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Old 30-12-2020, 13:42   #1
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Using two insulated backstays as inverted Vee for SSB?

On my Formosa ketch, rather than having a triatic stay between the mizzen and main mast, each mast has dedicated stays - the main mast having two backstays running back to outboard chainplates which are about even with the mizzen mast.



Currently I have an old Icom M-700 SSB feeding the starboard backstay via an AT-120 tuner, and old Icom IC-706MkIIG ham radio feeding the port backstay via an AH-4 tuner.

The insulated portion of each are approximately 40-42' long, but with the lead wire coming from the tuners adding about 10-12', they're effectively 52-54' long or so, and probably don't match very well across the MF/HF spectrum. They did work, though, as I was able to retrieve grib weather forecasts via Airmail stations in Virginia and Massachusetts while cruising in the Bahamas and the Caribbean as far south as Grenada.

I was thinking of upgrading both radios in the next year or two, and was thinking if the performance across the MF/HF spectrum would be better if I used them as an inverted Vee dipole, feeding it from the top of of the mast, and using a manual switch to switch the single tuner and control/power lead between the SSB and the ham radio.

Anyone have any experience with NEC modeling software, or with the antenna modeling package for MATLAB? I'd like to play around with some designs to compare what I have to an inverted Vee.

I could also see if it's worth mounting the tuner at the masthead, or at the mast base and use coax up the mast to a balun feeding each pole/stay.

I know - if it ain't broke, don't fix it, but I'd at least like to model it to see if I can improve it.
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Old 30-12-2020, 14:39   #2
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Re: Using two insulated backstays as inverted Vee for SSB?

At the risk of getting jumped on by those on CF who might provide a treatise on the subject, the AT-120 and AH-4 are both "unbalanced" tuners. There is only one output connection for some form of long wire antenna. I have no idea how they would behave if you connected the ground connection on them to the other leg of your dipole.

A non-resonant dipole is usually fed with ladder line and connected to a "balanced" tuner. There's a brief article on the ARRL website and you can find lots more discussion online.

I like the idea, I've always had resonant inverted-V 15 and 20m dipoles on board because I find them useful. I have to hoist them when I want to use them, and then take them down. Having a non-resonant antenna in that form factor that you can tune would be interesting.
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Old 30-12-2020, 14:40   #3
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Re: Using two insulated backstays as inverted Vee for SSB?

Describe your Counterpoise?
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Old 30-12-2020, 16:45   #4
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Re: Using two insulated backstays as inverted Vee for SSB?

An inverted V is a single band antenna. If tuned properly, no tuner will be required. However, you may need to install a BALUN at the feed point, plus a Common Mode Choke at the feed point. So, basically you will limit your operating to one band. If you decided to add a tuner, you might be able to tune outside the band but your efficiency will probably be compromised. As mentioned before, an inverted V, or for that matter any dipole, is a balanced antenna and you plan on feeding it with unbalanced coax. No big deal, everyone does that.

OK. So what do you plan on gaining by using an inverted V? Why not just a single band vertical dipole for the main band you want to work? Put that on one side of the mizzen and keep the multi-band on the other side connected the tuner. Now, if you want to switch between the two you will need to devise a cleaver scheme to switch between the two radios.

JMHO
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Old 30-12-2020, 16:53   #5
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Re: Using two insulated backstays as inverted Vee for SSB?

A dipole (which an inverted V is) needs no counterpoise. Each backstay would be one leg of the dipole.
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Originally Posted by AA3JY View Post
Describe your Counterpoise?
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Old 30-12-2020, 17:17   #6
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Re: Using two insulated backstays as inverted Vee for SSB?

Dsanduril is spot on. The setup you are describing would be an unbalanced dipole that would require a balanced antenna tuner.

From an antenna modeling standpoint...the angle in your inverted V would be much less than the recommended 90-120 degrees. On an inverted V the the angle between the poles changes takeoff angle and the SWR. For this small angle the takeoff angle would be near NVIS and SWR would be very high decreasing the overall efficiency of the antenna system.

WRT to other comments..
You would never want to mount an antenna tuner near the top of the mast. (added weight aloft, maintenance nightmare, etc.)
You don't need a separate balun for each pole of a dipole. You should use one 1:1 balun to prevent the coax from radiating as part of antenna system.
One pole of the dipole is the radiator, the other pole is the counterpoise.

