Folded Dipole Antenna
 

Hi Guys,

I'm fairly new to this topic so please go easy!!

I have been put in charge of a RSL broadcast here in the UK. The only thing
I'm not sure of is in which direction in relation to a folded dipole antenna
does the maximum signal radiate?

I know normal dipole antenna radiate outwards from their ends, but I'm not
sure about the folded version.

Please Help.

Many Thanks

Gavin.

Hi Gavin:

Folded dipoles and regular dipoles radiate the same. Your idea of radiating
off the ends is wrong. Both types of dipoles radiate their maximum signal at
right angles to the plane of the wire. That is if the wire (or tubing) runs
north/south the maximum radiation with be east/west. This assumes the dipole
is horizontal, if it's vertical then it radiates equally in all horizontal
directions. This is for a dipole in free space. Depending on the frequency
and the height of the antenna the pattern may change. Such as a dipole for
the 3.5-4Mhz ham band mounted 10m up, with radiate most of its signal
straight up. I don't know what RSl broadcasting is here in the US so I can't
give anymore detailed answers.


--
John Passaneau, W3JXP
Penn State University


"gavin" ##NOSPAM wrote in message
...
Hi Guys,

I'm fairly new to this topic so please go easy!!

I have been put in charge of a RSL broadcast here in the UK. The only
thing
I'm not sure of is in which direction in relation to a folded dipole
antenna
does the maximum signal radiate?

I know normal dipole antenna radiate outwards from their ends, but I'm not
sure about the folded version.

Please Help.

Many Thanks

Gavin.

Folded dipoles and regular dipoles radiate the same. Your idea of
radiating
off the ends is wrong. Both types of dipoles radiate their maximum signal
at
right angles to the plane of the wire. That is if the wire (or tubing)
runs
north/south the maximum radiation with be east/west. This assumes the
dipole
is horizontal, if it's vertical then it radiates equally in all horizontal
directions. This is for a dipole in free space. Depending on the frequency
and the height of the antenna the pattern may change. Such as a dipole for
the 3.5-4Mhz ham band mounted 10m up, with radiate most of its signal
straight up.

Thanks John that was a great help.


I don't know what RSl broadcasting is here in the US so I can't
give anymore detailed answers.


RSL is a basiclly a legal tempoary radio station broadcasting on either
87.7MHz or 87.9MHz FM.


Gavin

OK, so here is the situation.

I have a 10W FM transmitter set to 87.7MHz. I also have a folded Dipole
antenna and a 20 meter high mast.

Would it produce and great coverage area with the antenna horizontal or
vertical?

Many Thanks

Gavin.

It would not surprise me if the "real world" proved this wrong, however:
Vertical should give the greatest circular area coverage (no end
attentuation like a horiz. dipole).
However, most external receiving fm antennas are horizontal in
polarization--most portables have short pullup whips...
If you do go vertical, a 5/8 (at least in theory) drops the lobes of the
radiation pattern and gives you the most usable low angle of radiation
(direct correlation to max distance)...
The vertical is omnidirectional of course--a horz dipole is more
bi-directional...
If any of this is in error--I am sure it will be corrected...

Regards,
John

"Gavin" wrote in message
...
OK, so here is the situation.

I have a 10W FM transmitter set to 87.7MHz. I also have a folded Dipole
antenna and a 20 meter high mast.

Would it produce and great coverage area with the antenna horizontal or
vertical?

Many Thanks

Gavin.

"Gavin" wrote in message
...
OK, so here is the situation.

I have a 10W FM transmitter set to 87.7MHz. I also have a folded Dipole
antenna and a 20 meter high mast.

Would it produce and great coverage area with the antenna horizontal or
vertical?

Many Thanks

Gavin.

