SSB ground plate

[timevans2000]

My keel cooler for the fridge is designed to be a ground plate for the radio. I am only using an SSB receiver not a transmitter. The instructions for the fridge say connect the cooler to the engine negative. If I do this, can I still use the plate as a ground for the SSB receiver?

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[Talbot]

SSB ground plate is used with an Aerial Tuning Unit to match the aerial length for transmissions, so is probably not required anyway for a receiver only fit!

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[steve_cronin]

A lot of sets have the earth tag connected to 12volt -ve anyway so it should be OK. In fact, if the cooler is connected to -ve, your set is probably earthed already.

The reception improves startlingly when you do connect to earth but should your boat have it's master switch in the negative line (like Bavarias) and the "earthing" wire to the cooler is permanently connected on the battery side of the switch (which it shouldn't) then when the switch is of any circuits switched on will try to return down your puney SSB earth wire and, if of high load, burn it out. If any switch in the negative line totally isolates the battery from the cooler then all is well.

Steve Cronin

<hr width=100% size=1>The above is, like any other post here, only a personal opinion

 

[timevans2000]

I have been adviced that for receiving only,the ground plate is more important than the aerial itself?

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[Bergman]

Sorry but this really is not the case

The aerial on a boat is invariably an un-balanced aerial. This requires an earth in order to work, irrespective of any ATU. And the better the earth the better the aerial will work.

As for using an ATU on receive. Its true that a receiver will work without, when the transmitter will not. This is because the transmitter recognises the impedance mismatch and reduces (often to zero) the output power to protect the output amplifier. Where there is a mismatch, power is reflected back to the source and if unprotected will destroy the output transistors.

However, on receive there is still a mismatch and this will reduce the power of an incoming signal being fed to the receiver. Because noise is present across the whole spectrum noise will be passed to the receiver undiminished by the mismatch.

The effet of this is to attenuate the wanted signal and additionally to degrade the signal to noise ratio.

I experimented with this after a previous debate on the subject and on a "typical" boat aerial, 30ft wire sloping upwards, I found an average of 6dB loss of signal strength on signals between 3 and 4 MHz. The S/N ratio was decreased by a greater amount but I could not achieve a reliable measurement - best guess would be 9 or 10 dB.

I would use the fridge earth with some caution. If both set and fridge are connected to the -ve rail then there will be an earth loop. Put simply this means any noise on the -ve rail will appear as a slight difference in voltage between the 2 ends of the earth wire and this will induce noise into the receiver.

Ideally the radio should have one earth connected at one point. Not always easy on a boat or indeed in a house, but a keel bolt or the anode may be a better bet.


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[halcyon]

Is this why we have a 2 foot square earthing plate under water on the hull???. Seamed to earth the mast, and something else that's not there now.


Brian

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[Bergman]

Probably



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