coax dipole help please help

[kokokokokoko]

Hi i have been trying to simulate a simple dipole fed by a coax cable but i have always failed to acomplish it due to numerous mistakes with coax. I have read and search like crazy for examples and someof them like pdf of patch fed by coax is specifying some of the options for exicutions that my v10 hfss does not have or ......

I was wondering if someone could create basic dipole fed by a coax that actually works in simulation.

Let's say each arm of dipole would be a tube 2" in diameter and 1/4 of a wavelength for 125MHZ in length

the coax cable would be connected between two tubes and length of cable would be 1 wavelength in cable not air.

if anyone has time and is willing to do it it would help a lot i need to simulate a much more complex structure but i think i could do it if i get this example for hfss.


thank you

 

[WimRFP]

Hello,

You can only do this if your EM simulator accepts 3D dielectrics, as the dielectric in the coaxial cable is different from that of air.

You may that a coaxial cable has two independent circuits: the differential mode circuit and the common mode circuit.

The DM mode circuit is the current in the center conductor and a same but opposite current at the innerside of the braid/shield.

The CM mode circuit is formed via the current at the outside of the braid/shield.

You can simulate your setup in two steps. Draw the dipole and draw the screen of the coaxial cable. Replace the inner circuit of the coaxial cable with a localized source directly at the antenna. So the source is connected directly between the dipole halves. Note that the braid of the coaxial cable is connected to one of the dipole halves and you don''t draw the inner conductor.

Run the simulation and see what happens (look at the current distribution and modify the coaxial braid length).

Once you have S11 for this arrangement, put this in another (circuit) simulation to take into account the effect of the coaxial cable electrical length and loss. If the cable's characteristic impedance is same as the reference impedance, it is relatively simple as you only get a rotation of the S11 graph (when in Smith Chart plot mode) and some scaling due to cable loss.

You probably can't simulate a sleeve dipole when you can't have 3D dielectrics. The dielectric is inbetween the sleeve and the coaxial cable.

 

[kokokokokoko]

Sleeve dipole is what i want to simulate and what i need to determine is sensitivity of dipole when there is a floating feed line along dipole tube vs sensitivity of dipole when coax is directly connected to dipole tubes.

I think that direct hook up should be more sensitive for low power signals but i am not sure and that is what i need to simulate but i am having hard time creating coax cable
and trying to excite it. I was wondering if someone could do a quick example of coax cable attached to dipole tubes and that it works.

I have another question that is playing with my mind, if you have two antennas with same or very similar radiation patterns and similar size and same gain is it possible for one to be more sensitive then another. Do you think that in case of direct hook up of coax instead of floating coax you increase sensitivity (low signal cutoff) and therefore antenna would be able to receive low power signals.
Anybody knows relationship between gain and sensitivity?