UK Broadcast Transmission
Main indexMain GalleryFeaturesInfoTech Wiki
Send in your photosDesktop wallpaperMailing listsFAQsContact
The LibraryTeletextMHPTBSAstrohosts

HOW TO RECOGNISE DIFFERENT TYPES OF BROADCASTING AERIALS FROM A QUITE A LONG WAY AWAY

By Mike Smith

LESSON 3: DAB - DIGITAL AUDIO BROADCASTING

Band III is presently used for DAB and because the frequency of around 225 MHz is higher than the Band II FM broadcasts, the aerials used are significantly reduced in dimension as can be seen in figures 29 and 30.


Figure 29

Currently all UK DAB transmissions are vertically polarised. There are four reasons for this:

  1. Most receivers are in cars or use “whip” antennas which are normally set vertical.
  2. Single polarisation antennas are simpler and cheaper than mixed polarisation.
  3. Point 2 also makes it easier to share aperture on the masts used by Band II VHF-FM antennas (see below).
  4. There was no legacy of listeners using horizontally polarised antennas as with Band II VHF-FM transmissions.

However, it is interesting to speculate whether, sometime in the future as DAB receivers become more prevalent, whether mixed polarisation will become more common. This would match what happened with mobile phones where initially only vertically polarised antennas were used, but later, as people expected their handsets to work in any position, mixed polarisation was added.

It is now quite common for DAB antennas to make use of the same aperture on the mast as the Band II VHF-FM antennas. This means that it is possible to fit the DAB antenna high on the mast where otherwise it might have to be much lower down. The downside is that the when it is necessary to work on either the Band II or DAB antennas, the other service has to be shutdown too.

Wenvoe and Sutton Coldfield are two of the bigger stations that use shared aperture antennas. The mb21 pictures of the top of the Wenvoe mast show the extra vertical dipoles in the Band II aperture quite clearly. In this case, the DAB dipoles project from the “edges” of the Band II array.

In other cases, the DAB dipoles are mounted to the face of the screening mesh behind the Band II antenna elements.

Typically, there are 2 tiers of dipoles for each Band II dipole. This give the possibility of the DAB antenna being higher gain than the Band II (if the whole aperture is used), or of not fully filling the Band II aperture if the extra gain (and complexity) is not needed.

Sheffield uses a shared aperture antenna of this type. The mb21 picture of the Band II antenna clearly shows the 4 – tier DAB antenna using the same screen as the 2 tier Band II. Each Band II dipole has 4 DAB dipoles sharing its reflector screen.

On the same picture, the vertical dipoles off the corners of the antenna array are parasitic (i.e. unpowered) Band II radiators to improve the vertically polarised radiation pattern from the antenna.


Figure 30

 

LESSON 1:

UHF Television
LESSON 2: VHF-FM Radio
LESSON 3: VHF-DAB - Digital Audio Broadcasting

LESSON 4:

MF-AM Radio

Section Index

If you can add some extra detail or provide corrections
to inaccurate information please e-mail the site - Mike Smith.

Transdiffusion

EMC RMC TMC
PMC Bitstream Sounds On
HHG HTW ITW Irish TV mb21
sub-TV Transdiffusion TodayEMCRMCTMCPMC

HHGHTWITWIrish TVmb21MHPsub-TVTTLP

 
mb21 by Mike Brown
Hosted by Astrohosts

Top