Topband: 1/4 wave 160M wire antenna

[Joe Subich, W4TV]

On 2025-12-18 1:58 PM, Dave Cuthbert wrote:

> 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.
SWR is not minimum at zero reactance ... it is minimum where the
vector impedance ( (R^2 + jX^2)^.5 ) is closest to 50 Ohms (in equipment
calibrated to 50 Ohms).

Use a VNA or antenna analyzer in the complex impedance mode (R +/-jx)
and look for the minimum absolute value of jX to find the *resonant*
point.  An SWR bridge (or Wattmeter in reverse power mode) does not
measure jX ... only the vector product of R +/- jX.

73,

    ... Joe, W4TV


On 2025-12-18 1:58 PM, Dave Cuthbert wrote:
> 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
>