Aerial SPLITTER - useful, no use, problematic?

Salty John said:
Nigel, How can you criticise splitters and then recommend a stubby antenna with 20m of RG58! Performance will be appalling.
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RG58 should have about 4dB for 20m?
Stubby could have a bit of gain, but might be unity gain.
That sounds very workable.

There are plenty of installations out there with more than 4dB loss.
 
lw395 said:
There are plenty of installations out there with more than 4dB loss.
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I'm sure there are but that doesn't make it right. A 3db loss is half the signal. The offshore racing authorities say you should have no more than 40% loss in your antenna system - they don't recommend RG58 at all, they specify RG8X up to 20m, thereafter RG8U or RG213. I'd be happy to use RG58 of good quality up to about 20' (6m) with a proper 3' whip antenna.

But my point to Nigel was, as an upholder of excellence in safety equipment why support mediocrity?
 
Salty John said:
Nigel, How can you criticise splitters and then recommend a stubby antenna with 20m of RG58! Performance will be appalling.
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I didn't recommend it, just adding info to the other post. In fact I thought they both looked a bit cheap! I've added a disclaimer :)

I don't like spitters because they add at least four points of failure to a critical system: the VHF radio.

I think that stubby antennas would be ideal as a push-pit antenna for an AIS transceiver. Long range is not needed here, in fact it can be a disadvantage: in causing screen clutter and using up the available time-slots.

I would welcome suggestions for better quality models, but I don't want to hijack the thread.
Personally, I like Glomex, so would consider this one: http://www.glomex.it/product_details.asp?id=RA111AIS
 
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While I realise very few of us, if any, will be offshore racing, the rules they generate seem as good a guide as any to best practice. There is now a requirement for an AIS transponder aerial at the masthead and I am confused as how best to configure the masthead aerials.
 
awol said:
While I realise very few of us, if any, will be offshore racing, the rules they generate seem as good a guide as any to best practice. There is now a requirement for an AIS transponder aerial at the masthead and I am confused as how best to configure the masthead aerials.
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Yes, masthead mounting of more than one vhf antenna is a bit of a problem on the size of boat most of us sail. Ideally you would want the antennas to be 0.95m apart - half wavelength. Some say a quarter wavelength is OK - say 0.5m. Two typical stand off brackets on either side of a 180mm wide mast would only give 0.54m.
However, you could consider mounting one of the antennas upside down to give vertical separation of the antennas. The upside down antenna would be operating in the shadow of the mast for a small portion of its coverage. Another option is to put one antenna on a 1m extension. Yet another option is to mount one antenna further down the mast to give some degree of vertical separation as well as the horizontal separation.
A 1m flat bar with a 17mm hole at each end bolted across the top of the mast is the simplest way to get the antennas far enough apart, if not the most elegant solution.
 
So on a slightly related question...
When you go to the Chandlery and buy a blister pack containing the aerial, a mount and some cable in your experience what cable do they supply? Is it any good or should it be ditched?
 
Salty John said:
I'm sure there are but that doesn't make it right. A 3db loss is half the signal. The offshore racing authorities say you should have no more than 40% loss in your antenna system - they don't recommend RG58 at all, they specify RG8X up to 20m, thereafter RG8U or RG213. I'd be happy to use RG58 of good quality up to about 20' (6m) with a proper 3' whip antenna.

But my point to Nigel was, as an upholder of excellence in safety equipment why support mediocrity?
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1) Most people don't use all 20m
2) 40% loss is 2.2dB, so you are worrying about a maximum of 1.8dB.
That really is barely noticeable in they typical dynamic range.
Most systems that are noticeably duff are at least 20dB lacking.
If you want the last ounce of power then improving the match, upgrading the connectors etc is probably more fruitful.
But its easier for RORC to tick-box the legend on the cable than to turn up with a VNA and actually measure things.
Height of aerial is general worth the cable loss.

The biggest issue with coax is generally that the braid corrodes causing the loss to shoot up, not what the loss was when the cable was new.
Decent connectors that seal the cable are the best investment.

If cable loss was that important, we'd stick the power amp and first receiver stage up the pole, like an old style GSM car kit. But in practice, it's rarely worth it, unless you have a very long run to the radio and the weight of the cable matters. OTOH, I think I've got the bits to do it around here somewhere.....
 
nigelmercier said:
Actually, on the cheaper of these two you have to.

"Important Note: The 20m cable fitted to this antenna acts as a tuned ground plane and should not be shortened or lengthened"
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That needs further information to understand how it's going to work as a 'ground plane' if it's mostly inside a metal mast.
I think I'll avoid that model. Which is it exactly?
 
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