New Shark Fin Aerial Reception Strength Mystery

April 24, 2009

I've finally fitted a new shark fin aerial to my car. it was quite an ordeal (more later) - but I've not quite finished yet, because the reception is rubbish - it's very difficult to find, say radio1, and then it de-tunes and seeks.

However, and here's the mystery, if I take the aerial off of the roof, and connect it to the same connectors from under the headlining, but leave it dangling over the parcel shelf, the reception is perfect!

I've even tried lifting the connected, but dangling, aerial out of the boot and on to the roof - and the reception becomes rubbish again.

I've tried this with 2 of these aerials (I had a replacement sent) and the same happens...

any ideas???

April 24, 2009

any ideas???

Yes, but do you want my honest opinion? I'd send it back, get your money back and fit a standard whip (with pre-amp if you like).

I've never played with one of these fin antennas, but the physics/electro-mag theory screams at me that it just can't be very efficient, So I've never even considered it.

As a sweeping coverall statement, generally an antenna has to be a significant fraction of a wavelength long at the frequency(ies) it will operate at, to be any good. So, a fin will be OK at mobile phone freqs/GPS where the frequency is much higher and the wavelength much shorter, but at FM broadcast, it is just not big enough to be any good IMHO.

I suspect it is so cr*p as an antenna, that when you leave it "dangling" it is just picking signal up on the coax outer screen (which can happen under some circumstances).

Is it cr*p if you take it close to any metallic part of the vehicle body?

They fit pre-amps to these things and I expect all the work is being done in the amp, with very poor actual antenna efficiency. That is fine up to a point, but the amp will add noise and you always need more signal (from the actual antenna) than noise introduced by the amp, otherwise you are screwed. Signal to noise ratio is a key parameter in radio.

Having a quick scan on the web at a few fins, it does appear all of the gain is in the pre-amp and some of the claimed measurement figures are dubious at FM broadcast.

Of course, I'm willing to be proven wrong. People have be playing with fractal based antenna patterns and other stuff in an attempt to make small (compared to the operating wavelength) antennas but I've yet to be convinced of any real benefit.

Bear in mind, I occasionally build radio type stuff for fun, design radio related stuff professionally and grew up with a parent that did the same, so I hope I'm in a position to provide a valid comment.

Cheers

April 24, 2009

thanks for your response schm.

I suspect you are correct, and that the dangling aerial is infact using the bit if co-ax as an extra bit of antenna.

The fin has 12db gain on FM according to the pack. It's fine on BBC stations, but rubbish on capital/absolute/etc.

hmmm.

Where can I get a good 'whip' arial base?

BTW, I wonder what BMW do. They have fins? or do they have a separate radio antenna inside the car?

April 24, 2009

I'm interested in changing my 4.3's whip for something a bit shorter (bust the 4.2's with the roofbox, or something from Ikea) so I follow these, and I notice that a number of makers seem to be using fin or near fins - Volvo, Audi and VW IIRC. The point about the mobile/GPS vs radio is interesting, as is that about boosting it all - maybe that's what they're doing in order to make the car look pretty outside? So does that mean we use a shorter arial at our own risk unless we boost the signal?

I thought there might be the opposite effect with the roof as I remember something about cross-planes, but is that for transmitting rather than receiving?

April 25, 2009

I would concure fully with shcm... most local FM broadcasts stations operate at VHF, and a 'quarter wave' whip antenna works out to be approx 20", which is a good compromise for practical aerial length and performance. Those finn aerials mus be operating at something like an 1/8th wave (not too good!). The base of the antenna where the coax is terminated needs a good 'earth' to the car bodywork. The car bodywork (metal) then acts as a 'groundplane' and effectively supplys the other half of the aerial 'diplole'. The groundplane is effective for transmitting (TX) and receiving (RX). A 'halfwave' or 'full wave' antenna (twice and four times the phisicle length for a given wavelength/frequency) is not as dependant on a groundplane for good performance. It is possible to wind part of the antenna into a coil to reduce its length... but thats another story. :)

A car antenna is for all intents an purposes a 'vertically' polarised 'halfwave dipole' ---------------+-------------- (two quarter waves fed in the middle). One side is the metal aerial you 'see', the other is the vehicle metal body (groundplane). Get as much signal with the antenna as you can... then use pre-amps with discretion (if at all!)

That's why... if you are driving in a weak radio reception area, pull your (telescopic aerial) out to its full length for extra gain (fully extended is the optium tuned length for performance). Also, if you use a metal roof box or bike rack, keep them as far away from your aeral as possible, else the metal will 'de-tune' the aerial and decrease its gain (performance).

Blimy... I have not done this stuff for years! :rolleyes:

DaveH

April 26, 2009

The fin has 12db gain on FM according to the pack. It's fine on BBC stations, but rubbish on capital/absolute/etc.

I suspect that is all down to the amp and I'm also slightly dubious about their claims in that area too. Odd that it is not good on absolute. Think that transmitter is at Crystal Place with similar power to the Beeb stations. Possibly you are on a local relay for the beeb stuff.

The dB figure on its own is slightly meaningless. It is usually quoted against a theoretical isotropic (uniform in all directions) antenna (dBi) or perhaps a dipole (dBd).

hmmm.
Where can I get a good 'whip' arial base?

If you were after a comms antenna, I could point you at loads of places. I don't take an interest in broadcast radio ants much. I've found a few, but I expect it is a case of sit down and take a sharp intake of breath. I've never had a Mr T. antenna off, so I'm not sure of the mounting hole diameter etc or the connector. The connector wouldn't bother me, I'd just change it, but that might not be the case for you. I'd trust people like panorama not to sell you rubbish:

http://www.panorama-antennas.com/uk/products/dab_am_fm.html

http://www.siretta.co.uk/search_box.php?mcatid=67

http://cgi.ebay.com/ebaymotors/Toyota-CORO...emZ370188257892

BTW, I wonder what BMW do. They have fins? or do they have a separate radio antenna inside the car?

At one time, there was a trend to incorporate them into the rear screen or print them separately onto the glass on the side window. There were concerns of the tinting materials used for the glass attenuating the signal, but I believe it is reasonable compromise. Maybe they are in the fin, but my assumption is they were phone/GPS only, but I could be wrong.

To be honest though, from what I can see of the PCB on this, it looks laughable:

http://www.visualgarage.com/images2/Installation/11.jpg

but at least they admit its performance will not be as good as the standard antenna:

http://www.visualgarage.com/Product_How_It_Works.html

As the frequency goes up (phone, GPS etc), things like patch antennas (exactly that, a patch of metal on a substrate) become practical, which you could put under a fin. There are also things like helical antennas which can be made to perform OK at short lengths, plus as Dave says, a loading coil at the base can help compensate up to a point, for a short and therefore usually capacitive antenna.

Getting too technical now probably. For those that can understand, this provides a good explanation of the limitations of electrically small antennas. It talks mainly about transmitting, but most of the time, it is also applies to receive as well:

http://personal.ee.surrey.ac.uk/Personal/D...cles/radimp.htm