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CTStallion · #1 09-05-2010, 03:25 PM

Well, the XYL didn't have any "Honey-Do's" for me this Labor Day weekend, so I figured I'd take a few moments and model some of our favorite CB Antennas.

Please note that I am using EZNEC DEMO VERSION, which limits me to only 20 segments, therefore all antennas were modeled at 20' above REAL GROUND (at the feedpoints), using #10 awg insulated wire as the elements (exception: Dipole was modeled using #14 insulated wire).

FIRST ANTENNA: 1/4 WAVE OMNIDIRECTIONAL VERTICAL WITH FOUR (4) 1/4 WAVE RADIALS (again, this antenna is at 20' high at the base).

1st attachment shows the antenna and it's currents

2nd attachment shows TOA (Take Off Angle AKA: Elevation Angle) provides a maximum gain of 2.12 dBi (dB gain over a theoretical isotropic radiator) at a 42.0 deg TOA

3rd attachment shows only 1.0 dBi gain at a 12 deg TOA (Elevation Angle, Launch Angle... etc...)

So... how do we make the 1/4 Wave Vertical perform BETTER....

ENTER THE STARDUSTER (AKA: M-400), next installment

CTStallion · #2 09-05-2010, 03:30 PM

To make a 1/4 wave vertical perform better (i.e., lower TOA, better match to 50 ohm coax, less likelihood of common mode currents on the outside <shield> of your coax), we slope the radials downward to achieve the following model:

1 Shows the currents on a Starduster
2 Shows that by sloping the radials downward, we have lowered the TOA max gain down to 13.0 degrees with a maximum gain of 1.31 dBi.

But... how do we then INCREASE THE GAIN OF AN OMNIDIRECTIONAL VERTICAL?????..........

ENTER THE 5/8 WAVE OMNI (next installment)

CTStallion · #3 09-05-2010, 03:40 PM

The 5/8 wave vertical I am modeling here is 21 feet in length, and incorporates four (4) 1/4 wavelength horizontal radials.

Picture 1 shows the antenna and it's maximum current flows
Picture 2 shows that maximum gain (at a 20' feedpoint over real ground) is 2.86 dBi at a TOA launch elevation angle of 32 degrees.
Picture 3 shows that a a 10 degree TOA, the 5/8 wave beats BOTH the 1/4 wave groundplane AND the Starduster (M-400) by offering 2.35 dBi gain at a nice low 10 degree TOA.

Please do note however that a 5/8 wavelength radiator is NOT a direct match to 50 ohm coaxial cable; a matching device is necessary. This is WHY antenna manufacturers (i.e.: Maco, I-10K, Sirio, etc.) place a coil at the feedpoint to make it resonate (electrically appear to be an odd multiple of a quarter wave to the coax) in order to obtain a 50 ohm match. But why not just make the radiator length 3/4 wave? Because that would create unwanted high angles of radiation which we do NOT want. More on that later.

So... is there any way to improve on a 5/8 wave vertical with horizontal radials???

Enter: THE 5/8 WAVE OMNI WITH SLOPING RADIALS in the next installment.

CTStallion · #4 09-05-2010, 03:50 PM

The model below shows what would happen if you sloped your 1/4 wave radials on a 5/8 wave vertical. The TOA does NOT drop (like it did on the Starduster vs. 1/4 wave Vert), BUT the gain DOES increase somewhat, and the sloping radials will certainly help to decouple the coax.

So.... ever wonder what would happen if you added one of the radial kits to your IMAX 2000 or your ABS-1600??? You got it.... no drop in TOA, merely MORE OMNIDIRECTIONAL GAIN and less likelihood of TVI. (Both of which are good things <as Martha Stewart would say>.

1 is the Current profile of the antenna
2 is the Max gain and TOA profile
3 is the low angle performance

(Please note that these models do NOT apply to the Anttron or Solarcon 99 style antennas, Ringo Rangers, or any other kind of END FED HALF WAVES; merely 5/8 wave (21 - 24 ft radiators).

