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Installation of Yagi antennae

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rajsiyer

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I wish to know where to install antennae, on an oil platform. In the photo attached, the Yagi is mounted between decks. That is there is a lot of metal structure above and to the side of the antenna. I am not finding an "open to sky" location. There will always be metal structure above and beside the antenna. How does one manage. I am using LMR 400 on VHF using 158 MHz. How long could the coaxial cable be??
 

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WA0CBW

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The coax cable needs to be long enough to reach from the antenna to the radio! I'm sorry I couldn't resist that answer. To answer that question you need to know the attenuation per foot of the type of cable you are using. LMR400 has about 1.5 db attenuation per 100 feet. How much db loss you can afford depends on the gain of the antenna and the strength of the signal you are transmitting/receiving. 3db is a 2 to 1 ratio, 6db is a 4 to 1 ratio and 10db is a 10 to 1 ratio power ratio.
Are both antennas on the same/different frequencies?
BB
 

rajsiyer

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Yes the coax cable is now at @ 100 feet. It may increase to 200 feet, when it is moved to a new location. I think the present location (please see the pix) is not suitable as it has a roof of iron above and some structures to the right. The alignment direction will also be almost parallel to the hand-rail.

I am most concerned about the effect of the nearby structures. How badly would they affect the performance is what I wish to know. Question is.. is it worthwhile moving the antennae to a better place that will require more than twice the length of coax cable?

These Yagis have a gain of 10.5dBi, They communicate with an omni (gain of 3.0 dBi) about 5 kms away over the sea. Transmission power will be max 37dB (5w) but we've to run it at the lowest possible power. These are meant for telemetry data on 156 MHz, using an over the air baud rate of 19200. For BER < 1 ppm. the RSSI at both ends needs to be better than -75dBm.

Now coming to the 2 antennas.. They are attached to redundant modems. So only one will be active at a time. Does this mean that they could be mounted on the same pole.
 

bharvey2

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Yagi antennas

Can you move your radio closer to the antenna? If the connection to your radio is RS232 or better yet, RS485, you can run up to 1000' with the proper cable/driver circuitry. CAT5 could get almost as close. I'm guessing the the radio would have to be placed in a minimum NEMA 4 or 4X enclosure on an oil rig.
 

rajsiyer

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@bharvey2

Thanks,

But that can't be done. Main problem is the correct location of antenna.(Please see the picture) On the upper most deck is a helipad. So no antenna there. Below this deck, there are lots of piping and structures which come in the way of LOS if we attempt keeping the coax-cable length below 100' .A good antenna location for LOS and at a good distance from nearby structures is available. But placing the antenna there would mean the coax-cable length increases to more than 200 feet!

I have a hunch though, since the losses in coax-cable are a lot less than in free space, it may be worth the trade-off when you move the antenna to the better location.

Everything else ie. Power supply, radios, PLCs + HMI + all instrument connections are inside the Nema 4X panel, which is inside a MCC room, which itself is Flame-proof enclosure with pressure-tight doors. All cables + antennae leads enter this room through a MCT. Outside this MCC room it is hazardous zone. Either Zone-1 or 2.
 

popnokick

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Is it correct to assume you can't change the omni on the other end of the link? I'm guessing it's an omni handling multiple platforms from different directions (?)
 

davenlr

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If you look at the beam patterns for a yagi like you have pictured, you will find that they attenuate the signals off the back by about 15 to 20db, and the sides by >30db, so they should not be affected by metal above or to the side of the antenna, as long as you can point them at the omni with at least 10 degrees between the metal structure and the direction the antenna is pointed. If they are pointed correctly in your picture, they should be just fine where they are. If the signal is marginal, you might consider just getting one with more directors (added gain and narrower beamwidth).


The question I have is, if they are working now, why are you wanting to move them?
 

mmckenna

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So, at 100 feet of LMR-400, and at 158MHz, you are still getting almost 70% of your signal out to the antenna. With the antenna gain you are/should be doing pretty good on a 5KM link over the ocean. It shouldn't take much power to keep a good link on VHF. a few watts should do it. Keep in mind that installing a higher gain antenna on the other end will help. A 6 or 9dB gain omnidirectional antenna wouldn't hurt, either.

