Aerial connections - live chassis

[Bushbaby]

Hi All,
This may be a stupid question but I'll pose it.
I've been trying various indoor aerial set ups without thinking for one second whether the set has a live chassis. I then thought that wont be risk as the chassis must be earthed with regard to radios I have been working with e.g. Pye PE80. Am I safe to assume all is ok...........

Regards

Rob

[Joe]

A PE80 will be OK. Live chassis sets either don't have aerial sockets, eg the DAC90a, or have them connected via blocking capacitors. The aerial could be inductively coupled to the set so there'd be no direct connection at all. Bit beyond my skills though that. There's some info here: http://www.mds975.co.uk/Content/aerials1.html

- Joe

[Skywave]

That's not a stupid question, Rob, far from it. The mains supply has a nasty habit of not checking first: it assumes that we have!

Al.

[Steve_P]

First, there is no such thing as a stupid question. You may be unsure in one case or just want to make sure.

Any aerial and earth connections on an AC/DC chassis must be isolated from the chassis in case any fault occurs. I would use 'Y' rated capacitors here for added safety. as well as making sure the chassis is linked to Neutral.

Cheers,

Steve P.

[pwdrive]

I too would agree with what has been said, it can be so easy to forget the scenario of a possible live chassis, even for the long time experienced. If fitting blocking capacitors then don't forget that capacitors have an impedance so far as AC is concerned, the higher the value of capacitance then generally the lower the impedance at any given frequency. So far as your aerial goes Rob it depends what you want out of your radio, for MW reception then a frame or loop aerial takes some beating, but for SW an outside long wire as high and clear as you can get it is prefferable.
Never earth a receivers chassis if the receiver is of the AC/DC type unless it is run from a fully isolated supply eg: a suitably rated isolating transformer.

Lawrence.

[Yorkie]

In sets that had neither a frame aerial nor ferrite rod, it was commonplace to have either a throw out aerial or an aerial socket, isolated by a capacitor. The Portadyne Princess that I'm working on just now has such an arrangement - a 15ft 'throw out' aerial, isolated by a 0.002uF cap. (See attached circuit). Same will apply to many similar sets - Champion, certain Little Maestro's etc.

Hope that helps Rob.

[AlanBeckett]

They didn't spend a lot of money on that
If you really want to improve the safety put two Y caps in series.
Alan

[Yorkie]

True enough Alan!

I think they designed a set that seemed to work after a fashion, then removed components until it stopped working, then put the last one back!

[Skywave]

So true! I've spent a lifetime in professional electronics and even in the most revered establishments such a practice was not unheard of - sometimes bluntly referred to as 'building it down to a price'.

Al.

[Aerodyne]

It's probably worth mentioning the need - in respect of the relatively few sets that it applies to - that in the case of gram (PU) sockets fitted to AC/DC chassis (the odd Philips set, among others) that these too should be isolated by capacitors that are above reproach, such as 'Y' rated types. The original wax-coated ones fitted would be absolutely bog standard and liable to leak after the years have taken their toll. Anyone not wishing to have any form of capacitive connection for an aerial could make a purely inductive connection quite easily by wrapping a number of turns of wire around the aerial coil, either onto tape insulation or by the use of ready-insulated wire. Extended, this could then safely be used as a throw-out aerial.
Tony