SSB Antenna

geem

geem

Well-Known Member
Joined
27 Apr 2006
Messages
9,007
Location
European side of the pond
Visit site
I need to wire my SSB transceiver to the back stay. What cable specification do I need for the 40 ft run from the SSB to the back stay via the autotuner? I seem to remember when I did this on the last boat that it was 600v screened cable with copper core but it was 12years ago so don't want to rely on my dodgy memory!
 
Ours was just copper core with no screening. From the ATU to the closest point where the aerial wire leaves the boat should be copper foil. Don't forget the ground it's important because it makes up half the aerial.
 
Everything from the stud on top of the tuner onwards counts as antenna, so you want to minimise the length of random wire between this and the backstay. This is why the Icom tuners are waterproof - the one on Stavros was mounted on the outside of the wheelhouse right next to where the shrouds passed by from the deck. Though most yachts do mount them internally (in cockpit lockers etc) for belt-and-braces waterproofing, which makes sense. You also need to be careful how you run the cable - as straight as reasonably possible, no wrapping up excess length into coils. Where it goes up the un-isolated lower section of backstay, it should be stood off from it by three or four inches to reduce coupling - the traditional technique is to cut short lengths of narrow plastic tube with notches in each end, and run a cable tie through the tube with one end looped round the backstay and the other round the aerial feed. I've also seen it suggested that if the bottom section of backstay and its chainplate is grounded, and can be relatively easily disconnected from that ground, then you should do so.

Sorry if any of this is teaching you to suck eggs.

Pete
 
prv said:
Everything from the stud on top of the tuner onwards counts as antenna, so you want to minimise the length of random wire between this and the backstay. This is why the Icom tuners are waterproof - the one on Stavros was mounted on the outside of the wheelhouse right next to where the shrouds passed by from the deck. Though most yachts do mount them internally (in cockpit lockers etc) for belt-and-braces waterproofing, which makes sense. You also need to be careful how you run the cable - as straight as reasonably possible, no wrapping up excess length into coils. Where it goes up the un-isolated lower section of backstay, it should be stood off from it by three or four inches to reduce coupling - the traditional technique is to cut short lengths of narrow plastic tube with notches in each end, and run a cable tie through the tube with one end looped round the backstay and the other round the aerial feed. I've also seen it suggested that if the bottom section of backstay and its chainplate is grounded, and can be relatively easily disconnected from that ground, then you should do so.

Sorry if any of this is teaching you to suck eggs.

Pete
Click to expand...

Hi Pete, I am happy with the installation process it's justo getting the spec of cable to the backstay right.
I will run 50 mm wide copper tape in the bilges as well as connecting up to the ground plate. This worked well last time. I will also ground the rig as this helped last time to.
 
I used standard coax for mine. The length from ATU (located in cockpit locker aft) to just above stay insulator is about 1.3m. The coax is RG8 Super XX. (Italian) similar to RG8Mini.
The coax exits the hull through a gland on deck, and via standoffs from the pushpit ends up just at the bottom end of the insulated backstay.
Both the inner and braid are connected at each end, so not using this as coax, just as a cable.

IMG_4716_zps527987bd.jpg


The standoffs are plastic or rubber tubing about 100mm, and using two cable ties they form a convenient doublended clamp from stainless rail to cable.
Connection at the stay is by cable clamp, with tape over the top to run water off. The bottom of the tape is open to let water out. I change this cable every three years and so far its been surprisingly good (not wicking).
I don't use copper tape in the bilge, merely house wiring wire, 5 off, running from ATU forward, spread across the width of hull, one central, other 2 each side. They meet, but are not joined, at the bow.

ps, where was I?
 
The cable must be low loss Coax 50 Ohm and not tv coax which is 75 Ohm.
A good supplier is
http://wsplc.com/index.php/cable-connectors/rg-58-50-ohm-coax-cable.html
They can also supply the connectors and will make up leads for you if needed.

The coax cable used after the ATU should be as short as possible but does not need to be spaced off steel conductors as the rf signal is contained within the screen.

I use a stainless steel Bulldog clamp to connect the inner core of the coax to the backstay and waterproof the connection with self amalgamating tape. The shield screening should be stripped back to the height of the insulator so it doesn't start grounding the active part. Spacing this part off the backstay by 10cm is a good idea but probably not important.

