VHF Aerial on the side of the mast

[RestlessL]

Without going into the reasons why, I am considering mounting a stub VHF aerial on the side of, and half-way up the mast (just above the spreaders). I will probably stand it off by about 6".

My question is, will there be sufficient refraction of the signal round the mast to give 360 degree coverage? Given a radio wavelength of about 2m, and the mast diameter only a tenth of this, I would have thought so, but has anyone experienced this ?

John

 

[Salty John]

I can't say from personal experience what the effect will be but I can say that theoretically you will have a large shaded area, a blind spot, caused by the mast. The usual recommendation is to locate a vhf antenna at least one meter, 2 is better, from any vertical obstruction.

 

[graham]

I once had a VHF aerial on a standoff bracket on the front of a mast above the forestay attatchment on a fractional rig.

If there was a blindspot I never noticed it.

 

[Evadne]

I have stood on the deck of a steel ship with a hand held vhf and talked to someone inside the ship, on the other side of several bulkheads. The signal may well be distorted or attenuated but the effect of a thin conducting pipe in the near field ill not arrest the radio waves.

 

[Billjratt]

Not just the blind area to consider, but also the potential reflector function that the mast could have. You may, depending on the gap, get quite a narrow lobe out to one side. Having said that, I've seen lots of large yachts with all sorts of aerials on their (large) spreaders.

 

[MM5AHO]

Theoretically the mast would be like the reflector in a directional antenna. The "yagi" type antenna is a bit like this (TV ant are usually multiple element yagis). The reflector is longer than the "driven element" (your whip in this case). The actual results will depend on many factors including there being a connection or not between the 2, like distance between, diameters etc

But in practice you probably will not notice much difference. But this would not be a wise move for your primary vhf marine band antenna.

 

[Beadle]

I've just been running this through the antenna simulator.

First the good news the loss in the direction of the mast is not too bad. The 3dB loss point are about 110 degs either side of the antenna. Maximum loss a bit over 6 dB.

I could live with that.

The bad news is that it shows as impossible to match to 50 Ohms. The SWR is in 4 figures.

I suspect that the antenna is in effect trying to drive power into the mast which is not a good radiator for VHF - Very high capacitive impedance

Broadly speaking you will be using most of your RF power to warm the mast.

I will have another tinker later tonight but I suspect even if I can find a simulation solution the practical implementation will involve so much careful trimming and measurement (pratting about) as to be impractical

The simulator assumes a parallel aluminium pole and a 5mm copper wire for the antenna - neither of which is actually the case but near enough to demonstrate the principle

 

[RestlessL]

Thanks for the responses.

I already have a Metz (from saltyjohn) at the mast head as my primary VHF aerial, and am considering the options for an AIS aerial. So far I have considered:

(1) 2nd aerial at the mast head - concerned about 25W from the VHF frying the AIS unit (unless I can find a suitable suppressor in the AIS feed, or a means of isolating the aerial when the PTT is pressed).

(2) a small stub VHF aerial at the mast head, inverted and in line with, but underneath the primary aerial. Each would be in the other's blind spot, but I want to keep the mounting simple, and would probably end up with the stub alongside the mast anyway.

(3) As I proposed, mount the stub aerial on a stand-off bracket half way up the mast. This seems the simplest option if I can get away with it.

(4) Mount the AIS aerial on the pushpit (where it is at present). However, I may fit an AIS transponder next year, and am not happy to have a transmitting aerial just behind my left ear.

(5) Fit a radar arch over the cockpit and stick everything up there - probably the best solution, but seems a bitt OTT at present as all I want is an AIS aerial.

John

 

[MM5AHO]

If its only a receiving antenna, then put it where you suggested. It'll hardly see any difference, and impedance matching isn't very important.

 

[Beadle]

I agree

Didn't realise we were talking receive only

RTFQ Beadle!

 

[RestlessL]

[ QUOTE ]

Didn't realise we were talking receive only


[/ QUOTE ]
Sorry - I should have made it clear - I want to be able to fit an AIS transponder - transmit and receive. I think they push out about 4W.

Not happy with an aerial which transmits regularly close to the cockpit, and could do with the additional range, which is why I want to replace the current pushpit mounted aerial, and mount the replacement higher up.

John

 

[Nick_Pam]

4w from an AIS transponder at VHF frequencies on the pushpit will expose you to about the same radiation levels as using a VHF Handheld at 1W by your head (i.e. none at all) and considerably less (allegedly) than a mobile phone working at near microwave frequencies by your ear!

As the AIS signal is digital, it will work more efficiently over the same distance than an analogue (voice) signal and therefore for a given efficiency level, will work at a lower antenna height....I'd leave it where it is as you are likely to blow the output transistors in the transmitter mounting an aerial so close to a great lump of metal....see previous poster re "warming the mast"

 

[Salty John]

If you intend to transmit, here are the MCA guidelines for installing the antenna of a shipboard AIS system:
'2.2 VHF Antenna Installation

2.2.1 Location

Location of the mandatory AIS VHF antenna should be carefully considered. Digital communication is more sensitive than analogue/voice communication to interference created by reflections in obstructions like masts and booms. It may be necessary to relocate the VHF radiotelephone antenna to minimize interference effects.

To minimise interference effects, the following guidelines apply:

The AIS VHF antenna should have omnidirectional vertical polarisation.
The AIS VHF antenna should be placed in an elevated position that is as free as possible with a minimum of 2 metres in horizontal direction from constructions made of conductive materials. The antenna should not be installed close to any large vertical obstruction. The objective for the AIS VHF antenna is to see the horizon freely through 360°.
The AIS VHF antenna should be installed safely away from interfering high-power energy sources like radar and other transmitting radio antennas, preferably at least 3 m away from and out of the transmitting beam.
Ideally there should not be more than one antenna on the same level. The AIS VHF antenna should be mounted directly above or below the ship’s primary VHF radiotelephone antenna, with no horizontal separation and with a minimum of 2 m vertical separation. If it is located on the same level as other antennas, the distance apart should be at least 10 m'.

This refers to a ship's system and the masts and booms are of that type and size. Still, the principles should be adhered to as far as possible.
Personally I'd go for a pushpit arrangement and have the appropriate patch leads to make this antenna my emergency antenna in the event of losing the mast, but that may not be a consideration in your case.

 

[RestlessL]

Again, thanks for all the advice.

I will go with a stub aerial on the pushpit.

John

 

[hightech]

You will likely end up with an SWR value that is not acceptable due to reflection back into the antenna, even if the antenna/cable match is perfect.