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This blog is the replacement of the my preview Topband blog here http://topband.blog.cz/ because of an agressive advertising and flash banners inserted by the blog provider. For older posts access You can visit the old blog link which will exist still... but it will not be updated anymore.

Effective from 1st Jan 2017 please paper QSL via OM-bureau only.

Friday, February 26, 2016

W7IUV preamplifier for LB antennas - Revision 5 (built 2003)

Hi all,

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.
Final quick measurement in the lab confirmed it's parameters. The gain is slightly lower than expected. Also be careful which power supply you are using. In some cases you can get strange noise from the PSU (mainly switched unit) I am using linear PSU with enough power and proper filtering from AC mains.


Hope it helps.

73 - Petr, OK1RP

Monday, February 1, 2016

CQ 160m DX, CW contest 2016 at OK5T

Hi all,
if you worked OK5T this weekend in CQ160 contest on 160m then there I am sending my operation time schedule from OK5T log...

I was on key as follows:
30Jan16 03:30-06:30UTC
30Jan16 19:15-21:30UTC
31Jan16 04:30-07:30UTC
31Jan16 20:00-21:30UTC
The condx was not as good as we expected. We worked much less US friends and signals were really weak. We used 330m long Bevs to 320/30/90 degs. They wkd very nice even although the VK Bev was so low. (1m up only)

Mr. Murphy visited us when I was on key first morning at 05:00U when I lost NA Bev > stopped work so I was so deaf for last 2hrs of opening to USA. We repaired it during the daylight few hours after. My apologize to all US guys!

We really tried to catch our kangaroos friends in this event too. I am really sorry but no way to work VKs in this CQ160...

I was both evenings on time around 1820-1825kHz. Our VK Beverage sounded also quite well with no usual high noise level.

I really looked for famous Ron, VK3IO and both Steves, VK6VZ and VK6IR. Unfortunately the band was extremely crowded and I am pretty sure that even although we used narrow filters (only IC7600...) our receiver/AGC has been knocked down from so strong nearby signals so I was very deaf even with 330m long Bev to VK.

Well the only single spot I realized on cluster with VK6IR on 1833 on Sat evening but I did not hear Steve unfortunately on this frequency. No VKs readable at our QTH even with Bev to 90deg at all.

As Ron, VK3IO already mentioned on Topband Reflector - http://www.contesting.com/_topband  "This year's contest was almost a total wipe out as far as VK was concerned, due to summer storms on the east coast and the usual topical storms up north and a cyclone off the west coast made it impossible."

Never mind, it is just hobby. I was really happy to work XE2X, YV1KK, HK1X, BA1CW, B1Z. Many thanks for your time and effort and hope to work you soon outside the contest on quiet band.

Many thanks for nice QSO to all members and hope to work you soon again.

73 - Petr, OK1RP