EZNEC Models of Your Favorite CB Antennas

[jazzsinger]

We run vertical on CB because we want to work other bases and mobiles which are vertically polarized as well, and we want to take advantage of ground wave propagation for local / area work.

one minor point,groundwave barely exists at 11m as pointed out in another thread by shockwave,it is line of sight that predominates at higher hf frequencies.

a better term is line of sight or surface wave which takes in both ground wave and line of sight as opposed to skywaves used for dx.

on the whole a very interesting piece of work.might see slightly different patterns if you include the transmission line,but i realise you are limited within the confines of the modelling program you used and that wouldn't have been possible,but it does give people a good general idea of whats going on.

another minor point would be the halfwave dipole is only a good dx antenna when in free space,below that height ground reflections tend to cancel out low angle radiation and leave high angle radiation which isn't as good for dx.

like shockwave i'm also curious the effect sloping radials has on the 5/8 wave swr/impedance.i remember back in the early 80's a couple of 5/8 wave antennas having sloping radials but sadly never got the chance to try one.

i'd also be curious to see what effect upward sloping radials has on a 5/8 / .64 wave as modelled in your sigma 4 model.

 

[W5LZ]

Sloping radial.
It's fairly logical to see how they might affect things. Take a 1/4 wave vertical with horizontal radials (90 degrees). The input impedance is something on the order of 20 - 30 ohms. Start sloping those radials downward and the input impedance goes up till they reach straight down (180 degrees), which makes that input impedance equivalent to a center fed 1/2 wave, about 75 ohms. The greater the angle between vertical element and radials increases impedance.
But what if we start with a high impedance at the feed point, such as an end fed 1/2 wave? Increasing the radial angle from 90 degrees to 180 degrees would also increase the input impedance to something on the order of a full wave center fed doublet, probably around 150 ohms. So, go the other direction, make the angle between the vertical element and the radials less than 90 degrees. The input impedance will start decreasing. It would finally be zero ohms impedance when the radials get to zero degrees (short). Somewhere in between that 90 and zero degrees you'll find 50 ohms. Basically what happens with a 'J'-pole, isn't it??
- 'Doc


(Commonly called the 'Monroe Doctrine'. 'Marilyn Monroe', that is! Tight skirts, more 'impedance'. A blown up skirt, much less impedance! Right?)

 

[HomerBB]

Only thing I can reply to is the SWR peice of 5/8 with sloping radials. never did it with radials straight out:

Final SWR:

28.305 2.0:1
27.855 1.2:1
27.405 1.0:1
27.185 1.0:1
26.965 1.0:1
26:515 1.2:1
26.065 1.5:1

From thread:http://www.worldwidedx.com/home-brew-mods/33039-my-first-attempt-build-5-8-wave-homemade.html

 

[jazzsinger]

Sloping radial.
It's fairly logical to see how they might affect things. Take a 1/4 wave vertical with horizontal radials (90 degrees). The input impedance is something on the order of 20 - 30 ohms. Start sloping those radials downward and the input impedance goes up till they reach straight down (180 degrees), which makes that input impedance equivalent to a center fed 1/2 wave, about 75 ohms. The greater the angle between vertical element and radials increases impedance.
But what if we start with a high impedance at the feed point, such as an end fed 1/2 wave? Increasing the radial angle from 90 degrees to 180 degrees would also increase the input impedance to something on the order of a full wave center fed doublet, probably around 150 ohms. So, go the other direction, make the angle between the vertical element and the radials less than 90 degrees. The input impedance will start decreasing. It would finally be zero ohms impedance when the radials get to zero degrees (short). Somewhere in between that 90 and zero degrees you'll find 50 ohms. Basically what happens with a 'J'-pole, isn't it??
- 'Doc


(Commonly called the 'Monroe Doctrine'. 'Marilyn Monroe', that is! Tight skirts, more 'impedance'. A blown up skirt, much less impedance! Right?)

Not neccessarily so on .5/.625 and .64 verticals doc,they all have one thing the 1/4 wave gp and 1/2 wave dipole don't have,a coil to match base impedance from a very high value.

i would imagine on these antennas you would have to compensate on the coil tap point for the changes induced by changing radial angle.

as you point out a j pole has a zero degree radial,but it doesn't have 0 ohms feedpoint so do skeleton sleeve monopoles,but their feed impedance ain't 0 ohms either,your thinking may be logical but it is flawed when applied to antennas.

as for your marilyn monroe analogy,i think at zero impedance their is still a serious danger of getting a slap if you tried to utilise that lack of resistance.

