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Old 12-10-2011, 18:23   #1
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Emergency VHF Antenna Question

Lets say my antenna gets broken and all I have left is the coax

Could I make an antenna out of say, the 10 feet of coax cable that leads to the base of the antenna?
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Old 12-10-2011, 18:37   #2
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Don't know for sure but id like to think u could...is a good question ..id like to know if I ever get in a pinch
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Old 12-10-2011, 18:49   #3
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Re: Emergency VHF Antenna Question

The rule for a 1/4 wave antenna is 468/freq. in Mhz/2 = antenna lenght in feet.

468 / 160 = 2.925 / 2 = 1.46', or about 18" of stiff wire should work just fine.

Your radio needs a 50 ohm load, so if you have to make a antenna for your VHF, don't even try to make a gain type antenna (i.e. 10') because for that to work you have to calculate 5/8 wave stacked with 1/4 wave, and it isn't easy to produce that 50 ohm load if you don't know what you are doing. You will only lose 3~5db by going to a 1/4 wave for a homebrew replacement. The easiest way is choose your favorite operating frequency, i.e. Channel 16 (156.8 MHz) cut 5 chunks of coat hanger or other stiff, solid wire, to the above specs, burnish/clean one end of all 5 rods for soldering and then use a SO-239 chassis mount antenna connector for the common PL-259 connector, solder the ground radials to the holes for chassis mounting and bend these four rods 130* to the vertical radiator that you will solder to the center connector. This will be a 50 ohm antenna and your VHF coax will plug right into it. For greater ranger, hoist up with a halyard but don't allow it to get too close to the boom.
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Old 12-10-2011, 18:54   #4
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Re: Emergency VHF Antenna Question

We have an emergency antenna made by signal mate that consists of a coiled length of flat ribbon connected to a PL-259 connector. Just uncoil it and connect for an antenna.

Shakespeare makes a small stow-able emergency antenna.

Another thought is to get a short pigtail that connects an SO-259 to whatever the connection is on a handheld ducky antenna and use one of those.

Best of all, keep a handheld around. They are useful for lots of things.

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Old 12-10-2011, 18:54   #5
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Re: Emergency VHF Antenna Question

18" is just what the signal mate emergency antenna we have is.

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Old 12-10-2011, 18:59   #6
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Re: Emergency VHF Antenna Question

So, what would be the procedure for stripping the wire off the coax cable....wouldn't the sheath prevent transmission?
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Old 12-10-2011, 19:15   #7
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Re: Emergency VHF Antenna Question

If it is the coax you want to use AS the antenna, don't bother trying to strip the outer braid. Not necessary. Just short the outer and inner conductors together, secure it as best you can, preferably with a PL259, into the antenna connection CENTER pin on the radio antenna output and cut to resonant length. The outer insulation of the coax will prevent shorts or RF burns. Alternative is a metal coat hanger cut to length and easily bent to insert into the ant center pin connector.
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Old 12-10-2011, 19:36   #8
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Re: Emergency VHF Antenna Question

Just remember 468 / freq in Mhz / 2. Should you ever burn out your antenna tuner for either an HF vertical or your backstay. Lets say you need to operate 14.3 Mhz, then 468 divided by 14.3 equals 32' 8 1/2". If you don't have antenna dipole wire, coax will work if you follow S/V Illusion's advice. Cut two at these dimensions and use an egg insulator or anything you can fabricate, to tied physically but not electrically together, connect center to one of the 32' leads, the braid to the other, and put a egg insulator at the end of each lead to connect any small line you have, hoist with halyard up and tie off the lines to try and create a 90* angle to the two down sloping leads. If you have never used a resonate inverted Vee antenna on 20 meters, you will be in for a pleasant surprise as this antenna will perform much better than a non-resonant backstay being fed by a tuner that its only purpose is to fool the radio into thinking it is feeding a resonant antenna at a proper 50 ohm load.
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Old 12-10-2011, 19:37   #9
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Re: Emergency VHF Antenna Question

Heck, in past years I have used a clip lead hanging on a door for an antenna. And it worked. Sure, maybe there was a standing wave caused by mismatch creating a high VSWR but who cares if the antenna works? Nothing in life is perfect anyway.

