Friday, March 1, 2013

Secrets of Successful Skimming - WZ7I

For several years now, WZ7I has been one of our more successful Skimmer contributors to the RBN.  He consistently tops all US East Coast Skimmers in the number of spots he posts during contests, so I asked him to tell us his secrets.  Here's his story:


Pete asked for a note describing my RBN receiving system but a precautionary note is in order -- I am not an engineer and my views are not authoritative.  My location is an advantage.  I am in a semi-rural environment 100km west of the New Jersey coast.  The vertical antenna is in the woods where the radials are in wet soil part of the year.

The Antenna

I was introduced to SDR units by playing with a KB9YIG SoftRock kit and I became persuaded of the importance of controlling the impedance of the receive antenna.  When Alex introduced Skimmer Server I decided to purchase the QS1R but was perplexed by what kind of omni-directional antenna system to install that would have reasonable impedance on all of the bands, including 160m and the WARC bands, if possible.  

 he active vertical antenna similar to the Clifton Laboratories Z1501D was an obvious possibility but Jack's antenna wasn't quite ready at the time and I had a bias toward putting up a vertical that could serve as a transmitting antenna if my interests changed.   

The excellent study of HF vertical performance in 2000 by N0AX and K7LXC excluded several multi-band verticals from my consideration.  I liked the HyTower verticals installed in my 80m array and decided to install that antenna's "little brother", the HyGain HyTower Jr, a 39 foot (12m) vertical with wire "cage loading" on 80 and full size wire elements on 40, 20, 15 and 10 meters.  It is a design that has been around for decades and it is simple enough that I understand how it works so I can repair it when it breaks.  Mine is not guyed.   

Of course the key to making this work is the radial field so I purchased four 500' rolls of #14 insulated solid house wiring and cut each roll into 8 equal pieces resulting in 32 radials, each 19 meters long.  I use the DX Engineering radial plate and feed the vertical with about 90 meters of Davis RF  Bury-Flex through a Polyphaser lightning protector.   After the coax losses this system has an impedance between 23 and 78 ohms on all ten bands from 1.8mHz to 28.3mHz with the exception of 18.1mHz where the impedance is about 148 ohms.

The Pre-Amp and Receiver

I tried using the QS1R with no preamp but it was obvious that other RBN receivers were hearing stations above 14mHz that I could not hear so I added the Clifton Laboratories Z10040A 11db amplifier for a big improvement in the number of spots I detected.

The Computer

The initial computer was a Intel Q8300 Core 2 Quad running Windows XP which worked fine except for the CQ WW DX contest when it failed at about 2800 decoders.  This has been replaced with a Windows 7 64 bit Intel I7-3770 which has yet to be truly tested in the CQ contest, but seems to work fine.  I use the W3OA Aggregator to change bands during the day.

2 comments:

  1. Is this antenna also used on 160m ? - your system rcvd my sigs with 33 db/sn. this morning on 160m band cw, nice... hr. running 1kw and the Cushcraft MA-160V..
    So your system really picks up sigs, thanks. Vy 73 de OU2V / OZ1FJB

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