Mounting UHF Military Double Discone on Roof

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blantonl

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So, I purchased this Military Discone antenna about 6 years ago on Ebay... was super excited to get it until I realized how big it is. It is a double discone antenna, and the mast stands about 5 feet tall and the whole thing put together weighs about 50 pounds. It arrived from Turkey in the middle east, covered in customs forms, coated in Desert dust, and smelled like gunpowder. I bet this bad boy has seen some action.

Anyway, I'm looking for suggestions on how to mount this on the apex of my metal room, and properly ground it. I already have a run of LMR-400 ready to go, and grounding at the base of the room ready with the home electric service panel and pool equipment just below it... but need suggestions for hardware to properly mount it. I would prefer to not use any guy wires. The pictures show the apex of the roof where I plan on installing it, along with the picture of the mast without the elements installed. The base, as you can see, would be screwed into another mounting base etc.

I'd love for some suggestions on how to best accomplish this. This antenna is an absolute unit. The first thing I'm probably going to do is to build a temp base out of wood and haul it up there just to test the receive ability of the antenna and see if it works well from 30-950 MHz. If all goes well, I'll go for a permanent install.

Thoughts? Suggestions?IMG_9193.jpgIMG_9194.jpgIMG_9195.jpg
 

mmckenna

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What's your tolerance for ugly?



If you don't want guy wires, you're probably going to need to through-bolt it to keep it in place.

And I suspect that's a biconic antenna.
 

Ubbe

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The first thing I'm probably going to do is to build a temp base out of wood and haul it up there just to test the receive ability of the antenna and see if it works well from 30-950 MHz.
Yes, do that first.

I don't know why these antennas has been named double discone when they have nothing to do with a discone. It is a wide band dipole antenna. It is theoreticly two solid cones with their tips pointing to each other and as the bandwidth of a dipole are dependent of the diameter of the elements these are widebanded as the diameter of the cone are huge at the ends. It's impractical with a solid cone so multiple elements are instead used. It will of course work best at the middle of its frequency band as will give unwanted directional loobs the more the further the frequency are from the elements wavelenghts. It will still give low SWR but performance will be bad at 30MHz and 950MHz. So actual live tests are a must before putting too much work into it.

/Ubbe
 

prcguy

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So you were the one that got that thing! I was curious about it but passed. I believe it was made for the 100 to 400MHz range and it would not work very well at all below 100MHz but should work fine to at least 500MHz. Since its two stacked Discones it could have up to 3dB gain at some frequencies.

If it were my antenna and my roof I would visit a scrap metal joint, pick up a chunk of surplus 1/4" thick steel maybe 1ft X 3Ft and have them put two bends in it so you have a flat spot in the middle just wide enough for the antenna base then two wings bent to match your roof angle so it can sit at the apex of the roof. Then drill and tap holes for the antenna base and maybe a couple at the ends of the wings for a few small screws to hold it down to the roof.

Of course clean, red primer and paint to match the roof. Then pop a beer, float in the pool and enjoy the new view. Here is a view of my Texas antennas from the pool although the antennas are quite boring compared to yours.

TX pool.JPG
 
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blantonl

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Yes, do that first.

I don't know why these antennas has been named double discone when they have nothing to do with a discone. It is a wide band dipole antenna. It is theoreticly two solid cones with their tips pointing to each other and as the bandwidth of a dipole are dependent of the diameter of the elements these are widebanded as the diameter of the cone are huge at the ends.

/Ubbe
This antenna is not two cones pointed at each other. It is two cones stacked on top of each other.... like this

/|\
|
/|\
 

prcguy

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Yes, its two individual but identical Discone antennas somehow stacked on top of each other with a continuous mast up through the bottom unit and an unknown type of feed between them. Not to be confused with the commercial scanner antenna made by Nevada called a double Discone, which was actually a Bicone, but a really cheesy one.

This antenna is not two cones pointed at each other. It is two cones stacked on top of each other.... like this

/|\
|
/|\
 

blantonl

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I think I might get it out tomorrow and put it together, bolt it to a piece of wood, and start testing it for performance
 

RFI-EMI-GUY

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That is too nice a roof to get all messed up. I would get some stout pipe, weld or thread a flange on it to mate with your antenna and stand it in concrete right at the side of the house. Use uni-strut to secure to the eaves. Paint the lower section to match the house.
 

MDScanFan

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I too am very interested to see how it turns out. That’s an unusual antenna for a home installation. The coolness factor is high.

Out of curiosity, how is the feed implemented for this type of stacked broadband antenna? Is it a coax running from each discone that is then brought together with a broadband combiner.
 