I dont think its a good idea to ground the lower part of the backstay as the insulator may break down when really wet which could be just the time you need it to work perfectly. l cant see what advantage you would get by connecting it to the RF ground. Keep it simple.
 
GHA said:
Not between the tuner and the backstay, GTO15 high voltage neon/ignition cable is common to use for there.

I agree you can use a non screened cable between the ATU and the backstay but it does have to be spaced well away from any conductor and as short as possible. But using coax avoids any risk of RF loss.
Click to expand...
 
HunterWanderer said:
GHA said:
Not between the tuner and the backstay, GTO15 high voltage neon/ignition cable is common to use for there.

I agree you can use a non screened cable between the ATU and the backstay but it does have to be spaced well away from any conductor and as short as possible. But using coax avoids any risk of RF loss.
Click to expand...
As the cable between the tuner and the backstay is part of the antenna rf loss is exactly what you do want. Otherwise you are effectively screening part of your antenna. Spaced so it doesn't get lost into the lower part of the backstay.
Anyway, such a short length of screen will do next to nothing. Coax will work as will any old wire but Gto150 is common as it's rated up to 15,000v, cheap, has good RF conduction properties, easy to get and easy to work with.
Click to expand...
 
Last edited:
GHA said:
HunterWanderer said:
As the cable between the tuner and the backstay is part of the antenna rf loss is exactly what you do want. Otherwise you are effectively screening part of your antenna.
Anyway, such a short length of screen will do next to nothing. Coax will work as will any old wire but Gto150 is common as it's rated up to 15,000v, cheap, has good RF conduction properties, easy to get and easy to work with.
Click to expand...

No you dont want rf loss untill you get to the backstay. If you transmit rf power below the guard rail for example it will absorb the Rf as it would work like a fraday cage.

The short length of Coax does not transmit rf till the screen is removed. But " any old wire" will transmit as soon as its connected. The backstay is exactly that.

Its better to have a slightly shorter antenna than losses at the base.
Click to expand...
 
HunterWanderer said:
GHA said:
No you dont want rf loss untill you get to the backstay. If you transmit rf power below the guard rail for example it will absorb the Rf as it would work like a fraday cage.

The short length of Coax does not transmit rf till the screen is removed. But " any old wire" will transmit as soon as its connected. The backstay is exactly that.

Its better to have a slightly shorter antenna than losses at the base.
Click to expand...
Disagree, you won't have a shorter antenna, your tuner sees the whole length and tunes to that, what you think happens with coax is like covering the bottom bit up. Bad idea.

And irrelevant as a screen floating won't do anything anyway, the core and the screen will be coupled capacitively . Doing nothing to shield rf.
Click to expand...
 
The ATU is tuning everything that's "downstream" of it, and that includes the wire connecting the ATU to the stay. That I used coax is irrelevant (I did mention that I had braid and centre conected together, that makes it merely a wire).
If coax were used what use would the screen (braid) be? If earthed back at the ATU (say connected to the counterpoises), then this would short circuit some RF reducing erp. It can't be helped that this forms part of the antenna, but minimising losses can be done, for example by reducing the coupling into the pushpit, but spacing it off.
The pictured setup is common on many installations and works.
In RF, its often the case that if something works its OK.
 
MM5AHO said:
The ATU is tuning everything that's "downstream" of it, and that includes the wire connecting the ATU to the stay. That I used coax is irrelevant (I did mention that I had braid and centre conected together, that makes it merely a wire).
If coax were used what use would the screen (braid) be? If earthed back at the ATU (say connected to the counterpoises), then this would short circuit some RF reducing erp. It can't be helped that this forms part of the antenna, but minimising losses can be done, for example by reducing the coupling into the pushpit, but spacing it off.
The pictured setup is common on many installations and works.
In RF, its often the case that if something works its OK.
Click to expand...

The coax braid should be earthed to prevent the Rf being transmitted till it gets to the backstay. Most base stations have long coax feeder cables from the antenna tuner to the antenna with no rf power loss.
 
