................

 

right there with you mike5511.

 

He said in defense of of his indefensible position.
If you see something I said that's in error on some other post/thread you should counter it there not snipe from the weeds!
Go ahead run, hide. Every one will assume you are wrong and I win by default.Because you wont defend your position.

Interesting fact: I read the whole page. You can't scare me with higher math, 3 semesters of calculus and differential equations .
Integration and differentiation aren't hard to do.
I still didn't see any thing in there that supports your position.
Or has the slightest thing to do with dielectrics or coatings.
.

Nether is a frog but high enough magnetic fields can levitate a frog ( I wonder what the dielectric constant is for a frog).

I applied the same technique to a 694 segment car with whip model over ground. and the SWR didn't change even the .01 of the dipole models.
You must think theat the models are misrepresenting reality in some way, so what part do you doubt the NEC code written by Lawrence Livermore National labs for the Navy?

or maybe the derivation of the effect of the dielectic constants effect on antennas
described by L.B. Cebik in ‘www.cebik.com/amod/amod83.html'
(if that is still a good link)

Code:

[B][FONT=Arial]L = 2e-7 * (diel * R/r) ^  (1/12) * (1 - 1/diel) * Ln(R/r)[/FONT][/B]
 
 [FONT=Arial]L           Value for  distributed inductance in Henry/meter[/FONT]
 
 [FONT=Arial]diel        Dielectric  constant (as specified in the LD7 command)[/FONT]
 [FONT=Arial]Ln         Natural  Logarithm[/FONT]
 [FONT=Arial]R          Radius of of  wire plus insulation (as specified in the LD7 command)[/FONT]
 [FONT=Arial]r            Radius of  wire (as specified in the corresponding GW command)[/FONT]

Maybe you think my models are flawed ?
I'll happily post the models I used and then any one can check them for themselves.

I didn't see his post! Are you trying to say that the OP Pilgrim007 is the engineer firestick.consulted ?

Perhaps you are just basing your position on info released by firestick, cause we all know that manufactures are always right and they never mislead their customers.
You didn't argue for #3 being true so you must agree that firestick is selling snake oil there.
But #58 is right because the engineer doesn't have time to argue the point?

And BTW I never claimed to be an expert or guru.
My formal education is not in electronics or RF theory.
It's just a hobby for me so my job doesn't depend on me being right.
If you prove me wrong I'll freely admit to being wrong and won't insult you and run away with

Run Chicken !
Or argue your point. If you don't maybe I'll follow you around and post "CHICKEN" with a link to this thread in response to your future posts.

 

Why should every CBer know this?

 

A very good question......(but I like to read these kind of threads anyway......)