Mounting UHF Military Double Discone on Roof

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vagrant

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Put it up for UHF mil air and enjoy it...
If given a choice for 225-400 Mil Air, would you go with this antenna, the AT-197A with an amp, or something else? Actually, I think you mentioned a vertical in the past that has two antenna ports as it handles VHF air as well.
 

MDScanFan

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I gave the feeding a bit more thought. Based on the comments above it sounds like there may be a coax connecting them in a series configuration. From the picture it looks to be around 2.5’ between the feed points. That sets the min possible cable length between them.

Picking a cable length that gives one wavelength, ~39”, at 300 MHz seems like the only viable option. At 300 MHz there will be a nice constructive pattern with gain higher than a single discone along the horizon. At 200 MHz and 400 MHz the phase is around -120 and +120, which gives nulls at +/- 10 degrees from the horizon, depending on the frequency. Along the horizon the roll off of the pattern due to the nearby null would be significant and is likely worse than a single discone. The sweet spot seems to be 225 to 375. Regardless, above 300 MHz there is an omnidirectional null that moves from zenith towards the horizon with increasing frequency. The numbers change a bit depending on the antenna spacing but the general takeaway still applies.

A parallel feeding scheme would mitigate the phase slope issue but comes with it its own issues for transmitting. A parallel feeding scheme would mitigate the phase slope issue and seems a better option for a receive-only antenna. What makes you guys think it’s a series fed design and not a parallel fed?


l
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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Due to the band width, I suspect they are feeding a broad band power divider with equal length cables to each antenna. That would minimize any frequency specific phasing problems in the feed, leaving the spacing between antennas as the major variable for pattern and gain.


I gave the feeding a bit more thought. Based on the comments above it sounds like there may be a coax connecting them in a series configuration. From the picture it looks to be around 2.5’ between the feed points. That sets the min possible cable length between them.

Picking a cable length that gives one wavelength, ~39”, at 300 MHz seems like the only viable option. At 300 MHz there will be a nice constructive pattern with gain higher than a single discone along the horizon. At 200 MHz and 400 MHz the phase is around -120 and +120, which gives nulls at +/- 10 degrees from the horizon, depending on the frequency. Along the horizon the roll off of the pattern due to the nearby null would be significant and is likely worse than a single discone. The sweet spot seems to be 225 to 375. Regardless, above 300 MHz there is an omnidirectional null that moves from zenith towards the horizon with increasing frequency. The numbers change a bit depending on the antenna spacing but the general takeaway still applies.

A parallel feeding scheme would mitigate the phase slope issue but comes with it its own issues for transmitting. A parallel feeding scheme would mitigate the phase slope issue and seems a better option for a receive-only antenna. What makes you guys think it’s a series fed design and not a parallel fed?


l
 

blantonl

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So I got this chonker setup and running.

Here's a picture of it all assembled.
IMG_9198.jpg

The preliminary performance results are interesting. It turns out that this antenna is a 50 pound 50db attenuator that looks freaking cool, mean, and gives the neighbors the impression that I've set up a military special operations forward air controller post. The reality is it doesn't receive ****, and can barely get NOAA weather radio on 162.550 while a whip antenna on the same radio gets S9+. 800 MHz is completely deaf. I'm not sure if there is corrosion inside the coax to antenna connections or what, but this thing is a paperweight. And it's going to stay there for the next week or so until I can decide what to do with it.

It will be fun to see what the lawn guys think of it on Monday morning. My pool guy raised both eyebrows when he saw my Trivec UHF Satcom antenna installed on the side of the house. He's former Army and casually mentioned he used to carry that same antenna on his back. He probably thinks I'm some crazy nut.

Back to the drawing board.
 

prcguy

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Its beautiful. Send it to me. I'll refurb it and love it and stick it in the bed of my truck and parade around with it.

In the mean time stick an antenna analyzer on it, that will tell you if something is open, shorted or if its actually an antenna.
 

