Friday, November 5, 2010

More on HF – SSB - Antennas

Carrying on from my previous blog on engineers antennas HF and SSB, in this second part I will more fully describe our emergency antenna setup.


I draw your attention to a very useful set of resources available free on the internet:

In the previous post I worked out the length of antenna for a given frequency that would provide the best chance of effective communication.  This certainly does not mean for a given frequency if the length is different you can not communicate, it just means that there is more chance at the lengths specified.

So how is it all going to work on the boat.

Situation one – the antenna is broken, but the tuner is intact.

Cut a new antenna from a piece of wire of a length greater than 7m as specified for our ICOM tuner. 

One end of the antenna would need to be lifted towards the sky, if the mast or part of it is still standing the wire could be hoisted on it, otherwise some other pole, like the prod, kite pole or boom could be used. The resulting antenna would have the characteristics of this sloping wire antenna show below:


sloping wire

Sloping Wire – source: US Marine Corps (1991) “Field Antenna Handbook” pg 4-34

Situation two – the antenna is broken, the tuner is broken

This is the situation where  having the antenna length appropriate for the frequency you want to transmit of is important. Normally the tuner, tunes between length suitable for the frequency and the actual antenna length using a series of capacitors and inductors. Without a tuner each time you change frequency, to optimise the chance of being able to communicate the length of the antenna must be changed.

The length of antennas required for international emergency frequency range between 32.6m for 2182 Hz to 11.6m for 6125Hz.

32.6m is greater than distance between the back of Coppelia and the mast head. Therefore the sloping wire arrangement used above can not be used. Moreover, it is likely that if we have broken the tuner, we might have suffered damage to the mast, and therefore we would not even be able to make a 11.6m sloping antenna.

The answer is a different sort of antenna, a vertical half rhombic antenna.


Vertical half rhombic antenna – source: US Marine Corps (1991) “Field Antenna Handbook” pg 4-37

The plan onboard would be to make some sort of pole in the middle of the boat. A shielded cable would then be run from the radio to the back of the boat, and a second wire from the ground plate to the same location. A piece  of wire equal to the length of antenna required for the frequency to be used would then be attached to these wires, and hoisted aloft in the middle, then fastened at the bow. The resulting antenna setup would form a vertical half rhombic antenna.

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