Emergency VHF Antenna Question

[svHyLyte]

Originally Posted by Chief Engineer

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?

Chief,

I made a very effective emergency antenna that can be mounted on the rail and connected to the radio with a pre-measured length of coax using a Metz Manta-6 Whip Antenna and a removable Fast Rail Mount Bracket against the possibility that we might loose the masthead whip or, worse, the mast. The whole business was less than $75.00 and the Metz has a much longer reach than the "Emergency" antennas, particularly near sea-level. It takes about 5 minutes or less to mount.

FWIW...

[donradcliffe]

Years ago I was on a boat which lost its mast, and used one of the emergency antennas which consisted of a piece of steel measuring tape hooked to a PL259 connector. I had clear communications with the coast guard from 35 miles offshore.

[deckofficer]

Wotname has the best and easiest method for the VHF antenna. Foggysail, I switched gears and was talking about HF. The Atlas 210 was the 1st solid state final of HF radios with 100 watt output. It did have a VFO for frequency selection. My first transmitter was a Knight Kit T-60, a tube radio, 60 watt output, CW and AM only, and back then cyrstal controlled. But as a novice HAM in 1963, that was all the regs allowed. There just has to be someone that remembers the Atlas 210 SSB HF from the mid 70's on this forum. It WAS the hot ticket for cruising back in the day, ran on 12 volts.

[psneeld]

Originally Posted by deckofficer

Wotname has the best and easiest method for the VHF antenna. Foggysail, I switched gears and was talking about HF. The Atlas 210 was the 1st solid state final of HF radios with 100 watt output. It did have a VFO for frequency selection. My first transmitter was a Knight Kit T-60, a tube radio, 60 watt output, CW and AM only, and back then cyrstal controlled. But as a novice HAM in 1963, that was all the regs allowed. There just has to be someone that remembers the Atlas 210 SSB HF from the mid 70's on this forum. It WAS the hot ticket for cruising back in the day, ran on 12 volts.

Why go to all that trouble when the true antenna is intact and all that is broken (which is 99 percent of the cases) is the glass shell that holds it upright?

Unless there was a break in the coax inside the "antenna" it's still fully intact as if from the factory.

[deckofficer]

Originally Posted by psneeld

Why go to all that trouble when the true antenna is intact and all that is broken (which is 99 percent of the cases) is the glass shell that holds it upright?

Unless there was a break in the coax inside the "antenna" it's still fully intact as if from the factory.

I thought the question was for a broken VHF antenna? If the antenna is fiberglass and is longer than 18", then it is a gain type antenna. The only way for a omni antenna to have gain is for its design to pull unwanted skyward RF closer to the horizon. To do this requires vertical phasing of multiple wavelengths (or fractions of). Depending on where and how the antenna was broken, keeping good SWRs will be a problem, so all the good folks here have done (and wotname has the best method), was to instruct how to build an antenna. If all anyone takes from this is to store "upstairs" for future reference is 468 divided by the operating freq in Mhz divided by 2 for a 1/4 wave antenna.

[SoonerSailor]

Even easier. Use Wotname's instructions but run the coax braid straight down - 180 degrees from the center conductor. Ideally the intact coax will form the base of a horizontal "T" and come away at 90 degrees. That will give you a vertical dipole and pretty closely matched to the 50 ohm impedance of the radio. Vertically polarized and omnidirectional just like it should be. The length of each leg should be 17.9 inches long for resonance on Ch 16.

[sailorchic34]

I remember seeing a photo / writeup many years ago about using a piece of coax, where the shield was carefully removed whole from around the feed.. Not cut but shield was allowed to hang down and became the opposite side of a vertical dipole. No counterpose required.

That would be my way to do it should the mast / antenna ended up separated from the boat. Well if my vhf and tri-band handhelds were not working either, that is. ;-)

[Wotname]

Originally Posted by SoonerSailor

Even easier. Use Wotname's instructions but run the coax braid straight down - 180 degrees from the center conductor. Ideally the intact coax will form the base of a horizontal "T" and come away at 90 degrees. That will give you a vertical dipole and pretty closely matched to the 50 ohm impedance of the radio. Vertically polarized and omnidirectional just like it should be. The length of each leg should be 17.9 inches long for resonance on Ch 16.

Doh! Now why did I forget the dipole, brain must have rusted on to a 1/4 wave somehow. Thanks for the reminder SS.

[deckofficer]

sailorchic34 brings up another good point that some knowledge tucked away upstairs could come in handy. If ever left to just a handheld in a emergency, and it is bitterly cold, your low on battery juice, and you see a ship on the horizon but each time you attempt to transmit the handheld shuts off due to the low battery, what do you do? Remove the battery and place in your arm pit for about 5 minutes, then re-install the warmed battery and get your call out. This simple knowledge could have saved countless hikers caught on mountains during bad storms with their low battery cell phone.

