FM reception snags

[peteandthira]

Has anyone with radio nollege got any tips for this? I have crap reception on FM stereo. I have tried a splitter from the VHF masthead aerial, not good. I now have one of those domestic ribbon-type, best results seem to be had from putting both wires into one and into the aerial socket with no ground, but still not good enough. Should one of the wires go to the sea? I do have a grounding plate under the hull for the generator earth.

Next plan is to try a splitter from the telly aerial up the mast, but the frequencies are very different, aren't they, therefore unlikely to work?

BTW, in the car park nearby I get excellent reception on all stations in my car with a standard car aerial. I have even tried swapping the radio, to no avail.

HELP!

Pops

 

[SwedishLass]

I use one of the little bendy type car aerials fixed through the bulkhead into the cockpit.
IIRC In a car the aerial is usually earthed to the set via the bodywork may be worth trying. The fluorescent light often plays havoc with reception. The length & resistance of the aerial is also an issue.

 

[michael_w]

Try coonecting the coax to a stantion base. That way, if you have bare wire guard rails the whole thing works as an aerial. If no bare wire try the toe rail.

I did the former and it works a treat.

 

[ems]

Hi pops, many years ago in the days of RDF (ie pre decca gps etc). We always used a lashing to secure one end of the guard wires. The problem was, I believe, that a continuous wire to pupit and pushpit acts as a radio signal screen. Certainly in those days RDF reception was much improved by breaking the continuity of the guardrails. I have noticed on todays production built tupperware tonka toys that the guard rails are generally fitted with rigging screws not lanyards. Hope this helps.

 

[steverow]

OK the telly splitter wont work effectively.
The frequency of FM Band 2 (mid band) is 100Mhz That works out to a wavelength of 3 metres. Antenna length is directly proportional to wavelength, to get satisfactory reception.
There are essentially two ways to do this, either to use a half wave dipole or a quarter wave vertical.
Sounds like you already have a domestic plastic flexible dipole (T antenna).
Make sure that each arm of this is about 75cm and the whole thing streched out is 1.5 meters or a little under.
try and rig this so that niether end is touching any metalwork and is roughly horizontal, then connect one arm to coaxial inner and one to outer braid, and put your plug directly into car radio socket at back...I assume you are using a car radio type set??

The other way to do it is to use a vertical whip antenna of the type you see on taxis, and this should be roughly cut to 70cm or a little over.
Mount this using a bracket on a convenient guard rail or something metal and earthed. Coax cable is used, inner to centre and outer to the mounting earth.
This is exactly similar to the way you would mount a VHF Marine antenna.
Try and get it as high as possible is the golden rule.
Again plug other end of coax into car aerial socket.
If you are not using a car radio type reciever then there can be diferences.
What is the socket you are plugging into...the FM radio will nearly always have a 75 ohm socket of some sort, either car radio type or belling lee coax.

Hope this all helps

Steve.

 

[peteandthira]

Steverow et al

thank you all for your tips - I will try them all and report back later! This is more important than the rigging. If swmbo doesn't get radio 2 soon I will have to get floats fitted to the car.

Pops

 

[William_H]

VHF is notoriously variable when among buildings etc so don't be surprised if you move the boat a little and it all comes good.
There is the possibiliity of interference just check with every thing else not operating. You could try for a test using the VHF com aerial without a splitter.
When you use your wire dipole as steve suggests try it both horozontal and vertical it can make a difference. I don't think you will do much good at VHF with the lifeliines etc. good luck olewill

 

[Vara]

I was going to suggest going down the DAB route or Freeview route,then I saw the Cruising area,still might be all right if you stay in the Clyde. COVERAGE DETAILS

 

[jrt]

It depends which transmitter the VHF signal is coming from. Some are vertically polarised, some horizontal and one (I think only one) is slant polarised.

When you assemble the dipole it may be an idea to slowly rotate it from vertical to horizontal.

