Radio wave experts - Radar reflector on whip aerial

[lw395]

That type of antenna is usually a 5/8 wavelength whip. These are either stub tuned or coil tuned at the base. It is not advisable to hang any metal from them as it will affect the tuning. This can have the effect of a greater SWR causing power to be reflected back into the transmitter, if its too high it could lead to damage of the final transmit stages of the VHF.
Would I do it ? No is the answer as it will affect transmit / receive to a greater or lesser extent and it may just make the difference between being heard or not in the case of an emergency.

If it's a 5/8 whip, then the section of aerial parallel to the radar refelector is simply a straight conductor. So the reflelctor is at worst, a short conductor close and parallel to the antenna. Both ends of the reflector are open circuit, no part of it is grounded. It's small compared to a wavelength. The effect of it will be trivial.
Bear in mind that the bigger unknown when design a marine VHF antenna is exactly what the boat under the antenna looks like in RF terms. So the antenna is a best guess compromise. Hopefully, it's optimised to be OK with anything from a steel ground plate to a random lump of plastic under it. Unlike some serious antennas which are tuned to account for the ship or vehicle that is under them.

 

[Gumpy]

Depends on how the 5/8 is tuned, if series tuned with a coil then I agree totally, however if tuned using a shorted or open stub this comes further up the fibre glass and may be affected. I just don't think its wise to possibly compromise your VHF.

 

[lw395]

Depends on how the 5/8 is tuned, if series tuned with a coil then I agree totally, however if tuned using a shorted or open stub this comes further up the fibre glass and may be affected. I just don't think its wise to possibly compromise your VHF.

Fair comment in an ideal world, but unless you are putting your antennas where they have no metal (or carbon/water-based lifeforms) in sight above the ground plane, it's all compromise. The reflector would quite likely have more effect if you moved it a metre away from the VHF aerial.

 

[Gumpy]

Sadly I put antennas as high and as clear as I can get them, even to the point of installing a large arch over the wheelhouse to get them well above the roof line.

 

[lw395]

Sadly I put antennas as high and as clear as I can get them, even to the point of installing a large arch over the wheelhouse to get them well above the roof line.

Same here. I even went to the expense of mounting my VHF aerial on a 15m aluminium pole. Now my yacht won't go under some bridges, but it saves on diesel as I can use sails.
Sadly this means my GPS antenna is sometimes compromised by the mast being in the way, likewise, unless I can mount the radar reflector on top of the VHF aerial, that will be hidden from some directions. My AIS antenna is roughly parallel to the mast and a few lambda from it, so it ought to be acting as a yagi-type reflector giving some directivity biased towards things coming up from astern?

All that wire rigging is bond to be having an effect. However it all seems to work well enough...

Apart from the DAB radio which seems to get interfered with by 'adverts'.

 

[Gumpy]

Even sadder I have been known to work out coax losses and height gain to maximise range, not worth it on the boat but at home on other frequencies.........

 

[superheat6k]

The technical answer I requested says it is a bad idea, so I will remove it and attach some lines so I can hang it from the stub mast if I find myself in fog.