Sirio Thunder 27 on balcony - Lots of noise on all channels

[mutley]

Hi,

I've installed a Sirio Thunder 27 on my balcony and it's giving me grief.

It's sold as a self contained 1/4 wave dipole so doesn't need a ground plane. I've cut my coax to the exact length specified (8.48M) and terminated it correctly. I've also checked continuity of center pin to center pin and of shield to shield and all is fine. I've also checked insulation resistance and all is also fine.

When connected up to the radio and just leant up against my coffee table, I get good SWR, but as soon as I mount it on my balcony guard rail, SWR goes right into the red.

If I ground the aerial to my balcony guard rail, the SWR comes down to 1.5 but I get loads of noise (over 9 on the signal meter) on every channel.

As it's a dipole, I didn't think it should need grounding as it already has one

My balcony guard rail runs the whole length of the building on my floor (probably 80M) and may well be earthed to every floor of the building. Could the ground plane be too big?

Could the noise be interference from electrical appliances inducting from everones flats?

Is there a way to suppress this?

The aerial seems to be working well, apart from the noise, and have heard stations from all over europe calling, but they are fighting over the ammount of backgroud static being picked up.

Any Ideas?

Cheers.

[Sniper_Sid]

If they're specifying co-ax length then its using the co-ax as the missing half of the antenna which also means you're going to be having RF coming into the house. I certainly wouldn't want to be touching the co-ax if you TX with more than a few watts. Its only 96cm long which doesn't come anywhere near to a 1/4 wave dipole, no matter how you wind the wire, and there's no such thing in the first place - or at least no such thing that works very well. I suspect that when you stick it on your balcony, the presence of lots of metal on the "ground" side is detuning it. To confirm this, shove something insulating the antenna completely from the balcony (remember to include the U-bolts as well.

You're going to get more interference on a vertical than a horizontal and you're going to get electrical noise due to the close proximity of the sources to the antenna.

If you've got a bit of wire spare, knock up a 1/4 wave groundplane antenna. You'll need 5 bits of wire 2.7m long. Connect one to the centre of the coax and hang it vertically. Connect the others to the braid and lay out in a cross pattern or if you've not the room, fanned out equally spaced in whatever room you have. See what you get then. SWR might be a bit off cock but you'll get an idea of what noise levels you should be getting.

[mutley]

Thanks for the reply Sid.

I think you're right about it using the co-ax as the other half of the dipole.

I decided to take it down, but used the mount to attach a 1/4 wave stinger to the balcony guard. I've now got it SWR'd to about 1:1.1 and have almost completely lost the excess noise which the sirio was picking up (which was probably all my electricals in the flat)

As the Sirio and the Stinger are both 1/4 wave, I may have a go at rigging them up, end to end and isolating the balcony ground to give me a real 1/4 wave dipole to see how that performs.

Cheers,

[Sniper_Sid]

There is really no such thing as a 1/4 wave dipole. A half wave dipole is two quarter wave lengths fed from the middle which is what you're constructing. It should work just fine although proximity of nearby metal (balcony rails, window frames, re-enforcing rods in the concrete walls etc) will have an effect on the radiating pattern so don't be disheartened if you can't get to or hear people in a certain direction.