ON1BES Labo 2015          Modified SDR Stockantenna's

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Dear SDR friends.

Recently I have developed some little antennas for SDR , mostly for indoor , for testing to receive SDR .
The aim was to actually modify the supplied rod antenna with the DVB - T stick and improve the receipt.
Affter a long search on the Net, there was verry little to find abbout it. I dit a few tryouts with modifications on the stock-antenna with reasonable results .
Earlier tests with all that, one of the best choices was a self-made discone antenna, demonstrated in our Radio Club.
It was tested for the VTS open house in 2013. Which was very good on most bands , but also stood outside and on the roof !
Those who still have less
time, with less work , and something needs simpler , find here quite a few tips .
Elsewhere on my website there are also some that are made from cable TV coax , and costs almost nothing , and they perform very well from 60 to 500 MHz .
These attic antennas are very suitable , especially if you can put them on a window
Interesting frequency's: most voice modes are below 500MHz, above that , you sit in a hole different domain , sitting there are only digital modes ,
and you get need additional decoding to further to do anything. (e.g. ADSB to track aircraft)
Back to class : here are a few modified indoor rod antennas ( with magnet foot ) . Later follow a few others .
The first one is the simplest : the rod has been replaced by a small pull-out telescopic antenna from an old FM transistor radio .
If necessary,
at the base on the small antenna , re-tapping the hole with M3 thread to screw it on the magnetic base.

Place the small antenna
always on a metal plate or box like in the picture !

Telescopic rod pull-in
Telescopic rod pull-out
Click photo for bigger version.
The second one, a advanced version, is a sudden variation of the J-Antenna, or "Slim Jim", designed for 70cm.
These can be perfect calculated for a given frequency. Look for a J-pole calculator.
Made of pleated "Welding rod" or bras wire of 2mm diameter and 1m long. It's a bit more work, but it is a very good antenna.
This is actually made for the 70cm band (432.500MHz), but do not panic: it works even for much lower frequencies, down to the FM radio band.
In the photo, he shows very long for such a little stand, but he is only 50cm high.
See the detail in left foto , the original cable
come  through a extra hole in the foot and must be soldered on both sides of the Welding rod.
This is on the 75 ohm point, because the cable is also 75 ohm.

Slim Jim 1
Slim Jim 1
Slim Jim Feed Point - 75 Ohm
Slim Jim
Click photo for bigger version.
As mentioned earlier, these antennas provide all reasonable results indoor, and much better than the standard supplied stock antenna.
But the party starts when you have the opportunity with your Laptop or PC on the 1st floor or higher to sit down, and preferably still on a window!
Until the next time.

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