as I received several emails with questions about the preamplifier which I am using for my LB external RX antennas like W2PM MiniDiamond loop, EWE loop, K9FD loop etc. I decided to post over here some informations about this unit.
The basic design is inspired with his permission by Larry, W7IUV. The complete article about his preamplifier was published on his web here:
Please read his entire document before starting to build this project. There is data contained within that is necessary to obtain maximum performance from this circuit. Be sure that you downloaded the latest - updated revision.
I started with building this preamplifier in the end of 2002 as Christmas project but I finished it in Jan of 2003. The current version of the W7IUV document was at that time Revision 5 copyright 1998-2009.
It is very important to follow the inctructions for building the preamplifier and use the parts values as described and recommended by Larry in the document.
The size, shape and terminals has been choosen according to my alu-case available...
The most important part of this design in my opinion is the transformer and working state of the transistor 2N5109. You must follow the winding rules as same as core material for transformer and finally you must use prescribed current of the transistor in order to reach expected performances.
If you are interested in the suppliers for Amidon, Fair-Rite cores or even you have problem with finding the 2N5109 there are some links which I used...
The heatsink on the transistor is a must. Do not change the resistors in order to decreasing the collector current of the transistor! These conditions are key parameters for high IMD immunity / OIP3 performance...
Testing the assembled preamplifier is straight forward. Before applying power double check your work. The most common problems are solder shorts and resistors inserted in the wrong place. Once you are satisfied you have correctly assembled the circuit place 50 Ohms terminations on the input and output.
Apply the Vcc terminal and ground. Measure the current drawn with a digital ammeter. It should be 75 mA +/5mA. Remove power and then remove the 50 ohm terminations. Re-apply power and measure current again. It should be the same as it was with the terminations in place. If not the device is oscillating and you need to fix that before proceeding.Oscillations are usually in the GHz region and are caused by poor layout and/or poor quality capacitors. Bypass caps with very long leads could also be a cause. In fact I used in this version thru hole components but I can recommend to use high quality SMD parts.
With power applied and the specified current being drawn the transistor should be very warm to the touch but you should be able to hold the tip of your finger on it indefinitely. If you cannot either you have insufficient heat sinking or the device is drawing too much current...
If you have proper alu-case then you can use it as the big heatsink.
Hope it helps.
73 - Petr, OK1RP
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