FM omni aerial "garbage" ?

Richard Dower

According to: http://www.aerialsandtv.com/fmanddabradio.html



The round FM “Omni” type antennas do not perform as well as the half wave dipole and this reflects their design, which gives a theoretical minus 3 dBd gain figure. That`s why we think they`re crap and we don`t stock them.

To be honest I`m at a loss to understand why there are so many around, but, of course, just because there`s a lot of something around it doesn`t necessarily mean it`s any good. Take chipboard for example. A building material which has negligible physical strength and isn`t even waterproof. It`s absolute crap, yet it`s used bleedin` everywhere.....



^ i wonder what tvtrade.ie think about this?....i'm in the market for a combined FM/DAB aerial so was wondering......

zg3409

To be fair, any rooftop outdoor aerial will operate better than an indoor aerial. Also the radio stations you may want may be in different directions.

Most installers do not have a meter for pointing a directional FM aerial.

However yes, a good installer would know if these aerials would work, and if they are in a difficult area where one would be needed for a popular weak station.

It also is required that the radio actually has an FM socket (only fancy HiFis seem to have them) and that a seperate outlet is provided for this that filters out TV signals.

So yes, they are not great, they beat an indoor aerial, but they are fast to fit and often solves the problem. I think in many cases they are not even needed but installers use it as an excuse to charge more for large stuff in the skies.

By the way I think DAB aerials are just band III TV aerials so there should be plenty of spares shortly.

watty

Not only that, the Halo is horizontal only and ALL FM & DAB is vertical, slant or circular in this country,

The Halo (and -3dB is generous) was for pre 1975 UK which was all horizontal VHF-FM in 1955 for roof aerials only. Later with portable FM and Car FM the vertical polarisation or slant/Circular made more sense. Irish FM is from about 1962.

Halos should not even be imported. It's a disgrace that they are on sale. There is no use for them at all.

A vertical whip will perform far better for VHF-FM or DAB and is inherently omnidirectional.

A vertical dipole is harder to mount than a whip, but is also omni. Nearly 10dB to 20dB better reception than a Halo because a halo is -4 due to design and -3 to -18 due to being wrong polarisation!


A horizontal dipole has two nulls, but there is never a need for one for Broadcast FM Radio or DAB

lawhec

Before the first (legal) commercial station in the UK opened in October 1973, the BBC's radio broadcasting monopoly meant they were the only stations on FM - a two or three element horizontal polarised Band II aerial pointing at the transmission site was usually efficient as few people could pick up other services unless they were near other countries e.g. Ireland or France, or they were trying to receive pirate FM broadcasters whom at the time were rare (RNI did have a service on 100MHz that reached London).

ILR then came in to the UK, and transmitters were at sites different to those used by the BBC using slant, mixed or circular polarisation allowing better reception on car aerials and many portables. In response the BBC started reconfiguring their FM network for their transmitters at the time to be upgraded from horizontal to mixed polarisation starting in 1982 and finished in the early 90's.

In Ireland - north & south - no FM broadcaster AFAIK transmits with the power of their vertical polarised component less that their horizontal one.

A vertical dipole for Band II can provide omnidirectional coverage though ideally to do this at least one half of the dipole needs to be above any other conducting object e.g. pole. Otherwise careful positioning of the aerial can use a aluminium (or other metal) pole whereby you can achieve a cardioid pattern. Using the same dipole aerial horizontally polarised will give a rough figure-8 pattern with fairly broad forward lobes and sharp null at the tips. One advantage in using a dipole horizontally is that if multipath interference is a problem, receiving signals on their horizontal plane as opposed to vertical reduces multipath problems. As an example, FM radio where I live from Brougher Mountain suffers from harsh multipath because of the reflection of the signal from the southern face of the Sperrins - adjusting the aerial on a portable so it lies 'flat' rather than pointing straight up kills most of the multipath problem even though face of the mountains is almost 180 degrees from the transmitter site. This is less useful however from sites which only broadcast with vertical polarisation.

Halos as already pointed will likely work better than an indoor aerial - however a 30 inch piece of insulated wire taped to the pole and connected to the inner core of the coax downlead would likely work just as well. At best halos have a -3db "gain" or lose half the signal in the aerial already before transferring it to the balun (if one is fitted, otherwise direct to the coax). At certain points the hulls are very deep; I'm told that the point where the tips nearly meet has the biggest null in that direction.

I've yet to see a combined FM/DAB aerial, mainly because they work in two separate frequency bands that aren't terribly close to each other. In countries like the USA and Australia where analogue TV has been broadcast in VHF Bands I and III (DAB uses Band III) with service available in many locations on both bands, log periodic aerials have been used though in Europe it's been a more common preference to install aerials separately for each band. A VHF Band III TV aerial that's been used in Ireland to cover channels H, I and J (particularly J, for now) should work fine for DAB reception.