VHF Radio Reception on the Thames (Tidal!)

vandy

vandy

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Hi,

Just a very daft question, my VHF radio receives all annoucements from PLA and even most ferries and sailing boats moving behind Thames Barriers (my boat is moored in Kew) but I do have problem broadcasting from Kew to PLA.

Yesterday, I had to reluctantly ask for Radio check on Channel 14 and PLA operator said the signal is a bit distorted....

Does anyone have similar problem or is it normal not being able to broadcast on a marine VHF radio when I am in Kew? and shall I be closer to PLA in central London?!

Thanks.
 
The PLA will be using a series of very efficient land based aerials high above the
the water using at least a Tx of 25 watts,probably a lot more.Most leisure craft will be using some budget equipment probably running at less than 13.8v and pushing out a few watts.By the time this has traveled down a few meters of cable via a couple of lossy pl259s and into a short bit of wire and aerial which is halving whats left of your TX output and guessing virtually no groundplane under it .......... reckon they are listening to you putting out about 5w on good day.
get one decent building twixt you and the aerial and .......ta da............:)
 
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I can work London VTS all the way to Teddington with no problems at all
even with the antenna down at deck level but still vertical (its on an arch)
Think you may have a problem or need a decent antenna.
 
Thanks, I actually bought a new 5ft Antenna and a New VHF radio!

Not the connections seem to be OK, and all I can think about is that since it is mounted on an A-Frame on a rib, it is not high enough, and considering so many bends in the river!

Need to sort this by next week as we are planning a night cruise from Teddington to Gravesend next Friday...!

Boo!
 
VHF range is line of site. Large lumps of land or buildings will block the signal.
Practical range is 1.4 x square root of height of transmitting antenna plus height of receiving antenna. With your transmitting antenna at, say, 6' high and the receiving antenna at, say, 75' high the range would be 13nm.
Obviously, large building or lumps of ground in the way to block the signal and range is drastically reduced.
 
vandy said:
Need to sort this by next week as we are planning a night cruise from Teddington to Gravesend next Friday...!
Click to expand...

Thinking again, you say its distorted could it be RF feedback into the set
how far is the set / mic from the antenna?
What is the antenna /set?

With the antenna down on mine its about 6ft above the water however the boat is steel so a large ground-plane
 
VHF Reception in the Thames

I would suggest asking a suitable engineer to check the SWR. (Standing Wave Ratio) This will indicate how much power you are trasnmitting. It is a very simple task, but I do not know what the coast will be !!!
PLA & other organisations using marine band VHF are limited to 25 Watt output & as other users have suggested "bends in the river" make a big difference !

Good Luck

Yeoman
 
I have found that London VTS fades out about Hammersmith or so certainly never received it up at Teddington, however receiving is fairly easy just use a piece of wet string is the old radio ham saying. Transmitting is a different ball game, from Kew it must be nearly 10 miles to the nearest reciever then all the buildings and bends in the river its a big ask from an average boat aerial, normal range from a low boat mounted aerial would be 12miles at sea unless the recieving station is particuarly high up. As has been said get someone to try an SWR meter on it then you will know for sure.
 
Make sure the antenna is vertical. I had a problem broadcasting near the barrier with it lowered to 45 degrees. Could receive fine.
 
repeaters

PLA use a number of repeater stations to pick up and receive transmissions. During the winter while on the hardstanding at Harleyford I could hear PLA clearly. On a good day I can hear PLA via a scanner and aerial on my roof in WGC Hertfordshire. It is most likely that the problem is your set up. Do make sure that your aerial has at least a one metre length of aerial lead and try to keep the aerial above the radio. You can also use a ferrite ring on the aerial lead which will help to reduce interference.
I do have a SWR metre for that frequency range but this will only tell you if you have a problem as most fibreglass type aerials cannot be adjusted.
 
Cervinia said:
A thought from another thread
Check the voltage on Transmit if its low this will affect the TX signal
It needs to be checked at the radio not at the batteries.
Click to expand...

