Home Brew Milair, Off Center Fed Dipole

[digitalanalog]

@ rbm
Don't want to listen to Pirate's...
Just Milair.

 

[popnokick]

Be patient. As you noted earlier, milair is somewhat quiet in your area. Fact that you're hearing military satellite comms is a very good sign the antenna is working well. MILITARY SATELLITE COMMS ARE IN THE MILAIR BAND. You're not getting spurious signals or some sort of receiver overload. THOSE SATELLITE SIGNALS BELONG EXACTLY WHERE YOU HEARD THEM. Look it up. If you don't believe it you might as well take the dipole down now. But I think a lot of us are following this thread to see the results.
How are you searching for milair comms? Have you also entered some "usual activity" freqs in your scanned channels?

 

[nanZor]

Understood 8 1/2" for 1/2 wave, still don't understand why the coax has to be perpendicular, not seen any other mention of this in reference to any other antenna.

For most dipoles, you do this to minimize the coupling to the coax shield, perpendicularly is best. You want to run it away perpendicularly for a half-wave (about as long as the milair dipole is long) to also avoid skewing the pattern - in other words instead of being an omnidirectional pattern, with close spacing to a conductive mast or draped coax, you end up with a cardiod pattern instead.

Some users with towers will actually do this to change the pattern to their needs.

From your pics I see that you have chosen the "sleeve" dipole method. Ok, the trick here is that you want to center the coax and not let it touch the bottom element. This means putting a spacer inside to have an air gap all around it. One easy thing to do would be to squirt some self-sealing foam inside and let it set with the coax centered. Again, you want to try and avoid coupling the antenna to the coax shield by putting as much space between them, of you may skew the pattern.

As a side note, from your pics, if this were a transmit-capable antenna, and you put a healthy amount of RF into it, where the coax touches the inside of the bottom tubing would be the first place the coax would arc-over.

As well, I don't want a 1/2 wave antenna, I want a Full wave.
Am i correct a Full Wave Antenna will be a better receiving antenna?

For 99.9% of the people, this is the WRONG choice. I kind of wish I hadn't brought it up. Stick to the half-wave dipole as your first mil-air antenna.

 

[nanZor]

This is going to be put up today, problem is there is so little milair traffic in my area, plus it's the weekend, so it may take some time before I hear anything to be able to tell if it's working or not.

Unless you have a known good list of frequencies already on hand, due to the very short and to-the-point military comms, in addition to the enormously wide spectrum, you could very well miss 90% of the traffic and mistakenly conclude that there is no activity.

Be sure to visit the other subforums here for tips, as well as the RR database for air freqs. Other than that, it takes a different mindset when searching milair. Many times one will spend a week or more just scanning a 2 mhz spread for a week before moving on the next 2 mhz spread, and so on. There are other techniques, but scanning the whole spectrum from 225-380 mhz in one go involves a lot of crossed-fingers.

In other words, your antenna may be working far better than you think it is! Note that we're starting to get up into the UHF range where it pays to use quality coax with low loss for long runs.

 

[digitalanalog]

For most dipoles, you do this to minimize the coupling to the coax shield, perpendicularly is best. You want to run it away perpendicularly for a half-wave (about as long as the milair dipole is long) to also avoid skewing the pattern - in other words instead of being an omnidirectional pattern, with close spacing to a conductive mast or draped coax, you end up with a cardiod pattern instead.

Some users with towers will actually do this to change the pattern to their needs.

From your pics I see that you have chosen the "sleeve" dipole method. Ok, the trick here is that you want to center the coax and not let it touch the bottom element. This means putting a spacer inside to have an air gap all around it. One easy thing to do would be to squirt some self-sealing foam inside and let it set with the coax centered. Again, you want to try and avoid coupling the antenna to the coax shield by putting as much space between them, of you may skew the pattern.

As a side note, from your pics, if this were a transmit-capable antenna, and you put a healthy amount of RF into it, where the coax touches the inside of the bottom tubing would be the first place the coax would arc-over.



For 99.9% of the people, this is the WRONG choice. I kind of wish I hadn't brought it up. Stick to the half-wave dipole as your first mil-air antenna.

I have centered the coax running up through the bottom tube using foam rubber, so this should help. I also mounted it on solid plastic, and the mast is PVC, i did this to cut out the antenna to mast issues.
The two tubes are only touching plastic,the mast mounting bracket is also countersunk from the back so it's not touching the antenna either.

I am hoping it all works out.

Thanks for your comments, I appreciate it.

 

[digitalanalog]

Be patient. As you noted earlier, milair is somewhat quiet in your area. Fact that you're hearing military satellite comms is a very good sign the antenna is working well. MILITARY SATELLITE COMMS ARE IN THE MILAIR BAND. You're not getting spurious signals or some sort of receiver overload. THOSE SATELLITE SIGNALS BELONG EXACTLY WHERE YOU HEARD THEM. Look it up. If you don't believe it you might as well take the dipole down now. But I think a lot of us are following this thread to see the results.
How are you searching for milair comms? Have you also entered some "usual activity" freqs in your scanned channels?

Yes I understand the satcom freq's are in the milair band, perhaps I should have worded my statement better.

I know patience is a must with milair, I will scan and keep tract of what i hear.

Thank you for your comments, I appreciate it.

 

[digitalanalog]

Unless you have a known good list of frequencies already on hand, due to the very short and to-the-point military comms, in addition to the enormously wide spectrum, you could very well miss 90% of the traffic and mistakenly conclude that there is no activity.

Be sure to visit the other subforums here for tips, as well as the RR database for air freqs. Other than that, it takes a different mindset when searching milair. Many times one will spend a week or more just scanning a 2 mhz spread for a week before moving on the next 2 mhz spread, and so on. There are other techniques, but scanning the whole spectrum from 225-380 mhz in one go involves a lot of crossed-fingers.