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Old 12-10-2011, 19:46   #10
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Re: Emergency VHF Antenna Question

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Originally Posted by Chief Engineer View Post
Lets say my antenna gets broken and all I have left is the coax
Could I make an antenna out of say, the 10 feet of coax cable that leads to the base of the antenna?
Probably but why? K.I.S.S. - I keep one of these in the tool room with my spare parts. If the main antenna gets knocked off I undo the antenna lead to the vhf, attach the portable emergency antenna cable and stick the antenna outside.
Stowaway Portable VHF Antenna
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Old 12-10-2011, 20:42   #11
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Re: Emergency VHF Antenna Question

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Heck, in past years I have used a clip lead hanging on a door for an antenna. And it worked. Sure, maybe there was a standing wave caused by mismatch creating a high VSWR but who cares if the antenna works? Nothing in life is perfect anyway.

Foggy
If you tried that on an old Atlas 210, the 1st solid state HF transceiver offered back in the late 70's, you would have fried the finals and then you would be without HF. Modern radios will back down the RF output when it sees a lousy match. So not only are you losing signal by going into a non-resonant antenna, but your transmitter might feed 1/2 watt into your wet noodle of an antenna after backing down. You might be inclined to KISS for most all other things, but it isn't too effective for resonant antennas. Every anchorage I've ever been in, I was the one with the strongest HF signal. Always
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Old 13-10-2011, 02:37   #12
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Re: Emergency VHF Antenna Question

Quote:
Originally Posted by Chief Engineer View Post
Lets say my antenna gets broken and all I have left is the coax

Could I make an antenna out of say, the 10 feet of coax cable that leads to the base of the antenna?
Quote:
Originally Posted by Chief Engineer View Post
So, what would be the procedure for stripping the wire off the coax cable....wouldn't the sheath prevent transmission?
Well Chief, you now have a lot of answers to choose from and a bit of extraneous RF theory thrown in to boot.

Time to answer the question you actually asked .

Take the end of your coax (that use to go to the antenna), strip back and discard 18.75 inches of the outer insulation leaving the outer braid in place for a moment. Now extract the centre conductor (and its insulation / dielectric) from inside the braid. Hold the centre conductor vertical as high as you can and hold the braid horizontal. This will give you a 90 degree angle between the two conductors. You are finished and ready to transmit.

To improve on this, take some time to separate the outer braid into 2 (or even better 3 or 4) pieces and arrange them all horizontally equally spaced apart in a circle. The centre conductor will still be vertical and is the radiating element with the braid strips forming the ground plane.

Couldn't be easier!
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Old 13-10-2011, 06:50   #13
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Re: Emergency VHF Antenna Question

Thank you all for your advice...I am sure there are others who are going to "tuck this one away"
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Old 13-10-2011, 07:06   #14
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Re: Emergency VHF Antenna Question

If you break your antenna...I'm assuming you mean the fiberglass shell...the antenna still works fine...just duct tape or reglass or hoist the tip or strap a non-metallic splint on it.

If you have one with the full copper element (more expensive ones) versus the cheaper (just coax to the tip)...then just make sure the element isn't broken/bent too bad and follow above methods....as an emergency.

I've re-glassed tubes and they have worked fine for many years...but when resonable...replace and upgrade to the better antennas as your VHF can be a lifeline.
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Old 13-10-2011, 07:54   #15
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Re: Emergency VHF Antenna Question

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If you tried that on an old Atlas 210, the 1st solid state HF transceiver offered back in the late 70's, you would have fried the finals and then you would be without HF. Modern radios will back down the RF output when it sees a lousy match. So not only are you losing signal by going into a non-resonant antenna, but your transmitter might feed 1/2 watt into your wet noodle of an antenna after backing down. You might be inclined to KISS for most all other things, but it isn't too effective for resonant antennas. Every anchorage I've ever been in, I was the one with the strongest HF signal. Always

Been doing this stuff for a long time but never heard of the old Atlas 210. The transceiver I had on the boat at that time if I remember correctly was a Texas Instrument, time frame was around 1979-1980, it was one of the first that offered multichannels rather than individual crystals. I do agree that the semiconductors back in the 70's were highly susceptible to failures. Many of them were germanium, very fragile and very low voltage. I sometimes wonder how they even worked because they were so sensitive to temperature.

But getting back to using a clip lead. That presents no worse load (VSWR) to a transmitter than working into a shorted transmission line which is a common occurrence on boats. And the reason I resorted to the clip lead is because The mast had just been installed and the riggers shorted out the coax somehow. I got through the season by later installing an antenna on one of the railings until the coax in the mast could be replaced.

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