desktop discone... a joke?

[mmckenna]

I had a co-worker that lived in a deed restricted area. He was not allowed to install antennas.

But flying the flag was OK. So up went a 30 something foot flag pole with a genuine made in China US flag on top.

Of course a close look showed that the metal base was insulated from the ground, and there was a wire leading to a planter box next to it. Inside the planter box was an antenna tuner.
He'd run AM on the 80 meter band just fine, and no one knew.

While you don't need a 30 foot tall antenna, you can think outside the box. Look at what your neighbors have and see what you can use to disguise an antenna.

 

[JELAIR]

Lol yes
I will be in the thinking-box and look around for some type of balcony 'decoration' that looks innocent but practical (and one that won't interfere with the antenna's reception, so I'm assuming; no metal)

 

[krokus]

Lol yes
I will be in the thinking-box and look around for some type of balcony 'decoration' that looks innocent but practical (and one that won't interfere with the antenna's reception, so I'm assuming; no metal)

Unless the decoration is the antenna. Chicken wire can be used as a fan antenna.

Sent using Tapatalk

 

[JELAIR]

Well, I'm currently thinking about getting a discone and putting a table-cloth over it so it looks like a small table

 

[Ubbe]

Try to get a discone that starts its frequency range at 100MHz. It's only at its lowest frequency it will be more or less comparable with a 1/4 wave GP antenna. Double up from its designed lowest frequency it looses 3dB and 4 times up it's down by 6dB, on a perfect ideal theoretical discone.

/Ubbe

Example from a AOR DA3200:

 

[JELAIR]

Try to get a discone that starts its frequency range at 100MHz. It's only at its lowest frequency it will be more or less comparable with a 1/4 wave GP antenna. Double up from its designed lowest frequency it looses 3dB and 4 times up it's down by 6dB, on a perfect ideal theoretical discone.

Thank you.
I'm currently leaning towards getting the "diamond d130j" and not adding the top-whip to it.

I assume this means it will have slightly less sensitivity at the mil-airband around 300-400 MHz, since that seems to be where the lobes begin to split into 2 (one oriented up and one oriented along the horizon, resulting in lower sensitivity along the 35-45 degree angle)
But it will hopefully still be an improvement over the current 'bunny' setup I use

 

[SDRPlayer]

I have some questions for the forum:

1) Why was there a move away from the original "disc-cone" design? (eg wind resistance or cost).
2) Is there any electrical disadvantge in the manufacturered "wire" design over the solid original?

I ask because i notice the better quality discones have so many radials, they aren't far off being solid.
Thanks.

 

[Ubbe]

Wind concerns, as a discone at VHF are so big. They still make solid discones for GHz. When you have 8 or more elements as a disc or as a cone you will not see a dramatic difference to a solid cone. Using solid elements will have a perfect omni direction and going to 8 elements will give a slight dip at the directions between the elements seen from the center. Using only 4 elements will have bigger dips that are noticable.

If you build your discone, or modify a cheap 4 element one, you could put a solid disc, brass or copper foil, on top of the elements and solder it to the elements and for the cone you could add a skirt of metal net, the more solid the better but will have a huge wind resistance. One advantage is that when the original elements are soldered to a more solid component it will be next to impossible that one element will drop off or break. So it the mast and/or guy wires can handle it it is much prefeered.

/Ubbe

 

[SDRPlayer]

Thanks Ubbe, so if (for arguments sake) i made the cone section from what we here call rodent wire (1cm x 1cm square grid wire) for wind resistance reasons, is better than radials (dependant on the number of radials...about 8) or about the same?

 

[Ubbe]

I'm currently leaning towards getting the "diamond d130j" and not adding the top-whip to it.

If you do not install it outside and exposed to weather you don't have to get the most expensive discone alternative. They all perform equally it's just the mechanical design and material used that differ. Look at something like this: https://www.ebay.co.uk/itm/Scanner-Receiving-Antenna-Discone-25-1300MHz-/121508352707

And if you are non-interested in stuff below 250MHz you could optimize it by cutting all elements, both top and bottom ones, down to half size. It will be easier to handle in a room if it's only 50cm high and can hang from the ceiling or stand on the floor.

/Ubbe

 

[Ubbe]

Thanks Ubbe, so if (for arguments sake) i made the cone section from what we here call rodent wire (1cm x 1cm square grid wire) for wind resistance reasons, is better than radials (dependant on the number of radials...about 8) or about the same?

It will of course be better the more solid it gets but by how much? I haven't seen any data of that. The professional mil discones often use 16 or more elements. Probably what is practical to mount to the "crown" without running out of space.

