Gimbals mounted antenna?

[gshaw]

Looking today through a few old mags on radio stuff I came across a short article in a USA radio mag on mounting a VHF high gain antenna in a gimbals mount. This was to keep the antenna vertical despite boat movement. High gain antenna are NOT recommended for small boats as the narrow pattern of radiation can cause the beam to "miss" the target (Rx on shore/ship). Has anyone seen or know of such a device or is success?

 

[lenseman]

I think I can answer this problem as I wrote large parts of the original MPT 1250/1251/1252 specifications regarding marine radios for installation in ships and leisure craft including EPIRBs.

Firstly, the maximum Effective Radiated Power (erp) allowed for VHF radios is 14dBW (25watts) and the marine radio manufacturers make marine transceivers with this power at the antenna terminal.

You are not supposed to use an antenna with any form of gain and this is why a ¼-wave antenna (19 inch) is typically specified.

Having said that, if Ofcom did a survey of your installation and found that you had an antenna with gain you would need to explain and prove any losses in your installation between the radio antenna output and the antenna input. At best, you might have losses of ½ to1dB due to cable and connector losses and therefore your output at the Tx terminal would need to not exceed this power loss.

You can have your equipment confiscated, licence revoked and up to a £5,000 fine so it is quite a serious offence. This is all due to the fact that the radio spectrum is a finite resource and has to be shared by everyone

It is a similar offence if you start to increase the gain on any other fixed (moulded-in) antenna which will change the performance. These types of radios (transceivers), including WiFi and short range devices (446MHz), usually are set to transmit the maximum power and changing or altering the antenna can cause problems like intermodualtion products and spurious emissions and can overload other radios in the vicinity, marina or harbour which would certainly be frowned on!

I used to work as an radio communications engineer (Telecommunications Technical Officer) for the Radiocommunications Agency and latterly Ofcom. We were extremely hot on radio interference problems with an investigation officer sent to resolve the problems. As you can imagine, interference to the RNLI and Coastguard services would not make you the most popular sailor in the marina.

I hope this helps?

ps: They do things different in the USA and their laws are different from ours including N.I.

 

[Ubergeekian]

Firstly, the maximum Effective Radiated Power (erp) allowed for VHF radios is 14dBW (25watts) and the marine radio manufacturers make marine transceivers with this power at the antenna terminal.

You are not supposed to use an antenna with any form of gain and this is why a ¼-wave antenna (19 inch) is typically specified.

Having said that, if Ofcom did a survey of your installation and found that you had an antenna with gain you would need to explain and prove any losses in your installation between the radio antenna output and the antenna input.

I'm a little confused, probably through not listening hard enough to lectures on antennae <mumble> years ago. I thought that "antenna gain" referred only to directional effects: the power radiated in a particular direction compared to a hypothetical uniform spread. If you feed 25W into an antenna (without active circuitry) you surely can't get more than 25W out. Or am I missing something?

 

[lenseman]

I'm a little confused, probably through not listening hard enough to lectures on antennae <mumble> years ago. I thought that "antenna gain" referred only to directional effects: the power radiated in a particular direction compared to a hypothetical uniform spread. If you feed 25W into an antenna (without active circuitry) you surely can't get more than 25W out. Or am I missing something?

Antenna gain does not have to include active circuitry.

Don't forget we are talking about an antenna which is both a receiving antenna and also a transmitting antenna. Active components or circuitry are usually associated with received signal enhancement and if you are talking about active components in the transmitting side, then you are talking about linear amplifiers.

Most TV antennas on roofs of houses have 'gain', lots of it, 9dBi to 48dBi gain is not uncommon but there is no active component or circuitry in the antenna. You can of course use a mast-head amplifier but that is not the antenna.

 

[JimC]

Still Confused

Is Lenseman saying that, having restricted the power sent along the wire to your aerial to 25 watts, you will be breaking the law if the aerial focusses this power into a particular directional zone. If this is so then most of us are breaking the law.

The typical masthead aerial achieves a 3db gain by focussing the signal in a broadly horizontal way. Some of the longer aerials for powerboats, which don't have to allow for heeling and can concentrate the power into a narrower angle, have gains of up to 9db e.g. http://shakespeare-marine.com/antennas.asp?antenna=5018

Surely if these were illegal they could not be sold.

 

[Billjratt]

It's not illegal to sell these things (there's a lot of equipment especially radio. that is out of UK spec on the market) It is illegal to USE them in a way that breaks the law.
Theoretically, if you have a high gain antenna, and good quality cable, you should drop the power at the transmitter, but I bet you can't adjust it to the correct output, even if you had the gear to measure it.
Better to get a boat with high mast. 25 watts is adequate as it's more or less line of sight anyway. Calculate the horizon for you aerial height and have a think about it.

 

[lenseman]

Is Lenseman saying that, having restricted the power sent along the wire to your aerial to 25 watts, you will be breaking the law if the aerial focusses this power into a particular directional zone. If this is so then most of us are breaking the law.

The typical masthead aerial achieves a 3db gain by focussing the signal in a broadly horizontal way. . . . Surely if these were illegal they could not be sold.

Yes, I am afraid I am saying that! The antenna you quoted in the URL is for teh American market (FCC) and is illegal to use in this country and within Europe (Harmonised Standards) if it is to be used with a transmitter. You are perfectly entitled to use it solely for a receiver with no transmitting capability.

The other oft misquoted specification is the dB gain of an antenna. Most advertisements fail to mention whether it is referring to dBi (gain relative to an isotropic radiator) or dBd (gain relative to a dipole). I strongly suggest it is dBi. These gain figures can only be confirmed on a 30-metre Open Area Test Site using calibrated dipoles and a calibrated RF signal source, the work of which I am very familiar.

 

[JimC]

Probably the most widely used masthead aerial in the UK is the V-tronix, often fitted with the Hawk wind indicator. It is quoted as having a gain of 3dbi on the manufacturer's website, http://vtronix-antennas.com/antennashow.asp?cat=sail&show=ywx. I have this aerial myself, coupled to a radio with a 25 watt output. Can you say in plain language if I am breaking the law by transmitting via it. V-tronix are in Fleetwood, trading quite openly. Some of their other aerials have higher gains

 

[lenseman]

. . Can you say in plain language if I am breaking the law by transmitting via it. . . . . . Some of their other aerials have higher gains

Hiya Jim - the ¼-wave antenna which you use is perfectly legal as are any other antennas which that company supply. Your marine transceiver has a maximum theoretical output power at the SO239 of 25 watts (14dBW) and there will be losses from the final stage of the transmitter right the way up to the antenna including all the connectors and cabling and the SWR in the cable. Therefore you are in effect transmitting LESS than the 25 watts from the transmitter probably in the order of 18 watts (12.55dBW) or more simply stated 1½dB loss from radio to radiator.

If you use an antenna with a greater amount of gain then you would need to run a correspondingly less amount of transmitted power into that antenna. For example, if you had a transmitter which only was capable of running 1 watt of transmitted power, then you would be perfectly entitled to connect it to and use an antenna with a gain of 14dBi.

You would not be prosecuted or have your equipment seized with the set up you are using but you might get inspected if your equipment started to interfere with other radio frequency users and might be told to shut down your station until the equipment is repaired or replaced.

I hope this helps?

 

[JimC]

Thanks very much David for that comprehensive reply.