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  #41 (permalink)  
Old 09-20-2013, 12:16 AM
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Default Re: Home Brew Milair, Off Center Fed Dipole

It may, but it's far from optimal, as the sats are circularly polarized. With a linearly polarized antenna, you lose around 3db off the top.

Mark

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Old 09-20-2013, 2:05 AM
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this would be nice to see on youtube you making this antenna what frequency range is this antenna
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Old 09-20-2013, 4:36 AM
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Quote:
Originally Posted by popnokick View Post
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Does the ST-2 receive the satellites?
No.
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Old 09-20-2013, 4:37 AM
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this would be nice to see on youtube you making this antenna what frequency range is this antenna
I built this antenna for milair 200-400mhz.
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Old 09-20-2013, 4:40 AM
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sat coms keep coming up in topic.
while I am getting sat com interference in the 240. - 270. mhz range, this is NOT what I intended this antenna to be for.

Just wanted to make that clear.

Thanks to all for your comments, I appreciate them all.
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Old 09-20-2013, 8:38 AM
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... And we appreciate your posts and pics! Lets everyone know what works and what doesn't. Valuable info!
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Old 09-20-2013, 10:46 AM
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digitalanalog, take a look at Airnav.com. There you will find milair frequencies listed under "airport communications" for a major airport near you. Example: Pittsburgh (KPIT). Although you probably will not hear ground coms, you may hear approach or departure flights heading to/from there. you should be able to hear aircraft 100+ miles away from you depending on their altitude. Also, click on the sectional chart, as this will give you a good picture of all airports in relationship to your location. Hope this helps.......PW
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Old 09-20-2013, 6:58 PM
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Yes I am aware of Airnav dot com,
My main problem and it is a big problem, is I am down in a whole(valley) and I am surrounded by 60-80 foot tall trees on 2 sides.
The closest airport to me is Youngstown, Ohio which is also home of the 910th Airlift Wing.
The airport is 26.09 miles from my home, not bad, BUT, The ground elevation is 87' higher then mine.
So, You see the problem, I have my antenna at 35' so I am still 52 feet lower in elevation and that is Ground elevation, plus as mentioned tall trees around my antennas.

This is why I use the GRE amp, The ST2 works well without the amp, but I am still missing alot of air traffic(milair) becuase of my surroundings, Ths is also another reason I am going to try a Full Wave antenna versus a 1/2 antenna.

In my situation I need a High Gain antenna.

Pittsburgh International Airport is 38.68 Miles from my home.
Cleveland Hopkins Airport is 67.10 miles from my home.

There are mil flights out of the three airports I listed, But hearing them is Not an Easy Task with my location.
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Old 09-21-2013, 3:33 AM
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Originally Posted by digitalanalog View Post
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
Ok, if you want to play with a full-wave centered around 300mhz, know that you can not feed it in the center, but a quarter-wave in on one side, and 3/4 on the other. While this is physically off-center, it is NOT your typical ocfd, and you can get away with a direct connect (no balun/transformer) to coax this way.

Ie, if I was going to do that with some small tubing, I would cut one side to 9-1/4 inches, and the other side to 26-3/4 inches. The total is slightly smaller at 36 inches, since you are using tubing and not thin wire.

Maybe you will be part of the .001% where a full-wave works for you.

Last edited by hertzian; 09-21-2013 at 3:35 AM..
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Old 09-21-2013, 4:25 AM
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Quote:
Originally Posted by hertzian View Post

Maybe you will be part of the .001% where a full-wave works for you.
Why is it only .001% works at full wave, I don't understand this at all.
please explain.
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Old 09-21-2013, 3:36 PM
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Well, it has to do with the pattern. Instead of the familiar "doughnut" shape of a half-wave dipole, with the full-wave, you have a cloverleaf pattern instead.

Essentially it looks upwards, and not outwards.

As revealed by prcguy, this is not always a real advantage when monitoring airband (civil or milair) since even at altitude, the planes are not really that far away, and there is very little absorption between your antenna and the aircraft, so having gain looking upwards is not a major benefit.

