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Old 04-05-2008, 00:12   #1
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Backstay Antenna Full Lenght (No Isolators)

I'm looking a boat and is has an antenna setup that I wasn't familiar with. In Particular, I could not find where the Antenna was since there was No COAX above deck and no Isolators in any of the rigging. I asked the current owner about this and I was wondering what you all thought about this.
"With respect to the SSB/Ham radio antenna: The Antenna Tuner is mounted forward in the Port cockpit locker. The antenna tuner has two connections. The antenna side is connected with #8 stranded wire to the chainplate for the main backstay. All the chainplates were bonded together (along with through hull fittings, etc.) with #8 stranded wire. The ground wire that was attached to that chainplate was removed and attached to the grounding terminal on the antenna tuner. What you end up with is a whole series of closed loops instead of a single separate isolated antenna wire and a large ground plane in the bilge. I cannot explain how it works, but it does.

Getting a shock seems to be the biggest question that we have received over the years, but neither one of us has ever been shocked from this arrangement. However, we have never tried touching the rigging while transmitting, but we used the radio so frequently, I am sure we have accidentally touched the rigging several times. Our biggest use of the radio was in sending and receiving email over the Ham bands which we did daily when on long passages and when anchored in remote locations. While in marinas, it is often hard to use the radio with all the other boat masts in close proximity and it was usually easier to use shoreside computer connections to the internet."
Let me know what you think,
-p
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Old 04-05-2008, 05:37   #2
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A good antenna tuner (coupler) can "tune" just about anything, i.e., can make the transmitter "see" the 50 ohms impedance it likes. This says nothing about the efficiency or effectiveness of the antenna system, however. At one extreme, an efficient resonant dipole antenna will put out a whopping signal, with a pretty high efficiency. At the other extreme, a dummy load will also present a 50-ohm load to the transmitter, but will radiate almost no signal (converting virtually all the output power to heat).

What may be happening with the setup the owner described is that he's created a kind of vertical loop antenna, with the #8 wire and backstay forming the base and hypoteneuse, while the mast and/or shrouds form the vertical component. Or, possibly, the tuner is just feeding the whole rig...depends on what's connected to what.

How well this works is hard to say, but if the owner got good results it might be a good idea to try it out for awhile and see. Comparisons over time with other nearby boats on all desired bands should tell you whether to leave it as is, or modify the antenna system to get better performance.

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Old 04-05-2008, 07:44   #3
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Bill's comment about tuners is spot on (as is his antenna knowledge in general).

Just when I thought I had seen most yacht antennas, up pops another one.

I can't determine from the PO description if all the chain plates are still connected together along with the through hulls and are therefore connected to antenna side of tuner with the ground side of the tuner going "somewhere else" in the bilge?? OR the antenna side is just connected to the main backstay while the ground side to connected to a wire that commons all the other chain plates and through hulls.

Either is unusal but (as Bill says) if the PO found it OK, try it out for awhile.
Heck, it a large notch in the vertical fin of a 747 can be made to transmit HF, this set up might just work as well.
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Old 04-05-2008, 17:32   #4
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Patrick,
I have done this type of installation on a few boats with good results. There is a compelling reason to try it before going through the expense of adding insulators to the rigging, of course.

This type of antenna can be a loop, as Bill mentioned. It seems to work with decent effective radiated power if there is a good conductive element run between the headstay chainplate (inside the boat) and run below the floors to where it can be connected to the backstay via a knife switch. The switch is normally left open with the tuner connections made between the so-called "ground" wire and the other to the backstay chainplate.

Using a noise bridge I measure the impedance of this connection first at various frequencies to get an idea if there will be a good reason to believe that there will be a useful effective radiated output power at such frequencies. It is usually a good indication.

There will be NO shock if anyone touches a radiating element used in the HF bands. One CAN get an RF burn, however, if the power is sufficiently high and relatively continuous. This happens on the skin when you merely brush up against it when transmitting. If you grab on with a good connection you actually don't get burned. I love this myth about getting shocked with SSB antennas.
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Old 04-05-2008, 18:45   #5
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Heck, it a large notch in the vertical fin of a 747 can be made to transmit HF, this set up might just work as well.
Negative...

I have flown 747s for a few years and I am here to tell ya that the HF antennas are in the wing-tips. We have 2 HF/SSB/AM radios onboard, each putting out 300 watts.
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Old 05-05-2008, 00:19   #6
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This is very helpful thank you. I am sure you can tune anything, I was just suspect about safetly. My concerns about shocks RF Exposure, etc., come from the FCC Ham Test material. But I know that at SSB\HF frequencies its not the same as in ranges approaching microwaves.

Sounds like I should give this a shot and see if there issues and if not leave it alone.

-p
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Old 05-05-2008, 01:33   #7
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Negative...

I have flown 747s for a few years and I am here to tell ya that the HF antennas are in the wing-tips. We have 2 HF/SSB/AM radios onboard, each putting out 300 watts.
Thanks for the correction maybe I was getting the it mixed up with a 707 (perhaps) - brain isn't as good as it used to be . IIRC both used a "notch" HF antenna, and I just recall seeing some notch HF antennas in the vertical stab. - must be another airframe. The smaller babies around here mainly use leading edge of the vertical stab. or long wire.
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Old 05-05-2008, 01:50   #8
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...This type of antenna can be a loop, as Bill mentioned. It seems to work with decent effective radiated power if there is a good conductive element run between the headstay chainplate (inside the boat) and run below the floors to where it can be connected to the backstay via a knife switch. The switch is normally left open with the tuner connections made between the so-called "ground" wire and the other to the backstay chainplate.

....
Rick, lets see if I understand this correctly, you are saying the one side of the antenna tuner is connected to the backstay chainplate and the other end to the headstay chainplate. If so, I can see the "loop" antenna set up; however it seems (to me) that Patrick's set up was having all the other chain plates AND the through hulls connected together going to the "ground" side of the tuner rather than just to the headstay chainplate - this is what struck me as unusal.
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