capacity hat expert needed

[Needle Bender]

i want to use a capacity hat to make my 4 foot piece of tubing work as a 1/4 wave on 10m. i have two 1/4 wave wires drooped down each side of the top of the 4 in 12 roof and the 4 foot tube sitting vertical and insulated from the ground plain wires.
how long will 1/8 inch rods need to be to tune flat? there will be four strait rods sticking horizontally out of the top of the 4 foot tube.
do they need to be the whole 4 feet 3 inches long for 28.4 or with 4 of them should they be a little shorter? they cant be bent into a clover leaf, i need them strait for looks.

 

[Captain Kilowatt]

I can't tell you how long they would have to be but I can tell you that if you are only using a 4 foot vertical you would probably be better off using a base loading coil to tune it to resonance. If you insist on using a top hat then experiment with it. Start off with each rod two feet long and see what it gets you and go from there.

 

[Needle Bender]

2 feet long plus the 4 foot tube is still only 6 feet which is 3 feet short of a 1/4 wave. Do you think 4 of those would be long enough?

 

[Captain Kilowatt]

You don't simply make up the rest of the length. A capacity hat does not actually radiate a signal but adds extra capacitance that tunes the antenna. Just how much capacitance would be needed to tune the 4 feet of tubing is a question mark right now.Either way you are going to need some form of base loading as the impedance will not be near 50 ohms using just a capacity hat and will be much lower. Why the 4 foot fixed length? If you went to about 2/3 of a full sized 1/4 wave , or about 6 feet you would end up with something near 30 ohms impedance when you are finished while 4 feet will end up being further off still and the minimum SWR will likely be over 2:1 at best.Remember, the top hat tunes the antenna to resonance and resonance is not necessarily 50 ohms.

 

[Marconi]

2 feet long plus the 4 foot tube is still only 6 feet which is 3 feet short of a 1/4 wave. Do you think 4 of those would be long enough?

NB, here is what my Eznec model shows for an antenna similar to what you describe above. One has 2 x 24" Top Hat radials, and the other has 4 x 24", with both using a 4' x .5" radiator, and 2 x 105" slanted down radials that are .5" in diameter.

Note that your 2 Top Hat radial setup is not resonant, while the 4 x 24" is resonant. However, both models show a similar low resistive value of 33 - 38 ohms respectively. At resonance such a resistive value should be marginally workable, but they are low just as CK suggested.

View attachment NB's Top Hat idea using a 4' radiator with 2 or 4 radials..pdf

You don't simply make up the rest of the length. A capacity hat does not actually radiate a signal but adds extra capacitance that tunes the antenna. Just how much capacitance would be needed to tune the 4 feet of tubing is a question mark right now.Either way you are going to need some form of base loading as the impedance will not be near 50 ohms using just a capacity hat and will be much lower. Why the 4 foot fixed length? If you went to about 2/3 of a full sized 1/4 wave , or about 6 feet you would end up with something near 30 ohms impedance when you are finished while 4 feet will end up being further off still and the minimum SWR will likely be over 2:1 at best.Remember, the top hat tunes the antenna to resonance and resonance is not necessarily 50 ohms.

Attached below is another Eznec model using the 72" idea that CK suggested. I also include a full 1/4 wave model with the same 2 radial design as noted above. I tweaked the radiator length to resonance as a comparison with the Top Hat model in order to achieve resonance. However, notice the Top Hat match, in this case, is much better than CK suggested it might be.

Remember these are just models, and I cannot be sure what experience you might have setting your radials on or near the roof. I would expect a noticeable reactive ill-response with such a setup, and I'm not sure additional tuning could compensate.

View attachment Captain Kilowatt's Top Hat idea vs. a full .25 wave.pdf

 

[Needle Bender]

NB, here is what my Eznec model shows for an antenna similar to what you describe above. One has 2 x 24" Top Hat radials, and the other has 4 x 24", with both using a 4' x .5" radiator, and 2 x 105" slanted down radials that are .5" in diameter.

