16 November 2012

23cm biquad simulation

Before assembling my 23 cm biquad antenna I wanted to know what happens when loop-to-reflector spacing is reduced. In theory, at least.

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
I use MMANA to simulate antennas. So I drew a biquad with a reflector spaced 1/8th lambda and simulated it (the .maa file). The result was encouraging; both Z and gain were within expected values.

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.
The comparison of far fields for both antennas (1/8th is in black, 1/16th in green) show a reduced gain and F/B ratio for the shorter antenna. Gain difference is 0.5dB (theoretical), which is negligible for my final, RX-only, application. F/B looses 3dB and doesn't worry me too much, even if I will mainly use the front lobe beaming out of the balcony... I am going to build the biquad right off a BNC female head screwed to the reflector panel (a copper clad board about 20x30 cm).

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?