Sticking to a standard half-wave dipole puts gain where it is needed most - towards the horizon.

But, I say go ahead and try a fullwave (fed properly at the quarter-wave point) - in your situation perhaps looking towards the horizon means looking straight into a mountainside or other absorbing obstacles that no gain will improve upon.

Given your location down in the hole, give the fullwave a shot...
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Old 09-22-2013, 4:06 AM
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ok, I do recall prcguy mentioning the low horizon affect, but as you mention, I think I need my antenna to look upwards, I am at least going to give it a try and see if it helps, This antenna is Not doing very well at all, would not consider it a failed attempt, just due to my location it's just not enough to get to the desired frequencies I am looking for, So Maybe just Maybe a full wave will do it, I am still monitoring with this recent homebrew, But nothing to report in the way of successful monitoring.

thanks hertzian for your reply, I appreciate it.
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  #53 (permalink)  
Old 09-23-2013, 2:50 AM
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Have you tried it without the tee to see if that makes any difference? I know you are feeding two scanners, and simple tees are not really ideal. 3db of loss right there.

Also, what type of coax are you running and how long? This can also be an issue at milair freqs.

Another possibility is overload / desense. Do you notice a difference if you have the attenuator switched IN? You may also want to look into milair bandpass filters like the JIM HPF-0706A, but no need to go there just yet unless you can determine that you are getting desense / overload from out of band transmitters...

Last edited by hertzian; 09-23-2013 at 2:56 AM..
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  #54 (permalink)  
Old 09-23-2013, 4:24 AM
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No I have not tried it without the "T", understanding it does lose 3 db, but it's run directly into the GRE amp then out to the "T" then out to two scanners, the GRE amp should take any loss out and it is defiantly putting some gain back into it, as I mentioned in a post prior, without the amp it does not receive at all or at least it doesn't seems to.

Coax, Is Commscope 18awg and approx. 50 feet, Should be o.k., it's good heavy coax, believe its from the roll of sat cable the neighbor gave me from his last sat install and the sat company left it behind.

It could be desensitizing with the amp, but like i said, it doesn't seem to receive with out it, but it's very difficult to tell, most transmissions are not long enough to switch off the amp to tell (amp is behind all my equipment).

I am receiving allot of good strong signals with this antenna in civilian air bands (in AM mode) which kind of is confusing since it should be way to short for those signals. But still not strong enough to pick up Any ground towers planes in the air is all I seem to be able to pick up, but I still don't understand why it's picking up civil airband at all, But it does. I have tried other bands as well (public service) and it does not work very well if at all.

Truly not sure what to do with it at this point, but cold weather is setting in and access to the roof will soon be gone in a month or so probably. I have an old discone (missing two disc. and two cone elements) I might put it up there for the winter, Just not sure........

Thanks hertzian for your comments.
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Old 09-27-2013, 4:22 AM
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After listening and scanning for a while now, I don't think this antenna or any other for that matter is going to be good enough to receive anything other then the air wing about 20+ miles from me.

As mentioned my location is very poor for this particular band.

Get good signals from "Vadar Ops" and a Mid air refuel but other then that, Just nothing being herd anywhere in the band, I have tried every available option I have with the equipment I have and nothing seems to improve. It is also very reasonable to think there is not much milair traffic in my area to begin with and this also is not to my advantage.

So the monitoring continues, but not getting excited about it..........

Winter is heading our way so i will more then likely take down this antenna (can still receive the little milair i do get with the 2 ST2 antennas I have up currently)and put up the Celwave i bought from prcguy several years ago for local LEO traffic.

Thanks to all for the comments,suggestions and etc........
Jim
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Old 10-27-2013, 2:23 AM
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OK if you built this antenna do you have any spec or photos of it digitalanalog ? anyone else have made this antenna
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Old 10-27-2013, 6:53 AM
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Quote:
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OK if you built this antenna do you have any spec or photos of it digitalanalog ? anyone else have made this antenna
Did you read the entire post/thread?, The info and pics are posted.
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