SSB antenna. whip or back stay ?

[prv]

Hope no-one minds an idle question, but is there any reason not to use the mast itself as an SSB antenna ?

I guess then you'd need to put isolators in all the standing rigging, near the top. They do this with some radio transmission towers where the tower itself is the antenna rather than just a structure to put smaller antennas higher up.

Surrounding your antenna with nearly-parallel wires probably doesn't help with transmission.

Anyone touching the mast could get an RF burn. Unlike most of the antennas we drape around our yachts, HF transmissions have some serious power behind them (necessary to talk over thousands of miles).

Dunno what your nav lights would make of it either.

Pete

 

[KellysEye]

>any reason not to use the mast itself as an SSB antenna ?

Yes there is a very good reason, when transmitting the power in the aerial will cook your hand from the inside out if you hold it and, as I said, you can light a cigarette on it, which is handy of you smoke.

 

[Jassira]

Thanks for all your advice, I think I'll go for insulators in the back stay, so I guess the next question is where about's in the back stay.

Shes a steel, centre cockpit, 16m cutter with a single mast and steel arch (goal post type structure) on the stern. The back stay is a little over 20m long but passes close (app 100/150mm) to the arch about 2m above the deck.

As I understand things, I'm looking for a 15m antenna measured from the output of the antenna tuner. This gives me a few options for the location of the lower insulator.

I can put it above the arch which will keep it out of reach of the crew but will mean the output cable from the tuner will be a nuisance, when moored stern too the back stay is a natural hand hold and having a cable strapped to it is likely to lead to damage. I could physically run the output cable inside the steel tube that forms the arch, is it ok to have what would effectively be 4 or 5 m of the antenna inside a 50mm steel tube ?.

Or do I put the lower insulator lower down the back stay, perhaps a foot above the deck, which leaves me a clean back stay from the perspective of people boarding from the stern but risks burning the crew if they're hanging onto the back stay at the wrong moment.

Last question (well last for now ) how critical is the antenna length, would somewhere between 14 an 16 be close enough ?

 

[whipper_snapper]

Don't get too hung-up about the length. The ATU will handle pretty much anything you ask it to, with a steel hull and a long backstay you should be able to get a fanatically well performing station - I am jealous.

Do not run the antenna feed (as you rightly point-out, anything after the ATU is antenna not feed) tightly strapped to any grounded metal, including the lower part of the backstay or you are throwing away most of your signal, you can make simple standoffs from 1" tubing. I would not worry too much about risks to your crew when you are transmitting, just stick your head out and let people know, I have never had problems with such an arrangement. But equally shortening the antenna by raising the insulators will do no harm, but equally equally, the feed will be almost as likely to burn as the bare backstay, but I suppose less likely to be grabbed for support. Personally, I would put the insulator above convenient grab height and feed with it somewhat ugly bits of insulated wire held off with some kind of spacer, I have used toilet outlet pipe drilled for cable ties. The bottom of the backstay won't look pretty but there will be an obvious and safe grab point on the backstay wire itself.

 

[tri39]

Def put the lower insulator above the arch. I have successfully used HT spark plug insulated wire as the feed. And held it off the pushpit and lower backstay merely with wine bottle corks and tape!! The offstanding wire looks a bit odd only to the uninitiated.

 

[Jassira]

How about if I mounted the ATU on the top of the arch, I guess it would need a water proof casing of some sort, does it get warm and/or need ventilation. Am I right in thinking that cables before the ATU don't need isolating from the surrounding metal work and could be run physically inside the tubing that forms the arch

 

[KellysEye]

>Shes a steel, centre cockpit

That's good news, ground the radio to the hull and you will have a stronger signal than any GRP boat.

>As I understand things, I'm looking for a 15m antenna measured from the output of the antenna tuner.

Typical installations are less the that, around 34 feet to 36 feet.

>I can put it above the arch which will keep it out of reach of the crew but will mean the output cable from the tuner will be a nuisance, when moored stern too the back stay is a natural hand hold and having a cable strapped to it is likely to lead to damage. I could physically run the output cable inside the steel tube that forms the arch, is it ok to have what would effectively be 4 or 5 m of the antenna inside a 50mm steel tube ?.

Not a good idea, see below.

>Or do I put the lower insulator lower down the back stay, perhaps a foot above the deck, which leaves me a clean back stay from the perspective of people boarding from the stern but risks burning the crew if they're hanging onto the back stay at the wrong moment.

That's the way to go. If you use the shorter length I mentioned you can have the insulator higher. Bear in mind that on long passages you usually only use SSB a couple of times a day, so just tell the crew that you are going to trasmit and get them to sit on the other side of the boat away from the aerial. We were two handed and that's what we did.

 

[Roberto]

deleted as duplicate with previous post, I am getting old

 

[blenkinsop]

As observed on a previous related thread, whatever antenna arrangement you end up with, check the voltage drop between the set and the antenna tuner.
We had transmission problems and upgrading the control cable from that supplied by the manufacturer solved the problem. Part of the clue was that the optimum voltage for the set was 13.1v, higher than normal for most sailing boats on passage, so by the time the voltage reached the tuner it was below the critical level, unless the engine was on which we did not want just for the sake of creating a transmission voltage.

