Jeff, I would build it as you describe then measure the feedpoint
resistance to determine the near-field radiation efficiency. Or, if someone
here can model your antenna with NEC-4 we will know beforehand. From that
you can decide if radial wires are worth the cost, time and effort.
EZNEC shows a 130' wire at 45 degrees to have a radiation resistance (at
1830 kHz) of 20 ohms. Anything measured higher is due to near-field loss
resistance. For example, if you measure 33 ohms the antenna system consists
of the 20 ohm antenna plus 13 ohms of ground loss resistance. The radiation
efficiency is 20/33 x 100% = 61% putting it 2.2dB down from the same
sloping wire over perfect GND.
The antenna system can be measured with a VNA, or even with an SWR meter.
Using an SWR meter sweep the band to find the SWR minimum. At that
frequency reactance is zero and you can calculate base-referred ground loss
resistance from SWR. Rr = 1/SWR x 50 ohms. For example, if 1.5:1 we get
1/1.5 x 50 ohms = 33 ohms giving a base-referred ground loss resistance of
13 ohms. Note that I am assuming the feed point resistance is lower than 50
ohms and I would be surprised if it was higher than 50 ohms. A check of
this is to disconnect the four GND rods to increase the base-referred GND
loss resistance. If the SWR decreases it means loss resistance is lower
than 50 ohms and if SWR increases it means loss resistance is higher than
50 ohms.
Dave KH6AQ
On Thu, Dec 18, 2025 at 6:15 AM Radio KH6O <radio.kh6o@gmail.com> wrote:
> Colleagues,
>
> I have an 80 acre ranch consisting of mountains and canyons. I'm developing
> one location: Rivera Canyon (34.75073, -118.55116). I've had a one-trip
> 20ft shipping container brought in, modified with windows and a residential
> door.* This will be the radio shack.
>
> This canyon is surrounded by steep mountains on each side. For 160M, I
> intend to use a 130ft wire attached to a pine tree growing on the side of a
> mountain; the angle of inclination will be about 45 degrees. The shipping
> container itself will be the counterpoise/ground -- its surface area is
> about 700 sq. ft. Grounding rods will be driven in at each corner.
>
> With this in mind, should I also extend radial wires from the container or
> will the 700 sq.ft. of surface area be sufficient?
>
> * A one-trip container is used for one ocean trip, then sold; it's free of
> rust and all pieces are welded together. Base price for a 20 footer is
> $2800: https://www.containeralliance.com/
>
> --
> 73,
> Jeff KH6O / 6
>
> NNNN
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