Go for a twinlead dipole about 5 feet long

Like I said in the post you replied to, I have about half this.

Try starting with a twinlead dipole of this length, and shape it into
a near-circle with around 6" between the ends of the arms.

This will give you a vaguely-omnidirectional pattern, using an antenna
which fits into a 2' circle.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Shakesphere makes good antennas for marine use. Just google (Shakesphere
Antenna) and that should get you to a link to their site. Rollie KD0FGX


"Nigel Molesworth" wrote in message
...
A little OT for this group, but I figure you guys are the experts!

I've got a boat, it has an VHF FM Car Radio with a car aerial. The first
problem is that there is no ground plane, the boat is GRP. It is also
lying
horizontally, it is too long to be vertical.

I want to replace the aerial something a bit more suitable. Modern cars
seem
to have much shorter aerials, so is there a coil involved?

Any suggestions on making or buying a replacement?

BTW, the radio has a DIN antenna socket, so 150 Ohms I think?

--
Nigel M
G1NDM (a long time ago)

On Mon, 29 Dec 2008 10:51:19 -0600, "Rollie" wrote:

Shakesphere makes good antennas for marine use.

In my experience, add the word "marine" and charge double the price.

I need to mount this antenna inside, so it does not have to be robust.

Nigel Molesworth wrote:
On Mon, 29 Dec 2008 10:51:19 -0600, "Rollie" wrote:

Shakesphere makes good antennas for marine use.

In my experience, add the word "marine" and charge double the price.

I need to mount this antenna inside, so it does not have to be robust.


You have not stated the frequencies you need this for.

On Wed, 31 Dec 2008 14:16:57 +0000, Dave wrote:

You have not stated the frequencies you need this for.

Is there more than one VHF FM broadcast band then? I guess 88MHz to 108MHz.

I don't think I've made my problem clear: I don't have space to mount a 1.5m
antenna, hence my OP "Modern cars seem to have much shorter aerials ..."

I've got about 75cm of headroom, so I need to use a short antenna, like they
have on cars. So how are they made to be short?

Nigel Molesworth wrote:
On Wed, 31 Dec 2008 14:16:57 +0000, Dave wrote:

You have not stated the frequencies you need this for.

Is there more than one VHF FM broadcast band then? I guess 88MHz to 108MHz.

I don't think I've made my problem clear: I don't have space to mount a 1.5m
antenna, hence my OP "Modern cars seem to have much shorter aerials ..."

I've got about 75cm of headroom, so I need to use a short antenna, like they
have on cars. So how are they made to be short?

Get a copy of the ARRL Antenna book. Look up VHF antenna's. You can make
a 1/4 wave antenna directly or you can add a coil of wire to either end
or the center of a shorter length of wire. The Antenna book will give
you the details and some antenna theory at the same time.

Dave WD9BDZ

Nigel Molesworth wrote:
On Wed, 31 Dec 2008 14:16:57 +0000, Dave wrote:

You have not stated the frequencies you need this for.

Is there more than one VHF FM broadcast band then? I guess 88MHz to 108MHz.

I don't think I've made my problem clear: I don't have space to mount a 1.5m
antenna, hence my OP "Modern cars seem to have much shorter aerials ..."

I've got about 75cm of headroom, so I need to use a short antenna, like they
have on cars. So how are they made to be short?


The whip(s) on my FM receiver(s) seem(s) to demonstrate a complete
disregard for polarization, lay it horizontal, it works well, stand it
up vertically--it works well, sometimes a positive change in signal can
be had, sometimes not, sometimes an improvement--sometimes not. I have
not done a thorough study, but it all seems completely random, location,
nearby metal objects, structures, etc., most likely, contribute to all
this, also ... but, I hazard to guess, that quite often, polarization
gets "screwed" up on the way to the receiving antenna.

Currently, I have a 1/2 wave DLM (distributed load monopole -- Robert
Vincent) up in the attic which provides input to a homemade preamp
utilizing a MMIC device. Most of the time, the preamp is not used ...
however, I do kick it in because, I am in a poor location to receive NPR
and it makes the difference in being able to receive a low power, local
college station satisfactorily.

Of all the antennas I have tried, this just simply works the most
satisfactorily for me. It would be possible to construct the DLM in a
dipole configuration and run it horizontal, but, like I stated, I just
don't have all that much faith in that paying all that much attention to
polarization, at FM broadcast freqs, will provide a great benefit(s.)

And, of course, others' mileage may vary ...

But, for a cheap quick fix, the folded dipole, already suggested, helps.

Regards,
JS

Nigel Molesworth wrote:
"I don`t have space to mount a 1.5 m. antenna--."

In any direction? Well Nigel is in luck anyway as a quarterwave antenna
worked against a counterpoise or groundplane workes about as well as a
halfwave vertical antenna. Automobile antennas work as a vertical over a
groundplane of sorts. If the antenna is too short, it picks up less
signal but the receiver usually is quiet and has surplus gain so the
problem is unimportant.

Only 75 cm or 30 inches is required for 1/4-wavelength which is resonant
at 100 MHz over a good ground of the right sort.

As few as two 30-in. wires laid horizontally in opposite directions may
serve as a counterpoise to a 30-in. vertical antenna.

Receiving antennas are usually very tolerant but need to be kept away
from large noise sources.

Best regards, Richard Harrison, KB5WZI

Nigel Molesworth wrote:
On Wed, 31 Dec 2008 14:16:57 +0000, Dave wrote:

You have not stated the frequencies you need this for.

Is there more than one VHF FM broadcast band then? I guess 88MHz to 108MHz.

I don't think I've made my problem clear: I don't have space to mount a 1.5m
antenna, hence my OP "Modern cars seem to have much shorter aerials ..."

I've got about 75cm of headroom, so I need to use a short antenna, like they
have on cars. So how are they made to be short?


http://www.elecfree.com/electronic/a...nna-by-mpf102/

Nigel Molesworth wrote:
On Mon, 29 Dec 2008 10:51:19 -0600, "Rollie" wrote:

Shakesphere makes good antennas for marine use.

In my experience, add the word "marine" and charge double the price.

I need to mount this antenna inside, so it does not have to be robust.

Most "MARINE" VHF antenna's are vertical coaxial dipoles.