For FM Broadcasting this is VERY IMPORTANT. Originally, FM broadcasters
used horizontal polarization (usually versions of "halos" ) for their
broadcast antennas. This was fine for fixed or home listeners, who used
horizontal TV/FM antennas or folder wire dipoles (included with many stereo
receivers) - BUT cars and mobile FM receivers tended to be vertically
polarized. In order to address this 2 solutions were used -- one was to add
a vertically polarized element to a large "phased array" on the side of the
tower. The other was to replace the antenna with one that could produce
circular polarization
http://www.starantenna.com/fm_broadcast_antenna.htm

http://www.scott-inc.com/html/fmant.htm

w9gb

Here is another circular polarisation article you may find useful.
Simple construction too.

You will obviously need to recalc the dimensions from 144MHz. (Multiply
everything by 144/87.7)

http://home.comcast.net/~ross_anders...dCollinear.htm

Cheers Bob VK2YQA


For FM Broadcasting this is VERY IMPORTANT. Originally, FM
broadcasters
used horizontal polarization (usually versions of "halos" ) for their
broadcast antennas. This was fine for fixed or home listeners, who used
horizontal TV/FM antennas or folder wire dipoles (included with many stereo
receivers) - BUT cars and mobile FM receivers tended to be vertically
polarized. In order to address this 2 solutions were used -- one was to add
a vertically polarized element to a large "phased array" on the side of the
tower. The other was to replace the antenna with one that could produce
circular polarization
http://www.starantenna.com/fm_broadcast_antenna.htm

http://www.scott-inc.com/html/fmant.htm

w9gb

For FM Broadcasting this is VERY IMPORTANT. Originally, FM
broadcasters used horizontal polarization (usually versions of "halos"
) for their broadcast antennas. This was fine for fixed or home
listeners, who used horizontal TV/FM antennas or folder wire dipoles
(included with many stereo receivers) - BUT cars and mobile FM
receivers tended to be vertically polarized.

I believe the trend in modern cars is getting away form vertical FM
receiving antennas and going to in-glass horizontal wires for the antennas.


Ed

"Ed" wrote in message
. 92.175...


For FM Broadcasting this is VERY IMPORTANT. Originally, FM
broadcasters used horizontal polarization (usually versions of
"halos"
) for their broadcast antennas. This was fine for fixed or home
listeners, who used horizontal TV/FM antennas or folder wire
dipoles
(included with many stereo receivers) - BUT cars and mobile FM
receivers tended to be vertically polarized.

I believe the trend in modern cars is getting away form vertical
FM
receiving antennas and going to in-glass horizontal wires for the
antennas.


Ed

I would agree that some manufacturers - Ford and BMW to name but two -
have in the last decade or two used heated rear window antennas, but
if you look closely you will find that they have vertical components
as well - the side connector strips and often a 'joining' strip down
the centre as well. If they did not, by definition, the horzontal
elements would make the aerial quite directional - as the
manufacturers found out the first time round in the '70's.

In Europe and certainly the UK (a) many transmissions are not circular
but of mixed polarisation (i.e. crossed diagonal) to give a vertical
component for car and portable use and a horizontal component for the
many still horizontal roof aerials fitted over here from the days when
all FM was horizontal, and (b) many cars over here have either a
conventional angled whip on the front of the roof (as with Citroen and
Peugeot) or the ubiquitous 'bee-sting' amplified whip on the rear of
the roof.

One does notice how many American import cars over here still have
wing mounted vertical aerials - perhaps that is why there is so much
difference between them and us?


--
Woody

harrogate2 at ntlworld dot com

Gavin wrote:
"I know normal dipole antenna radiate outwards from their ends, but I`m
bot sure about the folded version."

The propagation pattern from a dipole forms a toroid centered on the
middle of the dipole wire. Radiatiation is perpendicular too the center
of the wire. There are nulls at the ends of the wire.

Adding a wire or wires between enda of the simple dipole does nothong to
change its radiation resistance, antenna efficiency, directional
pattern, or directive gain. Drivepoint impedance increases with added
wires as total antenna current is divided among them. If two equal wires
are used in the folded dipole, its drivepoint impedance is four times
the impedance of the single wire dipole. If there are three wires, the
impedance is multiplied by nine. Impedance varies by the square of the
number of same sized wires in the folded dipole.

Best regards, Richard Harrison, KB5WZI