pyramidal horn with coaxial fed

[sssoftech]

hiiii i need help with my design plz.

i have designed pyramidal horn antenna with coaxial fed in CST 2006.

with 20dbi gain
frequency range 2-8 ghz
dimensions:
1. waveguide: a x b x l = 48 x 22 x 50 mm and coaxial feeding at (l/4) from back.

2. coaxial cable: d = 1.12 mm and D = 3.13 mm

everything result is good except the "impedance matching" real part of my z matrix is coming to be -ve and that's why smith chart is not forming close loop.

 

[WimRFP]

Hello,

To avoid confusion, can you post a link to your S11 result (smith chart with some frequency reference).

To be honest I don't think you will get a reasonable match over 2..8 GHz with this feed scheme. It is likely that you will excite a wrong mode at the higher end of the frequency range (this will result in strong reduction in gain and deformed radiation pattern.

If you really want this bandwidth in a single antenna you may consider a ridged horn design.

 

[jiripolivka]

As you wrote, your pyramidal-horn antenna has a waveguide feed. The important device now is the transition from your coaxial line to the waveguide.
I am not sure if 48x22 mm is close to a standard waveguide cross-section. You will possibly have to add a smooth or stepped transition from 48x22 to a standard waveguide, and purchase a commercial coaxial to waveguide transition.
Designing such coaxial to waveguide transition is not easy, this is why such commercial devices are expensive. Manufacturers have their know-how and sell it for good money.
Also, no standard waveguide covers 2-8 GHz, so you will have to design your waveguide section as "M" waveguide. Technically no rectangular waveguide can transmit 2 to 8 GHz with a pure dominant mode.

 

[sssoftech]

can u tell me what is a proper method for impedance matching in coaxial feed horn.

i will go for ridged horn design as u mentioned above.........but can u help me out with ridge locus equation as it varies from one ieee paper to other.

 

[ferdows]

hi
its very hard and impossible to achieve bandwidth 2-8GHz by a pyramidal horn with coax feed with ridge u can achieve this bandwidth

 

[jiripolivka]

I am sorry, I have no experience with ridged horns.
In your horn design you achieved a transformation from a 48 x 22 mm waveguide to the full aperture.
With 48 mm = a, the cutoff wavelenght is lc= 2a, therefore 96 mm, corresponding to 3.125 GHz. Your waveguide cannot therefore pass any lower frequency (your requirement was 2 GHz). The dominant mode can exist in your waveguide from ~3.5 GHz to 8 GHz. For frequencies 2 to 3.5 GHz you will need another waveguide cross section (and possibly another horn).

 

[WimRFP]

Jiripolivka: I agree that he may not find a pyramidal horn that will cover 2...8 GHz with descent gain/radiation pattern. He has to divert to ridged concept and/or flared shapes.

sssoftech: Why not drop the horn concept and go to a TEM travellling wave structure? There you don't have the cut-off problems as with waveguides. The design isn't easy, but it will give you an antenna for the required frequency range with descent radiation pattern and return loss.

If you want to go for the ridged version, best is to source such an antenna (they are used in EMC environments) and see how they did it and use it as input for your own design.

 

[sssoftech]

wimRFP is there any way u can help me with the effective way of impedance matching in horn antenna with coax feeding both with horn and without horn.

 

[ferdows]

hi
i designed ridge antenna before this and with out and it work from 1.5-10GHz

 

[WimRFP]

For to horn, I have no idea how to do it for your full frequency range and with maintaining the correct propagation mode.

Regarding non-horn wide band antennas, you may search for ultra wide band antennas. I actually did design flared transmission line wide band antennas, but I don't have a reference available.

The trick with these antennas is that you can avoid steep impedance transitions versus distance along the antenna. The difficulty is not in getting reasonable return loss, but more getting a descent radiation pattern over the full frequency range and (as always) getting reasonable aperture efficiency. Antennas (Kraus, Marhefka) do mention flared transmission lime antennas, but doesn't give you guidance in how to achieve your goals.