Best shortest mobile CB antenna??

[fourstringburn]

The combination of the cap hat and the top load coil were for tuning purposes,

I'm referring to the cap hat alone can't tune an antenna with no coil. Again your modeling program shows a 5 ft. antenna with a cap hat and no reference to a loading coil with or without and how much inductance is measured to tune at 11 meters. Important info left out of your sophisticated modeling program! A typical 5 ft. CB whip with a cap hat will tune well below 11 meters. Fact!

To say that a cap hat cannot be used to tune a shorter than 1/4 wavelength antenna is just plain wrong

That''s not what I said. I said "A cap hat on a whip with no coil won't tune properly" That means a straight whip less than a 1/4 wavelength with no loading coil won't tune up properly with or without a cap hat. I know because I have physically tried this and couldn't get a good match!

I repeat for a third time, a cap hat can't properly tune a NON- COIL LOADED WHIP that is LESS THAN A 1/4 WAVE. A shortened antenna has to have some inductance to match it.

It can tune a loaded antenna at 11 meters sort of speak but not without some pre- modifications to the antenna length and coil. It's not a case of a glass half empty or half full. Tuning a shortened antenna starts with the coil, not the cap hat. If you put a cap hat on a tuned 11 meter coil loaded antenna, some coil turns on the antenna will have to be removed due to capacitive cancellation of the inductance within the coil or shorten the antenna length or both.

That's why I suggested in an earlier post tuning a coil loaded antenna for 6 meters than add a cap hat and re-tune the coil by adding or subtracting coil turns to get 50 ohms. There is a point where a cap hat can be too big and it will reverse efficiency. Mobile Cap hats should be designed for a compromise of increasing efficiency but also accounting for size, weight, and wind loading.


It also appears the comparisons from the 3 antennas used the same antenna model throughout which proves your modeling program isn't unreliable, just the operator who entered the information. This is because you can't put a cap hat on the same antenna types as the other 2 and still get a match on 11 meters. I you disagree here, sorry to say but you haven't learned anything about cap hats yet.

Wow, do you really have that little faith in my modeling skills?

Apparently yes! I don't put much faith into modeling programs as the absolute end all of anything antenna related. They are useful tools but only as good as the operator uses it with the correct knowledge beforehand.

You left out some important details as I noted.

I have done far more experimentation than you seem to think, although in my case not directly with cap hats. I also have an AIM4170c VNA that I use when working with antennas

I'm sure you have and I'm aware of some of your nice equipment from other threads and I praised you on it.

Here you admit you don't have working experience with cap hats where I do and that makes a difference especially since I experimented with building my own antennas with and without cap hats. Nearly all my HF and CB experience comes from mobile operations.

Here is a question for you, have you ever made a cap hat that you can change the size of to tune the antenna?

I should ask you that question but I already know the answer is no.

Repeating myself again, Tuning a shortened antenna starts with the coil, not the cap hat.

Once a shortened loaded antenna is tuned, adding a cap hat will lower it in frequency to the next band or more. That's how I made a 20 meter coil loaded antenna resonate on 40 meters but it took experimenting with different coil turns to get it right. I also posted on how my screwdriver antenna that is designed for low bands down to 40 meters now tunes at 60 meters since adding the the cap hat. But...! it takes coil turn adjustments that's easy to do with a screwdriver antenna and that's why they are the most versatile mobile antenna to have for multi-band operations.

I'm sure you have to get the last word in so have at it.

I'll stand by and let some of the other knowledgeable people I know on this forum chime in if they care to.

 

[The DB]

OK then, time to go to other resources... How about the ARRL Antenna Book 22'nd edition, Chapter 9 pages 21 through 24.

Title of the section, 9.3.3 Other Methods of Loading Short Vertical Antennas...

First paragraph...

