Summary> This document describes a conversion of a stock Cushcraft (tm) MA5B three element HF yagi into a 'portable' two element 10,15, and 20M yagi. Improving your antenna is a huge win because you get a return on investment on both transmit and receive.

Background> I happen to live where an HF antenna installation is hampered by CC&R's. The next option is mobile/portable operation. I've used Hamsticks while mobile, and a pair as a dipole, with some success but wanted a more effective setup while operating portable (field day, lighthouses, etc). I purchased an MA5B yagi and converted it to a 2-element yagi and made a few mods to add to its portability and ease of setup. Since I don't operate on 12 or 17 meters, eliminating this capability from the MA5B is not a limitation.

Result> After making the mods described here, I have a portable tri-bander yagi that satisfies my operating and portablity requirements as well providing very good performance and excellent SWR. I've been able to single-handedly set it up at several locations including Pigeon Point Lighthouse to operate Spring Lights 2002.

Before

After

The Process>

Mod 1

Eliminate the Center Element. This element is a dipole used only on 12 and 17 and won't be missed. This makes the antenna lighter and provides some spare parts. The biggest win, however, is allowing us to use part of the center element to make the package more compact. More on that shortly.

Modify the MN5B Matching Network. I opened the box (four screws), unsoldered and removed one of the feedline sections. The two sections are identical and electrically symmetric so it doesn't matter which one you remove. I removed the grommet and patched the hole.

Mod 2

Reduce the breakdown length. By using the two center element tubes in place of the one longer reflector element tube, we can reduce the breakdown length from approx. 84" to approx. 46". Replace the one (reflector) element tube (EF) with the two tubes (ED) and the fiberglass insulator. We must 'bypass' the fiberglass insulator because this element must be shorted to the boom. I drilled one hole in each of the ED tubes to match the U-bolt holes in the insulator and mounted this element to boom just like it was the single tube EF. This is a substantial win in the portability department. It will now fit into a Pelican (tm) 1750 case.

Final Assembly

When adjusting the physical length of the reflector, remember to correct for the difference in tubes introduced by Mod 2. Adjust the two tubes 'EB' in the reflector so the each end extends 74" from the centerline of the boom. See Figure 2 of the instruction manual and do the math as a second check.

 

driven	x-hat
reflector

 

Notes and Noise>

1) The stainless steel (SS) hardware is great for a permanent installation. I replaced the U-Bolts with zinc-plated parts to eliminate dealing with galling that can occur with these SS fasteners when operated frequently. You may want to lubricate the SS threads with a powered graphite in an isopropyl alcohol mixture to reduce the galling tendency. I find it easier just to replace the U-bolts. Get a few extras for spares.

2) The Cushcraft documents refer to the elements as #1, #2, and #3. I refer to element #2 as the center element and the other two driven and reflector. In this project the center element will not be driven.

3) Cushcraft refers to their MA5B as "The Director". This is probably the marketeers contribution. If you look at the MA5B as two antennas in one, the two element 10/15/20M yagi and the other 12/17 meter dipole, then the concept of 'director' makes sense if you consider that the driven element is also the director.

4) Which way is forward? The driven element (the one the end in the unmodified MA5B) is forward. As stated above, the driven element is also the director, hence the name 'The Director'. The other element is the reflector.

5) The capacity hats are a real pain. They are time consuming to install (16 of them) and will become bent in a portable installation. I've added another nut on the screw to hold piece 26, 28 and the aluminum rod together. This reduces the assembly time and the number of small parts during assembly. I'm thinking about some spring-loaded hats to reduce bending. Bending will also be reduced if I improve the support scheme used during assembly.

6) I have color-coded each tube and trap to make identification during assembly easier.

7) Instruction Manual Errata:

The fiberglass insulators are identified as piece 96 in the parts list but called piece 98 in Figure 2 on page 5.

8) I have no financial interest in Cushcraft.