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Converting 3 element Yagi to 2 element

Aug 3rd 2011, 14:22

KE5WCP

Joined: Aug 22nd 2008, 13:37
Total Topics: 0
Total Posts: 0
Hi,
I've recently purchased a used 3 element Mosley 10 Meter monobander (CL-103). It's a to large for my 30 ft. push up pole. I would like to install only the driven element & radiator, but I do not know how well my yagi will work with out the director. Does anybody know if it will work, and, if so, how well. Here are the dimensions:

Distance between driven element & reflector: 81.5'
Reflector length: 220'
Driven Element length: 124'

Thanks,
KE5WCP, Stephen
Aug 3rd 2011, 16:55

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Removing the director will throw off the SWR. Often, the impedance of a 3 element Yagi is around 25 ohms, so a matching network is needed. With the wide spacing suggested, you can get an impedance around 50 ohms—once the driven element and reflector are properly tuned—which means you just need a split driven element and a choke balun to feed it.

You can use the Yagi for Windows software to design the new 2 element Yagi. It comes with the ARRL Antenna Book. I’d suggest modeling the 3 element antenna you have for practice. Ideally, you would be able to match the pattern that Mosley provides for the antenna. Designing a 2 element Yagi is pretty straightforward—the pattern is determined by the spacing and length of the reflector. You can lower the input impedance by reducing the spacing. Finally, you can adjust the driven element for resonance.

If you want to be clever, you might adjust the impedance of the 2 element Yagi to match the original 3 element Yagi—this may allow you to use the original driven element design with just a little tweaking of the element length.
Aug 3rd 2011, 20:37

W1RFIAdmin

Joined: Jul 25th 2011, 14:25
Total Topics: 0
Total Posts: 0
A good choke balun for 2 meters can consist of 3 or 4 of the ferrite split-bead cores, taped onto the feed line where it connects to the antenna.

As Zack points out, you can either remove the director and increase the spacing to get 50 ohms with the matching section removed and a balun installed or you can remove the director and decrease the spacing until the original matching network works.

In my experience with antenna modeling, there is a lot of interaction between spacing, resonance and feed-point impedance.

Most hams want more gain, but with a 2-element design, you will have a broader radiation pattern, making it easier for you to hear other stations and for them to hear you if you aren't pointed right at them.

73,
Ed Hare,
ARRL Lab
Technical forums moderator

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