Weak AIS Signal?

[BigBeakie]

John,

Something I've wondered about re which coax to use. Doing a new cat with 18 meter mast and then about another 6 meters to get to the VHF at the nav station, so when I look up the signal loss over that distance for RG8x, it is almost all the dB of the antenna if its a 3dB antenna. Thats why I thought LMR400uf would be the go. What am I missing?


Sent from my iPad using Cruisers Sailing Forum

[Paul Elliott]

You can ignore the dB gain rating of the antenna. For a monohull, you want a short whip. For a cat or powerboat you can use a longer, higher gain antenna if you like. The shorter antennas have a wider vertical pattern so they still work OK when the boat is heeled.

A short antenna is usually around 3dBi, which is gain in decibels relative to an ideal isotropic (point-source antenna), which does not exist in the real world. The longer antennas have more gain, but at the expense of a narrower vertical pattern. But you are stuck with these numbers by the antenna you have to use -- the antenna gain and coax loss are not really tied together.

I see a loss of 3.4dB for 24 meters of RG8X at 162 MHz. That isn't great, but it's not horrible either. I've got a run of RG8X almost that length to my short AIS whip on the upper spreader, and it works very well. If you use the LMR240UF that ka4wja suggested up-thread you can cut that loss down to about 2.5dB. That's better, but not so much that I would lose any sleep over it.

If your problem is indeed old, corroded, lossy coax, then replacing it with either 8X or 240UF will make a huge improvement.

[theway]

Does anyone know of the available Australian substitute for the Shakespeare 5215?

I’m at GoldCoast Whitworths and their supplier only carries the 5396 which is fiberglass and $395 aud.

I’m considering just have a friend bring me the 5215 from Hawaii which is $84 usd... convenient timing.

Thanks

[IslandHopper]

Originally Posted by theway

Does anyone know of the available Australian substitute for the Shakespeare 5215?

I’m at GoldCoast Whitworths and their supplier only carries the 5396 which is fiberglass and $395 aud.

I’m considering just have a friend bring me the 5215 from Hawaii which is $84 usd... convenient timing.

Thanks

Sounds expensive....

Try here >>> https://www.keoghsmarine.com.au/Shak...dB-Gain-SP5215

....there on the Gold Coast to

Unit 8 / 227a Brisbane Road,
Labrador Queensland 4215
Ph: 07 5529 2544
Fax: 07 5529 3676

https://www.google.com.au/maps/place...5!4d153.378137

[theway]

Originally Posted by IslandHopper

Sounds expensive....



Try here >>> https://www.keoghsmarine.com.au/Shak...dB-Gain-SP5215



....there on the Gold Coast to



Unit 8 / 227a Brisbane Road,

Labrador Queensland 4215

Ph: 07 5529 2544

Fax: 07 5529 3676



https://www.google.com.au/maps/place...5!4d153.378137

Awesome find!
I’ve been googling for days.
I’ll try them today.

Thank you much

[IslandHopper]

Originally Posted by theway

Awesome find!
I’ve been googling for days.
I’ll try them today.

Thank you much

No worries

I've used Keogh's quite a lot and been very happy with there prices and service.

Good Luck!

[theway]

Update:

I got the 5215 from Keoghs and connected it at deck level. The range increased immediately and drastically. But VSWR was still “OK less than 3:1”. So I climbed the mast. And checked the connection and the old antenna. The connection was in GREAT shape!!! (kidding, see image below). I didn’t have a replacement so I cleaned it with a light sanding and connection cleaner spray. (still looking for non soldering pl259)

I swapped the antenna and got a bit more range and finally got a “GOOD less than 2:1”

I was curious if the old antenna would be as good now that the connection was clean but it wasn’t, it went back to OK less than 3:1 and I wasn’t seeing updates from the boat the was 28NM away... as soon as I put the new antenna back on it updated... and back to GOOD less than 2:1.

I’ll get that mast pl259 connection replaced ASAP.

Thanks again for all the help and yes it was quite simple as many of you suggested it would be...

[IslandHopper]

Good stuff, looks like you are on the right track

As an aside, would it be to much of a task/cost to run a new antenna cable?

[transmitterdan]

That connector does not look so bad. Maybe a better picture would help. If you cannot replace with a good crimp one then I would leave it alone. Silver plated connectors turn brown/yellow naturally. That is not a sign of corrosion. Don't sand the yellow off. It is a good conductor of RF.

[continuouswave]

Re the PL-259 connector as it appears (in images posted above) at the masthead:

The first problem is the reduced that has been used to adapt the PL-259 to smaller diameter transmission line is poorly installed. It appears to have been threaded all the way onto the connector body.

Also, for decades I have been using Scotch 33+ vinyl electrical tape to wrap PL-259 connectors that are exposed to weather. Several wraps of Scotch 33+ tape onto the connector generally provide good weather protection.

There are also some newer tapes that tend to melt or bond together that can be used for antenna connections.

As for resurrection of that existing connector: I would not bother re-using it. The cost of a new connector is minuscule compared to the over project cost of installing an antenna.

Regarding loss in the transmission line: every bit of transmission line loss is lost signal, both on receive and transmit, so any decrease in transmission line loss will pay back immediately with more signal strength. Of course, you can to to extremes, but don't throw away one or two decibels of signal in the transmission line when you could get that signal back with a slightly higher transmission line cost. As a general goal, transmission line loss ought to be held to not more than -1 dB. On the other hand, antenna height is very effectual in improving communication range, so adding an extra 25-feet of transmission line (and its loss) to double the height is a good investment.

[transmitterdan]

All I am pointing out is that replacing a PL-259 at the mast head is a non-trivial task for most owners and even many yard workers. That connector isn’t the big problem as evidenced by the huge drop in VSWR by just swapping antennas.

If it ain’t broke don’t fix it; is pretty good advice for many situations. I think this is one of those.

[continuouswave]

Originally Posted by transmitterdan

All I am pointing out is that replacing a PL-259 at the mast head is a non-trivial task for most owners and even many yard workers. That connector isn’t the big problem as evidenced by the huge drop in VSWR by just swapping antennas.

If it ain’t broke don’t fix it; is pretty good advice for many situations. I think this is one of those.

I agree that sitting on a bosun chair at the top of a mast is no place to install a new connector. And under no circumstances would I recommend repairing a connector that has been out in the weather for several years to a length of transmission line that is unknown and has been flopping around inside a sailboat mast for years. Get rid the old transmission line and the old connector and the old antenna. They're on a sailboat that probably cost $200,000. Invest $500 to make this problem go away for a decade or two.

The corollary to the problem of the connector at the end of a transmission line that will be at the top of a mast: install it properly to begin with so you don't have any worries about it. Having the reducer only partially threaded into the main connector would be a rather clear sign something about the original assembly of that connector was not quite done properly.

ALSO: I meant to say in the earlier post (that can't be edited) that the REDUCER is NOT threaded into the connector all the way. That's not how they're supposed to work.