Why the difference in signal strength?

Post by **Bassman** » 09 May 2006, 18:11

I have a simple set up for SWL.A random wire to an ATU.straight into my Sangean ATS 803A.Works pretty well tho the ATU is home brew off Ebay.I decided to improve things by attaching the 'home end' of the long wire to a length of co-ax before bringing it inside the house.Better,you might think?Not so.The original set up brings in a stronger signal.There are no shorts to earth any where that I can see,so does anybody know why the coax has deteriorated the signal?

Andrew · Post by **Andrew** » 09 May 2006, 21:26

Depends on the impedance of the co ax. Is it 50 ohms or telly stuff, at 75? There can be other things like joints reduce signals, and signals done like travelling round corners inside co ax. Radio signals also get reduced by the construction materials used in building your house. Breezeblock eg, which also reduces mobile phone signals.

It could be something else entirely too.

Post by kc » 10 May 2006, 07:23

I had a similar problem when using the built in telescopic antenna on my Trio.
The antenna had no counterpoise despite an earth still being connected at the long wire connector. For some reason the RX did not work as effectively unless the earth & the antenna were both connected to the same point.
If you have not already tried, connect an earth to the braiding & see if that improves things.

Post by **Andy** » 10 May 2006, 08:40

I reckon it's like this: If your long wire is fairly short, it will be a high impedance (well above 50 or 75 ohms) and the impedance of the coax - which will look like a capacitor down to earth - will 'decouple' some of the signal to deck.
If you were working at the quarter-wave resonance of the wire, it's impedance would be low and the coax would do its job properly.
I think

Post by **Bassman** » 10 May 2006, 10:48

Okay.The coax is left over from a Sky installation(Dish to Digibox).The wire is about 40ft long and strung at about 4mtrs high between the gutter(bungalow) and a tree.The only difference between the two set-ups is the co-ax,about 10ft of it.Joints are soldered,self amal. tape is used.I'm not sure what the impedence of this co-ax is,but would it really make that much of a difference?

Post by **Andy** » 14 May 2006, 16:19

Yup, I reckon. I've just had a look at some cable specs, and W103 (the nearest I can find to CT100 satellite stuff) has a capacitance of 78pf per metre. So if you have 3 metres of it, you are plonking about 240pf across the antenna terminals.
If your wire was resonant (eg a quarter-wave or multiple) it would be a nice low impedance and the 240pf wouldn't matter much, but at other frequencies away from resonance the impedance will be high and the signal will be attenuated by the capacity of the coax.
If the aerial wire is 40ft long it should be resonant (as a quarter wave) at about 7MHz and you should get decent results around this frequency.

Just one q - does the coax feed into a 50 ohm socket on the radio or is the impedance unknown?

Post by **Bassman** » 27 May 2006, 12:48

Andy, sorry for not replying quicker.The coax goes to the 'phono'antenna socket on the back of the Sangean,which I believe is 50 ohms.I think the problem is the coax which I will replace in due course.Ta for advice.