Ubbe

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...and can barely get NOAA weather radio on 162.550 while a whip antenna on the same radio gets S9+. 800 MHz is completely deaf.
Then the specification of 220MHz-400Mhz might be correct and the two discones goes out of phase with each other at other frequencies and cancels out the signal. It works as a bandpass filter stopping any 150MHz pager signals and cellular towers. It must be a terrific mil-air antenna.

/Ubbe
 

blantonl

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My wife took one look at that antenna and said awwwww hell nah. That’s a hill I will have to die on at another date.

what’s a really good antenna analyzer. Probably should have on in my toolkit

note that UHF Satcom is completely silent as well on this antenna.
 

ra7850

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So I got this chonker setup and running.

Here's a picture of it all assembled.
View attachment 88887

The preliminary performance results are interesting. It turns out that this antenna is a 50 pound 50db attenuator that looks freaking cool, mean, and gives the neighbors the impression that I've set up a military special operations forward air controller post. The reality is it doesn't receive ****, and can barely get NOAA weather radio on 162.550 while a whip antenna on the same radio gets S9+. 800 MHz is completely deaf. I'm not sure if there is corrosion inside the coax to antenna connections or what, but this thing is a paperweight. And it's going to stay there for the next week or so until I can decide what to do with it.

It will be fun to see what the lawn guys think of it on Monday morning. My pool guy raised both eyebrows when he saw my Trivec UHF Satcom antenna installed on the side of the house. He's former Army and casually mentioned he used to carry that same antenna on his back. He probably thinks I'm some crazy nut.

Back to the drawing board.
If you decide to sell it, I'm interested.

Robert
 

Ubbe

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what’s a really good antenna analyzer. Probably should have on in my toolkit
You probably won't have any use for one, at least not with these types of antennas. All they do are tell you have much of the signal you send out from the analyzer are "consumed" by the antenna and doesn't reflect back to you. It tells nothing of how much it actually radiate and where the signal are directed and how it actually works as an antenna. The worse an antenna works the more it could look as a dummy load and give good SWR and return loss figures.

/Ubbe
 

blantonl

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I'm going to open it up and see if there is some corrosion or problem with the internal connections.

The thing is WAY too quiet to be not basically receiving anything more than what a piece of coax might. I suspect it was installed somewhere near a salt-water environment based on the condition of the external components.

I don't do any transmitting whatsoever... I'm strictly a receive only guy.
 

prcguy

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Or it was hit with a lot of power for jamming or similar and it burned up an internal power divider. I had a Trivec-Avant satcom antenna once that was bricked and I worked for a large aerospace company at the time. I called Trivec-Avant asking how to open the antenna as it was defective and told them it was part of our company inventory (it was actually mine). Since I was not far from the factory they told me to bring it by so I took the next day off and drove over with the antenna.

They ran the S/N and were very concerned how I (we) got the antenna and I said I had no clue on its past history, its just part of our inventory. I got a complete tour of the Trivec-Avant factory and antenna range and left the antenna with them. They called back a week later saying the internal 3dB hybrid was completely burned up and hinted that it may have gone through destructive testing at Ft Monmoth, NJ. They repaired it and shipped it back for free.

Lucky me.


I'm going to open it up and see if there is some corrosion or problem with the internal connections.

The thing is WAY too quiet to be not basically receiving anything more than what a piece of coax might. I suspect it was installed somewhere near a salt-water environment based on the condition of the external components.

I don't do any transmitting whatsoever... I'm strictly a receive only guy.
 

prcguy

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I bought the Trivec antenna from a well known mil surplus dealer and didn't find it was trashed until a year later. Hopefully Lindsay's antenna is an easy fix. Or maybe he can call the factory and explain (in Italian) that he is with an American aerospace company and they need the antenna to work or they will get a bad reputation. Or he can ship it to me and it will live a happy life at my antenna orphanage.

so were there locks on the dumpsters out back the next time you went out to Monmoth?
 
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