In other words, your antenna may be working far better than you think it is! Note that we're starting to get up into the UHF range where it pays to use quality coax with low loss for long runs.

I have this antenna split to two scanner.
One is loaded with channels I have been able to receive in the past, the other is set to scan,
as well two more scanner are also scanning other portions of the band (although not connected to this antenna,a ST2 antenna is being used), so a side by side comparison (so to say) will let me know how things are working.

As well, I am at work from 7am till 5:30pm and will miss this window of opportunity to listen, and this is probably a Good time to hear milair, But can't be helped, I thought about recording software, but just don't want to fool with it.

I shall Listen and waite...

 

[digitalanalog]

To cover the entire UHF mil air band I would make a 1/2 wave dipole and not a modified off center version and use fat elements in the 1.5 to 2" diameter range. You can make it a coaxial dipole where the coax exits the bottom of the lower element but you want the coax to be centered as it exits the element and a doughnut made of Styrofoam pushed up into the bottom element can work well for this.

Spacing between the dipole elements would have to be determined by experiment and a common mode choke on the coax near where it exits the lower element would help with tuning. The elements will also be shorter than calculated due to the fat diameter.

Making one of these has been on my list for awhile and if successful I'll post the info. Otherwise I already have a bunch of UHF mil air band antennas and would only be doing this for the curiosity.
prcguy

Would really like to see your build of this type antenna, Hopefully you will have time to.

Thanks for your comments,I appreciate it.

 

[digitalanalog]

Update 9/16/13

Well, here is what I have been receiving with the new antenna design.

249.325 Pirates
251.000 Pirates
265.475 Pirates
265.525 Pirates
265.5375 Pirates
275.875 Pirates
275.900 Pirates
296.700 Pirates
297.500 Pirates
379.200 Pirates

All received in the PM hours between 6 - 9 EST

 

[digitalanalog]

Still nothing but Pirates and they are all what seems to be be off frequency.
Hoping to hear something this weekend to be able to confirm the antenna is working
may take longer but I am hoping for the best.

 

[Markb]

Didn't see it mentioned. Just to confirm that you are aware that the SATCOM stuff is FM. May be why it sounds off freq.

Check out www.tacsat.com also. Lots of recordings and logs. Stuff besides pirates.

I have found that recording is the only way to go. I use a Thin Client and ProScan for the software. Makes a relatively inexpensive setup.


Sent from my Samsung Galaxy S4 using Tapatalk

 

[digitalanalog]

yeah forgot satcom is FM, Thanks for reminding me, that makes sense.

I have been receiving a lot of traffic for mid air refueling on 274.450.
New antenna at 35' using GRE Super amp.
ST2 scantenna with no amp at 30' (for comparison)

New Homebrew only receives the mid air refuel with the GRE Amp On, Turn the amp off and it does not receive it at all.

ST2 antenna received this pretty good without any help from the amp.
Switch the amp over to the ST2 and it receives Better then the new homebrew antenna does with amp.

Will continue to monitor, it may pick up better in a different area of the milair band.

 

[SCPD]

With the new US military sats up and now operational catching anything on the old UFO birds unfortunatly is going to soon be a thing of the past.

Spaceflight Now | Atlas Launch Report | Atlas 5 rocket on the launch pad for 40th mission

Im using a Diamond D777 for civil air/milair band stuff and a homebrew X-wing (thanks to prcguy) for UHF satcom work. I also use GRE Super amps on both antennas to boost signals.

 

[digitalanalog]

understood, I am not trying to listen to satcom,but I am getting allot ot satcom pirate interference.

 

[SCPD]

understood, I am not trying to listen to satcom,but I am getting allot ot satcom pirate interference.

Ok then lock out the UHF satcom portion of the band if your only interested in scanning milair activity. In most modern scanners you can custom edit the search range.

 

[digitalanalog]

That's a good place to start, locking out the portion of the band 240 - 270.
I have however been getting pirates outside this portion of the bands.
379.200 - 275.900 - 275.875 - 297.500 - 296.700 - 275.100

Thanks for your comments, They are Appreciated.

 

[Markb]

That's a good place to start, locking out the portion of the band 240 - 270.
I have however been getting pirates outside this portion of the bands.
379.200 - 275.900 - 275.875 - 297.500 - 296.700 - 275.100

Thanks for your comments, They are Appreciated.

That probably warrants some further investigation, as there aren't any sats in that range to my knowledge. I wounder if you are picking up images from another part of the spectrum.
Also, I would recommend locking out the known SATCOM freq's and not skipping that frequency range altogether, as it is not limited to SATCOM. For example, my nearest miltary airfield about 10 miles away uses 251.15 for their tower....



Sent from my Samsung Galaxy S4 using Tapatalk

 

[digitalanalog]

for my testing purposes, I completely skipped the 240-270 band.

So far side by side comparison, The ST2 with no amp is out performing this home made antenna with an amp. I received more on 274.450 mid air refueling and the ST2 beat this antenna Hands Down, No doubt about it and again with No amp,

Got a good copy on 311.525 this eveing and the same thing was true, ST2 was WAY better and crystal clear.

Will give it a while longer, but it does not look good for this antenna.

 

[digitalanalog]

I think I am going to try a Full wave dipole based on 300.00mhz, making it 39.36 inches and mount it vertical, won't be a FAT antenna just 3/8 tubing or something like that, A failed antenna will not stop me from creating something else, Who know's, I might get one to work like I need it to... I ain't scared... lol

 

[popnokick]

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Does the ST-2 receive the satellites?