 

[W5LZ]

Those antenna that have a coil, what if they didn't have that coil? The impedance would act like I said. That impedance (probably) wouldn't be very desirable, true, but it would act how I said.

Ah, but I didn't point out that a 'J'-pole had a zero degree angle radial. By moving that 'stub' further away from the vertical element things 'act' as the 'angle' is greater than zero. One way of thinking of that is "how much of that 'radial' can the vertical element 'see'. Because the vertical portion of that 'stub' appears shorter than it really is, it's the same as it being shorter, a matter of perspective. That analogy takes some imagination, but it works for me.

And the 'Monroe Doctrine' thingy. It's also a matter of 'perception'/imagination! You can get an 'impression' of a lot of things. Certainly doesn't mean that 'perception' is fact! Although, after some thought, a slap or two might be worth it!! I don't know about you, but I think I'd be sort'a proud of getting slapped by Marilyn, you know?
- 'Doc

 

[jazzsinger]

Those antenna that have a coil, what if they didn't have that coil? The impedance would act like I said. That impedance (probably) wouldn't be very desirable, true, but it would act how I said.

Ah, but I didn't point out that a 'J'-pole had a zero degree angle radial. By moving that 'stub' further away from the vertical element things 'act' as the 'angle' is greater than zero. One way of thinking of that is "how much of that 'radial' can the vertical element 'see'. Because the vertical portion of that 'stub' appears shorter than it really is, it's the same as it being shorter, a matter of perspective. That analogy takes some imagination, but it works for me.

And the 'Monroe Doctrine' thingy. It's also a matter of 'perception'/imagination! You can get an 'impression' of a lot of things. Certainly doesn't mean that 'perception' is fact! Although, after some thought, a slap or two might be worth it!! I don't know about you, but I think I'd be sort'a proud of getting slapped by Marilyn, you know?
- 'Doc

if they didn't have a coil they'd be a bitch to match with impedances in the thousands of ohms,i don't fancy running a 20:1 or more swr.but you may well be right how it would act.who knows? no-ones daft enough to try it without some form of matching network.

i know you didn't point out a j pole had a zero degree radial,i did.which was the point i was making.

a slap or two of marilyn,hmmm,can't argue with you there Doc.must be better than monica biting it off,lol.

 

[Marconi]

Homer, be sure and keep us posted on how your new V4 compares to your original 5/8 wave. I knew you could get it to working if you just stepped back from it a little. BTW, the best bandwidth I saw with my Sigma 4 was 1.8 mhz while using a coaxial choke directly at the feedpoint and I never did get my antenna really tuned well, it was set at the recommended dimensions.

CT, thanks very much for the straight forward approach with you modeling efforts covering all but a end fed vertical 1/2. I guess it would end up looking just like the Starduster model any way, so no loss.

Besides seeing how slanted radials affect impedance in the 5/8 wave that several have asked about, I would like to see a couple of your similar input results using more segments with Eznec+ from someone. Just to get and idea how much difference the number of segments makes to modeling with Eznec.

It may be too much to ask, but I would also love to see your Demo version model of the Starduster with the radials slanted down more, in the range of 25-30 degrees, and then with adding three or more radials. I request the Starduster model here, because the 1/4 wave radiator really needs the radials and IMO they may respond better to increasing numbers and sharper angles down.

Good work.

 

[bob85]

i can see where doc is comming from sort of,

if you are dealing with say a 5/8wave with high feedpoint impedance ( no matching ) as you sweep the radials up you create a transmissionline impedance which apears in parallel with the high antenna mode impedance of the 5/8wave, if you fold them up to where transmissionline mode impedance is 50ohms then almost all the current will flow in the transmissionline mode and little current flows in the portion of radiator above the radial sleeve ( unlike a j-pole ), you would have to extend the monopole to 3/4wave to lower antenna mode impedance in order to increase antenna mode currents in the monopole to make currents flow like a j-pole, the arrl open sleeve antenna article explains the situation in detail,

while the models posted here put the sigma where it belongs as the best vertical with the lowest radiation angle, what seems to be missing is the mast and feedline and their unavoidable commonmode currents that vary with mast and feedline length and spoil the textbook patterns shown here,

a properly adjusted vector has more of an advantage than indicated over a 5/8wave groundplane at low heights like 20ft above average ground and maintain an advantage at least up to 73feet above ground,

thanks for posting the models

 

