Thread: Finally planning to use US2

Originally Posted by Corker

In shallower water, the cones must not overlap.

Correct ...


Rickie

After more research, I can see that it's the cone angle that matters, not the frequency of the transducer. Most the HB head units operate at 83/200 kHz & so far it looks like 200 kHz is synonymous with a 20 degree cone angle (for HB.) Moving to Garmin, however, that relationship changes. The bottom end "Garmin echo 100" operates on a single frequency (200 kHz), but the bundled transducer has a 60 degree cone angle! If the head unit also causes the US2 transducer to cover a 60 degree angle, this would cause cross talk as shallow as 10'.

Let me extrapolate a little on how crosstalk inference works ...mainly to identify that "Frequency" is the major factor in possible crosstalk interference ...

A sonar ping of a certain (any) frequency is not proprietary toward brand or unit ... A 200kHz transmit signal has certain specific wave form properties that are the same ...regardless of what unit/brand transmitted that 200kHz transmit signal ...

Brand does not dictate frequency wave form shape properties - Science does ...

If there is a 200kHz sonar ping in the water (transmitted by any unit) ... ANY sonar unit "listening"'for a 200kHz return echo can pick up that 200kHz return echo ...

And that includes any reverberated echo (or stray echo) in that same frequency (double echo, triple echo, echo returns from the outer fringes of the sound pulse, etc,) ...

The problem arises in the scenario of 1 unit "listening" for a 200kHz return echo ... But that same unit did not "transmit" that 200kHz transmit ping ...

This makes it impossible for the "listening" unit to determine how much time has elapsed from the "transmit signal" to the "received return echo" (Duty Cycle)...of that specific 200kHz sonar ping = Unknown transmit start time + Known return time = confused unit = crosstalk interference ...

So while shallow water will reduce the likelihood of crosstalk interference (between 2 separate units transmitting the same frequency) ...and cone angle can have a factor in the likelihood of crosstalk interference ... It does not definitively eliminate the possibility of crosstalk interference ...

Because that 200kHz transmit pulse can be still in the water on the double and triple echo ...and picked up by any unit still "listening" for a 200kHz return echo ...

Rickie

Originally Posted by Corker

it's the cone angle that matters, not the frequency of the transducer

Originally Posted by rnvinc

So while shallow water will reduce the likelihood of crosstalk interference (between 2 separate units transmitting the same frequency) ...and cone angle can have a factor in the likelihood of crosstalk interference ... It does not definitively eliminate the possibility of crosstalk interference

There are several factors at work here and my wording wasn't the best. Let me try again.

Frequency of the transmitter matters if it matches the frequency to which the listening unit is tuned. The best choice for a two-finder setup would be for the two units to share no common frequencies. The 1197 operates at 83/200/455/800 kHz. If the yet-to-be-determined bow unit transmitted at 150 kHz, for example, it would hear the returns but the 1197 would not. Neither unit could be confused by the other unit's signal. Unfortunately, the bow transmitter (the transducer built into the US2) operates at 83/200 kHz and that means there is no way to eliminate 100% of the cross-talk. The best solution, then, will be to minimize cross-talk.

By selecting a new finder that operates on either 83 or 200 kHz, but not both, I will eliminate half of the sources of interference. Further reduction can only be accomplished by reducing the strength of the foreign signal that reaches the listening sides of my two units. I'm not aware of any way to adjust the output wattage of a finder (suggestions are welcome), so the only remaining option is the positioning of the transducers so they are not pointing at each other--either directly or by way of a "bank shot" off the bottom or suspended targets. This is where the cone angles become important.

The 1197's transducer operates with 20/74/84 degree cone angles--the 20 degree angle being the 200 kHz signal. For the sake of discussion, let's say the new unit will also operate at 200 kHz with a 20 degree cone angle. Both transducers will be set as close to a vertical alignment as possible, so each will transmit most of its signal toward a circle on the bottom of the lake. The diameter of these circles increase as water depth increases. At some depth, the two circles begin to overlap and that means each sending unit is now bouncing the edge of its signal directly toward the other unit's transducer--causing a significant increase in cross-talk. My only remaining option is to position the two transducers as far apart as possible and recognize that cross-talk could become a noticeable problem beyond a certain water depth.

Very well said ...

You can aim the xducers away from each other about 5° and reduce the likelihood of crosstalk even more ...

Rickie

Yeah, I thought about that. The US2 won't move unless the entire TM is tilted...not not a good thing. That leaves the side scan transducer. The 2D and down scan views won't be affected too much, but what about the side views?

the more I read about US2 the more I think it isn't worth the trouble to hook it up. I have a Maxumm 80 with US@ and a 'ducer mounted on the bottom of it for a 798 and it can be adjusted up down or rotated side to side, just loosen it a little bit adjust and re-tighten.

The US2 is an example of poor market research. There's no question that it's very desirable to eliminate loose transducer cables. Anyone who's ever broken a cable by over-rotating a TM would love to have the cable safely tucked inside the TM's shaft. Ten years ago, the US2 would have been welcomed with open arms. When it was introduced, however, its plain vanilla sonar transducer was no longer a state-of-the-art feature. Side-scan had arrived. By bundling yesterday's transducer with state-of-the-art top end TMs, M-K created an almost useless combo. Those who could afford the most expensive TMs ever sold were likely to be the same fishermen who could afford the most expensive finders ever sold. US2 would have been a better add-on for TMs with mid-range prices.