I do have EZNEC if you're interested in the actual modeled data.
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Old 30-12-2020, 17:59   #7
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Re: Using two insulated backstays as inverted Vee for SSB?

my first boat had both a SSB radio (Stevens) and a Ham radio (Kenwood).
The SSB radio came with an automatic tuner.

The boat was ketch rigged....the antenna was an insulated 1/8" wire that ran from a pvc pole at the stern of the boat up to the top of the mizzen, where it was separated from the mast by an insulator. The wire was probably 25' long or so.
The tuner was located at the base of the pvc pipe, inside the lazarette. The pvc pipe (about 8' high) was attached to the stern pulpit and used just to provide some height at the transom so nobody would run into the antenna. A basic end fed wire

I used the SSB to tune the auto tuner and had a switch that that could switch the antenna from SSB to Ham at the nav station. Once the SSB had tuned the antenna to a specific frequency, I simply flipped the switch to used the Kenwood at the same frequency, though some minor variation sometimes occurred as I looked for a free freq, but it didn't affect the signal that I could tell. The SSB radio had to stay on, as it powered the autotuner, but the the Kenwood was the radio in use.

It worked like a champ and I never had a problem getting anywhere. The orientation of the boat sometimes mattered, ie, whether I was n/s...s/n....w/e...e/w...

At one time, I had the antenna continue on to the top of the main. This gave me an extra 20' or so of antenna, but by this time, that portion of the antenna was at a very flat inclination and gave me no noticeable additional range so I removed it.

I later sold that boat and installed the identical setup on a sloop rigged boat, except off course, the antenna wire went to the top of the main mast. Coming from the pvc pole it did not interfere with any rigging or the mainsail.

As before, it worked like a champ.
It was simple, inexpensive, easy to install and worked like a charm.
My first boat had a steel hull, so a good ground. My second boat had two bronze grounding plates that acted as the counterpoise.

In both cases I never had a problem and could send and receive pretty much anywhere I wanted.
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Old 31-12-2020, 10:03   #8
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Re: Using two insulated backstays as inverted Vee for SSB?

I have done extensive modeling of a backstay antenna on a sloop. The simulations show that the mast, stays, lifelines and everything metallic are coupled with the backstay antenna and re-radiate. The antenna patterns change radically with frequency and boat heading. Your situation on the ketch is much more complicated and I would say impossible to predict. Keep your rig simple and try it.
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Old 31-12-2020, 10:47   #9
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Re: Using two insulated backstays as inverted Vee for SSB?

An inverted vee is just a dipole, and center fed dipoles are narrowband antennas by design. If you design and tune this antenna for any particular frequency, it will have very high VSWR at nearly all other frequencies (possible exception is at 3x the quarter wave frequency). The tuner will probably match the coax to your rig, but the high VSWR in the coax feedline for that long distance up to top of the vee will cause high transmission losses.

Putting a tuner at the feedpoint up the mast would partially solve the above problem, but certainly is not practical. And the narrow angle of the two backstays, as has been mentioned, means that you probably would be creating something more like a top-fed quarter-wave vertical antenna in terms of performance.

If you want to make any improvements, evaluate your grounding system and try to relocate your antenna tuners as close as possible to the point where the antenna connection exits the deck. You'll radiate less into the boat and more to the ionosphere.
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Old 31-12-2020, 11:33   #10
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Re: Using two insulated backstays as inverted Vee for SSB?

Quote:
Originally Posted by Discovery 15797 View Post
From an antenna modeling standpoint...the angle in your inverted V would be much less than the recommended 90-120 degrees. On an inverted V the the angle between the poles changes takeoff angle and the SWR. For this small angle the takeoff angle would be near NVIS and SWR would be very high decreasing the overall efficiency of the antenna system.
This is exactly the issue with what Beausoleil proposes. An inverted V needs to be as flat as possible. The idea of an inverted V is to use a single support instead of two. The sharper the angle the less efficient the antenna.
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Old 31-12-2020, 11:52   #11
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Re: Using two insulated backstays as inverted Vee for SSB?