Ubbe

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Ok, so it IS a double discone!

I wonder how those are interconnected as I can only think of that it would be frequency dependent and out of that frequency band they would fight each other with different phases and work worse than a single discone. The size indicates that it is nowhere to be 30Mhz as it lowest frequency and at 700MHz and higher the loob will be pointing up in the sky from both discones. As it is military it probably only works efficiently between 200MHz-400MHz. So testing it first is a must.

/Ubbe

This antenna is not two cones pointed at each other. It is two cones stacked on top of each other.... like this

/|\
|
/|\
 

blantonl

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So here's an interesting question.

For receive only, I wonder if there is a modification that could be done to make this a better antenna for 400+ MHZ

Maybe shorten the elements for the top discone and leave the bottom at the same length? The pictures in the Ebay listing above is the exact antenna.
 

vagrant

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I just noticed an interesting couple of plots on that page as well. I would sweep it and leave it as-is to enjoy whatever gain. Still, that would be interesting to have larger/smaller stacked discones handling lower and a bit higher UHF all-in-one out toward the horizon.

Temporarily mount it away from things, sweep it and then test it with a receiver. Your own data is the truth and the place to start. Testing the resistance with a multimeter would be prudent as well.
 
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dlwtrunked

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Ok, so it IS a double discone!

I wonder how those are interconnected as I can only think of that it would be frequency dependent and out of that frequency band they would fight each other with different phases and work worse than a single discone. The size indicates that it is nowhere to be 30Mhz as it lowest frequency and at 700MHz and higher the lobe will be pointing up in the sky from both discones. As it is military it probably only works efficiently between 200MHz-400MHz. So testing it first is a must.

/Ubbe
I also have one of those in my shed also and have never assembled it. Mine came from eBay from Greece several years ago. Yes, these are actual double discones not the bi-conical often mis-advertised as double discone. It is a 225-400 MHz antenna. Cost a bundle to get shipped from Greece. I am guessing it really only gives 3 dB gain over a single discone.
 

Ubbe

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The lenght of the connection, coax or hardline, between the discones makes it work with low SWR at 3 bands that are multiples of each other. 300MHz, 600MHz and 900Mhz and also 1600Mhz. The 300Mhz band are the broadest with a low SWR from 150MHz to 450MHz. It's named as a 200-440MHz 3dB gain antenna.

/Ubbe
 

prcguy

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The Discones look to have normal proportions, and an individual Discone of correct proportions has about an 8:1 usable frequency range, not counting the upward pattern shift about mid way in its frequency range. I would measure the length of a bottom skirt element including the cone hub and calculate what frequency that would equal a 1/4 wavelength. This would be very near the low frequency cutoff of the single Discone. Then verify if the dia of the top hat is around .67 to .70 dia of the bottom skirt length. That will tell you if its built to published specs.

Then you will know the basic frequency range and the question will then be what does the stacking do to the radiation pattern? The individual Discone patterns will converge at some frequency range so the antenna will achieve some gain but at other frequencies the pattern will be pulled up or down from the horizon. It would be nice to sweep it with an antenna analyzer then compare with another known antenna like a single Discone of similar size to see what the gain differences are.

If you ship it to me I will do the testing. You might get it back in a few years.


So here's an interesting question.

For receive only, I wonder if there is a modification that could be done to make this a better antenna for 400+ MHZ

Maybe shorten the elements for the top discone and leave the bottom at the same length? The pictures in the Ebay listing above is the exact antenna.
 

iMONITOR

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Non-Penetrating Ridge-Roof (Peak) Mount with 2 7/8″ OD (2.88″ OD) x 3′ Mast

88712
 

prcguy

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In the link, if you look at the return loss sweep from 5 to 1600MHz, then find the center frequency of 800MHz, then look at the lower half and find the center frequency of that you will find a consistent low return loss area that ends about 400Mhz and its a little hard to see where it starts but its above 100MHz and maybe around 200MHz. The antenna is listed on the label as UHF, so it probably is 200 to 440MHz as the advertisement states.

Since there would be a critical length of coax inside to combine the antennas and there is a specific spacing between them, in my opinion it is what its designed for and your not going to be able to modify it for anything else without starting from scratch with a new design. Put it up for UHF mil air and enjoy it, or trade it off for something wider band if that's what the OP really needs.

I’d swear I have seen that style of antenna discussed here on RR before. 30 to 950 MHz seems like a unicorn of an antenna, even if unity gain. I hope it works out.

 
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