HF Discone

[zguy1243]

I am kicking the idea around of building a HF discone. I am impressed with the discone design on VHF/UHF. I could only reason to believe that the discone built properly should behave the same performance on HF. With the first bit of research done I have come to some of these conclusions on design parameters. The Cone elements should be a quarter wavelength of your lowest operating frequency. The top hat elements should be 40% in length of the lowest operating frequencies quarter wave and the optimal slope angle for cone elements should be 60 degrees. With all of this said I do understand that I will not be trying to get down to 3 megs with this thing, size just gets out of hand. I still have many varibles such as element spacing and the effects of height above ground of the antenna. Anyone with knowledge on discone design please come forward. As of now I have the body of a old radio shack discone. I have put in place of the lower cone elements some 1"8-32 studs that have terminals to connect my lower cone wire elements to. For the top hat I have removed the small disc hunk of aluminum from the 3/8 stud and sourced a local machine shop to produce a beefier version that will accept 3/8" elements. The top hat elements will not be wire. They will be rigid and I am trying to keep them under 12 feet long. I would rather them be rigid enough not to need supports to prevent droop. Again this all is frequency vs. antenna size problems. I plan on using a 50 foot telescoping mast planted in some concrete to support the array. I think that a 8Mhz standard for my lowest operating frequency will cover my need for 8Mhz + listening and put the size of the antenna within reason for a prototype. Using 8Mhz in the calculations this puts the cone elements at 29.219 feet in length and the top hat elements 11.6 feet in length. Again right now this is all a experiment. I was inspired by doing some reading on these antennas. I also might add that I have no desire to ever use the antenna for transmitting. I am looking for pointers on my design and any info you can provide. Looking to learn and build a working antenna.

 

[zz0468]

I am kicking the idea around of building a HF discone. I am impressed with the discone design on VHF/UHF. I could only reason to believe that the discone built properly should behave the same performance on HF.

It would. There are commercially manufactured discones for HF use. They're huge.

With the first bit of research done I have come to some of these conclusions on design parameters. The Cone elements should be a quarter wavelength of your lowest operating frequency. The top hat elements should be 40% in length of the lowest operating frequencies quarter wave and the optimal slope angle for cone elements should be 60 degrees.

Yeah, that sounds about right. There's plenty of basic design information on the net on how to proportion the thing. Where you're going to run into trouble is in mechanical construction.

With all of this said I do understand that I will not be trying to get down to 3 megs with this thing, size just gets out of hand. I still have many varibles such as element spacing and the effects of height above ground of the antenna.

Do a google search on 'hf discones' and see what comes up. The feds and military use tons of them. The ones I've seen installed are pretty close to, or at, ground level.

Anyone with knowledge on discone design please come forward. As of now I have the body of a old radio shack discone. I have put in place of the lower cone elements some 1"8-32 studs that have terminals to connect my lower cone wire elements to. For the top hat I have removed the small disc hunk of aluminum from the 3/8 stud and sourced a local machine shop to produce a beefier version that will accept 3/8" elements.

Well, I'm hardly an expert on them. I'm not sure that even by getting a beefier hub for the disc machined, that the rest of a radio shack discone is up to the mechanical demands, but try it. I suspect there are better and cheaper ways of accomplishing that part.

The top hat elements will not be wire. They will be rigid and I am trying to keep them under 12 feet long. I would rather them be rigid enough not to need supports to prevent droop. Again this all is frequency vs. antenna size problems. I plan on using a 50 foot telescoping mast planted in some concrete to support the array. I think that a 8Mhz standard for my lowest operating frequency will cover my need for 8Mhz + listening and put the size of the antenna within reason for a prototype.

The thing is going to be monstrous. It seems to be a complex and expensive exercise to go through just for a receiving antenna. Are you hoping for some performance improvements or other types of receiving antennas? HF discones are generally used when there is a requirement to transmit at high power over a broad frequency range.

The idea that a discone could be used at HF is quite a valid one. But for receive only, I wouldn't expect any performance enhancements, especially if you're including cost effectiveness in you evaluation.

 

[prcguy]

An HF Discone can be built easily depending on what type of mast or tower you have. I can dig up existing plans in some ARRL publications buried around here somewhere if you want.
Basically the skirt would be made of wires which can be guy wires for the mast structure, 12 minimum and 16 would be a nice goal.

The top hat hub can be made from a thick aluminum sheet metal disk, 1/8" thick or greater and 2 or 3ft in diameter. From there you would attach the 12 or 16 top hat elements using thin wall aluminum tubing either bolted through each element near the center of the hub and at the edge or used a pair of U-bolts for each element.

For the top insulator you can use a short section of 4" ABS pipe with a toilet flange to interface to the sheet metal hub. Ideally if you had a Rohn 25 tower or similar with a flat rotor plate at the top you could make the top ABS insulator about 4" long with a toilet flange on both sides, one flange would bolt to the top hub and the other to the flat rotor plate. If you have a simple push-up mast, the 4" ABS can be several feet long and use 3 or 4 large hose clamps to attach to the mast leaving about 4" spacing between the top of the mast where the guy skirt wires attach and the top hub.