Since omnidirectional GAIN is a 'good-thing', is there any way to increase the gain of a 5/8 wave antenna? I'm glad you asked............

Enter the 5/8 WAVE VERTICAL WITH FOUR (4) SLOPING 5/8 WAVE RADIALS in the next installment.

CTStallion · #5 09-05-2010, 04:02 PM

Okay, this antenna is LARGE. The vertical radiator will be 21 - 24 feet in length, and each of the four radials will be the same length as the vertical element (21-24 feet each).

But, does it work?

I'd have to say that since it only offers an additional 0.03 dBi improvement in gain, and probably would be a bear to match to coax, I'd have to say it ain't worth it. Well, then WHY did I bother modeling it? Well, let's say you want to place your vertical on a 20' pole which you need to guy.... u could use 5/8 wave radial guy wires and take advantage of the extra gain offered in the process.

Picture 1 shows the current profile
Picture 2 shows the maximum gain & TOA profile

So.... having exhausted the 5/8 wave arena.... what say ye CTStallion about the .64 wave antennas (i.e.: IMAX 2000??? and the like).

.64 Wavelength antennas revealed in the next installment.

Shockwave · #6 09-05-2010, 04:12 PM

I'm curious as to the effect the sloping radials on the 5/8 wave has on the impedance and VSWR at the resonant frequency? Thanks for sharing your work in the forum too.

CTStallion · #7 09-05-2010, 04:18 PM

Okay, so does a .64 wave radiator really outperform a 5/8 wave antenna? If so, then WHY do all the so called antenna 'Gurus' out there claim that the maximum gain figure for a monopole element is at 5/8 wave?

The answer lies in the fact that there is little difference between 5/8 wave and .64 wave. Simple mathematics reveals that 5/8 is deconstructed as .625 wave. Therefore, how much difference is there between .64 and .625..... hardly any.

But, for all you IMAX 2000 lovers out there, I offer the following:

Picture 1 shows a .64 wave (24' long vertical radiator) with four (4) 1/4 wave horizontal radials (I know, I know... the IMAX 2000 doesn't have any radials, but I needed to model the antenna against a counterpoise). Besides, remember what we learned before on how the SLOPING radials will increase the gain, but not the TOA. Picture 1 also shows the rf current profile of the antenna.

Picture 2 shows the max gain to be 3.37 dBi at a TOA of 28 degrees. Hmmmm.... the 5/8 wave vertical had only a 2.86 dBi max gain at a TOA of 32 degrees, so maybe there IS something to a .64 (24' long) vertical radiator vs. a 5/8 wave (21' long) vertical radiator. WOW!!! (lightbulb in head glows brightly now). And, if I put SLOPING RADIALS ON MY IMAX 2000, there's no other omnidirectional antenna that can touch me (maybe)

Picture 3 shows that the gain at a low 10 deg TOA of my .64 radiator to be 2.42 dBi. How does that compare to a similar radial'ed 5/8 wave..... a mere 0.07 dBi increase... not even perceptible (which is why I use an Army Stick instead of an IMAX 2000)..... but I digress.

But again, as with a 5/8 wave radiator, 50 ohm coax doesn't like the .64 wave radiator, so what if we increased the length of the radiator to an odd multiple of 1/4 wave in order to see 50 ohms?

ENTER THE 3/4 WAVELENGTH MONOPOLE in the next installment.

CTStallion · #8 09-05-2010, 04:23 PM

In the model below, please find the 3/4 Wavelength Monopole (a beast approximately 27-30 feet in length) fed against four (4) 1/4 wave horizontal ground radials. Again, the feedpoint is at 20 feet above real ground.

Picture 1 shows the antenna RF currents
Picture 2 shows the awesome gain, BUT, it is all at HIGH TOA OF 55 degrees. This is pretty undesired.

How can we lower the wave angle of a 3/4 wave monopole to make this large beast useful on 11 meters????

ENTER THE SIGMA IV (aka Sigma 4, Vector 4000, LW-150, etc) in the next installment.

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