Question for the guru's

[ghz24]

If we take it a step further, then a 1/4WL antenna has around 35 ohm impedance, not the 50 ohm our rigs are designed for[/QUOTE
Where does 35 ohms come from ?

Last I knew a whip was resonant at about 70 or so ohms, SWR about 1.4 , about the same as a dipole. We could cut a little of and get a better SWR 1.33 or so but the reactive component increases as the real decreases.

I'm sure a multitude of variables effect this.
I guess it could be the perception of what is "low SWR" , to a 4-12 watt cb 1.3 -1.4 is quite acceptable

a number of base loaded CB antennas, like the Wilson and the Sirio, already have a shunt feed arrangement in their base, these antennas have the very matching device that Alan is referring to

But none of them out performs a whip with no match.
They only need the match because they are substantially shorter than they should be.
Just like you can't put a~ 60 foot whip on your car for 80 meters so it needs a matching device.
Just pointing out how the statement falls down at CB frequencies (and above).

 

[Captain Kilowatt]

Whoa . . . . Look at this . . . A piece of Radio Shack RG-8 coax from the late 70's with the cover peeled off . . . My, my, my . . .

I used a LED flashlight to get more detail. As one can easily see, that is not what one would call 'shielding/braid'. Better to call it 'lack of shielding'. Unbelievable.

Can you imagine using this stuff for 2m???? No foil and almost no braid = No chance. The spaces between the braid are much wider than the braid itself. Wouldn't even use it for the HF band, although it would barely pass for usable there at those freqs. Losses would be considerable @ 100 ft compared to just about any other coax.

This would be an example of really poor quality coax . . . Pure junk . . .

Late 70's hey? If it was Tandy cable I would have suspected it was better than that. I bought my first cable at R/S around 79 or 80 and it was much better than that. It was polyethylene insulated RG8. The RG8X mini foam seems to have stood up better over the years as opposed to RG8 and especially the RG-58. I do remember the RG-58 as being pretty bad. I may use the cable you show above for shielded audio cables or maybe shielded DC cables but even for HF it looks pretty bad. 2m?? Not in this lifetime or the next.

 

[W5LZ]

ghz24,
That 35 ohms isn't exact, but it's the ball-park input impedance for all 1/4 wave antennas. The actual input impedance ranges from between about 25 ohms to maybe 40 ohms. It's always lower than 50 ohms. Where does that come from? From measuring the input impedance of 1/4 wave antennas. Not SWR, but the actual impedance. An SWR of 1.4:1 can be either 35 ohms or 70 ohms. An SWR meter can't tell you which it is.
- 'Doc

 

[ghz24]

The actual input impedance ranges from between about 25 ohms to maybe 40 ohms. It's always lower than 50 ohms

So a 105 inch whip @ 27.18 MHz will be resonant at ~ 35 ohms not 70 ohms?
Cause I quoted a resonance point (well close to resonance) not just a SWR.
Element length =104.88
Impedance = 71.8 +j0.18
Would a starduster type antenna count as a 1/4 wave? What about a dipoe same question?

 

[northern35s]

So a 105 inch whip @ 27.18 MHz will be resonant at ~ 35 ohms not 70 ohms?
Cause I quoted a resonance point (well close to resonance) not just a SWR.
Element length =104.88
Impedance = 71.8 +j0.18
Would a starduster type antenna count as a 1/4 wave? What about a dipoe same question?

The angle of the radials on a GP antenna affect the impedance, horizontal ground planes would produce an impedance around 35 ohm, radials at approximately 45 degrees down will have around 50 ohm and of course a vertical radial (as in a dipole) will have around 70 ohm impedance.

Because a car body isn't an efficient groundplane, then anything much higher than around 35 ohms for a 1/4WL antenna means you have loss in the system, bonding the car body may well help lower the figure

 

[Captain Kilowatt]

Antenna impedance at resonance plus ground loss resistance equals total antenna impedance. If a 1/4 wave antenna has an impedance of 35 ohms and the system has 15 ohms of ground losses then you will see a total impedance of 40 ohms. Antenna impedance consists of more than one factor.

 

[W5LZ]

"So a 105 inch whip @ 27.18 MHz will be resonant at ~ 35 ohms not 70 ohms?"

How did you establish resonance?
There is no particular impedance that you will find at resonance, it can be almost anything. Nothing says 50 ohms means resonance. Resonance has nothing to do with impedance matching, or SWR.
- 'Doc

 

[JCH]

OK, so will painting the whip affect the receive? I painted it with some dark grey primer, looked real good and I've read on this site of other folks painting without affecting the antenna. thanks guys. I just sanded the primer off so I'll check it out for myself Tues.

http://www.homedepot.com/catalog/pdfImages/e7/e7c4ef52-4018-4226-bb55-5d48c1ffa906.pdf

Looking at the ingredients I'd guess the antenna could be affected negatively.

 

[ghz24]

How did you establish resonance?

That would be where the Imaginary part (the reactance) is 0

There is no particular impedance that you will find at resonance, it can be almost anything.

