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» Online Classroom   » Radar   » Public Discussion of Marine Radar   » Backstay mount: shadow ahead!

Author Topic: Backstay mount: shadow ahead!
Michael Delorenzo

 - posted July 12, 2005 12:54 PM      Profile for Michael Delorenzo           Edit/Delete Post 
I would like to know how users who have mounted a radome on the backstay find the shadow from the mast straight ahead - directly in the direction the vessel is heading.
Is this particularly disconcerting?
Is this a reason to go with a pole instead?

For both, the danger may be the same, a vessel on collision course with constant bearing in the shadow - either straight ahead (backstay mount) or off the port bow (starboard pole)?

What people found in practice? Does it matter?

David Burch

 - posted July 12, 2005 01:40 PM      Profile for David Burch           Edit/Delete Post 
I do not believe there would be any effective blocking at all from the mast to a backstay mount. For example, conservatively, consider a 10 inch diameter mast, located 6 feet from a backstay mounted radar. The mast is then blocking out about 4° [arctan (5/6x12) = 3.97] either side of dead ahead — providing the boat was precisely headed in the same true direction every 3 seconds that the sweep passed this point.

It is highly unlikely that this would happen many sweeps in a row, and consequently you would see ahead, having a 4° region blocked at various forward locations on consecutive sweeps.

I have not heard of any issues of this in the past.

This possible (and i would propose negligible) blocking would not be among the reasons one might choose a mast over a backstay... those pros and cons are discussed in the book Radar for Mariners (RFM).

Likewise, i would not anticipate any significant blocking by the mast when mounted on a quarter pole.

Furthermore, if you look at the suggested profile of the beam emerging form the antenna (Figure 7-20 and 7-21 in RFM), then you will see that the beam of even an 18" antenna would most likely be passing on either side of the mast before beginning to fan out into its eventual HBW, which would mean that the shadowing would not be complete at best (worst!).

In short, i do not think this is an issue, and there are several arguments to support that.

Either mount, however, if close to the mast (say 6 feet or less), might, on the other hand, lead to weak ghost images when traveling along a prominent close shoreline, as discussed in chapter 9 of RFM. This is unlikely to be a problem, since tuning can reduce these as discussed in the book, and again, i have not heard of any reports of this either.

The more powerful the antenna, the more likely ghost reflections from the mast might show up. A typical backstay mount would typically not be a larger higher powered antenna, so again, this is not likely an issue.... i would think of "larger" as starting at 2 feet diameter and 4 kW of power.

From: Starpath, Seattle, WA
Michael Delorenzo

 - posted July 13, 2005 05:33 AM      Profile for Michael Delorenzo           Edit/Delete Post 
Your answer is what I thought before I started asking people who had radar mounted on their backstay. One said they don't notice a blind spot dead ahead, another reports there is one, and, for instance, when they put a buoy dead ahead, it disappears from the screen. Another confirms blind spot. These are approx 40' sailboats.

Could be coincidence, misreading the radar, dropped target, paranoia(!), etc. I assume these where 2kw scanners.

Anyway, to me, the backstay mount looks cheapest, cleanest, and rigger says so problem with the backstay (excepts a pain to pull the rig).
Thanks again.

David Burch

 - posted July 13, 2005 10:25 AM      Profile for David Burch           Edit/Delete Post 
Do i understand right that you have two different boats that report a blind spot dead ahead from a backstay mounted radar? ... and it seems if they "put a buoy dead ahead" the implication is that they are underway moving. I would have guessed that even underpower on a steady course this would not happen, so.... If that is the report, then we should test this.

Can you obtain the specifics for these two radars... ie brand and model, and radome size. If you can provide me with this information, i will find similar units and rigs in one of our large marinas here and do a test.

One thing is to have a theory that something should not happen, and another is to actually see it happen. If this is the case, it is important and should be tested.

Note that there are actually options on some radars to electonically block out a specific region so as to prevent ghpost reflections. Not normally in a smaller radar, but such things exist, or there could be some other malfunction... or it could be physically blocked.

With your input of this data, we will check it.


From: Starpath, Seattle, WA
Michael Delorenzo

 - posted July 14, 2005 07:39 PM      Profile for Michael Delorenzo           Edit/Delete Post 

I will try to get the info to you.

Yesterday asked the owner of a 42 ft Alden yawl w/ the radome on the mizzen. He said no shadow from the main mast as far as he knew. He did not know the power of his radome.


Karl Koch

 - posted August 24, 2005 07:41 PM      Profile for Karl Koch           Edit/Delete Post 
the only time I had any problem with a backstay installation was when a scanner was mounted in line with the boom. This produced what we used to call a hollow body echo. Other than the signal going up the boom and reflecting spikes back at the scanner I have never, in fifteen years, seen any problem with this type of mount.

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