VHF Dead Spot?

[jusw]

I was anchored in Priory Bay (off Bembridge IOW) close into the shore a couple of weeks ago. Tried radioing Solent CG on both my fixed VHF and hand held - no joy.

After sailing away from shore a few hundred meters I had no problem raising them. Does anyone know if this is a black spot or do I have radio trouble (again?) ?

JuSw

 

[Bilgediver]

Were you below a high mounted shore antenna. We have a similar situation in the river Forth where boats have difficulty working the coastguard from a certain location yet boats 2/3 miles further away have no problem. This seems to be due to the shore antenna being high above the boat.

 

[jusw]

It's possible - I don't know where the antennae are - Anyone help with this conundrum?

JuSw

 

[jeremyshaw]

Surely not really surprising? VHF is (more or less)line of sight. It is some years since I used to tuck into Priory Bay (a lovely spot) but the ground rises quite steeply as I recall, so...

 

[maxi77]

VHF propogation and coverage is a funny thing. A very long time ago I spent a bit of time doing a coverag survey in the Inner Sound of Raasay for my base station in Kyle. I got good coverage in places that should have ahd poor and vice versa.

Yes VHF is generally line of sight, but it bounces of hard things like hill, big buildings and all sorts so manages to get round corners. There is though another problem, where you have both a bounced signal and a direct they will arrive at slightly different times and interfere with each other. If the timing mismatch makes one signal arriive completely out of phase with another then it will cancel out, woops a black hole.

 

[lenseman]

You are/were suffering from a classic case of Fresnel Zone null points and it is a very well known phenomenon when close in and under a transmitting antenna. You can sometimes clear this problem by only moving the receiver antenna a ¼-wavelength:

http://en.wikipedia.org/wiki/Fresnel_zone

 

[Beadle]

Agreed

An interesting example.I once experienced the same thing operating a mobile 2M station for a boy scout walk in upper Wharfedale. I parked in a pub car park and could receive nothing. Parked at the other side of the wall and perfect reception.

You can get a similar effect when a signal is transmitted simultaneously from several aerials. If you are in a spot with similar signal strength from 2 aerials they can effectively phase each other out. Occasionally get it on car radio with local radio stations. Don't notice it while moving but occasionally stop and very weak signal - move a yard and fine copy.

Funny stuff this wireless.

 

[NormanS]

Please note that there is no such place as the "Inner Sound of Raasay". there is the Inner Sound, and the Sound of Raasay, but they are different places.

 

[lenseman]

He did say it was a very long time ago, in olden days they might have been the same location (Here be dragons)! /forums/images/graemlins/grin.gif