FM Aerial Wiring

[Woody]

Er, no. Since the advent of VHF car radios 20 years or so ago most broadcast sites, main and relay, have been changed to 'mixed' polarisation - in effect diagonal - so that they have both vertical and horizontal components. Admittedly there are a few with very specific in-fill that have horizontal in one direction and vertical in another.

The only disadvantage of vertical polarisation - which is why it was not orginally used - is susceptability to electrical interference. However given the significantly increased sensitivity of modern receivers (giving limiting at much smaller signals) and the considerably lower radiated noise of cars, appliances, etc., it is no longer a problem.

[tony sayer]

In article , Woody writes

How do you reckon that Woody?...

[blah]

If you're looking at any yagi aerial (TV, FM etc.) it can sometimes help to imagine it as a telescope. The dipole (where the cable connects to) is your eye, the rest of the elements are like the body of the telescope. They guide and focus the signal to the dipole in the same way the body of a telescope does the same with light towards your eye.

[Dave Plowman]

This falls over with 3 element Band II types...

[Woody]

That was the original BBC theory.

I would guess that it is because the propagation pattern of a yagi is akin to a squashed cone. Therefore a horizontal antenna will tend to overshoot the ground and interference sources, wheres a vertical will have a pattern that includes the ground - any sources of RFI in it's path will thus be 'recovered.'

Perhaps this is why Bill Wright in uk.tech.digital-tv and Television magazine is always going on about RFI from cellular masts in line with vertically polarised TV antennae where it is less of a problem with horizontal? Could of course always be because main stations are usually horizontal and of much greater radiated power than relays?

[tony sayer]

In article , Woody writes

Sounds as good a theory as any;-)

Yep well as there not too far off UHF TV frequencies and the front end performance of TV's VID's etc isn't that good, then not surprising...