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Old 24-10-2005, 01:57   #1
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Emergency Antennae:

Copied from the Wheel-Tiller Thread:

Posted by: roverhi

... Off topic, but the radio is one of my concerns. What do you do if the mast comes down. Almost everyone has their antenna at the masthead or a stay. No mast, no antenna, no radio. Know that for a HF radio, you can use a tuned whip antenna. From experience HF radios are really finicky about tuning the antenna to the radio and proper ground planes. Without the proper match you simply don't get out. I understand Vhf radios aren't so finicky but without the mast height, the signal is probably greatly reduced. Next cruise, I'll test out the back up system for real just to be sure it works and I know how to make it work.

Peter O.

posted by: Gord May

A little frequency drift, but oh well, the Administrator isn't looking ...

I had a rail (pushpit) mounted 30" VHF antenna cabled back to my main radio location, coupled to another cable back to the cockpit (usually for my handheald).
Lose mast (& prime antenna), uncouple cockpit cable, and connect rail antenna to main radio (or not).
Also had an “emergency” stubby antenna, that’s supposed to connect directly to the radio - didn’t throw it away, but couldn’t see much utility in it.

Peter O. Is absolutely right about VHF range - it’s entirely antenna height dependant.
D = 1.22 x Root ‘Hft' -or- D = 2.21 x Root ‘Hm’
where: ‘D’ is distance, ‘Hft’ is antenna height in feet, and ‘Hm’ is in meters
To calculate radio range, we add the two (D1 transmit & D2 receive) antennae ranges together, thus:
Range = D1 + D2
See ‘VHF RADIO RANGE’, under Instruments & Communications.

Rigging an emergency HF antenna will require a little more ingenuity (I never had the luxury of contemplating that problem )

posted by: Kai Nui

Nice thing about HF, is it can be very forgiving. One year on Field Day a few of us got together and wanted to work 160meters. The contest guys were hammering away on all the good bands, and 10 was dead, so we dragged out some wire, paced off 80 meters, and cut the wire in half, soldered on the coax, and worked 7 states with the dipole on the ground. Bottom line is, if you need to, you can improvise. Something sailors are especially good at.
I have heard of all sorts of emergency rudder designs, from cabinet doors on spinaker poles, to sweeps through an oarlock. The back up for most hydraulic systems is a tiller. The back up for most wheel systems is a tiller, and all emergency rudder systems commercialy sold use a tiller. Might not mean anything, but just like keeping a hand crank for your engine, when the complex system fails, the simple system will work.


posted by: Rick

Should probably start a new thread

On the Vhf and Hf emergency antenna topic:
Roverhi said: ..."HF radios are really finicky about tuning the antenna to the radio and proper ground planes. Without the proper match you simply don't get out. I understand Vhf radios aren't so finicky..." Actually both types are equally "finicky" in that both HF as well as VHF amplifiers have built-in SWR detectors which automatically cut down on transmit power as the antenna/transmission line combination increases SWR. Both will operate into an open or a short, usually without damage due to the automatic protection.

Obviously an emergency VHF antenna is easier to stow than is an HF one. There are solutions to both. The reason that it seems that HF is more finicky is because of the sheer dimensions required in order to achieve a reasonable amount of effective radiated power, even with 150 Watts available at the transmitter (which is NOT available without a good antenna match). Even using a 16 MHz band on commercial SSB one "needs" to have dimensions approaching 20 feet in order to "get out" a reasonable amount of effective radiated power...where do you get 20 feet on a 30 foot boat without a mast? Of course using the lower bands the problem is even more difficult.
Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"

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Old 24-10-2005, 18:50   #2
Kai Nui

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OK, Here's another practical emergency HF antenna. Use a MOB pole, or a piece of PVC tubing, or if you have none of these, a fishing pole will work. A spool of insulated 22 ga wire, and an SWR meter. it will take a bit of fiddling unless you read up on antenna design, but you can build a nice emergency 1/4 wave verticle this way. For an emergency, if you are in good enough shape to have power for the radio, and not have it shorted out, you can probably build an antenna.
I am still a big proponent of the Outbacker antennas. 5' tall whip antenna, all bands, can be mounted on the taff rail. HF bands work quite well with the antenna at deck level. A good ground is very important, but the difference between a 1.5:1 swr and a 2.5:1 swr is not as bad as you might think. Especially considering how little difference there is between the ability to communicate with 50 watts vs 100 watts.
As for the length issue, radio waves only NEED electrical length. Physical length will provide more efficiency, but electrical length will provide functionality. A 3' long piece of PVC wrapped with wire can work fine for an emergency call for help.
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