Inductive loading is not the only, or even the best, way to compensate for reduced antenna height. Capacitive top loading can also be used as indicated in Figure 9.32 to bring a vertical monopole to resonance. Table 9-5 gives information on a shortened 3.525-MHz vertical using top loading. The vertical portion (L1) is made from 2-inch tubing. The top loading is also 2-inch tubing extending across the top like a T. The length of the top loading T (±L2) is adjusted to resonate the antenna. Again, the ground and the conductors are assumed to be perfect in Table 9-5.

Right there, capacitive top loading can also be used to shorten an antenna. There is no mention, yet, of using any inductive loading along with the cap hat.

Here is figure 9.32, with its caption:

I don't see any coils present.

And table 9.5:

This is the measurements for L1 and L2 in the diagram above to tune a shortened 3.525 MHz shortened vertical antenna with nothing but a cap hat.

Now on to the second paragraph.

For a given vertical height, resonating the antenna with top loading results in much higher radiation resistance RR — 2 to 4 times. In addition, the loss associated with the loading element will be smaller. The result is a more efficient antenna for low heights. A comparison of RR for both capacitive top loading and inductive base loading is given in Figure 9.33. For heights below 0.15 λ the length of the top-loading elements becomes impractical but there are other, potentially more useful, top-loading schemes.

There it is, resonating an antenna with top loading. As of yet, there is no discussion on using a loading coil to tune the top hat.

They go on to talk about using more wires to shorten the size of the top hat, and the various structures that can be used. They then go into a formula heavy subsection called "Finding Capacitance Hat Size" that tells you how to make a shortened antenna with noting but a capacity hat.

Now is where it gets interesting. The next subsection called "Combined Loading"

As an antenna is shortened further the size of the top loading device will become larger and at some point will be impractical. In this situation inductive loading, usually placed directly between the capacitance “hat” and the top of the antenna, can be added to resonate the antenna. An alternative would be to use linear loading in place of inductive loading. The previous section contained an example of end loading combined with linear loading.

That is the section. Finding the capacitance hat size which talks about resonating an antenna with nothing but a capacity hat gets most of a page, and the fact you can add a coil which, according to the ARRL Antenna Book, is just used to make the capacity hat smaller for a given antenna, is almost treated as an afterthought.

I have all but three versions of this book as well as many others, including some written by engineers to teach engineers, and they all talk about using such a device without a coil, nowhere does it say that you must have a coil based load to go with the capacity based load... I can quote them if you would like...

Still further, there are plenty of images on the www that show antennas with cap hats and no loading coil. Apparently those people got said antennas working.

I mean you no disrespect, generally you give very good advice, but this time you are clearly wrong. A cap hat, by itself, can be used to resonate a shortened 1/4 wavelength antenna. No matter how many times you say otherwise won't make it true.

It also appears the comparisons from the 3 antennas used the same antenna model throughout which proves your modeling program isn't unreliable, just the operator who entered the information. This is because you can't put a cap hat on the same antenna types as the other 2 and still get a match on 11 meters. I you disagree here, sorry to say but you haven't learned anything about cap hats yet.

When it comes to the models, yes, the three models are very similar, but they are not the same. The results clearly show that. If they were exactly the same how would they all have different SWR curves, much less the differing impedances when tuned to near resonance? They all use the vehicle model I made based on my Ford Explorer, and the antennas are all five foot long, but that is where the similarities end.

Apparently yes! I don't put much faith into modeling programs as the absolute end all of anything antenna related. They are useful tools but only as good as the operator uses it with the correct knowledge beforehand.

I agree, and I know much about modeling. I have gone though great pains to find out where this modeling software is accurate, and where it isn't. I also run multiple checks on all of my models to ensure they are as accurate as possible. It is a skill, and you can only really develop this skill by actually modeling. A good grasp of theory is a great help as well, I can tell you that from experience.

Here you admit you don't have working experience with cap hats where I do and that makes a difference especially since I experimented with building my own antennas with and without cap hats. Nearly all my HF and CB experience comes from mobile operations.

