ARLA/CLUSTER: Antenas de Emergência

[João Costa > CT1FBF]

More on HF – SSB - Antennas
 Carrying on from my previous blog on engineers antennas HF and
SSB<http://www.expeditioncoppelia.com/2010/10/engineers-antennas-hf-ssb.html>,
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:
Field Antenna Handbook - US Marine
Corps<http://frrl.files.wordpress.com/2009/02/usmc-antenna-hb.pdf>

Radio Operators Handbook - US Marine
Corps<http://hilltoparmyradios.com/usmc-radio-ops-hdbk.pdf>

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:

[image: sloping
wire]<http://lh6.ggpht.com/_rK1oaS0yOa8/TNNujQUNbNI/AAAAAAAAFTc/kuMS4ZHLEUg/s1600-h/sloping%20wire%5B2%5D.png>
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.
[image: Slide2]<http://lh6.ggpht.com/_rK1oaS0yOa8/TNNumH0uAaI/AAAAAAAAFTk/r08mwsGQ8lI/s1600-h/Slide2%5B1%5D.png>
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.

  *Emergency Antennas for VHF and UHF Use*

    Get a piece of RG-58 coax and using the chart below, strip the outer
jacket of the coax to that appropriate length, depending on the band you
choose to work.  Once the outer covering has been stripped, carefully open
the copper braid near the bottom of where you stripped the outer jacket.
Once the braid is open, carefully bend the center conductor over and pull
it out of the hole you opened in the copper braid.  Once you have the
center conductor pulled free, straighten it and either form a loop at the
top end for hanging, or solder on a terminal lug.  Remember, the antenna
length is measured from the very end of the look or terminal lug, and they
do add length to your antenna.  Trim as necessary.



 *MHz*
 *Length in Inches*
 *Length in Metric*
 52
 54"
 137.2 cm
 146
 19.2
 48.8 cm
 223
 12.5"
 31.8 cm
 445
 6.3"
 16cm



Fontes:  Expedition Coppelia e WCERC *VHF UHF Emergency Antennas*
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