Then 35 ohms is from the assumption that a car body most closely resembles a flat ground plane?

Nothing says 50 ohms means resonance. Resonance has nothing to do with impedance matching, or SWR.

Don't think I ever thought that let alone hinted at or stated it.
But it doesn't matter 35 or 70 either way it's a swr of ~1.4-1.3 so for barefoot CB the whip doesn't need an impedance match and it out performs it's competitors that do.

Or is that wrong?

 

[W5LZ]

"How did you establish resonance?
That would be where the Imaginary part (the reactance) is 0
... Right, but how did you extablish resonance or '0' reactance?

Quote:
There is no particular impedance that you will find at resonance, it can be almost anything.
Then 35 ohms is from the assumption that a car body most closely resembles a flat ground plane?
... Actually, it was by measurement of several different setups.


Quote:
Nothing says 50 ohms means resonance. Resonance has nothing to do with impedance matching, or SWR.
Don't think I ever thought that let alone hinted at or stated it.
But it doesn't matter 35 or 70 either way it's a swr of ~1.4-1.3 so for barefoot CB the whip doesn't need an impedance match and it out performs it's competitors that do.

Or is that wrong?
... Well, I would have to say that it isn't exactly 'right'. The difference in performance between a 35 or 70 ohm input impedance isn't going to be huge by any means. But the actual performance does depend on how 'tuned' the antenna is. That 'tuned' means BOTH resonance and impedance matching, not one or the other.
So, how did you determine resonance? If you want to get closer on the impedance match there are several ways of doing that which does not affect resonance.
- 'Doc

 

[Marconi]

I don't know if these models apply to the topic of this thread, but I see some guys claiming a 1/4 radiator has a 35 ohm impedance, but I seldom see that happen.

My real world testing of the 1/4 wave a few years ago didn't reveal that to be true as stated, and later when I learned to model using Eznec, it didn't prove that the 1/4 always shows a 35 ohm impedance either. I use to think the impedance at the end of a 1/4 wave radiator was always 35 ohms too. The argument that a 1/2 wave dipole showed about 73 ohms at resonance made a very convincing argument for the 1/4 showing about half of what a dipole did.

Since then, very few ever talked about feed point height, among other factors, that can effect the feed point impedance, so I did the following models to show how the feed point impedance for a 1/4 wave varies.

I'm not even getting into how the 1/4 wave radiator act on a mobile, because it is near to impossible to predict how that shape and size effects the ground plan that the 1/4 wave radiator needs to work predictably.

This fact probably helps explain why we hear so many stories about the success and failure for 102" SS whip installs on mobiles.

#1, is a resonant 1/4 wave whip with 1 suitable horizontal 1/4 wave radiator in the common L configuration at 32' feet. It shows a near perfect match at resonance, but a terrible pattern with a maximum vertical angle above 60* degrees. I think 'Doc is suggesting this weak design for a 1/4 wave with a very good match that doesn't talk worth a flip. Some autos might emulate such a pattern very well depending on install and how the ground plan works, yet it shows a super match which are often serious bragging rights for some owner/operators.

#2, model is a 1/4 wave with 4 x 1/4 wave horizontal radials. Again we see resonance, but the match is a bit high. The pattern is acceptable and workable, and we see the antenna dimensions get longer in order to produce resonance with a very low resistance part for the match. Again this match is still no where near 35 ohms, unless I set the model at a height of 17.75' feet above real Earth. Do you think this is where the idea for a 1/4 wave having an end impedance of 35 ohms came from?

#3, model is a 1/4 wave with 4 x 1/4 wave slanted down at 45* degrees radials that requires the antenna to become considerably shorter in radiator and radial lengths. Here we see a little better pattern, with a little improvement in gain, and again we see a near perfect match. However the maximum angle of radiation as actually gone up a bit, and this is contrary to many others who claims that slanted down radials on a 1/4 wave actually lowers the maximum angle. Not true again.

#4, model is similar to the original Starduster with slanted down radials at about 17* degrees. This configuration still has shorter elements than model #3, showing a bit better pattern with an improved maximum gain, but the angle is the same in the lower lobe and a bit lower in the upper lobe. So, in this sense only is the angle lowered as a result of slanting the radials down. Here you can see the results and not just read some words.

In order for me to tweak these models to show no reactance I had to tune using very small increments, maybe less that a .64 of an inch as I got close to Zero reactance. So when you get very close to perfect your adjustments are likely to be extremely small and maybe you didn't know that either. Here you could see the small differences if I posted the wires definitions...but that is just fodder that most don't understand and nobody has ever asked me one question on how to understand the data entry form for this software.

Now I ask you...did you see one 1/4 wave model here that showed a 35 ohm impedance? If I showed you a 1/2 wave resonant dipole, I would bet you would see an impedance near 70-73 ohms however. Go figure.