Here is a question for you, have you ever made a cap hat that you can change the size of to tune the antenna?

I should ask you that question but I already know the answer is no.

So it is a no for both if us then. We are at the same point experience wise. Good to know.

Once a shortened loaded antenna is tuned, adding a cap hat will lower it in frequency to the next band or more. That's how I made a 20 meter coil loaded antenna resonate on 40 meters but it took experimenting with different coil turns to get it right. I also posted on how my screwdriver antenna that is designed for low bands down to 40 meters now tunes at 60 meters since adding the the cap hat. But...! it takes coil turn adjustments that's easy to do with a screwdriver antenna and that's why they are the most versatile mobile antenna to have for multi-band operations.

Odd, because page 21-16 of the book I quoted from earlier clearly disagrees...

They increase efficiency by adding capacitance to that portion of the antenna above the loading coil, effectively increasing the overall electrical length.

Increasing the overall electrical length, thus to compensate the antenna will need to be physically shorter. And here it is again in the next paragraph.

The larger the hat (physically), the greater the capacitance and the greater the effective increase in electrical length. Since less inductance is then required to resonate the electrically longer antenna, coil Q losses will also decrease.

You say otherwise, I know, but what you are saying simply doesn't make sense. Why would you use one load to make a given antenna length electrically shorter, just to use another type of load to make the given antenna length electrically longer. That would be less efficient than using a single load to achieve the desired electrical length as each load, including a cap hat, will add its own inefficiencies. I'm sorry but that just doesn't make sense.

I'm sure you have to get the last word in so have at it.

I'll stand by and let some of the other knowledgeable people I know on this forum chime in if they care to.

Sure I would love to see someone else chime in here. I doubt they will though.


The DB

 

[DJboutit]

55 use to make this antenna

Tunes from 37" to 43"

 

[fourstringburn]

I'll go back on my word about standing by for just a few things.

Yes I have working experience on building antennas with capacity hats. I have stated this already and you admit you don't. I'm basing everything I state here off of personal experience not what books say or any antenna modeling.

A cap hat on a non loaded whip or mast will not give you 50 ohms on any frequency just as you know a 108 whip on 11 meters isn't 50 ohms either. I have physically tried this for different bands for myself and I used Rig exert analyzers which aren't as sophisticated as yours but it is good enough for simple measurements. All a cap hat is doing in this setup is basically adding to the antenna length and moving the frequency to the next band or two, but as you go lower in frequency with a shortened antenna, the feed point impedance drops even lower. That's a fact I can once again personally attest to.

What ARRL book says on 21-16 about me being wrong, I challenge you to make your own antenna just like I did. If not Look at DX engineering website for Cap hats for Hustler antennas and they have a guideline diagram on what size cap hats and how many radials needed to make a higher band resonator like 10 meters for example be resonate for 20 meters. And as far as my screwdriver antenna, you want me to make a video to show you the difference between using a cap hat and without so you can see how I can pick up 60 meters from a 40 meter screwdriver antenna? Yes you can pick up a lower band when using a cap hat but at the same time you will start to lose the upper bands. Again I'm talking about real world and personal experience and your quoting books and computer modeling and no real world use.

I'LL save you the trouble here for DX engineering. Click link and go to page 8 and look at the columns. Look at the RM10 resonator. To get to 20 meters you need 6 -24 inch extended rods and that shows a frequency 13.1 MHz. Now that's below 20 meters so this is just a guideline to get you close. Since the coil is prefab and can't be altered, the rods will need to be trimmed. That's why I made my own antenna with a taller mast and my own coils so I can match it up myself and not be as large of a cap hat.

https://static.dxengineering.com/global/images/instructions/dxe-hr-1p.pdf

Your ARRL diagram ( 9.32) is not a mobile antenna by the way and it doesn't give impedance values or anything. BAD EXAMPLE. The next diagram shows low impedance values, hardly a tuned antenna being well less than 50 ohms. Didn't you notice that?

