The Combe Raleigh Offset Centre Fed Antenna, design and build

I had grand plans for this antenna in 2012 when I first moved to this QTH. I bought the Baluns, Wire, Poles and antenna hardware, and promptly left them in the boxes until July 2020. I had Eznec designs and plots galore, no antenna though.

I made due with the Hustler 6-BTV for 8 years until I was playing with my portable Inverted V for 20 metres in the garden and switching between this two wires and a fishing Pole and the 6-BTV was marked. This prompted me to get on with this install.

Design

Based on a standard 33/66% offset from centre to locate a 200ohm feed point impedance and terminating with an 4:1 balun, this seems the ideal design for the site. The antenna has the following requirements :

  • Power handling of 400w (UK Legal Limit)
  • Antenna being Omni-directional, or biased to E-W.
  • Able to handle high duty cycle modes (PSK31 / SSTV / FT8)
  • Complement Hustler 6BTV vertical coverage
  • Provide some level of NAVTEX reception on 516Khz

The initial cut length to be used is the 28.43m / 13.76m split, to then identify the 200 Ohm feed point, thus 4:1 insertion point. 

Components

  • DX Engineering Maxi-Core® HighPower DXE-BAL200-H10-A Current Balun 4:1
  • DX Engineering Antenna Support Cord
  • DXE End Insulators
  • SSB Ecoflex 10 Co-axial Cable

  • Existing Push up Pole 12m

  • Palstar AT-2K Antenna Tuner

 

Location

 The antenna location is pivoted around the 12m push up mast, holding the substantial DX Engineering Balun and Antenna Centre. This extends two legs of weaved antenna wire (Kevlar Centered) of 28m and 13m between two trees. The tree points are as high as they will go with the centre at 7m (35ft is the best height for a 4:1 Balun)

A drop cable of Ecoflex 10, to a DX Engineering Feedline choke of 7m, and a run of 15m of Westflex 103 into the Palstar AT-2K.

 

*
1 - 30 Mhz Band Scan

 


3.5 Mhz Plot

 


7 Mhz Plot

 

Performance

I still can't get over the difference between the two antennas, obviously, the polarisation should account for -3dB but the signal meter on my FT-2000 tells me otherwise. I note the difference in audio more than the signal strength, and sometimes the signal is lower in quantative terms but better in most transmission reception. I will work with the antenna more over 2020 to get a feel for performance outbound. It feels better from the number of stations responding to my FT8 CQ calls anyway.

 

 

 

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