How do you make a temporary antenna?

Robin2

Robin2

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Perhaps someone on the forum knows the black art of antenna construction?

I want to make a cheap and effective antenna that is able to receive VHF (specifically AIS transmissions) so that I can try out my AIS engine in a different location without removing the antenna from the boat.

From what I have read on the web, all I need is some electric flex to make up a have wave dipole, which, presumably, should be connected at the centre to the core and screen of the coaxial cable leading to the receiver.

Does any know if this is correct, how well it should work, or whether there is a significantly better approach?

Would it also work as a temporary transmitting antenna (say, if one was dismasted)?
 
This pic was once posted here

emergencyantenna.jpg
 
My AIS works pretty well at home plugged into the house entertainment VHF distribution system, notwithstanding the fact that the antenna is not tuned for the right frequency and is pointed inland. System does have a broadband amplifier and is high up though.

Can pick up ships about 15M away from center of Ipswich with that arrangement.

<edit> would be a bad idea to try to transmit with this arrangement!</edit>
 
Well, perceived wisdom says that for the 156Mhz Marine band, a 5/8 Wavelength antenna will give a good gain.

Therefore, a non grounded 300/156 x 0.625 = 1.2metres.

Make it as described above from your coax, but just hoist it vertically on a halyard perhaps.
 
from my own experience, the AIS receiver will work fine from an aerial on the pushpit, however, range will be dependent on the aerial. (mine was actually fitted above the davit/solar panel gantry)

I started by using a vtronix short helical aerial (about 12 inches long) Ranges were more than a little disappointing averaging about 7 miles.

I then fitted a jgtech 1.1m whip - ranges more than doubled, and by fitting the correct plugs, I was able to swap aerials between VHF and AIS dependent on which I wanted to have the maximumm range.
 
That's a good point, but unfortunately a 5/8 wavelength needs a matching coil to look like 50 ohms impedence.

The AIS receiver won't worry too much, but don't be tempted to make a homemade antenna 5/8 wavelength long and transmit with it without some matching device at the base.
 
I have a 'Slimjim', basically like this
I made it from 8mm copper pipe clipped to a wood batten.

It is attached to an old Icom hand held in my kitchen: in high pressure conditions I hear Ushant radio. Don't know the transmission range.
 
Any chance of a photo, please ? Did you bend the tube at the corners, or use joints? (I have half a roll of 8mm copper - seemed a shame to throw it away!)
 
I just bent the pipe--crippled it into 90 deg. I have two slimjims, the other is made with heavy earth wire clipped to the batten, this is on a Belcom scanner. Car brake pipe is good, fencing wire--anything, it seems. One is stuffed into a piece of 1.5in sink waste, the other is in the open and has been for ten years.
The important thing is to get the overall dimensions right for the frequencies you want to use. I know the Icom is good for 5 miles, that is the maximum transmission I have attempted.
The local radio ham who gave me the info can receive Amersham (?300 miles) and transmit to Plymouth.
 
I just measured one. As far as I can tell, you start with an end, go up 920mm, right 40mm, down 1400mm, left 40mm, up 450mm,. this should give you a separation of 30mm between the ends.The core is soldered to the right, unbroken side, the screen to the left, broken side, both 90mm up from the base. I encased the soldered joints in silicone. Hope this makes sense. This centres on the marine VHF band.(?156 mega watsits?) No doubt someone will come up with the correct dimensions.
 
I've made a few of these : "J" antennas. a bit of stiffish wire ( I had a drum of 7core house wire for plugs etc.)
measure (from what will be the top) 1420mm, turn left 8mm,turn left again and go up 355mm. join the coax 35 to 40mm up the "U bend" from the bottom. doesn't seem to matter whether braid goes to long or short leg. If transmitting, set SWR by varying the 8mm gap till good then lay both sides of "U" on gaffer tape and fold tape over to hold position.This is my emergency aerial and can be rolled up and shoved in a locker. Would send piccy but no batteries in camera! You don't need to be technical if all you need is receive. A telescopic car aerial should do.
 
Just make a dipole from coax.. the inner becomes the element, the braid outer becomes the conterposie.
formula for length of each piece in metres is (300 / frequency in Mhz)/(4*.95)
the div 4 gives a quarter wave element length, the *.95 converts from free space to 'wire'.
you can use it as an emergency transmitter antenna too.
Joe.
 
Thanks for all the suggestions - lots of useful information. I think I will make one of the slim jims over the next few days.

I want to have an antenna with which to test my NASA ais engine with the assurance that, if it does not work, its not the antenna's fault. This could also be a useful check on whether a fault has developed in the antenna on my boat - such as rain-water getting in somewhere.
 
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