*Corrected the changed dipole dimension below to 17.25 feet. (thanks Dave)
**Added BALUN note using 40 m dipole on 20 m
Modeled a simple dipole on 40 and 20 meters, 33.5 ft each side and 33 feet (~ 1/4 wave) above ground.
These model figures are only approximations of what you might expect. They generally work out pretty close to actual measurements in the field. Plus j is inductive reactance, minus j is capacitive reactance.
EZNEC 5 shows the feed point Z to be about: 83.25 +j19.78 Ohms, slightly inductive.
Using Transmission Line Details, and the above Z as the load (feedpoint) end for RG-8X (mini):
Frequency: 7.1 MHz, at input (transmitter) end,
25 ft ~ 31.84 -j13.36 Ohms 0.182 dB loss
50 ft ~ 57.73 +j27.38 Ohms 0.464 dB loss
75 ft ~ 41.60 -j23.88 Ohms 0.617 dB loss
All easily matched for optimum power transfer with a good ATU with very little additional loss.
Same antenna same height
Frequency: 14.06 MHz, Antenna feed point Z ~ 3549 +j2504 Ohms
25 ft ~ 16.39 +j125.27 Ohms 6.73 dB loss (mostly SWR)
50 ft ~ 8.43 +j53.56 Ohms 9.16 dB loss "
75 ft ~ 6.72 +j23.83 Ohms 10.6 dB loss "
Some good tuners would match the low Z but the extreme SWR loss would still be there.
**A 1:9 BALUN might be useful at the feedpoint to mitigate the high Z to a lower value.
So, changing the dipole model to *17.25 ft each side and 33 feet (~ 1 wave) above ground
Frequency: 14.06 MHz, Antenna feed point Z ~ 78.13 +j4.39 Ohms
25 ft ~ 61.93 +j19.76 Ohms 0.313 dB loss
50 ft ~ 46.06 +j17.71 Ohms 0.614 dB loss
75 ft ~ 37.96 +j9.29 Ohms 0.903 dB loss
These values could be used without an ATU with probably less than 2:1 SWR. Easy match with any good tuner.
My ATU favorites are Z-Matchers some of which will match down to 3 Ohms resistive.
Chuck Carpenter, W5USJ
EM22cv, Rains Co. TX