HAM Radio of SM6FHZ 432 MHz Dual Dipole Choke Feed

SM6FHZ 432 MHz Dual Dipole Feed with baffle

1. Introduction

When looking in to all the different feeds in the 432 MHz feed survey, I got ideas from one feed that I wanted to try out on other feeds in order to see if the performance could be improved.

The Dual Dipole feed with circular is a good feed with good equality between E- and H-planes, good FBR and low cross polarization radiation. It has been and is still used by many EME stations with good results. A BFR would probably not make any good for this feed, so I tried to use the baffle (or choke if you like) from the Loop BFR with choke feed.

The result from the simulations show that we can improve the spillover characteristics by introducing the baffle around the circular reflector. It is probably diffraction from the edge of the original reflector that is reduced by the baffle and thereby reducing the spillover.

The simulation results from the original feed is also presented in this report as a reference.

The simulation models are simplified as much as I thought was OK without jeopardizing the radiation performance accuracy. The results from the simulations are probably on the optimistic side, as any shortcomings in a physical realization will introduce unsymmetries and errors that will make the performance worse than simulated. The feed performance data was put into W1GHZ "Phasepat.exe" (Ref. [1]) for further analysis of the "In Dish" performance.

The Phase Center data are all referred to the surface of the reflector. The Phase Center in the W1GHZ In Dish Performance graphs are referred to the Phase Center I arrived at in my simulation. See comments about Phase Center in the Conclusions part.

I hope this description will give the reader a good hint on the characteristics of the feeds, a lot of inspiration and aid in the choice of feed.

Please enjoy yourself digging in to the data. I have very much enjoyed making these simulations and arriving at the results and conclusions.

2. Disclaimer

Please bear in mind that the results below are from simulations and is not an absolute universal truth! But it is good enough to serve as comparison between the different feeds and to be used as basis for choosing a feed for a particular application.

This report is not detailed description to follow in order to build a these feeds and contains only the most important dimensions used. Some dimensions are intentionally left out, as the builder may have their own ideas on how to make these parts. In most cases they have not any major impact on the radiation performance.

I sincerely apologize for the mediocre picture resolution. As I did not find a way to get the full resolution when transferring the pictures to the web page composer, I have written all important data in the text and tables as well. See the graphs as a way to get a quick overview of the performance of each individual feed examined.

The results presented in this report are not allowed to be used for any commercial purpose without explicit permission from the author. It may be used for Ham, non-commercial, purposes if used together with clear reference to the source of the information. I.E. Normal polite journalistic standards.

3. Dual Dipole Feed over a one lambda circular reflector

Description:

Two half lambda dipoles quarter of lambda over a one lambda circular reflector. The dipoles are separated one half lambda.

432 MHz Dual Dipole feed model

Figure 3:1; 432 MHz Dual Dipole feed model

432 MHz Dual Dipole feed 3D pattern

Figure 3:2; 432 MHz Dual Dipole feed 3D pattern

Figure 3:3; 432 MHz Dual Dipole feed E-plane pattern

Figure 3:4; 432 MHz Dual Dipole feed E-plane phase pattern

432 MHz Dual Dipole feed H-plane pattern

Figure 3:5; 432 MHz Dual Dipole feed H-plane pattern

Figure 3:6; 432 MHz Dual Dipole feed H-plane phase pattern

432 MHz Dual Dipole feed efficiency table

Figure 3:7; 432 MHz Dual Dipole feed efficiency table for 10 wl dish diameter

432 MHz Dual Dipole feed in dish performance

Figure 3:8; 432 MHz Dual Dipole feed in dish performance Figure 3:9; 432 MHz Dual Dipole feed in dish performance (10 wl diam)

Illumination characteristics:
E-plane: -13.0 / -13.2 dB at +/- 64 degrees
H-plane: -12.8 / -12.9 dB at +/- 64 degrees
Calculated directivity: +10 dBi
FBR: 14.9 dB
Phase center (relative to reflector): +85 mm

Comments:
This is a classic 432 MHz feed for f/D 0.5 to 0.6 dishes. It is a derivative from the EIA standard gain antenna. It shows very well matched beam widths in the E- and H-planes that will give balanced illumination of the dish. Clean pattern with good front to back ratio as well as very good cross polar discrimination. A good choice even for dishes deeper than 0.5 f/D if you would like a low noise antenna at the expense of slightly reduced gain due to the under illumination that will result. A drawback could be that it is slightly more cumbersome to feed than a loop feed as it has two radiating elements.

4. SM6FHZ Dual Dipole Feed with baffle

Description:

Two half lambda dipoles quarter of lambda over a one lambda circular reflector comprising an 50 mm high choke (baffle). The dipoles are separated one half lambda.

