160 meter pre-amplifier

For the K9AY Terminated receive loop, I put together a small pre-amplifier to get some more signal when operating on 160 meter. Several had suggested a simple construction by W7IUV that gets the job done!.

To the construction I added a bypass relay. Rx Signal is routed through the amplifier when +12v is applied, and bypassed when no voltage is present.

T1 is a Ferroxcube 4A11 with a Al of 360 nH/n2. For Q1 I used a 2N3866 (sponsored by OZ7QF Finn - thanks!).

Temperature get pretty high of Q1. With only 10 volts applied, the pre-amp got over 100C with 3 minutes, and i cut power to prevent any damage.

With a semi large heat sink (se picture below) the temperature stabilizes around 86 C.

K9AY Gary, has has suggested replacing the emitter resistance (R5) from 10 ohm to to a higher value, e.g. 47 ohm to reduce current and thus reducing temperature of the transistor. The result is shown below.

R5 (emitter resistance) 10 ohm. R5 replaced with a 47 ohm.

W7IUV pre-amp construction diagram

The blue text have been added to the original diagram found a W7IUV Web site.

Rx Antenna controlbox, with pre-amp

Input: 2x RxLoop. 1 dipole. 12 volt supply is external.

Controlbox wirediagram

12 Powersupply filter

Another problem occured during construction as i was getting signals when no antenna was connected.

As it turned out the 12 volt linie (external power supply with 2 meter twin lead DC linie) picked up the RF signals. Well over S9 on 160 meter from local computer oscillators. The problem was traced back to the 100uH i used to supply 12v to the antenna k9AY relay. The RFC turned out to actually be a 2.2 uH i found in a plastic bag labeled as 100uH :=) However even with the recommended 100uH, the suppression was not enough. Although the signals where S0 you could still hear them on the receiver when using the pre-amp. So i ended up making my own RFC with at amidon FT50-75 with 20 turns ... about 800uH. That worked very well, and there where no more DC linie pick up signals.

Pre-amp strip print layout

On-Air experience

For the K9AY (with loopsize=26 meter) the pre-amplifier does makes a difference on 160 meter when trying to pick out signals with low signal strength. Basically i found that the CW audio peak filters in the RIG (FT897 and FT1000MP) works best when theres a desent amount of audio present. So, even though it is located in the radio shack and not at the antenna, it does has a value when trying to pick out weak signals (~ S0). The +20dB is pretty high and the amplifier "drags" in a lot of noise also, so for the next version 2.0 i would proberly put in a 6dB attunator in the output section.

Alternative solutions

The above project was initited with a request for information on the TopBand reflektor. Besides from the W7IUV pre-amplifier theres have been other suggestions:

DXEngineering Designed by W8JI and works superbly. Up to 17db gain [Dean W5PJR]
Wellbrook Communications manufactures a pre-amp [Andrew Ikin]
March/April 2003 issue of QEX, IK4AUY has a splendid preamp design [Steve KK7UV]
I use the W7IUV preamp now - it is available in kit form from Kits'nParts [Bill VE3CSK/VE3NH]
I have built the W7IUV preamp and used it with pennants with good results [SM0MDG Björn]
Take a Look at: http://www.hamlan.org/tech/w7iuvpreamp/
check out my preamp or bare boards at http://www.hizantennas.com [Lee K7TJR]
You can change R5 in the emitter circuit from 10 ohms to a higher value -- what you want is a current of about 40-75 mA. I use 47 ohms. [Gary K9AY]
RF preamplifiers. Several suggestions QRP.POPS.NET

Se Also:

2008 OZ1AXG