Standoff For Backstay For SSB

[RDW]

I have a Icom 802 SSB. I have a backstay antenna with a lower isolator that is 15-20 feet up the backstay. My cable from the tuner to the antenna has to run up the backstayt to get to the antenna. Right now it is directly wiretied to the backstay. I have heard that this is not the best and I shoud have some sort of standoff to get some separation between the two.
I would be interested in opinions on the need for a stand off.
I would be interested in suggestions on what to use for a standoff.
Thanks
RDW

[Hud3]

I used 3/8" ID PVC tubing. I first drilled holes the diameter of my backstay, equally spaced about 3" apart using a drill press and a jig to steady the tubing. Then I cut the tubes across the center of the holes with a bandsaw, so I ended up with a set of 3" tubes with a half-circle notched out of each end. I then used black cable ties to cinch the SSB cable to one end and the backstay to the other.

I did a test communication with a ham operator friend about 400 miles away before and after the installation, and he said he could tell the difference (better!).

[sy_gilana]

Absolutely not correct that way. From the hot terminal of the tuner, it is ALL antenna, and this cable should NOT be screened. The best wire for this is the silicone single core tinned that they use on Neon signwriting. Insulation to 30kv. The problem is capacitive coupling. Your tuner is probably matching to get your signal out but it could be better. The same theory as UTP cable, or twisted pair relates. To stand off, it should not look neat and be parallel to the uninsulated part of the backstay, (however I have to pause here, If your lower portion is grounded it is a problem if it floats potential free it is less of a problem. IE grounded to the rest of the boat or any equipment =BAD. If not connected to anything = not so bad.)
Let me continue...I use cable ties and sections of plastic hose. They are spaced unevenly, and cut unevenly so as NOT to be parallel. You thread them so they look like a ladder, take a cable tie pass it through the 4"or so of hose, round the stay, back through the hose, around the feedline, and zip shut. No 90 degree corners anywhere. and make the connection really good with a large area of connection clamped tight.

Happy QSO's
de mike QRT.

[fairbank56]

We did testing like this years ago. Tied a short string to the GTO-15 so it could be quickly pulled tight against the backstay and then pulled away all the while another person was in communications with another ham for the test. He noted no difference either by ear or on his S-meter. If you do a test with the wire tight against the backstay and then minutes later do a test again after installing standoffs, the test is meaningless. Propagation conditions via HF can change from good comm's to none in seconds.

If the lower end of the backstay is in fact grounded, then there will be some signal coupling, no matter how far away the feed wire is. Whether it's enough to make any significant difference is the question. In my testing, it didn't.

Eric

[btrayfors]

The "before and after" reports mentioned by Hud are almost meaningless, unless they were conducted nearly simultaneously. Not 5 or 10 or 30 or 60 minutes apart, but within seconds of each other.

You'll find reports that tying GTO-15 transmission line directly to backstays doesn't matter a whit. Others say it does. I'm a bit ambivalent, but believe there is indeed some potential for detuning/coupling -- particularly with grounded backstays -- and so generally use standoffs made of 5/8" nylon rod 2" long with a hole drilled in the middle and plastic wire ties.

The basic problem you're dealing with is that someone installed the lower insulator "15- to 20-feet up the backstay". That's unfortunate, but there's little you can do about it except take a breath and live with it.

I know of NO ONE who has ever been hurt by RF from antennas on a sailboat. At the power levels involved -- 100 to 150 watts output -- and the fact that the ONLY time one could possibly be injured is if they were holding fast to the antenna AT THE RIGHT HIGH VOLTAGE NODE ALONG THE ANTENNA and someone were simultaneously transmitting, it's damned unlikely to be a problem. If you should be hanging onto the antenna while someone is transmitting, you'll feel a distinct tingle and, unless you're brain dead, you'll let go and hang onto something else.

For many sailors, it makes good sense to feed the backstay from belowdecks, at the chainplate, with just one insulator located a few feet from the truck of the mast. Then, if you're worried about RF burns, put a PVC boot over the lower end of the backstay.

FWIW,

Bill

[btrayfors]

Oops...Eric posted while I was typing. I agree completely.

Bill

[keelbolt]

Just want to throw my $0.02 in here as I was just re-commissioning my SSB after being in storage for ten years (yeah, the refit was hell). My rig has a split back that is 15ft off the deck. Not ideal, but my radio ALWAYS worked really well (steel hull helps). When I re-attached my feedline to the backstay, I recalled that the original installation had standoffs, but I could not remember where I put them and I figured it didnt matter so I went ahead and tie wrapped the GTO feed line to the port lower back up to the insulator. I was recieving fine, but no one, on ANY band, could hear me. Tested the rig and tuner and both were fine. Went on deck, cut all tie wraps, and viola, I'm talking to Italy from NH, 5x9.

Now, every boat is different, and I'm sure we can get a HAM to chime in and explain all this, but hey, if works, do it! If don't work, do something else!

[Roy M]

I'm thinking about inserting my antenna inside a piece of Spectra and hoisting it up.

[Paul Elliott]

Originally Posted by Roy M

I'm thinking about inserting my antenna inside a piece of Spectra and hoisting it up.

That works very well.

I use 3" sections of PVC tube, with ty-wraps and notched like HUD3 describes.

I see no problem with having the feedline parallel to the backstay, as long as it's kept at a few inches distance. Even with unequal-length standoffs as sy_gilana describes, the wire will still be in the same plane as the backstay, so there will be some unavoidable coupling.

What we are trying to achieve with the standoffs is to reduce the capacitance between the feedline (which at this point *is* part of the antenna), and the grounded backstay. This should make the job of the tuner easier at some frequencies. At other frequencies it may make no difference. This is one reason why some tests show that the standoffs help, and other test show no effect.

Btrayfors suggests that the shock hazard of an uninsulated antenna is minimal. As he mentioned, this is also frequency-dependent, but I would still be careful. A 150-Watt transmitter can develop over 800 Volts at some points on the antenna. This will typically be at the top end of the backstay, but especially at higher frequencies high voltages can be developed near the bottom as well.