Installing SSB Antenna

[bbg]

I am completely ignorant of radio stuff. I have bought an ICOM IC-R20 receiver so I can (with luck) get forecasts when I'm offshore.

However, I am given to understand that radios work better with something called an "antenna". It came with a 1m whip but I am told that might not work very well mid-Atlantic.

My electrician has told me that all I need to do to get great reception is to attach a crocodile clip to the shroud fitting (below deck) and then my whole rig will become a wonderful antenna. Reading a little on the net tells me that a typical installation involves using the backstay as an antenna, but it must be insulated top and bottom, so I am not so sure about my electrician's advice. [Note - using a backstay is not an option because I don't have a permanent backstay.]

Another piece of advice was to take a piece of co-axial cable, withdraw the core for most of the length, then run the core up a shroud to the masthead, and run the outer part as far aft as it will go. Withdrawing the core from a 3-4 meter length of co-ax seems difficult verging on impossible. The co-ax would have to be intact for 2-3 feet (with an appropriate plug on the end) then split with the outer part going 3 meters aft.

The other installation I have seen seems to be simply an insulated wire run up the shroud.

Thoughts?

 

[Alpha22]

To transmit a radio antenna must be tuned to the frequency of the transmission in question. This means the length is critical.

To receive it does not matter as much. A long length of conductor is all that is required.
Shroud, back stay, mast, cable, anything you like, just think about what would happen in the event of a lightening strike?

A good earth (The sea) is useful with SSB reception.

Basic recommendations, but I'm sure a real expert will be along shortly.

 

[Neil_Y]

I used a simple insulated wire up the backstay, sheilded in coax from the radio to the exit from the back of the boat.

No earth

Biggest issue with ssb reception is interference from other electronics/screens/lights etc. I could get good reception with the wire and Nasa HF3 on the whole atlantic circuit. If you are testing it whilst sat in a marina reception will probably be rubbish, it got better as I got further offshore.

 

[KellysEye]

As you've found out don't use a crocodile clip on a stay, or a short whip aerial, you need a proper aerial. Bear in mind that the radiation in the aerial is powerful enough to light a cigarette and cause a serious burn if touched when transmitting. Aerial length is important but more important is the ATU - automatic tuning unit. This tricks the aerial into thinking it is the required length for every frequency, it is a must fit. Insulators must be used on a stay otherwise the whole rig risks being live. Since you don't have a fixed backstay best to run an insulated copper wire up the mast, the optimum length is around 33 feet. The ATU must be as close as possible to the start of the aerial wire.

Equally importanant is the ground, it makes up 50% of the aerial. The best ground is a metal boat which gives an idea of how big the ground should be for the very best transmission/reception. However a big ground is not possible on a non-metal boat so you need to wire together as much metal as you can inside the boat and then attach it to the radio's earth.

A ground doesn't matter if you are only receiving but if you are going ocean sailing then SSB is the lifeblood of cruising and will use both transmit and receive.

Also I strongly recommend you tell your electrician what I have said above, otherwise somebody could get seriously hurt. Also I'm intrigued to know what your boat is, I've never seen a long distance boat without a fixed backstay (or two) and you mention the mid Atlantic.

 

[bbg]

KellysEye - my radio is reception only. Rules of the race say that we are not allowed to have SSB transmitters (or any other communications beyond VHF).

Given that it is reception only - it has no transmit function, it is only a scanner:

1. I see that a ground doesn't matter, correct?
2. Is the length of the wire crucial? If so, is it measured from the radio or from where it exits the deck and runs up the shrouds? (I'll have to drill a hole in the deck for the wire.)
3. Is an ATU necessary? I don't think anyone else in the class has one, to my knowledge.
4. Should I leave the sheathing on the coax up to the point it goes through the deck?

I will mention the crocodile clips (no-no) to my electrician. He is an absolutely amazing electrician, but maybe doesn't know about the power of an SSB transmission.

If you want to see my boat, click on the link in my signature - there are some pictures. We sail with high-roach / square headed mainsails, so can't have fixed backstays. Take a look at the comparison with a J-24 (in the Gallery) and you'll see what I mean.

 

[MM5AHO]

You have said that you're receiving only..,. so

the crocodile clip is OK
The shrouds or stays could be a good receiving antenna, provided they are not in electrical contact to the sea
An insulated wire hoisted as high as you can get one end, other end to receiver could be good
the coax idea sounds so fiddly as to be not worth the effort over the above means of doing it.
if you have wire railings from pulpit to pushpit and provided thay are insulated from the sea, they could be good, even connected by a crocodile clip.
your mast might work
as long a length of metal as high as possible, but disconnected (electrically) from the sea will do.
Its not critical in length.
You don't need an ATU. In theory it might be better to have one, but you'd struggle to see any difference.
There's no radiation from a receiving only antenna, It won't light a cigarette, a light bulb, or even a gas burner!
Short lengths (like your whip) won't work very well. Length matters.

