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Author Topic: Airborne repeaters  (Read 532 times)
VK6AV
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Posts: 32




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« on: April 22, 2007, 11:04:06 AM »

I am looking to build a very lightweight crossband repeater (input on 2mtrs, output on 70cm) that could be used in a large radio controlled aircraft. I have seen articles in QST about repeaters being carried in weather balloons but no longer can find the articles. There would be a certain amount of battery power available. Also looking for ideas on lightweight, low drag vertically polarised antennas.
All help appreciated!
Harris VK6AV  Email vk6xxav at yahoo dot com. Delete the xx.
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KX8N
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Posts: 542




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« Reply #1 on: April 22, 2007, 11:14:21 AM »

There are dozens and dozens of examples on the net if you search with Google.  Way too many for me to list here.

Here's one, though, that a group put together to send up on a balloon.  There are very detailed pictures documenting what's inside.

http://www.eoss.org/hardware/crossband.htm
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KB1LKR
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Posts: 1898




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« Reply #2 on: April 22, 2007, 11:28:22 AM »

see also amsat.org for info on small, light receivers & transmitters for satellite use -- you should have some common design issues.
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KB4QAA
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Posts: 2379




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« Reply #3 on: April 22, 2007, 01:31:19 PM »

Regarding antennas, a simple wire whip bent back at say 45degrees is the most practical.  They have been used on general aviation aircraft since the 1930's.  Below 300 - 400kts I'm not sure drag is a significant problem, nor worth the additional weight penalty to create an airfoil shape.   However, you may be able to find airfoil shaped rod at an RC model aircraft shop.

Good luck with your project.

73.
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N6AJR
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« Reply #4 on: April 22, 2007, 03:17:16 PM »

I don't know if that would actually be legal.  you are limited in power even as a ham you can only run a couple of watts. and a repeater would probably overdrive any other rc controlers on that frequency.

please check the rules befor persuing this too far.
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KB1LKR
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Posts: 1898




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« Reply #5 on: April 22, 2007, 05:01:08 PM »

But there should be *no* R/C plane controls on/near either 2m or 70cm, only 72-73 MHz (4m), 51MHz (6m ham band) or perhaps 27 MHz (11m Class C CB band), plus surface models at 75-76MHz.

Further, R/C aircraft must tolerate ground based transmitters including part 97 FM repeaters, potentially at hundreds of watts ERP, plus, worse, ommercial Low VHF TV at *many* kilowatts ERP and much closer to their operating frequencies (particularly Channels 4: 66-72 MHz, 5: 76-82 MHz), & 2: 54-60 MHz).

He'd would need to avoid frequencies used by "local" coordinated repeaters, (local being defined by his output power and altitude) but that should be readily doable, just as it is for V/U satellites.

An airborn repeater wouldn't need much power anyway, 500mW would be plenty, perhaps even 50mW would be sufficient, as it's a line of site operation. The input antenna would not have to be particularly big or efficient as higher power (e.g. 5W) could be used on the input.  
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K9KJM
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« Reply #6 on: April 22, 2007, 10:26:15 PM »

As mentioned, All you need for an antenna is a 19 inch length of thin wire trailing behind the aircraft.
And a number of small hand held radios now available at swapfests for peanuts could be the "cross band" radio, Complete!

HOWEVER before you attempt to fly this thing, I would do some extensive testing on the ground to make sure your crossband repeat system does not overload the RC receiver on the aircraft, Lest you wind up with a runaway model plane when you operate the crossband repeater!
(Test with the crossband system running AND your RC controller the maximum distance from the plane you intend to operate at)
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KG6OMK
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Posts: 107




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« Reply #7 on: April 22, 2007, 10:48:45 PM »

A wire rod with round cross section is one of the worst draft producing devices you can make (for it's size).  Use epoxy and glass or carbon fiber to make a trailing edge about 4 or 5 wire diameters long.  Basically make an airfoil shape with a 3/32th  wire brazing rod used as the leading edge

As for the radio, I doubt you could do better then taking  one of those low power shirt pocket HTs out of it's case.  
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KB4QAA
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« Reply #8 on: April 23, 2007, 07:20:27 AM »

An additional thought.  I think that you will find that with the antennas so close together that obtaining enough frequency isolation will be a challenge in a small package.  
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NA0AA
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« Reply #9 on: April 23, 2007, 08:31:14 AM »

Build the antenna into the wings using lightweight rods or use copper foil on the wing surfaces?

I have to agree that the packaging is going to be a challenge but good luck!  Should be fun.

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K6AER
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Posts: 3518




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« Reply #10 on: April 23, 2007, 09:37:14 AM »

It is a neat idea but the laws of physics will be working against you.

Your line of sight coverage is limited by the useful flight height. You might be able to see the aircraft at a height of 500 feet and control it for a limited distance and as a result your LOS will be square root of two times the height AGL. This would be a distance of 31 miles for transmission coverage.

Because the antenna cannot be large you have lost any antenna gain and in some kind of aerodynamic function your actual antenna gain will be much less than a ¼ wave vertical.

The RC receiver will not tolerate an onboard transmitted signal in the VHF or UHF band. The 72 MHz receiver has no front end selectivity.

Weight of the on board battery to be able to run a 5 watt radio will be limited in operational time. Your R/C aircraft will need to be quite large to handle the radio payload.

Bottom line is a tethered balloon would be a better approach. Lifts more weight, doesn’t need a person to fly it, you can dangle a gain antenna and is cheep to build.
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K4VVX
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« Reply #11 on: April 23, 2007, 05:31:30 PM »



I'm missing something here--- I don't understand.
31 miles LOS doesn't sound unreasonable, but the square root of 2 is 1.414, times 500feet AGL= 707.  How did we get to 31 miles??
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K6AER
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« Reply #12 on: April 23, 2007, 07:22:10 PM »

At an antenna height of 500 feet the formula is the square root of 500 times 2 equals the distance to the horizon.

100 feet the horizon distance is 14 miles
200 feet the horizon distance is 20 miles
500 feet the horizon distance is 31 miles
1000 feet the horizon distance is 44 miles

And here in Colorado the horizon distance for 13,000 feet is 162 miles. Even a Falcon 2000 at 43,000 feet can only see 293 miles to the horizon.
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KB1LKR
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Posts: 1898




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« Reply #13 on: April 23, 2007, 08:31:53 PM »

The RC receiver will not tolerate an onboard transmitted signal in the VHF or UHF band. The 72 MHz receiver has no front end selectivity.

Oops, my oversight! That could be a problem. Hmmm.
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WB2WIK
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Posts: 20599




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« Reply #14 on: April 24, 2007, 01:34:43 PM »

First airborne repeater I heard of was deployed during the Arab-Israel "Six Day War" in June 1965.

The Israelis floated a VHF repeater in a balloon and used it to relay strategic traffic.  As I recall, some Israeli hams were involved in building it, and it went together in a hurry as did the entire war!

Any 4X hams out there know more details?

WB2WIK/6
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