View attachment Quarter wave radiator..pdf

If you don't understand a little about what these models are revealing here...then just ignore my post. Believe me when I say: "...before I learned to model with Eznec I didn't always know how to consider or what to believe about antennas either. Sometimes I found my ideas were confirmed however, but I could not really say in words anything that was really convincing to prove my point. I saw things happen in my real world testing too...that I could not put into words. Sometimes modeling reveals things in a way that I could understand, and then I present my models with a hope to pass on some that understanding, but often it is all in vain me thinks.

I hope this helps someone, it looks really clear and revealing to me.

 

[W5LZ]

Marconi,
Those examples do show that the angle of 'droop' of the radials of a typical groundplane antenna determine it's input impedance. Of those examples, the second one is probably the most commonly thought of what a 'groundplane' antenna looks like. It had an input impedance of around 23 - 25 ohms, right? As that 'droop' got larger the input impedance went up until it was 'close' to what a vertical dipole's input impedance is.
You have to realize that 35 ohm thingy is a generalization, sort of the average guess, it's not exact at all. If you average the high and low examples, what does the -average- or typical input impedance come out as?
I think you just 'proved' that 35 ohm -generalization-.
Another aspect of modeling programs is that they are ALWAYS a generalization, never exact unless you provide a huge amount of input data to show the antenna's environment and how the thing is constructed. Until you can provide that level of input data the results are never going to be 'exact' by any means. (Don't know about you, but I have no way of accurately finding all that data.) So, while the modeling programs are certainly useful, I don't think I'd bet the farm on them.
- 'Doc

 

[The DB]

Marconi, could you model a quarter wave vertical antenna over a "perfect" horizontal infinitely large and infinitely conductive groundplane and see what feedpoint impedance you get?

I seem to remember reading that this was the basis for the 36 ohm baseline figure that I personally use for quarter wavelength antennas. On my end I'll see about cracking open a few books and try to verify that.


The DB

 

[Marconi]

Marconi,
Those examples do show that the angle of 'droop' of the radials of a typical groundplane antenna determine it's input impedance. Of those examples, the second one is probably the most commonly thought of what a 'groundplane' antenna looks like. It had an input impedance of around 23 - 25 ohms, right? As that 'droop' got larger the input impedance went up until it was 'close' to what a vertical dipole's input impedance is.
You have to realize that 35 ohm thingy is a generalization, sort of the average guess, it's not exact at all. If you average the high and low examples, what does the -average- or typical input impedance come out as?
I think you just 'proved' that 35 ohm -generalization-.
Another aspect of modeling programs is that they are ALWAYS a generalization, never exact unless you provide a huge amount of input data to show the antenna's environment and how the thing is constructed. Until you can provide that level of input data the results are never going to be 'exact' by any means. (Don't know about you, but I have no way of accurately finding all that data.) So, while the modeling programs are certainly useful, I don't think I'd bet the farm on them.
- 'Doc

'Doc, are you telling us that CB guys that always claim an end fed 1/4 wave will show 35 oms of impedance are closer to the facts than what the licensed method of moments algorithms used with Eznec can produce even if the model was not produced and certified by LB Cebik?

You and I may accept the fact that this reference probably is a generalization, and that it is an accepted idea to be about 1/2 the value as stated for a 1/2 wave dipole. But I've never heard anybody that makes such a claim for any 1/4 wave ground plane say it has a 35 ohm impedance and that the value is "generalized." You just made that up in order to support an argument.

I'm not even making a claim that my Eznec models are perfect, but I think they can show the condition of an end fed 1/4 wave radiator a little closer to the facts...than some claims we often hear from CBr's.

Knowing you 'Doc, I would image you to be among the very first to Johnny on the Spot asking some CB'r where he got the idea that the feed point impedance as the bottom of a 1/4 wave antenna was in fact 35 ohms.

I'm just trying to make a similar point to what you did with in some other post, I think maybe even in this same thread: when you asked the poster, "...how do you determine resonance?", and after the guy tried to explain by saying in response, "...when the value of reactance equals Zero." I kinda' got your point on that one and did not raise some meaningless comment to confuse what you were trying to do.

Just to show how close such a claim might be noted by Eznec in another situation on this subject, and using the center fed dipole...here is a model at the same height of 32' feet, and one at 18' to compare as being fairly accurate with what theory reports to us about the feed point impedance for a center fed dipole at a couple of different heights. We don't see any need here for any "generalization" idea, do we?

View attachment Center fed .50w dipole.pdf

Do I think anything we do here will change any body's mind about the subject I tried to raise...not on your life 'Doc. CBr's hardly ever change their minds about anything they have in their imagination, and that too is a fact... that time and experience has taught me.

 

[W5LZ]

"'Doc, are you telling us that CB guys that always claim an end fed 1/4 wave will show 35 oms of impedance are closer to the facts than what the licensed method of moments algorithms used with Eznec can produce even if the model was not produced and certified by LB Cebik?"
No, that's not what I said at all. I'm not claiming any special knowledge that contradicts EZNEC or any of the other antenna programs. If you could ask Cebic he'd tell you the same things about his program and generalization. All those algorithms are by definition approximations!
As for changing your mind, I have no pretensions of being capable of that...
- 'Doc