 

[ProDave]

The OP is only receiving.

So a croc clip onto a shroud or the backstay will be fine.

It's a completely different game if you want to transmit as well, but that was not the question.

For reception, basically, the more metal you have in the air, the better.

 

[Barry Jones]

Normally I would get a rigger to put an insulated stay in for a semi-duplex SSB Transceiver.
Tis the proper way to do it.

If you just want receive, a bodge way to do it, is to tie wrap a piece of insulated copper wire up the stay (6-7m if possible), connecting the aerial to the input pin of the BNC connector on the R20.
Connect the outer of the BNC to the boat earth.

A neater way to do it is to fit one of these.

http://vtronix-antennas.com/antennashow.asp?cat=active&show=aa20

 

[ProDave]

A neater way to do it is to fit one of these.

http://vtronix-antennas.com/antennashow.asp?cat=active&show=aa20

Would you trust a product that says in it's spec that it's for 50KHz to 30 MHz, but describes itself as a VHF antenna ?

 

[Barry Jones]

Didn't see the misprint, but have used these before with good results for weatherfax.

ETA - you will need the AAX1 power supply as well...

 

[bbg]

OK thanks all. Looks like a crocodile clip might be the way to go. Ideally I'll fit it to the shroud fitting that projects into the boat (below the deck). As a backup I'll prepare a long length of cable that I can run up a shroud if I need to, but I'd prefer not to drill a hole in the deck.

I think the whole shroud / forestay / mast system is electrically isolated from the sea. I don't see how it could be connected as there is no grounding from mast to keel.

Now I need to source a couple of BNC connectors for 50 ohm. Can't find them in the local equivalent of B&Q.

BarryJones - no way am I fitting an extra antenna if I can avoid it. Extra weight = slower. If I can get away with 2m of cable and a crocodile clip, that's what I'll do. My electrician has just managed to remove 2 kg of cables from the boat. I don't want to put that weight back on board!

 

[NorthUp]

Would you trust a product that says in it's spec that it's for 50KHz to 30 MHz, but describes itself as a VHF antenna ?

from the quoted website-
"Frequency Range
50 KHz - 30 MHz;
88-108 MHz with AAX

V.S.W.R.
Receive only"


Broadcast VHF

 

[Barry Jones]

So your rigging (and presumably mast) is isolated from radio earth and your electrician has suggested you croc-clip to it below the deck.

How do you prevent lightning damage to the boat ?

OK thanks all. Looks like a crocodile clip might be the way to go. Ideally I'll fit it to the shroud fitting that projects into the boat (below the deck). As a backup I'll prepare a long length of cable that I can run up a shroud if I need to, but I'd prefer not to drill a hole in the deck.

I think the whole shroud / forestay / mast system is electrically isolated from the sea. I don't see how it could be connected as there is no grounding from mast to keel.

Now I need to source a couple of BNC connectors for 50 ohm. Can't find them in the local equivalent of B&Q.

BarryJones - no way am I fitting an extra antenna if I can avoid it. Extra weight = slower. If I can get away with 2m of cable and a crocodile clip, that's what I'll do. My electrician has just managed to remove 2 kg of cables from the boat. I don't want to put that weight back on board!

 

[Bilgediver]

I am completely ignorant of radio stuff. I have bought an ICOM IC-R20 receiver so I can (with luck) get forecasts when I'm offshore.

However, I am given to understand that radios work better with something called an "antenna". It came with a 1m whip but I am told that might not work very well mid-Atlantic.

Thoughts?

If the Icom R20 only has the whip and no external antenna socket then you can probably improve matters just by getting a long length of wire and stringing it up the mast and connecting with a crocodile clip as your friend suggests. Mind you many Japanese SSB pocket receivers seem to cope very well with the telescopic antenna.

The thing that may be a problem is the fine tuning. This radio might be designed for audio and so be difficult to tune to the requirements sometime needed for data.

I suggest before you head into the blue yonder you download JV Comm
fax program and connect the radios ear socket to you puter line in or mic socket and see what things are like before spending time on antenna systems.
You will find the software here and it is pretty easy to use

http://www.jvcomm.de/index_e.html

There are a few here who can answer questions if you get squiffy unclear faxes. The program runs fine in demo mode and will autosave faxes received.

Try 4608 8038 or 110883 on the dial. You will need to tune 1500 Kcs below the advertised frequencies.

 

[Barry Jones]

Actually it does in that you can remove the metal whip aerial as it is connected to the BNC connector on the top. You them just connect your wire and soldered BNC into the exposed socket.

I can't see the supplied whip picking much up inside the hull with all the other electrical noise flying around.

If the Icom R20 only has the whip and no external antenna socket

 

[bbg]

Bilge diver - weather fax not allowed. The only incoming communication I am allowed is VHF voice and SSB voice. No satellite, no weatherfax, no Navtex, no computers. VHF and SSB voice receiver only.