The diameter of the holes in a cross net are dependent of the formula dB=20log * wavelenght/2*diameter which gives the attenuate of the signal by the net, how much will pass and not be absorb/reflected. Holes with a diameter of less than 1/10 of the wavelenght will pass less than 1/10 of the signal and reflect 9/10. At a amateur 70cm wavelenght it would be sufficient with 7cm holes. If we concider the space between the lower elements lowest part as holes, half a hole anyway with half effeciency, at a 2m amateur frequency (145Mhz) the distance should be max 20cm to reprecent a 90% effeciency compared to a solid cone. If the distance between elements are 40cm the effeciency drops to 75%. The same calculations can probably be done to the disc so it might double the loss, which would be 50%, half the signal level. I don't know how many elements are needed to make a 20cm or a 40cm distance of the bottom elements of a standard discone but you get the general idea what could be expected if you try to close the gap between elements to get closer to a solid cone or disc.

/Ubbe

 

[SDRPlayer]

Thank you Ubbe, it's amazing how technical an antenna with very little (broadband) gain can become. Thanks for taking the time to explain things.

 

[dlwtrunked]

Wind concerns, as a discone at VHF are so big. They still make solid discones for GHz. When you have 8 or more elements as a disc or as a cone you will not see a dramatic difference to a solid cone. Using solid elements will have a perfect omni direction and going to 8 elements will give a slight dip at the directions between the elements seen from the center. Using only 4 elements will have bigger dips that are noticable.

If you build your discone, or modify a cheap 4 element one, you could put a solid disc, brass or copper foil, on top of the elements and solder it to the elements and for the cone you could add a skirt of metal net, the more solid the better but will have a huge wind resistance. One advantage is that when the original elements are soldered to a more solid component it will be next to impossible that one element will drop off or break. So it the mast and/or guy wires can handle it it is much prefeered.

/Ubbe

I have an FAA surplussed 100-400 MHz discone that uses either 10 or 12 elements (I do not recall which and can not get to it at the moment. One should keep in mind that the high frequency end is affect by the cone to disk spacing (smaller the better) and many discones are less than optimal in that.

 

[prcguy]

The spacing between the disc and cone will vary the impedance and not necessarily the high frequency response. Ideally the cone comes to a sharp point under the disc and compromised designs will have a flat top cone under the disc.

I have an FAA surplussed 100-400 MHz discone that uses either 10 or 12 elements (I do not recall which and can not get to it at the moment. One should keep in mind that the high frequency end is affect by the cone to disk spacing (smaller the better) and many discones are less than optimal in that.

 

[JELAIR]

If you do not install it outside and exposed to weather you don't have to get the most expensive discone alternative. They all perform equally it's just the mechanical design and material used that differ. Look at something like this: https://www.ebay.co.uk/itm/Scanner-Receiving-Antenna-Discone-25-1300MHz-/121508352707

And if you are non-interested in stuff below 250MHz you could optimize it by cutting all elements, both top and bottom ones, down to half size. It will be easier to handle in a room if it's only 50cm high and can hang from the ceiling or stand on the floor.

/Ubbe

It can get quite windy at the balcony (it's positioned in a way that leads to big pressure suction effects during some storms. A bit like a tornado effect, sucking/throwing things around violently)

And I do want the civil airband also, so down to 118 MHz at least.

The diamond seems worth the price (at least if it lasts me many years), so I think I will end up getting that.
I just want to make sure I have the proper things I need to disguise it before I actually get it.
A solid mount so it doesn't blow over when it's windy and some kind of tarp or table-cloth that looks natural on a 'table' (ooh, listening to radio almost feels like a crime with all these antenna rules that exist here. But rules are rules and of course I could get in real trouble by breaking them. That's the trouble we poor folks face I guess, since we can't afford our own houses in today's economy. But that's a debate for another day Thanks very much Ubbe )

 

[dlwtrunked]

The spacing between the disc and cone will vary the impedance and not necessarily the high frequency response. Ideally the cone comes to a sharp point under the disc and compromised designs will have a flat top cone under the disc.

For continuous coverage, detailed design parameters that I have seen disagree because all discones are "compromised" by reality and the cone does not come to a point at the disk-that is the problem. Generally I see the spacing defined as a percentage of the upper cone diameter (but many designs ignore that) and that diameter determines the "upper frequency limit". In essence, not being careful about the area near the cone, spacer, and disk are will result in less that ideal upper frequency response. One reference (not the original I read that I cannot find at the moment and was a book not the net) is:

https://books.google.com/books?id=P... design spacing between disk and cone&f=false

 

[prcguy]

The length of the cone or skirt determines the lower end frequency response and the ratio of the cone to the disc determines so some extent the frequency response, which is stated to be about 8:1 but can be optimized for a narrower range. Changing the spacing between the cone and disc is done mostly to achieve the best match across the entire working range.

