Sunday, February 20, 2011

Outdoor MF and HF Antenna


VE7BPO Antenna System

The schematic to the left summarizes the outdoor VE7BPO MF and HF receiving antenna system for summer 2007. Although modest for a big city lot, this antenna seems to pull in the DX and is relatively free of RFI. This antenna was just a case of "putting as much wire in the sky as possible" and the dimensions are indicated for interest sake only. The 27 meter long horizontal section is supported between 2 trees at a height of about 14 meters high. The weight of the vertical element wire plus slack in the horizontal wire droop it to about 13 meters high in the center. The vertical section is soldered to the horizontal wire 6 meters from the nearest anchoring tree and runs straight down to the antenna feed point which is about 1 meter off the ground. The feed point is a piece of copper-clad PC board (with isolated sections created with a hobbyist motor tool) and is bolted to a long copper pipe which serves as the first station earth-grounding stake. A transformer (T1) configured as a UNUN (unbalanced-to-unbalanced) is used to interface the antenna with 50 ohm coax that runs through the house and into the radio shack. Some rudimentary experiments with the UNUN and the earth-grounding system were undertaken.
The methods I used to potentially lower unwanted RFI to my antenna system are as follows:

  1. The receiver and power supply are independently connected to a single, central ground point (ground buss) in the radio shack.
  2. 6-10 gauge wire is used for my ground system (not including the radials which are bare 12 gauge wire).
  3. The ground wire connecting to my first earth stake to the station ground buss is just outside the shack window and is short as possible to provide a low impedance and low inductance path for MF and HF frequencies.
  4. There is a second ground stake located 1 meter from the primary ground stake (I will add 2-4 more in time).
  5. I have a large piece of steel buried underneath the soil tied in to my system as well as 3 bare copper radials. The radials are 3 - 7 meters in length.
  6. New RG58/U coax was used as the feed line.
  7. All wire splices in the grounding system are soldered and taped up. I used conductive grease (to prevent oxidation at the wire-stake interface) on any clamps connected to ground stakes. My ground stakes are ~ 2 meters long.
  8. The earth grounding area soil is moist and peat-laden and is watered regularly.
  9. I plan to maintain this ground system every 2 years.