ve noticed a couple of stations in birmingham using discone aerials lately, they dont stick out as much as the dreaded diploe... more money thou... aint sure about the gain on these,

I have seen a few of these around London, but never for fm transmitting. As far as I know, it's a wideband antenna mainly used for receiving many different bands.

There is another antenna called a ground plane (which looks similiar to above) which is often used for fm, do you mean that one?

There £25 each so hardly more expensive! And I don’t get the point of the post? Are you asking if they perform well or are they better then a single diapole? 20-30 stations in London are using these id say there nowhere near as good as a diapole but that’s just my opionion I have never used one they seem to chop a lot more and are not as strong on house stereos either but for £25 there ok and I suppose they look more official but if there coming to get you using this Ariel won’t stop them ! How many stations are still on the fm in Birmingham mate?

The gain from a discone will be at best flat and most likely a loss. That design is for a large range of frequencies, so if you have to change channels you will not have to retune the antenna. Most examples will transmit from 20MHz to well over 1GHz, so if you have any spurs or harmonics or 1/4f - 1/2f, they will be broadcast just as efficiently as your fundamental. You'd need a clean rig. If you are looking for something less conspicuous, consider an "all rounder" type: You'd have to hack out the balun circuit board and replace it with a 6:1 coax balun.

If I wanted a low profile aerial, I always used a ⅞-wave. From the ground, it looked like a CB aerial, and gives a really weak signal below it, so you don't get saturation problems in the block, and the numpties trying to steal your gear will think that it's elsewhere!

I have also clocked the aerials used in Birmingham occasionally - I think they are ground plane antennas rather than the discone aerials mentioned above.

they are groundplanes...

here is one

Looks like they've used bluetack to weatherproof it

If it works.....

We used to use "Iospon" car bodywork filler to waterproof aerials a few years ago!