I think the best easist obtained wire for the connection tuner unit to backstay is the centre core and insulation of RG8 coaxial cable. That is the 10mm diameter coax. This has very good high voltage insulation. One poster mentioned low loss coax transmitter to tuner. This is not so vital as the frequencies are low. Low loss is much more important at VHF.
It would be highly undersirable to have coax with the shield earthed after tuner unit and before the backstay. This will introduce a largish capacitance to the natenna as seen by the tuner. The tuner may connect capictance here internall but only as required to tune and match the antenna. Any added capacitance will be permanent to the tuner and may impede tuning at some frequencies.
I used to be involved maintaining Fokker F28 jets. They had HF SSB set which was vital to their operating area. The tuner was mounted in the luggage rack about 1/3 way down the cabin. It was found that the HF SSB would get into the PA system. The fix was to shield the 30cms or so of cable from tuner to the inner skin of the aircraft. This shielding was done with an elaborate aluminium box around the feed wire. Shielded but with a min spacing of about 5cms. Hence very low capacitance.
Shielding below the stern rail on a boat should not be needed or used. good luck olewill
 
HunterWanderer said:
The coax braid should be earthed to prevent the Rf being transmitted till it gets to the backstay. Most base stations have long coax feeder cables from the antenna tuner to the antenna with no rf power loss.
Click to expand...
Little loss perhaps, not zero.

When coax is used downstream of an ATU, (in base stations) both the core and the braid are in use, with current flowing in both, and therefore not open-eneded at the antenna end. Typically the braid goes to one leg of an antenna such as a dipole, a yagi or similar.
If coax is to be any use in this marine situation, the end of the braid near the antenna needs to go somewhere. To leave it open ended means just introducing capacitance (as mentioned above by OleWill), detuning it.
 
William_H said:
I think the best easist obtained wire for the connection tuner unit to backstay is the centre core and insulation of RG8 coaxial cable. That is the 10mm diameter coax. This has very good high voltage insulation. One poster mentioned low loss coax transmitter to tuner. This is not so vital as the frequencies are low. Low loss is much more important at VHF.
It would be highly undersirable to have coax with the shield earthed after tuner unit and before the backstay. This will introduce a largish capacitance to the natenna as seen by the tuner. The tuner may connect capictance here internall but only as required to tune and match the antenna. Any added capacitance will be permanent to the tuner and may impede tuning at some frequencies.
I used to be involved maintaining Fokker F28 jets. They had HF SSB set which was vital to their operating area. The tuner was mounted in the luggage rack about 1/3 way down the cabin. It was found that the HF SSB would get into the PA system. The fix was to shield the 30cms or so of cable from tuner to the inner skin of the aircraft. This shielding was done with an elaborate aluminium box around the feed wire. Shielded but with a min spacing of about 5cms. Hence very low capacitance.
Shielding below the stern rail on a boat should not be needed or used. good luck olewill
Click to expand...

I agree 100%.

I am afraid that Hunterwanderer is getting a little confused about how co-ax works. I can go into the details but others have sort of summed it up.

Thinking that the earthed braid "shields the inner and stops the inner wire radiating" is nonsense in this context. Co-ax when terminated with the correct impedance at BOTH ends is an unbalanced transmission line which (if things are perfect) doesn't radiate. In practice things aren't perfect and so there will be unintended emissions but the reasons for those are for another topic.

In simple terms, the ATU is (by an iterative process) adding capacitance and inductance to the 'random length of wire' so that the impedance it presents at the frequency chosen is 'seen' as 50 Ohms impedance by the co-ax from the transceiver.

Putting co-ax on the output of the ATU with an earthed sleeve just adds a random bit of capacitance to the tuner output and doesn't 'screen' the transmission line as Hunterwanderer believes.

I was an RF engineer in a previous career and with the greatest of respect to Hunterwanderer, it might be a case of a little knowledge is a dangerous thing. (His comment about 'Faraday cage' of the pushpit etc are another example. I know what he is trying to get at, but in practice at the frequencies of Marine SSB, it will make stuff all difference.) RF doesn't behave in such a simple way as he implies.

Its different where the connection to the backstay is parallel to a stray bit of rigging for a while. You might get some unwanted coupling there, and it makes sense to keep the antenna away from the rigging with some stand off insulators as already described. If its of any interest, its what we do with ours.

The antenna starts from the moment the wire leaves the ATU.
 
john_morris_uk said:
I was an RF engineer in a previous career
Click to expand...

You do get about a bit, don't you? RF engineer, Royal Marine padre, teacher... what next? :D

Mind you, our chaplain at school had had a varied life as well. Kicked out of the Royal Navy for more drunken fighting than is usually tolerated, seaman on container ships, found God while at sea, Army padre, then boarding-school chaplain and the TA. Probably makes for a better pastoral understanding I reckon.

Pete
 
You must log in or register to reply here.

Share this page

Top