If you need to move things, you should step up to a better cable to keep your cable losses low. While many are concerned about the loss in transmitted power, you also need to be concerned about your received signal.

200 feet of LMR 400 is going to result in losing a bit more than half your signal to the cable losses. Stepping up to 200 feet of 1/2 inch heliax is going to bring your cable losses back to a bit less than 70%.

70% of your signal making it down the cable isn't bad, doesn't matter if it's 100 feet of LMR400 or 200 feet of 1/2" heliax. Many mobile radio installations lose almost that much signal just between the mobile radio and the exterior antenna.

Placing your directional antennas in a better location with better coax should help. The more signal makes it to your antenna, the better. Honestly, though, from the photo, I don't see any issues with the way you have it installed.

1/2 heliax is going to run about $2.00 a foot, and figure on about $30 for each connector. It's a bit stiff, so routing it through the rig will require some wider bend radii than the LMR400. It's also bigger in diameter than the 400, so if it's in conduit, you really need something like 1 1/2 conduit for single runs.
 

rajsiyer

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Thanks MMcKenna, davelnr & others,

The required direction for is in a line that is parallel to the hand-rail. There are 2 Yagis. The one in the foreground is almost bang-in-line with the massive red pillar you see in the photo. This antenna in the foreground does not work at all. Both need to work reliably as part of a redundant pair. One at a time though.

The other antenna which is farther away, though not in line with the pillar has problems. Please observe that it is not exactly parallel to the required LOS, but rather turned away outwards.

This is because it NEEDED to be turned AWAY from the LOS. When aligned on the LOS - by sighting through the central beam- we failed to receive any signal. I thought that Yagis gave the best signal exactly along the LOS. However, with the massive iron structures around I am proved wrong!

Only when put in this direction, (approx 15 degs.outward of the LOS,) set by trial and error using walkie-talkies at both ends, did we obtain the best signal on the other end. The best RSSI we get at the buoy (5 kms. away) is only a measly -112 dBm with about 20% loss of data packets! This simply won't do. We need a stronger signal - RSSI > -75 dBm with no loss of data packets for a reliable link.

If I go for an additional 100 feet of LMR-400, The loss in the co-axial will no doubt increase to >3dBm. The 5W TX will deliver 37dBm with 33 dBm at the antenna. Let us now look at the Free-space loss.

Propagation loss in air for 156MHz & 5 Kms. =32.5 + 20*LOG10(156.0)+ 20*LOG10(5.0) = -90.3dB

Add the Coax cable losses of 3dB. So we get total transmitter power loss of 90.3 + 3 dB = -93 dB

Tx Power RTU at 1 watt => Transmit power of 30dBm (Assume lower power of 1 W and not 5W)

At the receiver, the received signal will be.. 30 - 93.3 = - 63dB. This means even at 1 watt transmitter power, the RSSI at 5Kms should be very strong..at -63 dBm. By this reckoning, since the required RSSI for reliable communication is as low as - 75dBm, we would still have about 12dB to spare! -Plenty-. This is to the coax-cable loss (in LMR 400 @1.6dB/100') for 7,500 feet!.

Note that I have not added the antenna gain of 10.5dBi for the Yagi and 3.0dBi for the Omni at the other end. Let this be set aside for the fade margin.

Coming back to the "Location" issue, the RSSI I get observe is only -112 dB. The coax + LOS path loss at present can account for only - 93dB. So the additional loss of about 17dB seems to come from the location. This causes me to think that even with a very long (200'-300') LMR we will be better off without the bad influence of the interfering structures.

I am in automation, not a radio engineer. But this telemetry link needs to be good. The formulas I used above, are from the Internet, where I seek answers for things I don't quite understand. Really appreciate all help. Am I right in thinking along above lines?

Best wishes..
 
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