Attaching Antenna Cable to Backstay

[fairbank56]

Originally Posted by Rick

Now for those who are concerned about "shock"; You cannot get shocked by HF you can only get burned and even then you can grab the backstay with max continuous power out (not what you get with SSB modulation, I'm just pointing out the worst case scenario) and not feel a thing..... So, it is not necessary to run a "High voltage" wire from below decks and attach it high up where theoretically someone might touch it.

So I guess the engineers at SGC are wrong with their warnings about dangerous high voltages existing inside their couplers as well as at the antenna terminal and their recommendation of using high voltage wire for the lead-in and the reason for using a porcelain insulator at the coupler output.

I'm a marine electronics service tech with 35 years of experience and remember getting frequent RF burns in the old days, before auto tuners came along, during the process of manually tuning these couplers.

I have also seen fairly recently where standard 14awg boat wire was used as the lead-in and was strapped directly to a grounded lower backstay and the high voltage nodes burnt holes through the insulation as well as melting the jacket around the holes.

Eric

[Rick]

High voltage is generated by resonance at high frequency by reactive components in the tuner in combination with what reactive components in the antenna. The voltage itself is not dangerous because the high frequency prevents penetration into your body of dangerous current. Voltage, like speed, never killed anyone. It is current through the tissues and deceleration that kill people.

Since you are a technician, Eric, you would know to not attempt to drive an element that is not anywhere near the desired quarter-wave length used in the HF band. What burned through the insulation was high frequency high voltage standing waves, an indication of a lack of tune or grossly incorrect drive element length for the frequency of operation.

The "danger" of HF energy is from a skin burn or, more importantly, a possible injury from pulling away from feeling a burn and falling or hitting something. Again, one cannot get shocked by what is generated by the tuner.

Modern tuners can "tune to a garbage can" literally, which does not mean that any effective radiated output power will occur even though the transmitter might be putting out full power into the tuner. It is the responsibility of the tech to test the radiator in any installation to determine whether or not realistic effective radiated power can be developed before the transmitter is ever powered up. This may be done by physical measurement or by using an RF noise bridge, or various other methods to determine if high energy standing waves will be developed in the desired band of operation. This evaluation must necessarily include consideration of the RF ground "seen" by the tuner which must be electrically significantly shorter than a quarter wave at the highest frequency of operation. I have inspected many installitions made by professional techs that had terrible grounds causing standing waves from the tuner ground stud all the way up the antenna.

[jlsail]

Hi Rick

Originally Posted by Rick

P.S. if you have a lower insulator just jump around it with the same wire as your stay. Use S/S cable clamps of same alloy and you don't have a dissimilar metals issue and, therefore do not need to tape over the connections. In fact you risk corrosion right where the tape ends and collects dirt. Keep it clean and exposed to air.

do I understand you right, I have my tuner on the outside of my doghouse, can I run a 4 mm stainless steel wire from the tuner, take up the force whit a connector to the edge of the doghousetop after 300mm and then connect it to my spreader 8m up in the mast (grp mast and spreaders)?

thanks
Johan

[Rick]

Not sure that I have the total mental picture yet from what I understand you can do that if the total length of what constitutes your radiator from the end of the tuner is on the order of 10 meters AND IF the RF ground that the tuner "sees" is good over the band of operation.

One caution is that the RF tends to conduct over the outside of the radiator and stainless is more than 40 times more resistive than copper you need to have a large circumference of rigging as possible to not have too much resistive a component. 4mm may be marginal for good effective radiated output power above 100 W yet the tuner will work.

[fairbank56]

Originally Posted by Rick

One caution is that the RF tends to conduct over the outside of the radiator and stainless is more than 40 times more resistive than copper

True but very misleading. The skin depth of stainless steel at HF is around 7 times that of copper so the overall effect is that the resistance per unit surface area including skin depth, of stainless steel is around 6 times that of copper. Not sure though about the differences in this respect between stranded and solid stainless steel.

Eric

[fairbank56]

Originally Posted by Rick

Since you are a technician, Eric, you would know to not attempt to drive an element that is not anywhere near the desired quarter-wave length used in the HF band. What burned through the insulation was high frequency high voltage standing waves, an indication of a lack of tune or grossly incorrect drive element length for the frequency of operation.

