Acr ais with sitex splitter swr.. 1.5 too high?

[Linwood4]

I installed a Cb2 ACR Ais transever to a back-up vhf system last year. My cockpit vhf is on the mizzen mast. The new Ais(backup vhf)is on the main mast. I ran it by it self on the now disconnect back-up vhf line. It worked ok, I sometimes had issues with and I would reset it. I bought a Si-tex active splitter so I could hook my vhf back into the system. It worked but kept throwing swr too high errors. I installed an Ais antenna to lower swr...no luck. So I removed the Si-tex and put in a manual switch for emergencies and left it until later...
So now is later... I bought a NanoVNA to measure Swr of the antenna. 1.8 on the Ais frequency. Which is goodish. So I bought and I installed new Anchor RG 213 cable with solder connectors and ran it from antenna to Ais removing the mast step joint. The cable is roughly 70'. So now the SWR reading is 1.5. I installed the splitter and read the SWR thru the splitter..its 1.7. So it's Ok-ish...I hooked up the AIS and it ran for ten minutes and then threw a SWR and shut down. I am running the AIS in long range mode. So now I have a new antenna...new cable...new splitter and new Ais...and no luck. I have been reading up on SWR tuning as I'm not in the bottom of the wave length on the SWR reading. i believe lose due to my antenna cable length of 70' is the problem and can't be worked around. I'm out of options🙄 any insight would be appreciated. Thanks

[smac999]

if your swr meter is showing 1.7 and your ais is alarming. the ais is probaly the issue... it won't alarm untill 3 or 4:1

the ais software on laptop will tell you what swr the ais is reading.

anything under 2 is totally fine.

cable length isn't the issue.

[Linwood4]

Good info. Looks like a phone call to Acr is in order.

[continuouswave]

When the VSWR is measured at the transmitter end of a long run of transmission line to the antenna, the measured value of VSWR is very much AFFECTED by loss in the transmission line. The effect of measuring the antenna VSWR through a transmission line with LOSS is to cause the measured VSWR to be lower than the actual VSWR at the antenna.

This can easily be understood with a simple example. Suppose the loss in the transmission line is 3 dB. Suppose the VSWR at the end of the transmission line (where the antenna is supposed to be) is infinite because there is an open circuit. In theory the VSWR should be infinite.

Suppose the transmitter output is 100-Watts. If there is 3-dB loss in the transmission line, only 50-Watts will arrive at the end. With an open circuit, all 50-Watts is reflected back toward the transmitter.

The line loss of 3 dB attenuates the 50-Watts to be only 25-Watts at the transmitter.

Measuring at the transmitter gives 100-Watts forward power, 25-Watts reflected power. This is a VSWR of 3:1.

The example is an extreme situation, but the effect is always the same: any transmission line loss on the antenna-side of a directional wattmeter will cause the measured VSWR to appear to be better than it actually is at the antenna.

ASIDE: For a good explanation of the relationship between VSWR and percent reflected power, see this article:

https://0x9900.com/reflected-power-and-vswr/

A further consideration: the amount of loss in a transmission line tends to INCREASE with increasing VSWR. A line that would have a 3 dB loss when terminated in a perfect matching load will then have more than 3 dB loss whenever the VSWR on the transmission line is higher than 1:1. The higher the VSWR, the greater the additional loss due to VSWR.

Getting back to the situation under discussion here, the actual VSWR at the antenna does not matter. The transmitter (in the AIS in this case) can only measure the VSWR at its end of the transmission line. This is why the VSWR seems below 2:1.

As others have noted, any VSWR below 2:1 should not cause the transmitter to shut down.

A more likely explanation is perhaps there is something loose in the antenna, or in the transmission line, or in a connector that creates a momentary open circuit or a momentary short circuit. This could immediately alter the VSWR to be greater than 2:1, and the AIS transmitter may then shut down to protect itself.

Or, as already proposed, the sensitivity of the AIS transmitter to high VSWR is too great, and it is shutting off when it should be completely happy to continue transmitting into a VSWR of only 1.5:1.

Or, there is some other element in the system that is disrupting the operation of the AIS transmitter.

A possible method to test the influence of the antenna and its VSWR might be to connect the AIS transmitter to a 50-Ohm dummy load. This MIGHT work, if the AIS just thinks it is in a region where there are no other AIS stations--because the receiver won't hear any other stations due to the 50-Ohm load--and just starts transmitting when it decides it is free to do so because it can't hear any other AIS stations to synchronize with. Let the AIS transmit like that for a few hours, and see if it shuts itself down for any reason.

[Wotname]

^^ good info -