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 do 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 518 Khz

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 Maxi-Core® Feedline Current Choke FCC050-H05-A 1:1
  • DX Engineering Antenna Support Cord
  • DX Engineering End Insulators
  • SSB Ecoflex 10 Plus Co-axial Cable
  • Nevada Kevlar Antenna Wire

  • 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 Plus, to a DX Engineering Feedline choke of 7m, and a run of 15m of Ecoflex 10 Plus into the Palstar AT-2K Tuner.

 

1 - 30 Mhz Band Scan

  

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 quantitative terms but better in most signal quality 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.

The OCFD tunes well on the bands, although as you can see on the scan above that I can operate on a number of bands without a tuner, or well within the Auto-tuner in the radio. (Rarely use that as I've got a Palstar AT2K)

Receive audio is noticeably stronger, Noise is largely less on the OCFD vs the Hustler 6BTV, with some noticeable exceptions of local electrical interference, but I can now switch polarisation with a flick of a rotary switch.

On FT-8, a station can be up to 2db better on the OCFD, and on 80m Phone, stations that are 1-4 db on the vertical, are 9 to +20db on the OCFD.

I'm happy with this antenna, as part of my station !

 

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