These are a few ideas on what I was considering for a Discone that would cover 40M through 6M with skirt wires about 34ft long and the top hat elements about 10 to 12ft long.
When you think about a mast that's maybe 40ft tall (much taller is better) with 12 or 16 symmetrical guy wires at 45deg to the mast and a hub at the top with a dozen or so 12ft elements its not that hard to imagine. Total cost could be $100 or so (not including mast) depending on the wire and where you get the aluminum tubing.
Good luck and I hope you get one up soon.
prcguy

 

[zguy1243]

I can visualize this. How is your feedpoint configured?

An HF Discone can be built easily depending on what type of mast or tower you have. I can dig up existing plans in some ARRL publications buried around here somewhere if you want.
Basically the skirt would be made of wires which can be guy wires for the mast structure, 12 minimum and 16 would be a nice goal.

The top hat hub can be made from a thick aluminum sheet metal disk, 1/8" thick or greater and 2 or 3ft in diameter. From there you would attach the 12 or 16 top hat elements using thin wall aluminum tubing either bolted through each element near the center of the hub and at the edge or used a pair of U-bolts for each element.

For the top insulator you can use a short section of 4" ABS pipe with a toilet flange to interface to the sheet metal hub. Ideally if you had a Rohn 25 tower or similar with a flat rotor plate at the top you could make the top ABS insulator about 4" long with a toilet flange on both sides, one flange would bolt to the top hub and the other to the flat rotor plate. If you have a simple push-up mast, the 4" ABS can be several feet long and use 3 or 4 large hose clamps to attach to the mast leaving about 4" spacing between the top of the mast where the guy skirt wires attach and the top hub.

These are a few ideas on what I was considering for a Discone that would cover 40M through 6M with skirt wires about 34ft long and the top hat elements about 10 to 12ft long.
When you think about a mast that's maybe 40ft tall (much taller is better) with 12 or 16 symmetrical guy wires at 45deg to the mast and a hub at the top with a dozen or so 12ft elements its not that hard to imagine. Total cost could be $100 or so (not including mast) depending on the wire and where you get the aluminum tubing.
Good luck and I hope you get one up soon.
prcguy

 

[prcguy]

The coax braid would connect to the common top skirt point and several inches of center conductor would run up through the ABS pipe and connect to the top hat hub with a lug. The spacing between the top hat and skirt can affect impedance and the general idea is to place the top hat hub where the skirt wires would intersect at a sharp point if you were making a perfect triangle. Instead you would end the skirt a few inches below the top hat (4" is probably OK for HF) and bridge that gap with the coax center conductor.

The skirt is an equilateral triangle with the length of the skirt equal to .25 wavelength at the lowest operation frequency. The top hat diameter is .67 to .7 of the skirt length.

I'm looking in a Radio Handbook from 1975 and they have plans for a 20M on up discone using 48 #12 wires for the skirt and 8 elements for the top hat with wire around the perimeter of the element ends and the midway point of the elements. I think 48 wires is a little overkill and 12 should be minimum.
prcguy

 

[N_Jay]

Sounds like a big project.

I second the "why" if it is RX only.

What bands are you most interested in?

 

[zguy1243]

Take a 1/4 wave antenna for say 440 Mhz and then tune down to 220 Mhz and try it out. It's not so hot.Now take a VHF/UHF discone and compare reception at 440 and 220 Mhz. The performance of a discone across a frequency spread is superior to a dipole. The discone also will be truly omni directional unlike that of a inverted V or dipole. The discone will meet bandwidth issues on HF and there is just alot more metal in the air.

Sounds like a big project.

I second the "why" if it is RX only.

What bands are you most interested in?

 

[N_Jay]

Take a 1/4 wave antenna for say 440 Mhz and then tune down to 220 Mhz and try it out. It's not so hot.Now take a VHF/UHF discone and compare reception at 440 and 220 Mhz. The performance of a discone across a frequency spread is superior to a dipole. The discone also will be truly omni directional unlike that of a inverted V or dipole. The discone will meet bandwidth issues on HF and there is just alot more metal in the air.

I guess it depends on the particular bands.

You could certainly design an antenna that is better the a discone at both 220 and 440, but not the frequencies in between.

That is why I asked the question that I asked.

 

[iMONITOR]

Here are a couple of pictures of my HF discone in my backyard.

Ok, it's not mine, but I thought it might give you some design ideas.

 

[zguy1243]

Yeah I have seen those out there at the silo. Those are amazing pieces of history. I hope mine to look like a downsized version of that one. I believe those were built and constructed by Collins.

Here are a couple of pictures of my HF discone in my backyard.

Ok, it's not mine, but I thought it might give you some design ideas.

 

[prcguy]

If you want to experiment with a medium size Discone I could probably set you up with one that covers about 26MHz on up. Its a nice military unit that needs the bungee cords inside the elements replaced which pull the elements together.
prcguy