I've made a few Discones in the past and have experienced varying the spacing then having to make a custom insulator once I found the best match. When you vary the spacing you can watch the VSWR change across the entire usable range of the antenna.

Many of the scanner type Discones do a fairly good job of bringing the cone to a point and one of the better examples I've seen is with the military surplus AT-197, which is optimized for the 225-400Mhz range.

For continuous coverage, detailed design parameters that I have seen disagree because all discones are "compromised" by reality and the cone does not come to a point at the disk-that is the problem. Generally I see the spacing defined as a percentage of the upper cone diameter (but many designs ignore that) and that diameter determines the "upper frequency limit". In essence, not being careful about the area near the cone, spacer, and disk are will result in less that ideal upper frequency response. One reference (not the original I read that I cannot find at the moment and was a book not the net) is:

https://books.google.com/books?id=P... design spacing between disk and cone&f=false

 

[Ubbe]

It can get quite windy at the balcony (it's positioned in a way that leads to big pressure suction effects during some storms. A bit like a tornado effect, sucking/throwing things around violently)

And I do want the civil airband also, so down to 118 MHz at least.

There will be no quality problem if you use the discone as a table on your balcony. The Diamond center top element are of no use to you so pay 1/3 and get the Moonraker or equal. Cutting all the elements will not get if dead on civil air. It will be much better than your current antenna but it will improve mil air a lot, double the gain or even more. You can always try that at a later time. If the rods are secured by screws, usually allen screws, you could buy extra rods at the hardware store, Biltema have those, and cut them to half size of the original rods. If they have threads you could probably find the proper ones also at Biltema as they have different sizes fully threaded 100cm rods for €2. Cut four of them in half and use as bottom element and use another four cut to 30cm lenghts for the top elements.

Try to let it stand on a wooden or MDF board and use screws to fasten the lower elements to it, it will help if you bend the ends in an angle. For the "table top" you could use plexiglass, also at Biltema 60cm x 60cm €15, cut to a circle and decorate with colored plast film and use electrical cable clamps to fix to the upper rods. The whole antenna will get tough and have extrem stability to handle any kind of storm winds. You will not need any kind of mast or base that would only complicate things.

/Ubbe

 

[Ubbe]

I've made a few Discones in the past and have experienced varying the spacing then having to make a custom insulator once I found the best match. When you vary the spacing you can watch the VSWR change across the entire usable range of the antenna.

The ideal would be to monitor signal strenght while adjusting an antenna, but it's a bit tricky to do.

It's the impedance that change when you alter the distance between cone and disc and hence the VSWR to a fixed impedance coax.

The SWR value could be deceiving as a matching dummy load will show a perfect SWR value. The more "lossy" an antenna are and worse Q value it has the better the general SWR will be. I have heard of some amateur shortwave antenna that has resistors in its design to keep the Q value down and present a good SWR value over a wide frequency range. A total waste of performance.

/Ubbe

 

[JELAIR]

There will be no quality problem if you use the discone as a table on your balcony. The Diamond center top element are of no use to you so pay 1/3 and get the Moonraker or equal. Cutting all the elements will not get if dead on civil air. It will be much better than your current antenna but it will improve mil air a lot, double the gain or even more. You can always try that at a later time. If the rods are secured by screws, usually allen screws, you could buy extra rods at the hardware store, Biltema have those, and cut them to half size of the original rods. If they have threads you could probably find the proper ones also at Biltema as they have different sizes fully threaded 100cm rods for €2. Cut four of them in half and use as bottom element and use another four cut to 30cm lenghts for the top elements.

Try to let it stand on a wooden or MDF board and use screws to fasten the lower elements to it, it will help if you bend the ends in an angle. For the "table top" you could use plexiglass, also at Biltema 60cm x 60cm €15, cut to a circle and decorate with colored plast film and use electrical cable clamps to fix to the upper rods. The whole antenna will get tough and have extrem stability to handle any kind of storm winds. You will not need any kind of mast or base that would only complicate things.

/Ubbe

Love the idea with the plexi-glass top. That should look natural and actually be able to function as a real table
I made a note of the elements you mentioned and will see what the local biltema has in store.
I might end up with 2 'tables' and an antenna-switcher

"What's that you got on the balcony?"
"It's the new "UbberCool-DiscOne" table. The first of its kind, hence the name DiscOne"
"With an antenna-wire attached?"
"Nah, it's just a storm protection cable, so it doesn't fly away when it's windy"

That could work