How to avoid lightning damage - ummm - not get hit by lightning? On a more serious note, I don't know what I can do to avoid lightning damage. If I'm in a lightning storm I hope I'll remember to disconnect the radio from the rigging, but if I do get hit, receiving weather forecasts is probably going to fall pretty far down the priority list. Regardless of whether or not the mast is grounded to the keel. I suppose I could wrap the anchor chain (which always lives in the bilge) around an internal shroud extension and one of the keel bolts.

 

[bbg]

Actually it does in that you can remove the metal whip aerial as it is connected to the BNC connector on the top. You them just connect your wire and soldered BNC into the exposed socket.

And where can I get one or two of these famous BNC connectors (male, I presume)?

 

[lenseman]

And where can I get one or two of these famous BNC connectors (male, I presume)?

RS Spares or Maplins for starters.

 

[bbg]

RS Spares or Maplins for starters.

Cheers, although I won't be in UK any time soon. I'll have to look locally or order online.

 

[William_H]

Receiving antenna

Fortunately most receivers these days are quite sensitive so reception is limited to the noise from atmospherics. The receiver will make some internal noise but this is usually quite low in level compared to that coming in the antenna. Obviously you can't receive a signal if it is less strength than the back ground noise. So no matter how long or good your antenna is providing the noise from atmospherics is much stronger than internal noise you can't do any better. Compare the background noise with no antenna to that with antenna connected.
This assumes that there is nothing on board that is generating noise. If there is then you need to have an antenna that receives from a point as far away from the nose source as possible. You then need to feed the antenna with a screened cable to stop pick up near the source. ie inside the boat. Hence a car radio always has an external antenna to pick up from outside the body while hopefully the screened cable and the metal of the car stop the engine noise source getting into the antenna. This of course does not work for a fibreglass boat.
So for a minitransatlantic sailor just start with the supplied whip antenna. Then try a piece of wire stretched around the cabin. Then compare with a wire stretched up the mast.

Now much depends on the impedance of the input to the radio. This might be reflected in the instructions on antennae. If it is designed for the whip antenna the input impedance will be very high. Impedance here means a bit like resistance in that the signal on the wire coming in has a high voltage with very low current. (high voltage here being still microvolts) a low impedance input means relatively high current compared to voltage. To get the best power transfer you need to have a similar impedance of the antenna and cable to the input circuit of the radio. The antenna will have a characteristic impedance itself. So an antenna near to 1/4 wavelength of the signal being received will have an impedance around 50 ohms. (low) Bear in mind that 33 ft long antenna has a resonant frequency (1/4 wavelength) of 7 mhz. 16ft being about 14 megahertz.

So in a sense the longer the antenna beyond the short whip length will give more power into the radio but not more voltage. So if the radio is designed for hi impedance input it runs just on voltage so longer antenna will not give better signal. (this does not mean shorter is better however as the radio needs some power from the antenna ie not infinitely high impedance input.) Complicated no but tricky yes.

Now most coaxial cables are designed with a characteristic impedance of 50 ohms. That means that power transfer with minimum loss is at 50 ohm impedance. (more critical at high frequencies) However it means that a short antenna compared to the frequency being received will be of high voltage low current format (relative to one another) so that the capacitance of a piece of 50 ohm coax will tend to bypass or lose the voltage. Further the capacitance of the cable can also detune the input circuits of the radio.

Car radios use a high impedance shielded cable. You might call it coax but it certainly is not 50 ohm coax.
if you wanted to use 50 ohm coax you would need to start with antenna 1/4 wavelength long so 50 ohms. (tricky with greatly different frequencies wanted) then feed it into a radio designed for 50 ohm input or fit a transformer to convert the 50 ohm signal to high impedance. (a bit like balun)

So to sum up. Check the manufacturer of the receiver to find what sort of antenna or cable is required. Some will have a high input impedance and a low input impedance option. If it has a low input impedance use a long wire and coax. If it is hi impedance then use a relatively short wire straight from terminal.
The inclusion of a coax connector for the antenna may indicate low impedance input the inclusion of just a terminal more likely means high impedance. Certainly the whip antenna means hi impedance even with a coax connector. In the end use what works best on the boat.

Regarding lightning strike. I would ground with heavy cable the bottom of the mast to the keel or any metal in the water. The cable should be comparable to that used for engine starter cables. Do not ground the stay wires. They are (relatively) high resistance wire which will not conduct much current before getting red hot and fusing. (100 amps will do it) A mast being a good conductor and larger will conduct thousands of amps without problem providing there is a similar connection to the sea. Chain from mast base to keel might be worthwhile use for anchor chain. Doesn't have to be well connected just close. However any poor connection will be a source of arcing and fire if it happens. The trick being to lead the current away from where it will do most damage. ie the moist hull or SS rigging.
Bear in mind that USA has more lightning than all the rest of the world put together. (there is a wild statement) Just be warned lots more than Europe.
good luck olewill