Calculating the impedance of a dielectric rod in free space to get a match to 377 ohms?


Neil Smith G4DBN
 

Certainly adding a choke ring lets me clean up the sidelobes radically, as does converting the design to a non-lensed Pickett-Potter or W2IMU, but my primary intent was to make a lens horn that can be mounted very close to a 5.6 GHz horn on the same dish, so keeping the outside diameter under 25mm would be good.

I think I've found another paper about characteristic impedance of an unenclosed dielectric guide, although it is densely mathematical, so I need a good night's sleep before reading it. If it makes any sense to me, I'll add a link to it on the UKuG wiki and write a covering page.

Neil

On 11/03/2021 13:22, Mike Willis wrote:
Try a pie tin (aka Chaparral ring) as it will help reduce sidelobes
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Mike Willis
 

Try a pie tin (aka Chaparral ring) as it will help reduce sidelobes 
--
Mike G0MJW


Neil Smith G4DBN
 

Simulation is working fine, but I had a feeling there was an analytical solution to calculate the characteristic impedance of a dielectric rod waveguide in air. Intellectual curiosity getting the better of me in late-night modelling sessions....

The extension rod does seem to reduce the sidelobes a little when it is about 6mm diameter.  I'm more interested in getting the edge taper right, and by adjusting the cone angle, I can get a nicely flattened lobe with a slight dip in the middle.  This is  in a 22 mm diameter tube but with a narrower lobe and no dip. It will be amusing to see if the real thing is anything close to the models

Neil G4DBN



Mike Willis
 

Simulate it perhaps? E.g. the HB9PZK design lens in 22mm circular waveguide.



 
--
Mike G0MJW


Dominique Dehays
 

Hi Matti ,


thx very much for this interesting report , it gives me some ideas...

73

Dom/F6DRO


Neil Smith G4DBN
 

Hi Matti! Very interesting document, many thanks.  I need to read it thoroughly, but it is late and I need to sleep.

Neil G4DBN

On 10/03/2021 21:30, Matti Lukkarinen,OH1DB wrote:
HI Neil!

Here you get some information. With this document, i have done my own polyrod antenna
https://hamwaves.com/dielectric.rod/doc/dielectric_rod_antenna.pdf

de. Matti oh1b

On Wed, 10 Mar 2021 at 21:22, Neil Smith G4DBN <neil@...> wrote:

I'm doing some simulations of dielrod/dielspike horns using a simple taper from a round waveguide down to a round rod.  The plan is to set the diameter of the round rod extension such that it has the same impedance as free space, so there is no discontinuity at the end of the taper, and hopefully to minimise reflections.

Can anyone point me at a way to calculate the diameter of a dielectric rod guide to get the impedance of the terminal extension close to 377 ohms?  At the moment, I'm just fiddling with the diameter to see if I can reduce the sidelobes a little, but I'd prefer to set the rod to a known size, then adjust the length of the rod and the taper angle.

The dielectric plastic is Rexolite 1422, with a dielectric "constant"  of 2.53. 

Neil G4DBN

-- 
Neil
http://g4dbn.uk


Matti Lukkarinen,OH1DB
 

HI Neil!

Here you get some information. With this document, i have done my own polyrod antenna
https://hamwaves.com/dielectric.rod/doc/dielectric_rod_antenna.pdf

de. Matti oh1b

On Wed, 10 Mar 2021 at 21:22, Neil Smith G4DBN <neil@...> wrote:

I'm doing some simulations of dielrod/dielspike horns using a simple taper from a round waveguide down to a round rod.  The plan is to set the diameter of the round rod extension such that it has the same impedance as free space, so there is no discontinuity at the end of the taper, and hopefully to minimise reflections.

Can anyone point me at a way to calculate the diameter of a dielectric rod guide to get the impedance of the terminal extension close to 377 ohms?  At the moment, I'm just fiddling with the diameter to see if I can reduce the sidelobes a little, but I'd prefer to set the rod to a known size, then adjust the length of the rod and the taper angle.

The dielectric plastic is Rexolite 1422, with a dielectric "constant"  of 2.53. 

Neil G4DBN


Neil Smith G4DBN
 

I'm doing some simulations of dielrod/dielspike horns using a simple taper from a round waveguide down to a round rod.  The plan is to set the diameter of the round rod extension such that it has the same impedance as free space, so there is no discontinuity at the end of the taper, and hopefully to minimise reflections.

Can anyone point me at a way to calculate the diameter of a dielectric rod guide to get the impedance of the terminal extension close to 377 ohms?  At the moment, I'm just fiddling with the diameter to see if I can reduce the sidelobes a little, but I'd prefer to set the rod to a known size, then adjust the length of the rod and the taper angle.

The dielectric plastic is Rexolite 1422, with a dielectric "constant"  of 2.53. 

Neil G4DBN