The standard spacing is 1/8th of lambda, while I want to halve it to 1.5 cm which is about 1/16th lambda.
Why? Because it simplifies building the antenna since the double loop can then be soldered directly to the back of a BNC female head. :-)
|3D model of biquad and wire mesh reflector|
Each simulation run takes 36 seconds in "free space" and 48 seconds if the antenna is said to be on "real ground", so I chose not to increase reflector density.
After saving to file the "far field" data, I reduced spacing to 15 mm (0.015 m) and re-run the simulation. In both cases the antenna was set to be over real ground.
|Comparison of 1/8th vs 1/16th lambda in free space.|
Next, since I had some spare CPU cycles, I ran an MMANA overview over +/-40 MHz from the center frequency (1295 MHz). Z/Gain/F-B were computed in 5 points, SWR is interpolated. Apart from an impedance discontinuity at 1315 MHz, all other relatively flat values give hope for a normally performing antenna ... especially taking into account all my mechanical bulding errors.
|Gain and other parameters comparison at various frequencies.|
|SWR interpolation over 80 MHz span.|
Someone may notice in screenshots that SWR is computed for 75 ohm and not 50. The fact is that the RTLSDR dongle is meant for TV reception, whose impedance is 75 ohm. Also I will use a SAT-TV coax to reduce losses, so why not reason in 75 ohm terms?