A Scenario..

The capacitive hat increases radiation by causing a redistribution of current in the vertical radiator. This current is increased along a greater length of the vertical element compared to that in one without a top hat, and it is the current in the vertical element which contributes to radiation. Current in the hat does not contribute to radiation because the currents in the hat elements are moving in opposite directions on opposite sides of the hat and thus their radiated fields cancel.

Mounting the hat close to the ground would minimize the vertical element length and also its radiated field while not adding to the radiation.

Neil

There is an antenna out there I've seen on the web developed for Ham radio tight spaces. It's basically a wire screen cube to increase capacitance, a loading coil and a screen counterpoise.

It's suppose to exhibit lower losses as the loading coil requirement is reduced and have a wider bandwidth than a simple rod antenna due to the effective diameter of the radiators large cross section.

Now, if I could just come up with the name...

I remember-I remember! It's the The Gap Super C. Check out this link to the Gap Super C antenna.

By using an inductor and a top-hat, you'll have made a tuned circuit. As the difference in reactance in the coil and the top-hat (capacitor with ground) approaches zero the antenna circuit will enter the neighborhood of resonance. With the proper combination of Xl and Xc, the antenna will become purely resistive. Short antenna resonant resistance values are usually in the single digits. This makes them very inefficient due to the low value of radiation resistance.

It is good to remember, a carbon resistor is purely resistive and makes a great dummy load. So, it stands to reason, there is another component necessary to make a great transmitting antenna. Radiator surface area is the missing ingredient. That's right, the vertical conductor between the coil and the top-hat produces the listenable signal at a distance from the antenna.

And finally, the closer an antenna radiator is to ground the more signal is absorbed by plain old dirt. An antenna can be to far from ground to work properly meaning there is an optimum distance for an antenna to be from ground. Also ground reflection can cancel a portion of the radiated signal causing reduced field strength.

Capacitance hats are added to short Medium Wave antennas to raise the radiation resistance, thereby raising the efficiency of the antenna to put more signal in the air.

BOTTOM LINE: I would encourage you to experiment. But, don't expect any huge gains in signal strength.

The Gap Super "C" looks like another variation of the Crossed Field Antenna (CFA) nonsense. An antenna that is very short compared to wavelength necessarily has very low radiation resistance, and therefore very low efficiency. There is no way around this. It is a fundamental law. As in the CFA and the EH antenna, look for a conductor that is not counted as part of the "antenna." This is the major part of the real antenna. It's the same as whip and mast.

As most of you guys know, my station is
off right now. In fact, the whole AM antenna
is down.

But I have experimented with large capacitance
hats. I can't remember how long, but this is
what I did.

The MICRO1700 AM system has always been a
ground mounted transmitter, buried radials
(the most I ever had was sixteen) and a roughly
9 foot copper pole with it's bottom end almost
on the ground.

I have had various antenna mounting schemes, but the
funniest one I ever had was where I attached
the copper vertical pole to the side of a --- how
do I describe this --- It was a little plastic kid's
play house that 2 or 3 five year olds could fit in.
You know - little windows, a little front door - that
sort of thing. My daughter loved it when she was
five years old. And it stayed in our back yard for
years. And the roof of the thing was about five feet
tall.

That's when I decided to attach the MICRO1700
9 foot pole to the side, add nonmetal guy wires,
plunk the SS-Tran on the ground, and attach it
to the 16 buried radials.

It worked pretty well. Then I decided to
make a top hat. I had lots of ideas, like pizza
cooking pans, chicken wire cut in a big circle,
spokes of wires, all kinds of things. What I
ended up doing, was fairly easy and not too
time consuming. I decided to have four horizontal poles
(copper pipe, just like the vertical section)
coming out of the top of the vertical pole, like spokes
on a wheel (not a lot of spokes though) one pointed north,
one south, one east, and one west. I wanted to make
them fairly long, and I did. Each top hat spoke was two
and a half feet long. Getting them to stay up there is
a whole other story. Then, I think I went even longer
than that, maybe each spoke was four feet or something
like that. I don't remember. I know that the longwave
Part 15 experimenters build huge top hats, so that is what
I went for.

My range improved. I think it was significant, although
my record keeping is poor. I know I heard it on the car
radio 2 miles away is several places. I remember one
day in particular being in front of my doctor's office
two miles from my house. The signal wasn't strong,
but it was completely listenable on the car radio.

So imagine if you will, a little kid's 5 foot tall plastic house with
a 9 foot antenna attached to the side, and 4 big horizontal
top hat spokes dwarfing the little structure. And lot's of guy
wires (very thick string/very thin rope. Then there was
a little yellow LED mounted
on the kiddie house to indicated that 18 volts was going
to the SS-Tran. Also, there was a little teeny sign over the
ittle teeny door that had my station name on it.

It was really hilarious! One local broadcast engineer practically
fell down laughing when he saw it.

But it worked.

Best Wishes
Bruce, MICRO1690/1700

My use of a top hat was to tune my homemade vertical as the coil and radiator resonated at a higher frequency than I needed. Since the radiator was already at the limit and I didn't want to rewind the coil, I put 6 radials above the coil which was in the center of the radiator.

Pruning the length of the radials did the trick bringing the resonant frequency down to 1500 kHz.

Hi MRAM. You did a great job on that antenna.
I've seen the picture.
Best Wishes,
Bruce, MICRO1690/1700

Another Scenario..

I'm just one of those guys that try to think outside of the box. I figured if one could get some high level reactance going on that it would make some noise in the air around it. Isn't AM just controlled static? LOL

Has anyone tried a top loaded antenna? What would the difference be in the coil compared to the loading coil that's used/built with the SSTran external antenna? Would the coil need to be more or less?

I have a design of an adjustable coil that I'm just itching to try! It can be adjusted at a good distance from the antenna itself so your body doesn't mess with the tuning. A field strength meter and a pair of binoculars will be my best friend..

Bruce MICRO 1700... Your playhouse antenna reminds me of Pee Wees Playhouse, which is the neatest looking house I've ever seen.

I don't know about top loaded antennas, but I have a question that splits the difference:

What if an antenna is center fed?

I have an SStran under the desk, feeding a vertical up to the ceiling. I was wondering what would happen if I added to the antenna going down below the floor into the basement.

They say a good ground radial creates a kind of mirror image of the vertical antenna, so what about actually having the antenna go below the ground (floor) creating its own mirror image?

The antenna I mentioned about putting the top hat near the coil is a center loaded antenna. Info I've read on center loading indicates the coil needs to have a little more inductance (more turns) compared to base loading.

The antenna I made worked fairly well considering the very poor counterpoise I put under it.

See the Antenna Story article on my webpage detailing the antenna.

Next time I think I will take the same center loaded approach but I will link feed the coil rather than end feeding from the bottom. That would make it a very short center fed dipole. Perhaps some capacity hats at each end of the dipole...

You're not far from where I work! (B&C Communications) I'm sure that you know Mr. Combs.. 😉