I am trying to understand an easy way to estimate the overall efficiency of my transmatch/coax/antenna system. Please read the following and tell me if I am all wet, and/or what I am missing..
ANTENNA SYSTEM DESCRIPTION
I use a homebrew Vertical Coaxial Dipole. The overall length of the dipole is 6.94 meters. This is strictly a single band antenna.
(I realize 15m band is not good right now, but I am using this method to evaluate on other bands)
The dipole is made of two pieces of aluminum tubing each approx 11 feet (3.4 meters) long, and a 4 foot (1.2 meter) long piece of PVC pipe. The 2 aluminum tubes are inserted into the PVC pipe, leaving about 3 inches (75 mm) so the tubes do not touch each other.
The feed line is RG-8 coax. The feedline goes up through the center of the lower length of aluminum tubing to the feed point at the center of the dipole, inside the PVC pipe. The center conductor of the coax is connected to the upper length of aluminum tubing (there is no balun device at the feed point). The outer braid from the coax is connected to the lower aluminum tubing. The lower half tubing is not electrically connected to anything other than the coax outer braid.
i.e. no ground connection.
The dipole is mounted in the vertical position, with the lower end of the bottom aluminum tubing 1 meter off the ground.
The feedline is 50 feet of RG-8 coax to a transmatch w/ SWR meter.
The transmatch is attached to the transmitter with about 1 foot of RG-58 coax.
The transmatch is adjusted to give SWR 1:1 (at the transmitter)
ESTIMATE THE POWER ACTUALLY RADIATED FROM THE ANTENNA
I estimate the power radiated by the antenna as follows:
1. Model the antenna with MMANA-G.
using the 'real ground' option
MMANA-G reports calculated SWR = 1.74 at 21.050 MHz
2. Determine coax loss using “Coax Calculator” at http://ocarc.ca/coax.htm
Input to Coax Calculator
Line Type: Belden 8237 (RG-8)
Line Length: 50 feet
Frequency: 21.050 MHz
Load SWR: 1.74
Power In: 100 Watts
Matched Loss: 0.41 dB
SWR Loss: 0.058 dB
Total Loss: 0.468 dB
Power Out: 89.782 Watts
3. With 100 watts transmitter output, the power radiated by the antenna is 89.782 watts.
Which means that the overall efficiency of the system is 89.782 %
My question is:
Is my approach to estimate power radiated by antenna realistic?
Is it reasonable to use the 1.74 SWR from MMANA-GAL, as the “Load SWR” in the “Coax Calculator”?
does the bottom end of dipole at 1 meter above ground introduce ground losses that my method does not include? or does it simply affect the radiation pattern from the dipole?
What am I missing? Please be gentle, but not kind.