Home |
Search |
Today's Posts |
#1
|
|||
|
|||
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. |
#2
|
|||
|
|||
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. |
#3
|
|||
|
|||
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 |
#4
|
|||
|
|||
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. |
#5
|
|||
|
|||
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. |
#6
|
|||
|
|||
"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 |
#7
|
|||
|
|||
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 |
#8
|
|||
|
|||
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 |
#9
|
|||
|
|||
"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 |
#10
|
|||
|
|||
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 |
Reply |
|
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Questions -?- Considering a 'small' Shortwave Listener's (SWLs) Antenna | Shortwave | |||
The "TRICK" to TV 'type' Coax Cable [Shielded] SWL Loop Antennas {RHF} | Antenna | |||
Question for better antenna mavens than I | Shortwave | |||
QST Article: An Easy to Build, Dual-Band Collinear Antenna | Antenna | |||
I need DIY plans for an antenna | CB |