There is certainly an " art" or " skill" to Ham radio. My girlfriend of the day, had a dad that was an avid old school ham, built most of his own radio's, antenna's, etc. He didn't want me traipsing around the globe with his daughter unless he was able to communicate with us.
To this end, he pushed me into getting my extra class license as the general class freq's were too busy for his taste. Knowing where we were all the time, he was able to tell me what frequency I needed to be on at what time. Off course, I knew most of this too, but not like him...though he had a variety of antenna's in his back yard, he invariable used an end fed long wire...but his wire was lo-o-o-o-n-g, mounted on a tall mast. He also used other hams in other places to reach me and vice versa.
Prior to leaving, I was a member of a local ham club. A few times a year we would head off to local farm and set up all sorts of antenna's to try and reach the far corners of the planet. I learned a lot.
My point being, that just having a Ham radio on the boat is just one part of the puzzle.
Knowing how and when to use this radio is also a part of the puzzle.
I've seen a variety of ham antenna's on boats...whips....dipoles.....tuned backstays....my own setup, etc...but my relatively simple setup worked marvelously. A simple end fed wire led to the top of the mizzen mast. There was no another shroud or stay anywhere near it.
But...to each his own.
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Old 31-12-2020, 12:36   #12
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Re: Using two insulated backstays as inverted Vee for SSB?

A question, would it act as a directional antenna?
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Old 31-12-2020, 17:05   #13
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Re: Using two insulated backstays as inverted Vee for SSB?

These days, it is possible to do computer simulations to answer the questions of antenna pattern, efficiency and VSWR, but when things get complicated, like with a rig with stays, etc. the computer models are hard to create. 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 usually from brass or other easily worked material (since their ships are steel - we could just use wood). The models were placed on and connected to a large sheet of lead, which turns out to have a similar conductivity as seawater (I donít know if they formulated a compound to get really accurate). Antennas were put on the models, and measurements were taken using frequencies at a multiple of the actual operating frequencies depending on the scale of the model. Instrumentation antennae were moved around to determine pattern, and efficiency and VSWR were easily measured also. Then, since they always found problems, antennas were moved around the ship and designs were changed to get good performance.

So, if you built a notional - it doesnít have to be museum quality) of your boat at say a 20:1 scale, and then measured at 20x your operating frequencies, you could quite accurately see the pattern and measure VSWR etc. One nice thing is that since you are measuring at pretty high frequencies, the instrumentation antennas can be quite small.

It would be pretty cool if somebody out there needed a fun project to while away our Covid-driven inactive time! Who knows - it could turn into a business.

The Navy lab at Pt Loma is really interesting to visit. The models are fantastic to look at - battleships, cruisers, aircraft carriers - all made out of brass and including all the radar and commas antennas all connected to allow measurements. The models are quite detailed, including railings, guns, etc. They also used the models to measure coupling between antennas to determine interference and selectivity requirements on radios. What fun for the guys working in that lab!
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Old 31-12-2020, 17:53   #14
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Re: Using two insulated backstays as inverted Vee for SSB?

Quote:
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A question, would it act as a directional antenna?
In short, no it would not.

The benefit of an inverted V dipole antenna as compared to a flat dipole antenna is that the azimuthal radiation pattern tends to become more omni-directional. But that comes at a cost.

A dipole antenna is somewhat more directional, and by sloping a dipole you are able to change the takeoff angle of the radiated signal to some degree.

However, all of this is somewhat of a mute point wrt sailboats due to the number of additional uncontrollable variables compared to a land based antenna.

If you really want to dig into antenna design I would recommend 2 ARRL books. Basic Antennas: Understanding Practical Antennas and Designs and The ARRL Antenna Book for Radio Communications.
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Old 01-01-2021, 18:02   #15
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Re: Using two insulated backstays as inverted Vee for SSB?

I just noticed this paragraph may have confused readers:

"I could also see if it's worth mounting the tuner at the masthead, or at the mast base and use coax up the mast to a balun feeding each pole/stay."

It should read "SINGLE balun feeding BOTH poles/stays"

Since the AH-4 tuner is meant to be mounted outdoors, it shouldn't be hard to make it salt-resistant. And the weight wouldn't be a factor at the top of the mast - it weighs less than 3lbs.

I need to digest the replies so far - lots of info...
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