I do that all the time with my 8' whip on the back of my vehicle. That's why we use tuners. Extremely inefficient on the low bands and yes, the reason for very high voltages on the antenna system. As for standing waves being an indication of lack of tune? Standing waves on an antenna is NORMAL, that's basic antenna theory Rick. We demonstrate that routinely in classes at the Naval Academy.

Eric

[SoonerSailor]

The difference between copper and stainless steel construction of antennas is negligible. Another interested ham used an NEC antenna modeling program to investigate:

Stainless Steel vs. Copper

[daddle]

Originally Posted by SoonerSailor

... Stainless Steel vs. Copper

Interesting. I plan to replace my backstay with synthetic and use a small copper wire alongside the stay.

The use of stainless steel grounding foil is interesting.

Wonder if the same NEC modeling program could debunk the copper foil grounding mythology. I had one serious HAM tell me that RF won't travel down a wire. I must use the foil. Hmmm....seems to work okay for antennas....feeds...and inside the transmitters and tuners....anyone?

[SoonerSailor]

My understanding is that NEC is not good for modeling and comparing ground systems, but I haven't jumped deeply enough into it myself to have an opinion.

[fairbank56]

Unfortunately, most of these common myth's, and there are many, come from the ham community.

Eric

[btrayfors]

Originally Posted by fairbank56

Unfortunately, most of these common myth's, and there are many, come from the ham community.

Eric

Yep, and many of them are repeated over and over and over again by well-meaning but misinformed persons, including those "in the industry". They appear in virtually every book on marine SSB ever published and in the factory-issued instructions for many/most SSB radios.

Here are a few:

- you need to have 100sq feet of metal as an RF ground/counterpoise
- you need to couple to seawater (directly or capacitatively)
- you cannot use wire because RF travels on the outside only; you must use wide copper foil
- stainless isn't good for antennas because of its resistance to RF
- external groundplates are good/necessary
- you must ground the radio as well as the tuner
- with a backstay antenna you're trying to get as close as possible to a 1/4 wave resonant antenna

Truth is, although there may be a grain or two of truth, for the most part these are pure bunk. But, like the Eveready Bunny, they just keep popping up and won't go away :-)

Bill

[Rick]

Sorry, Eric, I should have said that for a quarter wave element there is a minimum voltage at the drive point unless there is a mismatch. The standing wave, therefore, does not cause burning at the lower end and at the upper end, where the voltage is maximum, the element is hopefully dressed away from any metal.

[farotherside]

Ok - hey, you've got my curiosity piqued here! Sorry to revive an old thread, but I'm just about to have my backstay cut and I'm at exactly the point where I have to make decisions on just how to do it...

- I have a fiberglass-over-plywood trimaran, and I'm actively cruising. I'm installing a new ham system, my first ever, and I've read a tonne on this site about it and taken a great deal of different advice into account. I have ended up with the following setup:

- Icom IC-7000 radio
- SGC-230 tuner (and this tuner interface)
- KISS counterpoise

- I have a 3/8" backstay, so insulators are pretty pricy - I have mounted the SGC-230 with the antenna connector about 6" away from my backstay chainplate, and would like to attach directly to the chainplate instead of going up eight or nine feet with standoffs to attach to an insulated portion.

MY QUESTION: my backstay is split at the bottom! I've attached photos - if I were to attach directly to the chainplate of the starboard backstay leg, would the port backstay leg affect my antenna in a negative way?

I'd obviously only like to haul my backstay into the rigging shop once, and I figure one insulator is half the potential points of failure of two insulators, so there's definitely value there if I can help it... but even more obviously, I'd like the antenna to work the best it possibly can!

Your thoughts, oh gurus? Photos of the backstays attached!

[RDW]

got a way to show a picture?
rdw

[btrayfors]

Drew23,

Any chance you could use an "alternate backstay", i.e., an antenna secured, say, near the base of the Canadian flag? That would save you removing or cutting the backstay entirely, or spending money for a 3/8" insulator.

To see if that would work, bring the bottom end of a halyard to somewhere aft and as far away from the split standing backstay as you can. Now, if you have adequate clearance for the roach of the main, as well as for the boom, then you're golden. Make up an alternate backstay from 3/16" insulated s/s lifeline, haul one end up with a spare halyard, tie the lower end there in the corner, and run GTO-15 wire from the tuner belowdecks to the bottom of the antenna. That works every bit as well as an insulated backstay.

See, e.g., the five pics at the bottom of this page: Marine Antennas

Click twice on each pic for full resolution.

Bill