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
_________________
Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
|