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Author Topic: Receiver improvements and antenna build  (Read 4241 times)

AndyThomas

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Receiver improvements and antenna build
« on: May 18, 2013, 06:22:07 PM »

Hi

As you know the dongle is encased in plastic and has no hardware filtering: it is also rather deaf.

The problem will come when , say, a taxi nearby blasts out a radio message and it fuddles the reception of our satellite. This is a well known problem and already happens when pagers blast out - yes, pager transmisions still go on!

To counter this I have been designing simple improvements you can make to your dongle.

1. Encase your dongle in a a faraday cage. This is easily achieved by using  an aluminium box. This is the most critical thing you can do. I am mounting mine in a larger steel box with other bits (see below) and will run a USB extension lead out of the box to the pc/laptop, and cable up an antenna lead into the box.

2. I am putting in the antenna path a bandpass filter. I am using one obtained from the back end of a radio installation for which I paid 50p. I have tuned it for "maximum smoke" ie the strongest signal into the receiver at the Sprite frequency 437.240 MHz  - this is in the Amateur 70 centimeter band. Another solution can be found if you google "Amsat Mode J filter".

3. There is some loss through any bandpass filter, so in between the other side of the filter and the input of the dongle I am putting a small preamplifier. I shall use the Ramsay kit sold by Waters & stanton for about £20 (because I've got one). http:// www.wsplc.com . It'll need a 12 volt supply (eg a small battery pack - don't use a wall wart).

There are two other ways to improve the receiver:

a) AMSAT NA (North America) sell an integrated preamplifier for about $50 which is already tuned to the satellite band we are using. This also needs 12 volts.

b) Use an old Microwave Modules 70 cm band UHF to 28 MHz downconverter - if

i) you can find one - I have 3 available
ii) you can receive 28 to 30 MHz on your dongle. Many can't go down that far.
These need 12 volts but have no (significant) preamplification.

Finally, consider building a UHF antenna for the 70 centimetre band from a design on the 'net, for a "9 element Yagi for 70 cm" and attach it to a longer broom handle or similar that you can balance in your hand, or mount on a camera tripod. Why not fit the dongle in its box right next to the output of the antenna on the tripod, and stand the laptop on a stand nearby?

The art of this is to point the antenna to the elevation and azimiuth that you have predicted for the pass, and wait for the Sprite to fly into the direction of your beam. And as you have no idea what the polarisation of the Sprite antenna will be at the moment you are listening...so you might as well go for a linear polarisation.

best


andy thomas g0sfj
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