Balun for 437MHZ narrowband dipole

[Abhiram Deva]

Most of the balun types i have come across are not suitable for my design i.e a UHF dipole antenna for a nanosatellite. Surface mount transformer baluns have insertion loss and reflection which is normally acceptable but due to lack of power on the satellite we cant afford the loss. but these baluns are all wideband which is why they show bad performance. is there a way to design a balun for just one specific frequency where it gives excellent performance.
The closest thing i have come across is microstrip matching but turns out its not suitable for RF. Is there a solution for this situation.

P.S my dipole is precisely 50 OHM and so is the coaxial cable so no need for impedance transformation. I only need to eliminate the coax shield currents.

 

[davenn]

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P.S my dipole is precisely 50 OHM and so is the coaxial cable so no need for impedance transformation. I only need to eliminate the coax shield currents.

How did you achieve that ? ... dipoles are 75 Ohms by nature ( folded ones 300 Ohms) ... what impedance matching did you do ?


Dave

 

[chuckey]

See here :- https://www.google.co.uk/search?q=y...ebook%2Fantennas%2Fyagi_vhf_feed.html;360;503

More baluns then you can shake a stick at!!!
FRank

 

[jiripolivka]

Most of the balun types i have come across are not suitable for my design i.e a UHF dipole antenna for a nanosatellite. Surface mount transformer baluns have insertion loss and reflection which is normally acceptable but due to lack of power on the satellite we cant afford the loss. but these baluns are all wideband which is why they show bad performance. is there a way to design a balun for just one specific frequency where it gives excellent performance.
The closest thing i have come across is microstrip matching but turns out its not suitable for RF. Is there a solution for this situation.


If your special dipole is really 50 OHms, connect the coaxial line directly to it. To reject surface current on the 50-Ohm line, attach a ferrite bead, or add a quarter-wave shorted section to the "ground" end.
On a micro-satellite the radiation pattern is rather defined by satellite metal body (not a perfect ground), so do not worry about the surface current.

 

[Abhiram Deva]

How did you achieve that ? ... dipoles are 75 Ohms by nature ( folded ones 300 Ohms) ... what impedance matching did you do ?


Dave

a v-shaped dipole allows you to change the impedance but it took a while and quite a few simulations to get a 50+j0

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Most of the balun types i have come across are not suitable for my design i.e a UHF dipole antenna for a nanosatellite. Surface mount transformer baluns have insertion loss and reflection which is normally acceptable but due to lack of power on the satellite we cant afford the loss. but these baluns are all wideband which is why they show bad performance. is there a way to design a balun for just one specific frequency where it gives excellent performance.
The closest thing i have come across is microstrip matching but turns out its not suitable for RF. Is there a solution for this situation.


If your special dipole is really 50 OHms, connect the coaxial line directly to it. To reject surface current on the 50-Ohm line, attach a ferrite bead, or add a quarter-wave shorted section to the "ground" end.
On a micro-satellite the radiation pattern is rather defined by satellite metal body (not a perfect ground), so do not worry about the surface current.

It is 50 Ohms indeed, but achieving this requires a 96 deg angle between the arms which is pretty steep and causes complications for the mechanism involved in releasing it from a stowed position. And i am not sure if i should compromise and go ahead with a monopole which makes it easier on me but at the cost of performance, i would appreciate any advice. I ask you this cause you sound like you are familiar with antenna design on small sats. Also could you elaborate on adding a quarter wave shorted section to the ground end solution or point me to some source material.

- - - Updated - - -

Most of the balun types i have come across are not suitable for my design i.e a UHF dipole antenna for a nanosatellite. Surface mount transformer baluns have insertion loss and reflection which is normally acceptable but due to lack of power on the satellite we cant afford the loss. but these baluns are all wideband which is why they show bad performance. is there a way to design a balun for just one specific frequency where it gives excellent performance.
The closest thing i have come across is microstrip matching but turns out its not suitable for RF. Is there a solution for this situation.


If your special dipole is really 50 OHms, connect the coaxial line directly to it. To reject surface current on the 50-Ohm line, attach a ferrite bead, or add a quarter-wave shorted section to the "ground" end.
On a micro-satellite the radiation pattern is rather defined by satellite metal body (not a perfect ground), so do not worry about the surface current.

It is 50 Ohms indeed, but achieving this requires a 96 deg angle between the arms which is pretty steep and causes complications for the mechanism involved in releasing it from a stowed position. And i am not sure if i should compromise and go ahead with a monopole which makes it easier on me but at the cost of performance, i would appreciate any advice. I ask you this cause you sound like you are familiar with antenna design on small sats. Also could you elaborate on adding a quarter wave shorted section to the ground end solution or point me to some source material.

 

[chuckey]

Out of curiosity, how are you packing your dipole, i.e. arms parallel or opposed. Have you seen the arrangements for "pump up masts". How much power have you available to initiate the aerial erection process?
Frank

 

[Abhiram Deva]

there is no mast involved we cant afford the space or the weight on a nano satellite. this is what it will look like and the arms will be rolled around on a circular piece of plastic and tied up with dyneema which will be burnt melted to release the arms. the power required for melting it is around 2w.

 

[chuckey]

Pump up masts, are just a collection of concentric tubes with gas seals between them. Pumping air in to the bottom one pushes all the sections out. Sort of like a high class telescopic transistor radio aerials. Of course the "air" could be the gas from a chemical reaction or a small firework!
When I was a lad I used to make projectiles that used a tensioned elastic band as motive power, which was held by a short length of very thin copper wire. Passing too much current through the wire caused it to melt and release the elastic and my projectile then fired.
Frank

 

[davenn]

It is 50 Ohms indeed, but achieving this requires a 96 deg angle between the arms which is pretty steep and causes complications for the mechanism involved in releasing it from a stowed position.

Have you considered how this is going to adversely affect the radiation pattern of the antenna ?
its not going to have the pattern of a normal dipole any more

Dave

 

[Abhiram Deva]

Have you considered how this is going to adversely affect the radiation pattern of the antenna ?
its not going to have the pattern of a normal dipole any more

Dave

the distortion in pattern i observed from the simulation didnt seem so relevant but yes there was loss of about 0.5 db in max gain