[600MRG] 200 ohms

[Marshall Cross]

Hi Ben, according to Hagn: "Electrical Properties of Forested Media," contained in "Workshop on Radio Systems in Forested and/or Vegetated Environments," U.S. Army Communications Command, Feb.'74, AD-780712;the intrinsic conductivity of a tree is probably 0.01 to 1.0 mho/m (S/m), depending upon the type and condition of the tree. So a tree can be considered to be a poor conductor in the MF radio band. Many experiments have been conducted using trees as HF antennas, the most successful being due to Ikrath who used a Rogowski coil to excite the trunk with encouraging results. In Hagn's cited paper he mentions that Taylor studied the effect of a single tree upon a nearby open-wire transmission line and concluded that it loaded such a line as a lossy capacitor. So your intuition about your particular case (tree's capacitive coupling) could be correct. By the way, the use of trees as antennas was first proposed by U.S. Army Major (later Major General) G.O. Squire in 1905 and in a Franklin Institute paper in 1919! 73, Marshall, W1HK 

-----Original Message-----
From: 600MRG [mailto:600mrg-bounces at w7ekb.com] On Behalf Of Ben Gelb
Sent: Monday, November 06, 2017 11:25 PM
To: 600MRG
Subject: [600MRG] 200 ohms

My small city lot features a single 30 ft tree, which is currently supporting my 30' x 30' inverted L (other end of the horizontal segment supported by a 20' fishing pole strapped to a fence post ... so maybe 25'
in the air).

The tree is at the corner of the lot, so the radials are really just over a
90 degree sector (rather than a full circle around the vertical). I also only have 3 radials probably each about 20 ft long, with a 10' ground rod at the end of each one. There is also a 10' ground rod at the base of the vertical.

So, it ain't great. But is what it is on a small city lot.

Feedpoint impedance is ~200 ohms (and with 472khz.org antenna calculator estimating ~0.2ohm radiation resistance, thats an efficiency of maybe 0.1%, give or take).

So naturally I've been thinking about how to improve the setup. One thing is that the tree really is supporting the antenna (wire dragged right up over it). Its HV-insulated wire (20kV) to prevent any arcing to the tree and burning behavior (pre-HV wire version didn't fare too well, but the smolders have been successfully eliminated with the HV wire upgrade).

I guess I am wondering if the tree itself may be impacting the performance of the antenna (capacitive coupling to the tree?) and contributing to the high feedpoint Z, or if its really all just terrible ground that is to blame.

Your thoughts appreciated!

73,
Ben, N1VF
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