This is an excellent point regarding the pattern of shortened 40m beams
and it can easily been seen in a computer model. There are definitely
compromises but for the size they can work quite well.
John KK9A
To: <towertalk@contesting.com>
Subject: [TowerTalk] FW: Force 12 Delta 240 2 Elem 40M Yagi
From: "Matt" <maflukey@gmail.com>
Date: Tue, 15 Apr 2014 01:23:16 -0500
I will echo that the comments by all are valid.
I have the EF240X & an S402. Both are 2/3rds wavelength (~45') elements on
24' booms. The EF240X is linear loaded and the S402 is base loaded. Both
give an SWR bandwidth of about 250 khz and both perform about equally. Be
aware that the F/B bandwidth on both antennas is much less than the SWR
bandwidth and deteriorates quickly down to only about 5db as you get out
towards the band edges. This does not present any issue as a transmitting
antenna but receive performance suffers noticeably because poor F/B often
means lower S/N ratio - especially when your wave angle is not optimum. It
does not really matter if you plan to use Beverages for receive. As
transmitting antennas go, gain peaks at a little over 4dbd and decreases
down to about 3db or less at the band edge. So on both transmit and
receive both of these antennas are noticeably better than a dipole
(theoretically and observed in use). Mechanically, the S402 is built much
more stout then the EF240x, but it is also twice as heavy with almost twice
the wind load area.
I am currently converting the EF240X into a 3 el beam for 30m with full size
elements by replacing (and extending) the insulator at the linear load joint
with tubing. The loading wires will be replaced with non-conducting
supports. The driven element will be the new element and be
self-supporting.
I am planning to convert the S402 to a 40m Moxon in hopes of realizing full
SWR bandwidth and broader performance bandwidth. The models look promising.
I still have some work to do on the mechanical details but the element
stress analysis looks good so I think it is "do-able".
Hope this information offers some insight into the nature of the trade-offs
with the shortened 40m beams.
73
Matt
KM5VI
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