*Practice is the best of all instructors*

*-- Publius Syrus*

On a Log-Periodic, all the elements are driven, but each dipole is reversed by 180 degrees. To make it on a single sided PCB, will require some links to wire up the dipoles. I added a BNC connector and thin equilength coax to the feed points of the antennas.

**Stacked Log-Periodic 5GHz**

*By carefully stacking two of them 0.75 Lambda1 centre to centre (54 mm), one should get another 2 to 3 dB of gain for a total forward gain of 5 to 6 dBi.*

**SNR**

*The important thing in radios is not the signal strength, but rather the Signal to Noise Ratio (SNR).*A directional antenna reduces multipath reflections and noise from the surroundings and improves the SNR, so that a radio will perform much better with a directional antenna, compared to a simple omni-directional dipole.

**Antenna Calculator**

https://hamwaves.com/lpda/en/index.html

**Input**

Highest frequency f = 5200 MHz

Diameter of the shortest element ⌀ = 3 mm

Optimal relative spacing σo = 0.168

**Design**

Number of elements ⌊N⌉ = 5

dipole l1 = 0.036 m

Sum of all dipole lengths lo = 0.146 m Distances between the element centres and their position along the boom:

d1,2 = 0.004 m, i.e. l2 @ 0.004 m

Boom length L = 0.012 m

Length of the terminating stub l_Zterm = 0.009 m

**Dielectric vs Antenna Size/Frequency**

Bear in mind that the permittivity of FR4 (Fire Rating 4, glass and epoxy PCB) is about 4.3 and because the copper is in air on one side, the effective permittivity is less, about 3.3 - this conspires to make your antenna operate at about 20% lower frequency than the above (air dielectric) calculator indicates.

- VF = 1/sqrt(3.257) = 0.555

**Version 1: Single Sided PCB**

An antenna this complex, really should be built on double sided board, for best performance, but I managed to get the prototype together with 22 SWG hookup wire and RG316.

**Log-Periodic Wired Up**

If you look closely, you can see the hookup wire links under the coax.

**Performance Graphs**

Considering how it was built, this is not too bad. It needs a balun to clean up the VSWR ripples.

**Log-Periodic - Return Loss**

**Log-Periodic - VSWR**

**Log-Periodic - Smith Chart**

**Design Issues**

**Version 2: Single Sided PCB with J Balun for a Coaxial Cable**

This test unit is a single sided board design, which requires wire links and there is no way to make microstriplines, since there is no ground plane, so eventually, there are multiple reasons to go double sided.

**Log-Periodic with a Half Folded Dipole Balun for a Coaxial Cable**

**Log-Periodic With RG316 Feed**

**Log-Periodic with Shoelace Hook-up Wiring**

**Log-Periodic VSWR is Smooth, but High**

**Log-Periodic Smith Chart Impedance Mismatch**

**Log-Periodic Impedance Chart**

The graphs above show that the cable match is remarkably much better at the antennas. Next, I have to improve the power combiner/splitter match at the connector with a tapered microstrip line to improve the VSWR.

**Version 3: Double Sided PCB with J Balun and Tapered Power Combiner**

- Taper length: 18 mm
- Epsilon: 3.7 - FR4 glass-epoxy PCB
- Height: 1.5 mm - Standard double sided PCB
- 50 Ohm Width: 3 mm
- 100 Ohm Width: 0.8 mm

**Front Side - Microcoax and Tapers**

**Back Side**

**VSWR - Wide Bandwidth**

**Smith Chart**

**Impedance**

**Phase - A Log-Periodic is Complex**

**Version 3: Eight Elements on Double Sided FR4**

*Note that*

*I found a missing through hole link on the 2nd version above. It actually works much better than the above graphs show. However, the bandwidth is still not wide enough to my liking.*

*I think I have a better chance to improve the matching of the smaller 5 element antenna, than to fix this 8 element design. This one is just too complicated to get right and the elements are too close together to make them wider.*

**Version 4: Two Pieces of FR2, Back to Back**

Target Frequency Band: 4.5 to 5.0 GHz

Design target for single sided FR2 PCB: +20% = 5.4 to 6.0 GHz

Widened design BW for parasitics: 5.0 to 6.5 GHz

LPDA — https://hamwaves.com/lpda/ — v20180914

LPDA design 2020-07-16 14:11

INPUT

Lowest frequency f₁ = 5000 MHz

Highest frequency f = 6500 MHz

Diameter of the shortest element ⌀ = 1 mm

Characteristic input impedance Zc_in = 50 Ω

Taper τ = 0.880

Optimal relative spacing σₒ = 0.163

Chosen relative spacing σ = 0.060

RESULTING DESIGN

Number of elements ⌊N⌉ = 5

Dipole element lengths:

dipole ℓ₁ = 0.030 m

dipole ℓ₂ = 0.026 m

dipole ℓ₃ = 0.023 m

dipole ℓ₄ = 0.020 m

dipole ℓ₅ = 0.018 m

Sum of all dipole lengths ℓₒ = 0.118 m

Distances between the element centres

and their position along the boom:

d₁,₂ = 0.004 m, i.e. ℓ₂ @ 0.004 m

d₂,₃ = 0.003 m, i.e. ℓ₃ @ 0.007 m

d₃,₄ = 0.003 m, i.e. ℓ₄ @ 0.010 m

d₄,₅ = 0.002 m, i.e. ℓ₅ @ 0.012 m

Boom length L = 0.012 m

Length of the terminating stub ℓ_Zterm = 0.007 m

Required characteristic impedance of the feeder

connecting the elements Zc_feed = 144.7 Ω

Now build it on FR2 and see whether it is anywhere near the desired band…

*The half folded dipole feed makes a kind of balun with a 75 Ohm match, so it should be used with a 75 Ohm cable.*The trouble is that at 5 GHz, one cannot use Ferrites to make a balun - they only work up to maybe 2 or 3 GHz. I tried some beads and they had no effect.

*For a wide band receive antenna, it is good, since the VSWR doesn't matter much on receive.*