"That gain is broadside to the antenna, with nulls off the end."
That's only partially true. As I stated in my earlier reply to WC6Y,
EZNEC says that the nulls off the ends of the antenna aren't the worst
ones. The nulls off the ends are down about 10 db from the broadside
main lobes, but there are deep notches ONLY 27 DEGREES either side of
the main lobes (i.e., a total of four notches) that are 20 db down from
the main lobes. That's a very sharp pattern for an antenna that can't
be rotated. From my QTH 27 degrees is roughly the difference between
Spain and Norway, and 20 db is the difference between 10 watts and a
KW. In actual practice, it is often the case that proximity effects
cause the peaks and notches to not be as pronounced as claimed by the
model, but at 70 feet in the clear on 30m I suspect they are pretty close.
EZNEC does agree with you though that the antenna doesn't need ... or
want ... a 4:1 balun. The model says that if you get the dimensions
right the SWR can be less than 1.1:1 for 50 ohms at the shack end of the
ladder line. And as you say, of course, a common mode choke should be used.
73,
Dave AB7E
On 2/5/2020 12:47 PM, Matthew King - AK4MK wrote:
I've got a 20m EDZ that serves me quite well, so I thought I'd chime in
here.
The 4:1 balun isn't used on the EDZ, as the specific length of 450 ohm
ladder line specified by the calculator handles the impedance
transformation down to 50 ohms. You can use a 1:1 choke/balun to help with
common mode and convert from the end of the matching section of ladder line
to a coax feedline.
As far as the advantage of the antenna, it's two 5/8 wave elements fed in
the center, so it has about 3db gain over a standard center fed dipole on
the same frequency. That gain is broadside to the antenna, with nulls off
the end.
Hope that helps!
73
Matt
AK4MK
On Wed, Feb 5, 2020 at 2:35 PM <john@kk9a.com> wrote:
The double zepp looks to me like a dipole fed with balanced line. What
is the advantage to this antenna?
John KK9A
Ken Bauer WC6Y
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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|