[Shockwave]

I just wanted to add that in the field the Sigma maintains it's advantage over the 5/8 wave well beyond two wavelengths in height above ground. Where I work on FM broadcast this is only 20 feet above ground. While the 5/8 wave is not extremely popular on the broadcast band, I've replaced more then my share of them at heights between 6 and 10 wavelengths above ground to know the Sigma still beats the 5/8 wave in far field signal on the horizon at these heights.

 

[711]

Sorry, its a little off topic,but how did you learn to use the software? I'm trying to model a 3/4 element moxon but can't yet figure it out. The Moxon site has a calculator but it doesn't do a 3/4 element.

 

[distortion]

Very nice work! Thanks for the information.


ohhh and wtb: laser 400 plot

Josh

 

[CTStallion]

Sorry, its a little off topic,but how did you learn to use the software? I'm trying to model a 3/4 element moxon but can't yet figure it out. The Moxon site has a calculator but it doesn't do a 3/4 element.

Good question; I suppose that the 'strength' of a true MOXON (2 ele version) lies in it's superior front-to-back ratio, coupled with it's reduced size relative to a 2 element yagi, and it's 50 ohm match to standard coaxial cable with no need for matching.

I 'think' that if you extend a MOXON to either 3 or 4 elements, you may obtain greater forward gain, but it would be at the expense of a 50 ohm match to coax.

I just PLAYED AROUND with the demo software; I am by NO MEANS an expert at all. In fact, I've yet to model with tubing (I model all my antennas with WIRE elements, as that's all I ever build at home anyway: WIRE antennas). And, since the demo version only allows 20 segments, I'd never be able to model any kind of complex tapered tubing w/o the benefit of more segments. Therefore, all my models are extremely rudimentary.

Try it yourself and you'll get the hang of it in short order.

 

[Henry HPSD]

Hi there,

Another advantege would be that the high numbers are maintaind across the band where the Optimum 2el yagi will have a much smaller "peak" in FB.

The moxon can be calculated towards(for example) 4 or for example 6 elements:
Example:
4el moxon on a 3,5 meter boom (appox 11 feet) shows almost 9dBI and a FB 29dB
6el moxon on a 6,3 meter boom (approx 18feet) will show almost 10dBI and a FB of 27dB
Both antennas show 50 ohms.

Ofcourse the high gain on the short boom for the 3,5 meter version will reduce the bandwidth towards 500 Khz..but that is about the highest gain i have ever seen on such a boomlength for 11 meter.

It is the combination of the length of a element, its position and diamter that will give those results...but most of all...hours and hours of moddeling.

In aspect to the 3/4 element (think tubing?) in the normal eznec programs it is no problem to change diameter. I also notice there is a possibility to change diamter in the moxgen.
If you are refering to the length of the element (as the width of a moxon is approx 3/4 of the fullsize versio, well thats just a mather of placing "wires" rigth on the XYZ axes..
I am not sure i have answered your question. If not please be a bit more specific..
It is not you but my lack in understanding your language.



Kind regards,

Henry
11 meter Dx antenna systemx

 

[711]

I like the Moxon alot because I can put the Moxon on my roof and not have the roof distort the signal coming of the Moxon. It will be 50 ohms at the designed frequency without the use of a matching network. The Moxgen only allows a 2 element Moxon. I want a 3 or 4 element Moxon and the only way to build one is to use a modeling software. I also make wire antennas only because its cheaper and I can stealth the antenna on my roof with out the neighbors getting suspicious. I was able to build a 2 element and mount it on the ceiling of my garage and work Australia on SSB. What do I need to do to model a 3 or 4 element Moxon using EZNEC? I want to make it resonant to the Bowl 27.025 mhz. Thank you

 

[Henry HPSD]

you would need a normal vesion as there are to many segments involved.

Or you could download 4nec2 wich is free and dos almost equal.

However if your drop email ill be more than happy to share dimensions.
19sd348@planet.nl

Kind regards,
Henry

11 meter Dx antenna systemx