Lindenblad Circular Polarized Antenna

Lindenblad EZNEC model and plots for 146 MHz.

The Lindenblad is a circularly polarized array with equal horizontal and vertical components within the point-to-point radiation pattern. Its origins lie in the pioneering work of N. E. Lindenblad, who first proposed the antenna design almost off-hand in a broad article on television transmitting antennas around 1940.

Why circular polarization (before satellites)? Reduces fading on the fringes of coverage, and FM broadcasters like it since it reduces flutter (rapid drop outs / fading) for FM radio reception in cars, due to reflections / multi path causing cancellations and polarization shifts while moving!

The original design has a deep overhead null which helps the gain figures, and reduces receive noise from the sky. Similar to an 8JK with its close spaced dipoles!

Cebik modified the design to better suit satellite users, this filled in the overhead null and reduced gain at the horizon. This is determined by the dipole spacing and tilt angle!

• AO 16a: Horizontally Polarized Omni-Directional Antennas: Some Larger Choices
• Notes on Fixed Satellite Antennas
• The Practical Lindenblad

I found that interesting enough to model one and see what I could learn.

Starting with the EZ-Linenblad design (since it say's its easy), I found it didn't work when modeled, and I question the Mickey Mouse explanation about SWR and 75 ohm coax to get a match. Maybe I missed something, then is it really EZ? Plus coax carrying SWR has loss, something to think about if it matters.

So I start over with a dipole in free space, and add 3 more, set the tilt angles, and observe each dipole has a drive impedance of around 86 ohms and resonant (no reactance)!

1. This is close enough to use 75 ohm coax (any set of equal lengths) to a common point (since they are fed in phase).
2. When combined that results in around 17 ohms and resonant (no reactance)
3. Transform that back to 50, and it's simple?
4. Folded dipoles with balanced feed to common point would land right around 50 ohm for a direct feed!

I wondered how did I end up with something that just worked with out whacky matching systems?

I pulled the model apart to find I had somehow started with a dipole resonant at 150 MHz! The mutual coupling effect (interaction between elements in phased arrays) resulted in a compensation effect that made the system resonant at 146 MHz!

Did I accidentally fix the Lindenblad? Maybe.

Models at https://github.com/lonney9/Antenna-Models/tree/main/VHF/Lindenblad. Note these have even segments for NEC5.

Here are some plots of a 2m / 146 MHz Lindenblad, 30 degree tilt (optimized for horizon coverage with the deep overhead null - the original design), at 10m height over real ground (NEC5):

Lindenblad Elevation Plot (EZNEC/NEC5) 10m height over real ground

Lindenblad Azimuth Plot (EZNEC/NEC5) 10m height over real ground

Has very nice omni circular polarization, with the opposite pol around -10 to -20 dB down!