- July 12, 2017 at 8:30 am #19718
Had a little time to do some research on shortened 40m yagis. I am thinking of a 2 element design with a 18’ – 24’ boom length. With full sized elements, the tip to tip length would be about 70’ – probably too much antenna to manage with our towers.
There are basically three ways to shorten the elements:
- High-Q coils mid element
- Moxon or other capacitive end or mid-end loading
- Linear loading
I think that we should look at/compare all three approaches in terms of pattern, gain, 2:1 bandwidth and mechanical design/weight. I decided to look at linear loading first. Here’s an example of a linear loaded 40m yagi:
The linear loading section shown above effectively connects the inner half of the element with the outer half via a loop-back section. The picture below shows a detail view of the point where the inner and outer sections of the element are joined together:
The painted area is actually an insulator which electrically separates the two sections of the antenna. An advantage of this design is that the loading section also doubles as a needed element truss arrangement. The antenna shown in the picture is a 4 element design. We would be looking at a 2 element version of this.
Here is a view of the side of the loading section that is closest to the boom (also acts as an element truss):
There would be some mechanical work to put this together for sure. It looks like DX engineering could supply all of the basic hardware including:
- Boom material
- Matching section (hairpin)
- Element tubing and insulating material
- Clamps, plates and saddles
- Phillystran trust guy material
We would need to make the mechanical arms for the loading section. This might be a make-it labs project. Does anyone know how to operate a vertical milling machine?
The next option would be to use mid-span load coils. Here’s an example of this approach:
Here’s a closeup view of the sort of coils we’d be looking at building:
There is some complexity to fabricating high-Q coils of this type. Not exactly mechanically simple either. There would also be the issue of damaging these during disassembly and storage. Also, we’d likely still need some sort of element truss arrangement.
Finally, there is the end-load (moxon like) approach. This idea is probably to the most efficient of the three designs:
This approach uses capacitive end-loading to shorten the elements. Mechanical construction complexity is simplest with this approach I think. The difficulty with this design relates to our use of falling derrick towers. Think of what we’d have to do to install such an antenna while the tower was on its side on the ground. The end loading elements would require that the end of the tower(mast) be quite high in the air to install the antenna on the mast. Perhaps we could install the two end-loading sections on the ground facing element at 90 degrees to their normal orientation initially, raise the tower part way, and then turn them straight up? The concern here would be working on the antenna on a partially raised tower. Maybe we could work out supports to make this safe but we’d need to put some real thought into how to do it safely.
In any case, all three of these approaches would work. They each have their pros and cons and each is within the range of being practical for us as a homebrewed yagi project. The EZNEC modeling analysis is the next step I think. I’d appreciate everyone’s thoughts about the various approaches before we proceed.
- Fred (AB1OC)
President, Nashua Area Radio Society
You must be logged in to access attached files.July 12, 2017 at 8:57 am #19727July 12, 2017 at 4:33 pm #19859
There are some interesting designs for Moxon antennas on the web. I spotted one that used a shortened element design and also offered a light weight solution – to be used on a city lot. The authors PDF presentation should provide more insight.
Here is the 40 Meter Mini-MOXON Beam Antenna presentation:
MikeJuly 12, 2017 at 9:43 pm #19868
I found this article on the High-Q design for a Yagi (they also mention that Crushcraft has such a design).
His conclusion is that they may not be worth trouble and that the learning curve is high.
Kind of makes it seem more interesting! 🙂
TonyJuly 13, 2017 at 4:45 pm #19877
Good starting point for 40-meter beam ideas. The Moxon design posted by Mike, K1WVO has elements just over 50 feet long. The modified Moxon that Fred shows in the picture is similar to one that Tony, K1KP built. I wonder if the T-configured end points function as capacitive loading to shorten the element length. I remember Tony saying that it was empirically derived according to the designer.
I suggest trying a knock-off of the venerable Cushcraft/MFJ 40-2CD antenna. I had one of these “shorty forty” beams for many years. The two elements are about 40 feet long on a 20-foot boom. Each element has capacitive spreaders in an X configuration on each element end. (The club had a couple at one time that were used every Field Day.) The manual for the antenna should be available on line at a number of sites. Attached is a pdf of the coil data when I had to rebuild one after a lightening surge.
Whatever you end up with, remember (1) weight is critical, and (2) fewer parts the better; it’s Field Day, remember!
73 de TE
- Ed (K2TE)
President Emeritus, Nashua Area Radio Club
You must be logged in to access attached files.July 19, 2017 at 12:11 am #20013
I’m a Machinist . The bulk of my experience(25 + yrs) in the the lathe dept but I can safely run a vertical mill.
Jim Lajoie K1BRMJuly 19, 2017 at 8:23 am #20015July 24, 2017 at 12:20 am #20633
Imagine the tower laying flat with the antenna attached but rotated 90 degrees on its own long axis. Now imagine a latch mechanism on the boom that fixes the antenna in a horizontal position once the tower is raised. When the tower is up far enough to clear the elements you rotate the antenna to its latched position. The latch would need a release mechanism for rotating the antenna when the tower is dropped.July 25, 2017 at 1:46 am #20639
Looks like we are leaning towards the end-loaded approach. We will need a means to add or reposition the director/driven element once the hosting tower is part way up. I will try to model this design when I get some time.
Keep the thoughts and ideas coming!July 30, 2017 at 10:09 am #21071
Not a problem. This sounds like an exciting project and a way to really increase our field day performance. Glad I can bring something to the table to help out on this one.
Jim Lajoie K1BRM
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