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Old 12-01-2021, 09:53   #16
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Re: Using two insulated backstays as inverted Vee for SSB?

Quote:
Originally Posted by rondelais View Post
...The Navy had a great way to handle this problem on warships that were way too complex to model on a computer, like the tens of antennas on masts with all kinds of metal around. The lab was located in San Diego (Point Loma).

What they did was build scale models of the ships...
Thanks for this interesting account of the Navy Laboratory at Point Loma. I will put it on the list of places to visit someday.

Curiously, when the NEC code was first available to the public, I read--a long time ago and cannot cite the source--that the development of the NEC code for modeling antennas was, in part, to create the ability to model antennas that were part of complex conductive structures, specifically on ships.

The WIKIPEDIA article on NEC code mentions:

Quote:
NEC...was written by programmers at Lawrence Livermore National Laboratory (LLNL) under contract to the Naval Ocean Systems Center and the Air Force Weapons Laboratory.
Cf.: https://en.wikipedia.org/wiki/Numeri...s_Code#History
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Old 12-01-2021, 10:00   #17
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Re: Using two insulated backstays as inverted Vee for SSB?

Re the inverted-Vee antenna: the included angle between the two legs of an inverted-Vee in the plane they lie in is generally assumed to be about 90-degrees. The description of the the mast stays suggests the included angle will be much more acute, perhaps only a spread of 10-feet, the beam of the boat, with an angle of maybe only 5 to 10-degrees.

With such a narrow Vee angle, even if modeled in free space, I don't think the input impedance or radiation pattern of the antenna would resemble the usual horizontally-polarized inverted-Vee center-fed half-wavelength doublet antenna pattern.

As for feeding the antenna with a tuner atop the mast, that sounds very impractical.

I suggest you first try feeding the two backstays in parallel from their lower end. You will create a sloping vertical antenna, and some sort of counterpoise will be needed.

An alternative might be to connect the backstays together at the mast top, then feed them as a loop antenna by connecting each end to a balanced feedpoint at the lower end of the stays.
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Old 13-01-2021, 14:32   #18
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Re: Using two insulated backstays as inverted Vee for SSB?

Sorry - been busy the last couple of weeks...

My original post included a link to a diagram of the setup in Google Photos, but it didn't render correctly. Here's one that should work:



I should note that it's not entirely accurate - the mainmast backstays terminate at the forward mizzen stay chainplates, not aft.

And not that it makes any serious difference, the distance between the port and starboard mizzen chainplates is about 14 feet, not 10 as someone suggested. Connecting the stays at the masthead and at the foot to create a loop is technically doable, but would be right near my main VHF and AIS transceivers, and the autopilot computer. Probably creating more problems than solving...

When I get time, I'll do a bit of antenna research - my old copy of the ARRL Antenna Book was lost in a previous move...
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Old 19-01-2021, 02:24   #19
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Re: Using two insulated backstays as inverted Vee for SSB?

You can download a .pdf copy of the ARRL Antenna Book on the web - it is a great resource and highly recommended. Hams definitely know a lot more about antenna design than boaters. My basic understanding (I may be wrong) is as follows:

1. On a boat you want to use a vertical antenna with seawater as ground. Seawater is a perfect ground, so you definitely want to leverage this free resource.

2. The antenna radiates perpendicular to its axis but the angle of radiation is very broad, say 160 degrees. So, on a boat most of the radiation goes towards the horizon (which is what you want, for DX at least) and the part that goes towards the water gets reflected as well, which is good.

3. All the other antennas, dipoles, directional Yagi's etc. need a lot more space and also they need good separation from the ground (sea), i.e. 30-40 feet. You can imagine this becomes impractical on a boat. Also, it does not get you anything useful. A lot more people will be using verticals on land if only they could have good ground. We have a perfect ground at sea, so we should use it by means of a vertical antenna.

4. The rig definitely complicates things but it is too complex to worry about. Just try to get as much separation as possible. The most important thing though is to have the antenna tuner as close to the feed point of the backstay as possible. Two feet is ideal. Most of the energy radiates in the beginning of the wire, so you do not want to waste this.

5. In the end, the antenna/ground may not be as important as we think. Clearly, the better the antenna and the more solid the ground, the more efficient the radiation pattern will be. In the old days of voice communication you needed a good signal to noise ratio to be heard. These days people use mostly digital modes (PSK31, JS8, FT8). These modes have 12-15 dB advantage over voice. Even CW is a lot more efficient than voice. Once you get away from the RF noise of the marina, have relatively OK ground, a tuned antenna and well calibrated soundcard or Pactor modem, you will be in much better shape than most hams on land.
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