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  #21 (permalink)  
Old 03-08-2009, 1:16 PM
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Quote:
Originally Posted by carmelof View Post
Hello Matt which model SDR u got.

Regards Lino.
It says version 4.5 on the board, but I know there is only versions 1 and 2 so I am not sure.
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Old 03-08-2009, 8:38 PM
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Originally Posted by DickH View Post
Vertical element = 23", the other 4 = 24" for 120MHz

Antenna works like a champ! Did you use epoxy on the top to hold the vertical in place or is that hot glue? I'm having trouble getting the vertical to stay in place.

FYI......I had to drive a half hour to find a radio shack that had a PL-259 to F adapter to attach to the USRP daughter board, but it was worth it.
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Old 03-08-2009, 8:42 PM
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Originally Posted by matt_wi View Post
Antenna works like a champ! Did you use epoxy on the top to hold the vertical in place or is that hot glue? I'm having trouble getting the vertical to stay in place.
Use solder.
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Old 03-08-2009, 8:53 PM
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Originally Posted by DDan View Post
Use solder.
And particular solder recommendations? The stuff I originally tried didn't work for squat.
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Old 03-08-2009, 9:00 PM
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Originally Posted by matt_wi View Post
And particular solder recommendations? The stuff I originally tried didn't work for squat.
Just standard rosin core solder like you would normally use for electronics. What are you using for elements?
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Old 03-09-2009, 2:53 PM
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For an aircraft receive antenna, you will likely not find a better budget antenna... Anything greater than a 1/4 wave ground plane will likely result in a much narrower radiation pattern, closer to the horizon... A rather large amount of the radiation pattern in a quarter wave ground plane and half wave dipole goes nearly straight up (towards the aircraft silly lol)...
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Old 03-09-2009, 5:48 PM
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Originally Posted by IdaScan View Post
A rather large amount of the radiation pattern in a quarter wave ground plane and half wave dipole goes nearly straight up (towards the aircraft silly lol)...
Besides, the signal sent from ground is directed to the sky, so you can see it's better.
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Old 06-11-2009, 5:20 AM
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If you are looking for another easy to make antenna for airband, I did just that centered on 127 mhz:

http://www.radioreference.com/forums...ing-great.html

Since I'm not near any towers, and my look angle at the horizon is blocked by buildings and even nearby hills, I tried something that has an elevated angle, but fills in the overhead null.

It is made out of nothing but small speaker wire and optionally a 300:75 ohm balun if you don't want to cut anything. Direct connect to coax is best, but in real world tests, the balun works pretty well if not exactly optimal. It is so easy to make that you can find out in a jiffy if it is not applicable to your location. Because of the higher look angles, there is no point to mounting it high up, other than to clear nearby obstructions.

Works very well for me, and I'll trash an antenna faster than I can put it up sometimes.

Hint: if you are using wire, the elements should actually be a bit longer - 67 inches and 23 inches. 66 and 22 are just easy to remember, and are actually more precise when using tubing.
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Old 06-11-2009, 8:41 AM
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A 1/4 wave groundplane and half wave dipole orientated vertically will have a large null straight up with the main lobe slightly higher than the horizon. For best aircraft reception you want your gain at the horizon because within any range where the aircraft is at a high angle it will be very close with no obstructions. An antenna with -30dB gain will work great at high angles but at 100 or 200mi out the angle to the aircraft will be very low.
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Originally Posted by IdaScan View Post
For an aircraft receive antenna, you will likely not find a better budget antenna... Anything greater than a 1/4 wave ground plane will likely result in a much narrower radiation pattern, closer to the horizon... A rather large amount of the radiation pattern in a quarter wave ground plane and half wave dipole goes nearly straight up (towards the aircraft silly lol)...
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Old 06-11-2009, 3:17 PM
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Originally Posted by prcguy View Post
For best aircraft reception you want your gain at the horizon because within any range where the aircraft is at a high angle it will be very close with no obstructions. An antenna with -30dB gain will work great at high angles but at 100 or 200mi out the angle to the aircraft will be very low.
This is very true. But what if you don't live in open area, and your targets aren't down at the horizon? Or behind a hill or building down at the horizon? What if you just physically can't get your antenna mounted high and can only achieve just a few feet off the ground? A dipole or quarter wave would have most of the receive rf coming through miles of obstructions, if you could hear it at all.

I try to make the antenna fit the environment. For most, the textbook approach of getting the angles down low works very well - no arguing with that. For me however, most of an antenna's capabilities designed for low look angles is wasted.

With the antenna I describe and use, I'm not concerned with gain. And the big issue that makes this antenna work better than you would think is that the feedline is actually part of the antenna, and fills in the overhead nulls and actually brings the higher look angle down lower.

In real-world use, many dipoles mounted up high are not performing as well as they should if one doesn't decouple it from the feedline with a choke, or has interactions with the mast.

So yes, I agree with you wholeheartedly providing the low-angle antenna suits your environment.

Since antennas are so easy to build, especially the one I describe, try it and see if it fits the environment.

I've seen many budding enthusiasts afraid to actually experiment for themselves and just see if what we are talking about works or not, limiting themselves to standard issue rubber duck because they are afraid that their antennas aren't textbook perfect.

Get out there and string up some wire!
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