 

[Roberto]

As observed on a previous related thread, whatever antenna arrangement you end up with, check the voltage drop between the set and the antenna tuner.
We had transmission problems and upgrading the control cable from that supplied by the manufacturer solved the problem. Part of the clue was that the optimum voltage for the set was 13.1v, higher than normal for most sailing boats on passage, so by the time the voltage reached the tuner it was below the critical level, unless the engine was on which we did not want just for the sake of creating a transmission voltage.

apart from cables, battery capacity plays a big role

a 100-150W ssb on transmission can draw 20-25-30 Ampere, any small battery bank would immediately react with a big drop in voltage, with a larger bank say 400-500Ah the voltage drop would be a lot less important, hence providing more reliable operation

r

 

[KellysEye]

>a 100-150W ssb on transmission can draw 20-25-30 Ampere, any small battery bank would immediately react with a big drop in voltage, with a larger bank say 400-500Ah the voltage drop would be a lot less important, hence providing more reliable operation

We had a 150W SSB which used 25 watts on transmission and a 100 amp deep cycle battery. We had no problems with the battery but it does help if you have solar panels or a wind geneator, which we had.

 

[whipper_snapper]

I agree, SSB should not make heavy demands of any reasonably healthy battery. The wires should be sized for peak current demand and run direct to the battery; current draw will very significant, but in brief bursts. A 100W transmitter sending voice SSB does not use anything like 100W for most of the time. Data is different, but power is usually set much lower.

 

[Roberto]

FWIW, I have 500Ah of new batteries, at my max 100W on data (so maybe what? 50-60-70W continuous?) I measured on average a voltage drop by about 0.5-0.7v with fully charged batteries, more with batteries say down to 60% charge.

 

[BlondeBimbo]

I plan to fit an HF radio, probably an ICOM 801E, in the not too distance future, but right now I am replacing the standing rigging.

As I understand things I have a choice of using the back stay, which is about 20m long, or fitting a whip antenna of about 7m.
I'm a little concerned about comprising the back stay with insulators, although I have to say, I've never heard of any problems and I assume I'd get better performance with the longer length of the back stay, but a whip antenna should still be there (hopefully) if the mast fell off.

I would appreciate your opinions and experiences on the antenna and/or the radio

I would go for the back stay

But don't forget - realistically you have to have two or three antenna for an 801 (and other GMDSS HF/MF transceivers)

You have the main TX/RX - the backstay - connected via the ATU, then the short GMDSS whip - which is connected to the transceiver itself.

However there is always the possibility that you are using the GMDSS for a loss of mast! - in which case you have more than likely lost the main antenna (even if it was a large whip on the blunt end!)

So it would be prudent to fit the brackets etc at the stern - but then carry a temporary spare whip - nothing huge or expensive

The antenna length (total from the ATU) needs to be between 7 and 15 metres - the longer toward 15 the better - and always insulate the backstay our of arm's reach - forgetting to warn the crew or the crew forgetting - just for a moment can lead to serious burns.

As stated - volt drop is an issue on the supply, so site the transceiver as close to the batteries and use very heavy cable for the supply. the controller can go anywhere you want to have it

BB

 

[KellysEye]

>Power output is more important than the ariel.

No it isn't, the most important things are the aerial installation, the ATU and above all the gound which makes up half the aerial, if it's bad the signal will be weak. Most Marine SSB's have the same power output

 

[ccscott49]

I use a 5 meter whip for a SSB receiver only, works great.
For what its worth, with an arch already fitted to his boat, I would be tempted to fit a whip on the arch, saves all the insulator bull and still have the backstays as grab rails.

 

[BlondeBimbo]

Decided to have a real think about this one, as clearly the results whip or backstay will be different - so I plotted the radiated energy transmitted for each - typical installation

So - The whip (5 metre) - this elevation plot shows the radiated field for the RT distress frequencies (omnidirectional of course)



Whilst the backstay equivalent plot below is of course directional (not shown in this elevation)



Which is not better or worse - just different

So say we are transmitting at 2-4MHz since the D layer would adsorb all the signal we must use ground wave - so a typical elevation would be 10 deg - the whip radiates -0.64dBi, whilst the long wire radiates -0.53dBi - so about the same during the day.
At the higher frequencies say 8/16 MHz we must use sky wave so a typical elevation would be 35 deg - the whip radiates -0.23dBi/-0.95dBi, whilst the long wire radiates 0.37dBi/5.0dBi - so the long wire is much more efficient (almost 4x)

Of course at night the D layer isn't there so we would use the lower frequencies in sky wave again at 35 deg so say 6MHz - the whip radiates 0.05dBi, whilst the long wire radiates 0.24dBi - so about the same

Note the plots always show 0dB as the outermost scale - read the text below the plot or the note to see what 0dB represents in dBi

For completeness whilst the whip is omnidirectional the long wire isn't so the azimuth plots for 10 deg and 35 deg are shown below



Interesting?

BB

 

[Turnbuckle]

Hope no-one minds an idle question, but is there any reason not to use the mast itself as an SSB antenna ?

Boo2

It would need to be insulated from stays and boom and you wouldn’t be able to mount a vhf antenna On top without a lot of separation.