[psneeld]

Most VHF antennas and unless the major manufacturers changed in the last few years...all they are is a fiberglass shell with coax glued to the tip. The coax IS the antenna and NO CHANGE is necessary if you fix the fiberglass shell or break it off and hoist the coax.

I fixed or replaced hundreds in my time with a marine electronics firm.

[deckofficer]

On other non-cruising forums I would let slide certain misconceptions, but here we need correct information. Below is the most typical gain type VHF vertical, and by no means is it just coax glued to the tip. It doesn't take an FCC class 1 license to know or understand this, but it helps. If you passed the HAM General and knew the material, then this should come easy. As noted in the specs below, you are dealing with phased 5/8 wave elements to get the gain over a 1/4 wave. Some manufacturers will construct these co-linear 5/8 wave out of coax, using the outer shield for a given length and energizing the inner for a distance using caps, and others will use windings in the glass lay up. If this is what you meant by coax glued to the tip, then your right, but exposed to weather, the alternating length of coax with its precise L/C will be all over the map as salt moisture intrudes and VSWR become un-acceptable.

Length: 8ft
Type: Collinear 5/8 wave elements.
Gain: 6dB
DC Continuity: Yes
Bandwidth: 3Mhz within 2.0:1 VSWR

This design of a vertical achieves its omni gain by reducing skyward and downward RF, concentrating it at the horizon. This works great if you are not heavily heeled over.

[psneeld]

Originally Posted by deckofficer

On other non-cruising forums I would let slide certain misconceptions, but here we need correct information. Below is the most typical gain type VHF vertical, and by no means is it just coax glued to the tip. It doesn't take an FCC class 1 license to know or understand this, but it helps. If you passed the HAM General and knew the material, then this should come easy. As noted in the specs below, you are dealing with phased 5/8 wave elements to get the gain over a 1/4 wave. Some manufacturers will construct these co-linear 5/8 wave out of coax, using the outer shield for a given length and energizing the inner for a distance using caps, and others will use windings in the glass lay up. If this is what you meant by coax glued to the tip, then your right, but exposed to weather, the alternating length of coax with its precise L/C will be all over the map as salt moisture intrudes and VSWR become un-acceptable.

Length: 8ft
Type: Collinear 5/8 wave elements.
Gain: 6dB
DC Continuity: Yes
Bandwidth: 3Mhz within 2.0:1 VSWR

This design of a vertical achieves its omni gain by reducing skyward and downward RF, concentrating it at the horizon. This works great if you are not heavily heeled over.

Quote all the specs you want..but go to a marina or marine electronics store to see what's inside one...there's never been any wire in the glass that I have found.

The cheapos have wire...maybe not exactly the same coax but just a wire just the same...the more expensive have a copper element as I posted before.

But I don't need a HAM license to know what's in one and how to repair one...done it many times and they work as good as new if all that's broken is the glass cause it's got nothing to do with the wire inside other than holding it vertical.

[deckofficer]

No need for an argument as long as others take away the fact that if no damage was done to the coax stubs and caps inside, and only the fiberglass shell was compromised, then repairing the fiberglass for both structural and environmental integrity will work. Fair enough?

[psneeld]

Originally Posted by deckofficer

No need for an argument as long as others take away the fact that if no damage was done to the coax stubs and caps inside, and only the fiberglass shell was compromised, then repairing the fiberglass for both structural and environmental integrity will work. Fair enough?

You are right...no need to argue because that's EXACTLY what I said all along....

[hellosailor]

Bosun, I keep forgetting the name but there is in fact a specific technical name for an antenna made that way, and I have one made up fro a coil of coax just for that purpose, with an eye on the end so it can be tied aloft to whatever there is to hang it from vertical.

Marine VHF is pretty much the same as two-meter ham radio, which has a quarter-wave length of about 19" in copper wire. (The actual wavelength-versus-frequency has to be corrected for copper wire as opposed to what the wavelength is in a vaccum or air, if you want to get picky. You can find the specifics on the web if you want to run the calculations.)

So you peel back about 21" of the insulation, and then start to work the coax shield down and back over itself until there is 19" of the center conductor (leave the insulation on it) sticking out. Smooth or stretch the coax until and equal 19" of it has been pulled back over itself. Use some tape or string to secure that outer jacket, and put an eye or tie something to the top, so you can literally raise the antenna.

You've actually got a quarter-wave dipole antenna which will perform as well as any "quarter wave" commercial antenna. Getting it aloft and keeping it away from any close metal contact is all you need to worry about now, unless the coax you've used has been internally damaged, i.e. torn by whatever pulled your regular VHF down. It won't take long to make and the 19" isn't super-critical, but it costs so little to buy 25' of decent coax, prep it and stow it, that you can see why it might pay to just make an emergency antenna that way, and stow it. Or cut one from the bottom end of the old coax, next time you're replacing your antenna line.