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) |
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!
Copyright © Zappy TV (PE9ZZ) 2003