The goal is to replace the Steppir 3E with a model with 2 elements on 40m. I compared 2 models the 36ft 4 element DB36 and the 3 element DB18E.
- 3 element Yagi (3E)
- 3 el 20m-6m
- 1 el 40m-30m
- 40m 1.8dBi 30m 2.1dBi 20m 7.4dBi
- 16 foot boom, 39 ft longest element
- 19.7 ft turning radius
- 8.1 sq ft wind load
- Weight 58 lbs
- DreamBeam DB18E
- 3 el 30m-6m
- 2 el 40m
- Three looped elements
- 40m 6.2dBi 30m 7.0dBi 20m 7.4dBi
- 19 foot boom, 39 ft longest element
- 22 ft turning radius
- 12 sq ft wind load
- Weight 110lb
- DreamBeam DB36 4 element Yagi
- 40m 7.2dBi 30m 8.2dBi 20m 9.2dBi
- 36ft boom, 48 ft longest element
- 26 ft turning radius
- 17.5 sq ft wind load
- Weight 160 lb
How big is 0.3dB? 0.3dB corresponds to a 7% change in signal power into the air and this corresponds to 5% of one S-unit. It would be very unlikely that that DX station would be able to tell that you are using a shorten dipole. You should feel very comfortable with the trade-offs Steppir has made getting a 40m rotate-able dipole into the 39ft element length.
[ 10^(-0.3/10) = 0.93 and 0.3dB / 6(dB/S-unit) = 0.05 S-unit]
The DB18E provides 6.2dBi forward gain which is a 4.4dB over that of the 3E and this corresponds to 175% increase in apparent signal power, and 2/3 of an S-unit at the DX station.
[ 10^(4.4/10) = 2.75 and 4.4dB / 6(dB/S-unit) = 2/3 S-unit]
Finally, the DB36 provides an additional 1dB of gain over the DB18E, which is a 25% increase over the DB18E and an additional 1/6th of a S-unit.
[ 10^(1.0/10) = 1.25 and 1dB / 6(dB/S-unit) = 1/6 S-unit]
Consequently, the DB18E is more of an step up from the 3E than the DB36 is from the DB18E. The additional cost of the DB36 would have to be motivated by other factors.
Compared to the 3E which has a back lobe of equal gain to its front lobe, the front to back ratio of the DB18E is 20dB. This means the DB18E will suppress of unwanted reverse direction signals by up to 3 S-units.
The DB18E should provide a gain of 2/3 S-unit on forward transmit and 3 S-units of QRM protection compared to the 3E.