Capacitance Hat's Don't add Audio

[loosecannon]

LooseCannon, another great post. What the hell did you eat today, your like butter on a roll.

LOL, thanks.

im at work right now, and my job requires me to sit in front of computers for long periods of time. If i have to be here anyway, might as well make it interesting!

im pretty sure MrCoily has stopped reading my posts though. LMAO!
LC

 

[Moleculo]

Great post LC. I've read this same section of the ARRL book about a dozen times also. It's amazing how some people can make it infer something it does not, although it is a technical subject so it's easy to misconstrue things.

Actually more like this, except with a 3 or 4 foot whip on top. (that's about as high as I can get without banging overpasses and such)
I am also going to redo the cap hat and put twice as many radials on it.
I'm still in the experimental stage.

When you add the stinger on top of that, it will reduce the number of turns necessary in that loading coil. Put an analyzer or SWR meter inline and you'll see it immediately. If you add more radials to the hat, it will add capacitance and cause the same thing. Rather than add elements (which is OK, BTW), it might be fun to try to make it more like the cap hats shown here:

The problem with the hat shown in your pic is that it is not symmetrical, which will probably result in some pattern distortion and some radiation from the hat itself which you don't want.

I've quoted some material from the hiqantennas.com site from their cap hat installation instructions below that is relevant to the discussion. Notice that the entire purpose of doing this is where it is impractical to use a full length quarter wave antenna:

When a quarter-wave antenna is mounted near the rear of a vehicle, the back of the vehicle and capacitance to the roadway act as the other required quarter-wave of a half-wave dipole. An antenna which uses the ground (earth/vehicle) as part of its resonant circuit is a Marconi antenna. An example of a Marconi antenna is a quarter-wave antenna, where the ground operates as the missing quarter wavelength. Most mobile antennas are a variation of the Marconi antenna.
At High Frequencies (HF) it becomes impractical to use full length quarter wave or half wave antennas for mobile operation. Therefore, it is often desirable to top or center load a mobile antenna. This top or center loading lengthens the antenna, but more importantly, it materially increases the capacitance between the top or center of the antenna and ground. As a result of this lengthening and the increased capacitance, a shorter antenna can be made to resonate at a relatively lower frequency than that of the unloaded antenna.
One method of achieving the loading of an antenna is to add a hat-type structure to the top or center of the antenna. Since this structure increases the capacitance and originally was shaped to resemble the rim of a hat, they were referred to as “Capacitance Hats”. This term has since been used to describe many shapes of load devices that have the effect of increasing the relative capacitance of the antenna with respect to the ground. A Capacitance Hat may appear as an “X”, a butterfly, a circular hat or a sphere. Basically the idea of a capacitance hat is to make up the length needed for resonance while limiting the vertical height.
Normally a vertical antenna has the lowest RF voltage/highest RF current flowing near the base, where most of the radiation occurs, and the highest RF voltage/lowest RF current at the top of the antenna, where the least radiation occurs. A capacitance hat placed at the top of an antenna will be exposed to the highest RF voltage/lowest RF current. Therefore due to the low RF current, the capacitance hat does not radiate a significant amount of signal. However, due to the increased effective height of the vertical radiator, the radiated energy is more evenly distributed along the length of the vertical radiator, resulting in an increase of RF radiation from the top of the vertical radiator with respect to a vertical radiator without the capacitance hat. Increasing the effective height of the antenna has the beneficial side-effect of reducing the losses caused by nearby shrubs and buildings, assuming your antenna is taller than these objects.
The symmetry of the hat causes any radiation from the hat to cancel and multiple elements are used to reduce the RF current flowing in each individual element. (If you use 1 radial element, it's called an inverted L, the radiation does not cancel and the radiation pattern is distorted.) The effective length of the vertical is then approximately the vertical length plus the length of the capacitance hat radials. It is also advantageous to use a “skirt”; joining the ends of the hat to maintain symmetry and ensure that a more balanced RF current flows in the hat section. In that case, the effective length is approximately the vertical length, plus the radial length of the hat, plus 1/2 of the length of the skirt joining the radials.
Adding a large capacitance hat at either end of a coil also reduces coil Q, since a large portion of the hat’s capacitance directly shunts the inductor. It will act like a big capacitor in lowering the resonant frequency of the antenna. The Q will decrease and your bandwidth will widen. Also, if we add a large capacitance hat at the top of the coil, (bottom of the whip), we change nothing below the coil.
A capacity hat will also raise the radiation resistance (the resistance that represents energy radiated from the antenna) for shortened antennas. If the radiation resistance is increased, it will increase the efficiency of the antenna system assuming other losses are kept the same.

 

[HomerBB]

Thanks, loosecannon,

It seems you've put together in a single post the things Shockwave, and others, were trying to get through my head.
I'm not sure I would have followed it as easily as I did without the previous assistance from everyone else.

