Here’s the flyer we send out to retailers to support the Metz Manta VHF antenna, when we’re wearing our Metz Europe hat.
VHF antennas are also used by AIS engines. You may see antennas described as AIS aerials (or antennas) and what this means is the VHF antenna is tuned to centre on the AIS frequencies, 161.9 and 162.0 MHz, rather than on the broader range of radio frequencies including the emergency channel, 156.8 MHz.
The Metz Manta covers 156 MHz to 163 MHz, so includes all radio and AIS channels.
Metz offers an AIS optimised antenna for dedicated AIS applications.
A bad antenna system will make even the most exquisite radio perform badly. To get the best out of your VHF radio you need a top quality antenna system. As radio buffs are always telling us: “A penny in the antenna is worth a pound in the radio”.
VHF antenna systems comprise an antenna (aerial), a feeder cable and a few connectors. If you have a really good quality antenna such as the Metz Manta and use good quality connectors, you’ll want the correct coaxial cable to complete the system.
Coaxial cable for VHF radio is 50 ohm – your TV cable is 75 ohm so you can’t just use some coax left over from that satellite dish installation.
Furthermore, marine cable needs to work in a hostile and constantly moving environment. That’s why it needs to be tinned copper to resist corrosion and the centre core needs to be stranded so it can bend without breaking. A good PVC jacket will keep sunlight degradation at bay.
But what size cable should you choose?
I reckon you need to aim at a transmission loss of no more than 40% in the run from radio to antenna. This is also the opinion of the offshore yacht racing authorities so I’m in good company.
You’ll want to restrict the line loss (known as attenuation) to about 2.5 decibels (dB).
The picture shows, on the left, RG213 coax and on the right, RG8X.
RG213 (or its slightly lower spec but similarly stiff cousin, RG8U) is a whopping 10.3mm diameter, nearly half an inch in old money. So it’s heavy and doesn’t like to go around tight corners, but it only loses 2.2 dB per 30m length. On the right is its less uptight little brother – RG8X. This flexible fellow is only 6.5mm diameter, much lighter and easier to work with – it loses about 3.5 dB per 30m.
(RG58, which sometimes comes with cheap aerials is 5.5mm diameter and is very lossy – about 5 dB per 30m. OK for short runs, perhaps, but not for masthead installations).
So, if you have a cable run of up to about 25m or so you can go with RG8X, for much longer runs you’ll need to wrestle with RG213/ RG8U.
There you have it.
The marine environment is a brutal judge of the suitability of materials intended for use in it.
Here’s a picture showing the current Metz antenna coil alongside an 18 year old version of the same antenna which was, until very recently, at the masthead of an ocean going boat. Both have stainless steel bodies and the coil is packed in epoxy – indestructible, which is why Metz can give it a lifetime warrantee.
Alongside the Metz antennas is a picture sent to me by a reader – a non-stainless antenna that has succumbed to the marine environment. It clearly failed the test.
It’s that time of year – masts are back up, boats are out on the water and VHF radio problems are emerging.
Tracking down VHF radio receiving and transmitting problems is a fairly logical procedure and I’ve written a short article about it for the Salty John website, in the ‘articles and links’ section:
When your VHF radio fails to communicate, defective cable and connections are the overwhelming favorites to be the culprits.
Seen on a tee shirt: Paddle faster, I hear Banjos