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DAB TX

I recently attended a club evening at the OH2TI hamradio club. The subject turned in to digital broadcasting and from there to DAB and broadcasting it. 
As many of the folks present have an affinity to commercial broadcasting, the idea came to constructing a suitable transmitter for commercial DAB broadcast on a hobbyist budget. 

This page exists as a reference for myself and possibly others, but mainly just for myself. 

Standards:
ETSI EN 300 401 v1.4.1 - Original DAB specification
http://www.etsi.org/deliver/etsi_en/300400_300499/300401/01.04.01_40/en_300401v010401o.pdf

ETSI TS 102 563 v1.1.1 - DAB+ enhancement specification
http://www.etsi.org/deliver/etsi_ts/102500_102599/102563/01.01.01_60/ts_102563v010101p.pdf


The exciter:

Thankfully someone else has already done experiments on DAB broadcasting with hobbyist level tools. 

Transmitting DAB with the 300eur hackRF
http://www.hackrf.net/2014/03/hackrf-dab-%E5%B9%BF%E6%92%AD%E5%8F%91%E5%B0%84-hackrf-dab-transmit/

Simple DAB transmitter
http://tipok.org.ua/node/41

Small Scale DAB by Rashid Mustapha
http://stakeholders.ofcom.org.uk/binaries/research/radio-research/Software-DAB-Research.pdf

Linear amplifier:

To conform with the ETSI and national standards for interference and harmonic suppression a highly linear amplifier needs to be constructed for use with the exciter. While amateur radio designs for the ITU Region 2 220MHz band could be adapted, it might be a better idea to design a new amplifer from scratch. 
The Rashid Mustapha transmitter uses a hybrid power module.

While the transission power limit is currently unknown and the legality of hombrew transmitters without type acceptance for DAB is also unkown.
However, for commecial FM broadcast (especially with a limited short time licence) homebrew non-typeaccepted transmitters are legal up to 50W power. Naturally the transmitters need to meet the specifications, but no costly certification is required. This suits the hobbyist :P

The OH2TI club also has some 170-230MHz high power isolators in the junkbox, so those can also be used to preserve the IMD performance of the transmitter. Even if the isolators introduce additional harmonic distortion which needs to be filtered out. These isolators are needed due to the tight transmit filters.

Transmit filters: 
The original idea is to use a simple lumped element band pass filter for the transmit filter. If building the filter with High-Q caps and silvered copper wire coils is enough then it gets used. However, if this is not the case, then it might be needed to construct something more serious, like cavity filters for the job. And doing so would rise the cost and difficulty of the project quite harshly. 

Lumped element filter construction:
Mica caps and silvered copperwire coils on Taconics TLC-32 teflon laminate or on Mystery 1.2mm teflon laminate. 

Page 168 of the ETSI EN 300 401 v1.4.1 standard has the transmit spectrum mask to compare the TX spectrum to. 


Legality:
This page talks about broadcasting DAB with a licence on either the commercial 230MHz band, or on the 1296MHz and 2300MHz amateur radio bands. 
Pirate operation is not encouraged. 

Transmit frequencies:

Initial tests are likely held on the 1296MHz "23cm" amateur radio band, as the 1.5MHz wide DAB transmissions fit there quite easily and don't bother anyone. This range can also be received with the cheap RTL-SDR sdr radio sticks so capable receivers are widely available. 
For 230MHz range specified in the band plan ( http://www.finlex.fi/data/normit/32658-TJT_suomi.pdf ), well it is still uncertain. 
But the frequency range has been allocated for DAB broadcasting and there does not seem to be other stuff there. 
So it should be possible to get a temporary permit to transmit there.

The exact ranges specified in the document (page xx) are 174 - 230 MHz, 230 - 235 MHz and 235 - 267 MHz.
The upper frequencies are likely off limits as 243 MHz is military aviation emergency channel and 240 - 267 MHz is specified for military use. 
This form for applying for a short term digital TV or voice broadcast permit:
https://www.viestintavirasto.fi/asioikanssamme/luvattodistuksettunnukset/lyhytaikainendigitaalinentv-jaradiotoimilupahakemus.html
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