If you pick up a good signal in the car using a vertical polarised aerial you should get a good signal on the boat using a vertically polarised aerial.

Borrow another radio that you know works, put your aerial on it. Does it still work ?

 

[steverow]

I think most main FM transmitters now use circular polarisation, which is why I suggested the horizontal approach as being easier.
Either single field, mixed or slant polarisation is rarer, although still used by some commercial operators such as Arquiva (NTL) for commercial stations in difficult areas and in initial testing and RSL phases.

Steve.

 

[Bergman]

Re: circular polarisation

Eh?

 

[jrt]

Re: circular polarisation

Thanks, steverow, that's brought me up to date. /forums/images/graemlins/blush.gif

 

[jrt]

Re: circular polarisation

What's the theoretical loss of picking up a circularly polarised signal with a vertical aerial?

 

[steverow]

Re: circular polarisation

Dunno..I'd really need to get the books out for that one.
There will be some of course, as generally vertical polarisation is always to be preferred.

Off the top of my head I would say probably about 1.5 to 2dB..easily compensated for with higher power, however high power broadcast FM is not my field, and they do have some wierd and wonderful arrays at their disposal. The issue more and more these days is about footprint and demographic, and targeting with the least signal overspill, so it does get extremely complex.

Steve

 

[Bergman]

Re: circular polarisation

3dB

 

[peteandthira]

Re: circular polarisation

I lost most of you there. I might just try an old coathanger stuck into the socket at the back!

Thank you for all of your tips, even those that made me say "yerwot"!

Pops PS can't one of you just come up and sort it?

 

[Vara]

Whatever you do don't call in the experts you'll end up with this;

Stick with the coathanger!

 

[Bergman]

Re: circular polarisation

Pity W Scotland is so far or I would.

Basically fit a vertical aerial as high as possible.

Try moving boat about a bit, it is possible to get fairly small blank spots in coverage, and you may be in one.

By a bit I mean a few yards.

The rubber duck type for cars would probably be as good as any.

Use decent co-ax direct from aerial to radio - don't [--word removed--] about with splitters.

Make good, well soldered connections and use decent connectors

Protect all connectors with self amalgamating tape.

But

Before doing any of this check with neighbours to see if they get decent reception. If you are in a complete blank spot they will have same problems and no amount of pratting about with aerial will help if there is no signal to receive.

Hope that is adaquately de-anoraked to be of some help.

 

[Gordonmc]

Erm...
Circular polarisation is almost exclusively used for satellite transmission, while linear polarity is used for terrestrial FM by both the BBC and commercials stations. Broadly, vertical polarity favours in-car reception while horizontal will provide more robust reception in-home, especially with rooftop aerial arrays. Most stations transmit using both linear forms (mixed polarity).
The reality is that polarity is affected by reflection from building surfaces, so by the time the signal hits a receive aerial the polarity can change.
Industry advice is that dipole receive aerials should be installed with a 45 degree slant to get best advantage of both linear forms.
None of this really helps Pops.
I have tried all sorts of aerials on car receivers on the boat, including rubber ducks, powered aerials etc and found the only thing that really worked was a mag. mount on the coach roof on a piece of steel plate, about a foot square. This replicates the effect of the car as ground plane.
Not pretty or convenient, so I now use a Perstel personal DAB receiver with a tape deck adaptor into the car stereo. The boat is in the Clyde and picks up between 12 and 20 stations according to location.

 

[Bergman]

Erm

Not precisely.

Can't speak for independants but BBC use a mixture of vertical and "mixed" polarisation.

Not clear whether the "mixed" is true circular polarisation or not - I suspect not.

In general the large high power stations are mixed and the small low power ones are vertical.

Hence my recommendation that Pops uses a vertical aerial - it will work with both.

I suppose something like 5/8ths vertical would be best but don't know if anyone makes them for broadcast band - somehow I doubt it.