How VHF marine actually works.

[Ronje1]

A few bits and pieces that some might find useful.

What range can I get? What aerial should I use? Is it really only line of sight like everybody says?

The facts behind the fantasy.

121179

[tropicrows]

A few bits and pieces that some might find useful.

What range can I get? What aerial should I use? Is it really only line of site like everybody says?

The facts behind the fantasy.

121179Link not working

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[Ronje1]

Now that we know some basics, how do we extend the range to make VHF more useful?

VHF marine repeaters

121180

[Ronje1]

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Anybody else got the same thing?

[Tracker]

works for me,dl a pdf.

[Dignity]

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Bruce, check for downloaded files

[Ronje1]

Here's the third and final bit about VHF marine in Aus.

Bit of light reading and (I hope) interesting in how some govt departments failed to take on safety of life obligations that they had and how people power triumphed over bureaucracy gone mad.

Happy to talk about nitty-bitty stuff related to VHF marine if anybody wishes.

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[tropicrows]

Something I found interesting/disturbing a friend had a GME aerial that stopped working, so he decided to cut it open. What a surprise, the antenna only went up a 1/3 of the actual length. So you buy the long aerial thinking its better and its actually NOT.
The GME rep said they did that so people could match the length of the other aerials on there boat for looks, seriously, what a joke.


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[Tracker]

good read. when I lived in bankstown dick smith started selling am radio for around 90 bucks. you needed to fit crystals to set to operate on certain freqs.
we made our own aerials and they worked well.
what area do you live in Ronje?? may catch up for a beer one day.macca

[Ronje1]

Live in Rockhampton Qld.

With those older am 27meg radios, the pro fishermen used to swap the tx crystal and rx crystals in the opposite holders so that instead of tx/rx on 27.24, 27.88,etc they used to work on whatever the nominated channel was (say 27.88) 455khz higher or lower thus 27.88 became 27.425 mhz. Gave them "secret channels" (so they thought).

But 27 mhz marine filled a vacuum when it was needed and operated reasonably well but was still in the HF band so still noisy particularly when sunspot cycles allowed "skip" transmission.

[Ronje1]

Tropicrows

They obviously believed that there was a market for the "cosmetic" look.

Those fibreglass antennas were designed to operate without a reflecting metal ground plane under them like on a fibreglass vessel and were known as ground-independent antennas. Worked OK.

The thing to really watch out for are the claims made about the "gain" (power multiplying effect) of the VHF antennas on the market.

A ground plane antenna (remember the vehicle mount example I gave you), coaxial dipole type antenna, 1/2 wave ant each have a real gain of 2.2 dbi. They are relatively short antennas (maybe 1 metre or so).

A proper high gain vertical antenna suitable for small vessel is one which has a "stacked element" inside the fibreglass tube and is about 1.8 - 2m in length and which has a true gain of 5.2 dbi.

Creating confusion about the gain figures has been going on for years

You'll find all sorts of claims about the gain figures most of which are b.s.t.

The real gain figures are 2.2 dbi for the shorter antennas and 5.2 dbi for a stacked element VHF antenna. Look for the i suffix after the db term. i stands for isotropic and is part of the radiated power figure of EIRP - Equivalent Isotropic Radiated Power.

Just because a fibreglass antenna may be about 1.8 - 2m long, don't assume that the radiating part of the antenna inside the fibreglass is a real hi-gain antenna.

Here's one. https://www.gme.net.au/catalogue/antennas/vhf-marine-antennas-and-whips/aw368v.aspx ... an antenna that's nothing more than its claimed to be. A dipole type and note that they quote a gain of 3 dbi. Note the i. Somebody being honest. The fibreglass tube is 2.4m long so the top of the dipole is at the top of the fibreglass tube thus giving it more height above the water so it'll have a slightly better radio horizon. Remember the calculation?

Here's another one.https://www.ebay.com.au/p/1579101701?iid=333562874619 but this time look at the claim about the antenna gain. To have the gain claimed as 6db for this antenna, it would need to be about 5.8 metres long. Because of its length and the apparent presence of the the coil at the base for "matching" purposes, I'd say this is simply a 5/8 or 1/2 wave base loaded whip which isn't a high gain antenna at all. The feed arrangement simply means that the antenna radiation pattern has been compressed into a lower radiation angle.

Would work quite well but its not a true high gain antenna despite the claim.

Oils ain't oils, sol.

The introduction of vhf repeaters meant that the elevated repeater site could make up for the deficiencies found in most vessel antennas.

They were busy, significant and tumultuous times in the maritime industry and I was happy to be a part of it.

Here are a couple of copies (but modified) of those segmented circular stickers that I was talking about that were in use from the early 1980s in regional Qld but in more easily understood livery.

Only a non-operational public servant could dream up these modified contributions but that's what happens when non-operational people get mixed up in things.

.121199

[tropicrows]

Thanks, and very informative Ronje.

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