I used EZNEC+ to model the antenna as you described and got an SWR dip
at 10.7 MHz and 13.55 MHz, although the SWR values were different than
yours. Part of that may be due to a 4:1 balun not necessarily being 4:1
if not being fed with the impedances it was designed for, but the dips
match pretty closely with what you see in practice.
In any case, I lengthened the elements to 66' 10" and got a 50 ohm SWR
less than 1.1:1 at 10.12 KHz without the 4:1 balun ... i.e., directly at
50 ohms. This doesn't mean that exactly 66' 10" will work for you, but
you clearly need to lengthen the elements and I'd try it with and
without the balun ... substituting a common mode choke for the balun
when you don't use the balun.
By the way, EZNEC says that the two opposing main lobes have over 9 dbi
gain, but those two lobes are only 26 degrees wide at the 3db point and
the adjacent notches only 27 degrees either side of them are -20 db.
There are four other lobes (and two other notches), with those four
lobes down about 5 db from the main ones and the two notches down about
10 db. This antenna is going to be fairly picky in terms of orientation
and if you get it wrong it's going to be quite wrong.
Hope this helps a bit.
73,
Dave AB7E
On 2/4/2020 4:05 PM, Ken Bauer via TowerTalk wrote:
I thought this would be easy, but I’ve screwed it up. I’m hoping to get some
insights from this group of antenna experts re the easiest way to proceed! Perhaps someone
could model it for me?
I used the calculator at West Mountain Radio:
http://www.westmountainradio.com/antenna_calculator_zepp.php
Looking for resonance at 10.120mhz. I ordered supplies from Davis RF: their
balanced line / coax center insulator, 14awg solid copperweld and 16awg
19strand copperweld 450 ohm ladder line. I ordered a Balun Designs 4:1 current
balun.
The WMR calculator recommended a matching stub of ladderline 10’ 2.2” and each leg of the
antenna as 61’ 3.1”.
I thought I would add a half wavelength of ladderline to the stub thinking that this would present the same impedance
present at the recommended stub length. Figuring a .91% velocity factor, I added 44.22’ to the stub for a total
ladder line length of 54’ 5” between the center insulator and the balun. I temporarily set the antenna legs
lengths at 61’ 5” and wound about 18” of overage back on the antenna wire leg towards the center
insulator on each side.
I hoisted this up to about 70’ , pretty much in the clear, other than some pine
branches at each end. 10’ of RG-213 to my RigExpert AA-55 Zoom dangling at about eye
level.
The SWR plot shows a dip at 10.9mhz w SWR of 3.7:1 and a dip at 13.02mhz w
SWR of 2.7:1.
Thanks in advance for any insights!
73,
Ken WC6Y
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