Mucho gracias,
Homer

 

[Radio75]

Thanks for the info Moleculo. I perused the HiQ website, and they sure make some impressive antennas! Until my budget improves I will have to stick to my cheapy experimental antennas, but I am having fun!

Those fancy hoop type cap hats are beyond my fabricating skills I'm afraid, but I believe that I will make a hoop around the ends of all my radials when I make the second experimental cap hat.

Good thread, I have learned a lot!!!

 

[mackmobile43]

Mr. Coily is sounding more like *MPOWER with every post.

 

[BOOTY MONSTER]

well............ mr. coily may not have contributed anything worth while himself to this thread . but his evasive fiddle farting around did inspire some good info here .......but it isn't in support of his claims for his products .


now ..... a question . lol
on a base antenna , is there any negative effect to just having a disk or ball for a top hat (no coil) ?
something to spread out ground charges that may attract a lightning strike , like a small pizza pan . put some big holes in it to help prevent it from planing in the wind ..........

 

[Moleculo]

on a base antenna , is there any negative effect to just having a disk or ball for a top hat (no coil) ?
something to spread out ground charges that may attract a lightning strike , like a small pizza pan . put some big holes in it to help prevent it from planing in the wind

You're talking about a corona ball. Depending on how big you make it, it will add capacitance to the antenna which can affect the tuning. You don't do that with a flat, round surface, btw. Corona balls are fine and do serve a useful purpose, although they're certainly not mandatory.

 

[loosecannon]

booty, here is an article i really like by k0bg that talks about corona balls:
Controlling Static

im for sure going to put one on my 102" whip. he also talks about DC grounding the antenna through a coil.
that sounds like something i might consider also.
i drive in the city every day and static is my enemy!

the way i think about corona balls is, if the corona ball gets too big, it starts becoming a capacity hat.
LC

 

[jazzsinger]

.i know more about antenna building and design then i like to let out...

i'm certain that claim is plagerised from the king of amp builders (AM POWER) with the word amplifier cunningly substituted with antenna building/design.

 

[Moleculo]

And just because I couldn't let the thread die , I'll remind everyone of this monstrosity:

It's relevant to the discussion, though. This is the perfect example of not understanding both what the coil does AND what the cap hat does and then using them in a way that is absolutely laughable. This is the ultimate in CB antenna snake-oil.

 

[GnG8d]

^^ Is that a MrCoily? ^^

 

[pro151]

I see a street light in the background. You are telling me this abomination is mounted on a vehicle, right?

I know that Booty and others try to learn by experimenting with antennas and now I am feeling inspired to do the same. I think I am going to try to build my own antenna with every kind of coil, corona ball, capacitance hat, ground radials and any other doo-dads I can think of and see how it tunes and talks.
I will not post any pics, claims or results until after I have obtained a patent for it so I can make everyone pay me top dollar for the technology.
I will then spend the rest of my life scouring the internet for pictures of anyone copying my patented design and sue hell out of them for stealing my design.
Sounds like a plan to me.
I need to go buy a new calculator now so I can start calculating my soon to be realized wealth.

 

[Marconi]

And just because I couldn't let the thread die , I'll remind everyone of this monstrosity:

It's relevant to the discussion, though. This is the perfect example of not understanding both what the coil does AND what the cap hat does and then using them in a way that is absolutely laughable. This is the ultimate in CB antenna snake-oil.

Moleculo, I'm just curious, but in general what does this coil do for the antenna, and would the coils location affect the antenna in a similar way (noted to be a bad idea when right below a top hat) if there was a straight stinger installed above the coil rather than the top hat?

 

[Moleculo]

No, that is a Merlin 55 base antenna. It's a 1/4 wave ground plane antenna that has been butchered. Even sites that sell it don't know WTF it is, though. Check out this quote:

[FONT=Verdana, Arial, Helvetica, sans-serif]High grade aluminum center fed half wave base antenna[/FONT]
[FONT=Verdana, Arial, Helvetica, sans-serif]
[/FONT]
[FONT=Verdana, Arial, Helvetica, sans-serif]Features:
[/FONT]
[FONT=Verdana, Arial, Helvetica, sans-serif]
[/FONT]
[FONT=Verdana, Arial, Helvetica, sans-serif]Balanced feeding point that reduces common mode current that is a common cause of interference.
[/FONT]
[FONT=Verdana, Arial, Helvetica, sans-serif]

Merlin-Antenna

LMAO.
[/FONT]

 

[GnG8d]

I need to go buy a new calculator now so I can start calculating my soon to be realized wealth.

Don't bother, I'll donate this one for 5% of the take. I've had it since the late 80's but it's still in fine working condition