UHF Log-per Antenna (LPA) for Digiboxen

A DVB-T Antenna

With the start of DVB-T transmissions in the Netherlands ("Digitenne") I felt the need to construct a wideband UHF antenna to be able to receive the local multiplexen here in Amsterdam, on UHF channels 21, 24, 34, 57 and 64. As I am a sucker for Logarhythmic-Periodic ("log-per") antennas I searched on the web (using alltheweb as Google always redirects to a Dutch site which narrows my search dramatically) and found an Australian site describing just what I was looking for. And I don't even have a digibox as yet!

The Construction
The antenna is based on a double strip of fibreglass PCB material and 1.5 mm diameter enamelled copper wire. The PCB acts as transmission line and substrate for the elements attached to it. With the sandwich construction the impedance seems optimized for 50 ohms, but the antenna works just fine with 75 ohms RG59B/U cable attached to it. As the antenna's output is symmetrical but connected to a coaxial cable a ferrite bead is needed to kill the shield current. The original design also uses a loop as an inductor, but I didn't feel the necessity of that. The ferrite bead was liberated from a scrap computer monitor. The PCB strips were glued together using epoxy cement. The strips provided some difficulty as I had to remove a layer of copper from them as I only had double sided material. Using a phat soldering iron this turned out to be easy...

The elements were cut with extra length. The enamel was filed off from a 1 cm long part, and then soldered directly onto the copper of the PCB. The copper was polished and treated with flux to make that easier, and also to give a nice shiny finish. When all the elements were attached they were cut to length and bent straight. Finally the coax was connected and the ferrite choke fitted. All in all it took me about an hour to finish it, excluding drying time for the glue and the flux. The design parameters, copied directly from VK2ZAY's site:

Element Tip-Tip Length (mm) Spacing (mm to next element)

1 343 75

2 275 60

3 220 48

4 176 37

5 141 31

6 112 25

7 90 20 (to coax)

Element	Tip-Tip Length (mm)	Spacing (mm to next element)
1	343	75
2	275	60
3	220	48
4	176	37
5	141	31
6	112	25
7	90	20 (to coax)

The Results
After mounting the antenna onto a piece of wood I tried it. All five multiplexes could be seen. At first I did not onderstand the direction I had to point the antenna, but a quick glance on the map showed that it was correct. So I stuck the antenna betweeen the stems of my Fiscus plant and connected it to my trusty HP8558B spectrum analyzer. The level seems a bit low, but remember that a spectrum analyzer is decidedly deaf and the antenna gain is only about 3dB over dipole. The Channel 21 spectrum shows quite a bit of distortion (multipath?) but the Ch34 spectrum is a lot cleaner. In any case, the DVB-T specification specifically allows for reflections. Now I only have to get one of those digiboxen!

Success!
It was to be expected, but even so it was quite a thrill to see the borrowed digibox acquire lock! Even if this was a horrible Zinwell affair... And of course it did not have the required Conax (hacked wide open) conditional access, but the radio is (still) free to air. We'll see if we can kick Fuck UPC's butt!