I think you misunderstood, I said I don't have faith in your antenna modeling skills and that how I answered your original question.

Things would make more sense if you would get your hands dirty and try some of these antenna projects like this for yourself.

Time for other to chime in...

 

[fourstringburn]

55 use to make this antenna

Tunes from 37" to 43"

Nice! Very Nice! Did you make that cap hat yourself?

 

[fourstringburn]

That pic of that mobile antenna is a good example of what I'm talking about. Notice it has loading coils on it along with the cap hat. I would assume that is well designed and would give a near perfect match on it's resonant frequency at or near 50 ohms.

10 to 1 if you took those coils off, your impedance would change to the point where it is no longer a good match. Capacitance alone can't effectively tune an antenna without some inductance.

Why, because it is too short to be a 1/4 wavelength and needs inductance to bring the impedance up to 50 ohms. The cap hat then reduces the losses in the necessary loading coil increasing the radiation efficiency and the cap hat also adds to the antenna mass therefore given the illusion of being a much taller antenna in a shorter package.

The drawbacks to antenna like this is due to the rigid mass, one tree branch or low clearance, there goes your antenna. I have tried the best SS steel potbelly spring on my cap hat but the wind loading is too much at highway speeds to keep it steady.

If height isn't an issue for you, I would recommend this antenna for a very effective CB antenna.

 

[fourstringburn]

Here is my current cap hat on a Little Tarheel HP antenna that tunes 6 thru 40 meters without the cap hat and using the 4 ft. stock whip. Now with the added caphat, I lost 6 meters and 10 meters is almost at the top of the coil but I gained the 60 meter band with about an 1 inch or so of coil left over.

But the ARRL hand book says this isn't possible.

 

[The DB]

Why is it I notice things after I am done posting... Anyway, revisiting something already stated.

Once a shortened loaded antenna is tuned, adding a cap hat will lower it in frequency to the next band or more. That's how I made a 20 meter coil loaded antenna resonate on 40 meters but it took experimenting with different coil turns to get it right. I also posted on how my screwdriver antenna that is designed for low bands down to 40 meters now tunes at 60 meters since adding the the cap hat. But...! it takes coil turn adjustments that's easy to do with a screwdriver antenna and that's why they are the most versatile mobile antenna to have for multi-band operations.

What you are saying here is exactly correct, and I have been saying exactly this the entire thread. A cap hat is one device that can do just this.

I think there is confusion with the terminology between us. What we are saying on some level is essentially the same thing. At first glance, some of the terminology you are using sounds backwards from how I would normally expect to hear what is being said and I think it threw me off. I wonder if the same is happening for you.

So lets say you have a 1/4 wavelength 6 meter antenna (yep, your example), but you want to use it on the CB band at 11 meters. You can use a cap hat to do that, you said it yourself, "adding a cap hat will lower the frequency...". Shorter antennas are naturally resonant at a higher frequency, so to make them resonant on a lower frequency a cap hat will work beautifully. That is exactly what I was saying, just worded differently.

To take this a step further, the model that you are determined to say is wrong is demonstrating your words here perfectly, "adding a cap hat will lower it in frequency..." In essence, your words are describing what the modeling software is doing perfectly. As long your words here are correct, the model is also correct.

Now here is where I am getting confused by your words. While the cap hat is 'lowering the frequency' of the antenna, it must be tuning the antenna to a lower frequency. To have one you must also have the other, you cannot lower the frequency of an antenna without also being able to tune the lower frequency of the antenna. Lowering an antennas frequency would be a perfect example of tuning a 6 meter antenna to the CB band, yet you are saying with a cap hat just such a situation won't, and in fact can't work. Which is it, you cannot have it both ways, will a cap hat work or won't it?

