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Author Topic: Directional antennas  (Read 573 times)
XY3LON4
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Posts: 84




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« on: April 14, 2007, 06:25:12 PM »

I want to know what the best directional 2 meter antenna is for mobile. I found out that the international space station transmits on 2 meter and I would like to try to talk to them. Until I get an amp I will be at 65 watts. What would be the best antenna to use for this? Compactness is a bonus. I have seen these bazooka looking helical antennas somewhere but I can't seem to find them now.
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K0BG
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« Reply #1 on: April 15, 2007, 05:47:54 AM »

A beam might help if you have a way of pointing it while under way. Devices exist to do this, but I'd bet they're outside of your budget restraints.

In the three cases I have worked them mobile, I was using my regular dual band antenna, an NMO2/70BK. The limitation here is (was), they almost have to be dead overhead, with a dose of good luck thrown in.

Alan, KØBG
www.k0bg.com
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KA1MDA
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« Reply #2 on: April 15, 2007, 02:55:19 PM »

You may want to keep things simple and try it with a regular omni antenna first. Very few passes go directly overhead, and many of then tend to arc low over the horizon.

Although not directional, you may want to consider home brewing a turnstile antenna. It consists of a pair of crossed diploes mounted 3/8 of a wavelength above a ground plane. The pattern is wide, and peaks around 35-45 degrees above the horizon- which is where many of the passes will be located. Gain goes down overhead, but it isn't a problem, as that's when the ISS will be closest and have the strongest signal.

Around 10 years ago, I used to work some of the FM sats mobile running nothing more than an HT, a 35 watt 70 cm Tokyo HiPower amp, and a Larsen 2/440 dual band mag mount antenna stuck on the roof! It took some planning and effort, but worked.

Also, please remember that since the space station is travelling at around 17,000 MPH, the contacts are extremely short, usually not much more than exchanging call signs and some short pleasantries. Most passes offer only a 5-10 minute "window". With so many other stations trying to work the ISS, attempts at long-winded QSO's can sometimes be seen as "hogging" the frequency.

73 and good luck, de Tom, KA1MDA
www.ka1mda.org
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XY3LON4
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« Reply #3 on: April 15, 2007, 04:59:20 PM »

I thought that they were in a fixed orbit. What about the Quadix?
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K0BG
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« Reply #4 on: April 15, 2007, 06:27:19 PM »

Fixed orbit? This would require them to be in synchronous orbit, some 22,500 miles up! This is well beyond the range of the space shuttle, and most "small" rockets.

Alan, KØBG
www.k0bg.com
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KO1D
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« Reply #5 on: April 16, 2007, 05:32:39 AM »

A few thoughts. Check in with the AMSAT gang and they will be glad to help you with more technical and elmering support.

Now for working the space shuttle and space station you can visit the following options:

- My buddy at college worked MIR with 50w into a brazing wire 1/4 as outlined in the ARRL Technician Class book (back then at least) and later antenna handbooks. Bad part of that was, I was sick in my dorm room when I got the excited phone call....sigh

- I have heard S9+ the packet robot on the shuttle before running nothing more than a rubber ducky in a small to medium sized East Coast city. Coulda worked him had I also had a similarly sized computer and TNC set up with me at the bus stop

- Often times a few of the guys I know in AMSAT will grab a HT and an ARROW antenna to work the birds. This would not be a mobile antenna so much as a portable antenna. Its a handheld beam you can use to work 2/440 or even get the 2m only one. AMSAT conventions usually find about a hundred of these things about whenever a pass is overhead

- Mobile wise I would recommend a simple 1/4 wave antenna. You do not need a lot of power to work the shuttle or satellites. The shuttle is in a very high orbit and can hear quite well with modest power. I think they only run about 25w at the most up there I could be wrong. For working most other satellites running more than 10-25w would get you very dirty looks from the Control Operators and Ground Station Crews. More power in drains more power out of the transponder on the output. Space communications is about efficiency not about power. (This would change if you wanted to work say Mars but until we get a station there you needn't worry. I assure you portable antennas would not work for that.)

Hope this helps and please do get a hold of some AMSAT folks. They love helping new people get into Satellites.

Dan S
KO1D
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KE4DRN
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« Reply #6 on: April 16, 2007, 06:51:55 PM »

hi shawn,

Here is a link for you

http://spaceflight.nasa.gov/station/reference/radio/

You won't need an amp to make contact with the ISS,
so you can use your funds for low loss coax.

This link has pass predictions for the ISS
it uses Java.

http://spaceflight1.nasa.gov/realdata/sightings/

Take a look at the Turnstile antenna

http://cebik.com/vhf/ms2.html

73 james


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