Thanks. I think its either I am too far from them OR interference between Antenna and the Radio.

The later is quite likely as yesterday I was testing the signal using a handheld radio as "Receiver" and when I was broadcasting with fixed radio (which is attached to the antenna) at High Power (25W) the handheld radio could not pick any signal but when I broadcasted at Low Power (5W) it was nice and clear.

Will try other people's suggestions (vertically, etc.) and hopefully I will have a radio!
 
teddington_lock said:
We receive London VTS loud and clear at Tedders. We have a 20cm indoor aerial mounted in the lock office .
Click to expand...

Thanks. But can you also broadcast to THEM?

As I said, I can easily receive London VTS and even some ferries and boats behind Thames Barrier from Kew!!!

Also, do you happen to know where London Coastguard's Antenna is?
 
vandy said:
when I was broadcasting with fixed radio (which is attached to the antenna) at High Power (25W) the handheld radio could not pick any signal but when I broadcasted at Low Power (5W) it was nice and clear.
Click to expand...

If it works on low power but not on high then it could be a problem with the power supply. Probably the call for more power causes the voltage to drop too low to transmit. Check for poor connections and try measuring the voltage at the set when transmitting.

Of course, there could be a problem inside the set associated with the switch between low and high power but I would check the external wiring first.
 
vandy said:
Thanks, I actually bought a new 5ft Antenna and a New VHF radio!
Click to expand...

So what make of VHF did you buy and have you consulted the tech support people for the manufacturer/importer and explained your problem AND IN PARTICULAR THAT YOU HAVE INSTALLED ON A RIB!

Almost all the responses on here have been informed conjecture - intended to be helpful but a bit of a lottery when it comes down to finding out exactly what the problem is.
I happily receive VTS well upriver at Windsor or beyond on a good day but I wouldn't expect my transmissions to reach them.

One particular concern I would have would be earthing as well as ideal type and installation of antenna.

I would strongly recommend you get an expert to look at this problem with you.
 
boatone said:
So what make of VHF did you buy and have you consulted the tech support people for the manufacturer/importer and explained your problem AND IN PARTICULAR THAT YOU HAVE INSTALLED ON A RIB!

Almost all the responses on here have been informed conjecture - intended to be helpful but a bit of a lottery when it comes down to finding out exactly what the problem is.
I happily receive VTS well upriver at Windsor or beyond on a good day but I wouldn't expect my transmissions to reach them.

One particular concern I would have would be earthing as well as ideal type and installation of antenna.

I would strongly recommend you get an expert to look at this problem with you.
Click to expand...

I am alsmost sure about the radio unit. It is an I-COM.

I know I am being a bit greedy to expect my signal to be received by VTS from Kew, but the radio check confirmed that "My Broadcast was LOUD but DISTORTED". So that worries me now.

When you say Earthing; what do you mean by that? The radio is connected to the battery using + and - lead. They are brand new.

And the antenna cable is a coax cable (both antenna and cable are new). I will test to make sure there is no short circuit in the antenna between sheield and the core, and perhaps it would be a good idea to test the impedance between shield and the - side of the battery which I expect to be zero or very low for a proper signal broadcasting.

I will update and will hopefully with some pictures!
 
Transmitting at high power from a fixed radio to an adjacent portable is never going to give representative results, even if the quality of the transmission is perfect.

The sheer power of the transmission, along with the relatively high power of any spurious signals from the transmitter are going to cause the handheld to distort badly and effectively closes down most of the front end circuits within the receiver.

You need to get the receiver a reasonable distance away.

At the risk of telling you something you know, a radio signal consists of two main components, the basic radio carrier and the audio signal containing the message.

If the report from VTS is correct then:

LOUD = Your carrier signal is fully quietning their receiver
DISTORTED = Your audio (your voice) has a problem.

If, when you have carried out the test to a handheld further away, you get distorted audio on high carrier power, then is still sounds a bit like a DC power problem.

Martin
 
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