Another way to tune a 6 meter antenna to the CB band is to use a loading coil. You have agreed above that this will work without comment, although the wording was different. You also had no issue with either of my models that showed the same thing happening.

Would it benefit you if I took the five foot antennas used on my models and converted it to a 1/4 wavelength 6 meter antenna (about 5 or 6 inches shorter than the antennas I actually modeled so I was actually close to this anyway, what luck) and show a 6 meter antenna being made to use a 'lower frequency' that just so happens to be the CB band with nothing but a cap hat? It wouldn't be hard to do, in fact it wouldn't be any different than taking a 20 meter antenna and using a cap hat to get it to resonate on 40 meters, or using a cap hat to resonate a 40 meter antenna on the 60 meters band.


The DB

 

[The DB]

+foursringburn

I really wish you got to reading my second reply to a previous message of yours before you got around to posting again. I will save any further reply until after you replied to that specific post as that is really the post I am interested in hearing your comments on at this point. If you want to continue several pointless conversations I am happy to reply to other replies of yours, I'll hold off even though by your response you clearly misunderstood some of what I was trying to say...

Two night owls tonight huh?


The DB

 

[Captain Kilowatt]

Just my 2 cents here but I believe there is some confusion here about tuning an antenna and matching it. A single large cap hat can very well TUNE an antenna to resonance however that antenna will still need to be matched to 50 ohms as the impedance will be much lower than 50 ohms. I haven't read everything in the last several posts as I am getting ready for work but I believe that the models show the resonant frequency as well as the impedances do they not? This would confirm the cap hat tuning idea HOWEVER adding a matching network is still required and this may throw off the value of cap hat needed to tune the antenna back to resonance. Again just my 2 cents worth and I am off to work. C Ya.

 

[StrangeBrew]

.... It blends surprisingly well into the background, too!

I've always thought the same. A 102" whip while definitely not stealthy is a very unobtrusive antenna, the long thin line they present fades into the background much better than the various lumps and coils used to tune a shorter antenna.

Also I have never used one myself but these mounts look interesting: http://www.geotool.com/

 

[Tallman]

Good golly Miss Molly! Talk about Hijacking a thread!
They blinded me with SCIENCE!
The answer for the original poster is one of two possible answers.

1) 102" S.S. whip with a spring and a ball mount
OR
2) Model: KW7-? 7 Ft (213cm) 3/4 wave Rated @ 700 watts $31.99 (MSRP*) copied from Firestik web site. Also mounted to a ball mount.

 

[unit_399]

In any antenna, the rf voltage and current vary over the entire length. Where the voltage is highest, the current is lowest (and vice-versa). The point at which the rf current is the highest is where the antenna radiates. In a whip antenna, the length should be adjusted so that the radiation point is as close to the top of the antenna as possible (for maximum height and range).
If I need a shorter antenna, I always pick a top load because it represents the best compromise between performance and length. In a loaded antenna, the signal radiates from the top of the loading coil. So, all things being equal, a top load will give the greatest range because it radiates at a higher point above the vehicle than a base or center load.
This is why I use a Firestick when a whip antenna isn't practical. I usually use a 5' stick because it is the longest antenna that doesn't look obnoxious on a car, and still offers good range. The drawback is wind drag, so if you run a mag mount you need a really strong one.

My 2 pesos worth. 73s.

- 399

 

[fourstringburn]

large cap hat can very well TUNE an antenna to resonance however that antenna will still need to be matched to 50 ohms as the impedance will be much lower than 50 ohms.

That's what I've been hammering away at DB over and over. A cap hat alone can't effectively tune an antenna without some inductance. The inductance could be in the form of a loading coil either base load or center load or a shunt coil at the feed point or both.This is what will match the antenna.

But the you say it is a contradiction though.

 

[fourstringburn]

Good golly Miss Molly! Talk about Hijacking a thread!
They blinded me with SCIENCE!

Guilty as charged!...carry on Gentlemen.