432 MHz Dual Dipole feed with choke model

432 MHz Dual Dipole feed with choke model

Figure 4:1; 432 MHz SM6FHZ Dual Dipole feed with baffle (choke) model

432 MHz Dual Dipole feed with choke 3D pattern

Figure 4:2; 432 MHz SM6FHZ Dual Dipole feed with choke 3D pattern

Figure 4:3; 432 MHz SM6FHZ Dual Dipole Choke feed E-plane pattern

SM6FHZ Dual Dipole choke phase E-plane

Figure 4:4; 432 MHz SM6FHZ Dual Dipole Choke feed E-plane phase pattern

Figure 4:5; 432 MHz SM6FHZ Dual Dipole Choke feed H-plane pattern

432 MHz SM6FHZ Dual Dipole Choke H-plane phase

Figure 4:6; 432 MHz SM6FHZ Dual Dipole Choke feed H-plane phase pattern

432 MHz SM6FHZ Dual Dipole Choke feed efficiency table 7.92 wl

Figure 4:7; 432 MHz SM6FHZ Dual Dipole Choke feed efficiency table for 7.92 wl dish diameter

SM6FHZ Dual Dipole Choke feed efficiency table 10wl

Figure 4:8; 432 MHz SM6FHZ Dual Dipole Choke feed efficiency table for 10 wl dish diameter

432 MHz SM6FHZ Dual Dipole Choke feed efficiency table 15 wl

Figure 4:9; 432 MHz SM6FHZ Dual Dipole Choke feed efficiency table for 15 wl dish diameter

432 MHz SM6FHZ Dual Dipole in dish performance

432 MHz SM6FHZ Dual Dipole Choke inDish 10wl

Figure 4:10; 432 MHz SM6FHZ Dual Dipole Choke feed in dish performance (7.92 wl diam) Figure 4:11; 432 MHz SM6FHZ Dual Dipole Choke feed in dish performance (10 wl diam)

Figure 4:12; 432 MHz SM6FHZ Dual Dipole Choke feed in dish performance (15 wl diam)

Illumination characteristics:
E-plane: -13.2 / -13.4 dB at +/- 64 degrees
H-plane: -13.8 / -14.0 dB at +/- 64 degrees
Calculated directivity: +10.3 dBi
FBR: 17.9 dB
Phase center (relative to reflector): +85 mm

Comments:
This feed shows good equality in beam width between the E- and H-plane. The FBR improves with the baffle. The spillover is reduced compared to the Dual Dipole feed without the baffle. This makes it a very good feed for dishes with a f/D from 0.4 to 0.5. The complexity is somewhat higher with the baffle.

6. Conclusions

Table of summary:

Feed type

Level at +/- 64 deg (E-plane) [dB]

Level at +/- 64 deg (H-plane) [dB]

Directivity [dBi]

FBR [dB]

Efficiency, max [%]

Spillover, at max eff [%]

Spillover, at max eff [K]

Efficiency at 0.433 [%]

Spillover, at 0.433 [%]

Spillover, at 0.433 [K]

Phase center [mm]

Comments

Dual Dipole feed

-13.0/-13.2

-12.8/-12.9

14.9

72.3 / 0.48

13.8

70.5

10.6

+85

SM6FHZ Dual Dipole with choke

-13.2 / -13.4

-13.8/-14.0

10.3

17.9

73.7 / 0.48

11.5

71.6

8.3

+85

Efficiencies in the table referees to a 10 wl diameter dish. The efficiency values are read directly from W1GHZ PhasePat data and are subject to the note that Paul has in his chart; "REAL WORLD at least 15% lower". The spillover noise temperature figures have been estimated by using VK3UM EMECalc, just to give another reference to the improvement.

The evolution in performance in a 10 wl dish can be clearly seen below starting with the Dual Dipole feed going to the SM6FHZ Dual Dipole feed with baffle.

432 MHz Dual Dipole feed inDish performance in 10 wl dish

432 MHz Dual Dipole feed inDish performance in 10 wl dish

432 MHz SM6FHZ Dual Dipole feed with choke inDish performance in 10 wl dish

It has earlier been demonstrated that in order to equalize the the beam widths in E- and H-planes one can use one more dipole fed in phase, a Beam Forming Ring (BFR) or a choke (baffle) on the reflector. The two first ones makes the H-plane more narrow to match the E-plane. The choke in the Dual Dipole case cleans up the unwanted side lobes in the rear hemisphere of the feed, including the back lobe.

As seen from the table, the maximum efficiency increases from the Dual Dipole feed to the SM6FHZ Dual Dipole feed with choke with almost no change in f/D for maximum efficiency. At the same time the spillover both at maximum efficiency as well as at a constant f/D (0.433) improves. At the constant f/D it improves from 10.6% to 8.3%. This corresponds approximately to an noise temperature contribution change from 23 K to 17 K from the spillover.

The improvements are not dramatic but still a step in the right direction for a relatively small effort. Both efficiency and spillover improves at f/D's from 0.42 to 0.6. At f/D's from 0.41 and less the efficiency stays the same and the spillover is reduced.

The center of rotation for all feeds is in the center of the reflector (X,Y-axises) and in the plane of the phase center (Z-axis). The position of the phase center has been empirically determined by running simulations on different center of rotation (Z-axis). The Phase center has been chosen as a best compromise between E- and H-plane phase error. The Phase center data in this report is always referred to the front surface of the reflector. From experience with sun noise measurements on my XE1XA feed in my 0.37 f/D dish; if you are within 3 to 5 cm from the phase center you can hardly see any deviation in sun noise. This is at the 11 dB level on 432 MHz.

8. Acknowledgments

I would like to send a big thank you to G3LTF, Peter, VK3UM, Doug and W1GHZ, Paul for the support and discussions. Your support and comments have been indispensable along the way.

9. References

[1] W1GHZ Antenna Book on line: http://www.w1ghz.org/