70mhz DX Yagi Ideas

As people may have gathered I love 4m (70MHz) so here is an antenna design I’ve been playing with.

Model of the 70.200MHz Optimized antenna

Model of the 70.200MHz Optimized antenna

All elements are insulated from the boom!
Boom length 4m (just under)

Directors:
Dir Length Spaced Boom position Gain Gain
(no.) (mm) (mm) (mm) (dBd) (dBi)
1 1950.7 320.3 1204.4 4.8 6.9
2 1934.2 768.7 1973.1 6.5 8.6
3 1918.8 918.2 2891.3 7.8 9.9
4 1904.6 1067.6 3958.9 8.9 11.0

Reflector:
2092.9 mm long at boom position = 30 mm

Driven Element:
Single dipole 2028.1 mm tip to tip, spaced 854 mm from reflector at boom posn 884 mm (full sized not bent driven element)

Now normally this would require a Balun, but instead we attach the feeder to the driven element and run it forward towards D1 for a quarter wave, before dropping down, this actually does the same job as a balun, without the failure issues.

Results:

As you can see from the 3d far field plot this is optimized for DX and has very low take off lobes
3d Plot

2d Plots

2d Plots

From the data above you can see the theoretical Front to back ratio of 24.94dB and the SWR of 1.5 @ 50Ω obviously this was modeled at 15m because the VHF antenna’s need to be in the clear! I now plan on building one of these and comparing it to an LFA feed and an OWL feed, this should lead to some interesting experiments.

Other things that could be done would include using tapered elements so that it could be used for field day and packed away. Another option I am considering is using Fiberglass for the boom and elements and using Copper tape on the elements. However this is beyond most modeling programs (that I have found to date). As I experiment I will update the site.

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