Note that your 2 Top Hat radial setup is not resonant, while the 4 x 24" is resonant. However, both models show a similar low resistive value of 33 - 38 ohms respectively. At resonance such a resistive value should be marginally workable, but they are low just as CK suggested.

View attachment 5925



Attached below is another Eznec model using the 72" idea that CK suggested. I also include a full 1/4 wave model with the same 2 radial design as noted above. I tweaked the radiator length to resonance as a comparison with the Top Hat model in order to achieve resonance. However, notice the Top Hat match, in this case, is much better than CK suggested it might be.

Remember these are just models, and I cannot be sure what experience you might have setting your radials on or near the roof. I would expect a noticeable reactive ill-response with such a setup, and I'm not sure additional tuning could compensate.

View attachment 5926

marconi, thank you for running your program. this antenna is for a friend who has height restrictions and can only go 48 inches tall. the edge of the roof is right next to it so can you try running it with one radial strait down instead of the two at angles and with four 20 inch tophat radials? i dont think he can go longer then 20 inches out so lets see what it will expect from that thanks again

 

[Marconi]

marconi, thank you for running your program. this antenna is for a friend who has height restrictions and can only go 48 inches tall. the edge of the roof is right next to it so can you try running it with one radial strait down instead of the two at angles and with four 20 inch tophat radials? i dont think he can go longer then 20 inches out so lets see what it will expect from that thanks again

How high is the roof where the antenna is center mounted?

 

[Needle Bender]

its a 3 storie apt so about 35'

 

[Marconi]

marconi, thank you for running your program. this antenna is for a friend who has height restrictions and can only go 48 inches tall. the edge of the roof is right next to it so can you try running it with one radial strait down instead of the two at angles and with four 20 inch tophat radials? i dont think he can go longer then 20 inches out so lets see what it will expect from that thanks again

Here is the new model at 35' feet to the mount. Consider however that this antenna is not close to anything like a building, which could mess up the pattern badly.

View attachment NB's idea 110711.pdf

 

[BOOTY MONSTER]

FWIU the ground elements on a 1/4wgp only have to be sloped to get a 50 ohm tuning . they can be horizontal and still get to 1.3 which is still very good , even 1.5 is still quite adequate with a amp .

id try a 36 ft mast with a 108 on top and see if anyone notices , if they do then put a 4 FG mobile on it or try a dual coil 10K ........

 

[Moleculo]

Another thing you can do to increase the capacitance that the hat exhibits is to attach a wire to the end of each of the hat elements, effectively making it into a circle with spokes (like a wagon wheel).

 

[gamegetter]

needle, have you considered a new top one or an old astro plane if you could fine one? sounds like an ideal situation for one.

 

[Marconi]

FWIU the ground elements on a 1/4wgp only have to be sloped to get a 50 ohm tuning . they can be horizontal and still get to 1.3 which is still very good , even 1.5 is still quite adequate with a amp .

id try a 36 ft mast with a 108 on top and see if anyone notices , if they do then put a 4 FG mobile on it or try a dual coil 10K ........

BM, excepting that slanted radials do tend to affect the feed point impedance for such setups in a positive way, I disagree with your understanding here. Can you describe the basis for your thinking or experience?

 

[Marconi]

needle, have you considered a new top one or an old astro plane if you could fine one? sounds like an ideal situation for one.

GG, I think NB and his buddy are looking for some effective setup that does not draw attention, and the Top One, New Top One, or the AstroPlane are not such antennas. They are effective and short, but they are also big and very visible.

NB, how will your buddy mount this antenna up so high on a 3 story apt.? Is it a flat roof?

 

[RatsoW8]

Ok I'll ask. Since you alread have 1/4 radials and are about to add another 4' of top hat, why not just add the additional 4' 3" to make the vertical section 1/4 wave at 28.400 Mhz?