A couple of months ago I modeled and built a double extended Zepp for 40 CW. It
is about 193 feet long and is fed with 17 feet of 450 ohm ladder line. I
connected the coax feed line through a 1:1 balun at the bottom of the ladder
line. The antenna is about 40 feet high.
Eznec 3.0 shows the impedance at the 1:1 balun feed point to be about 43 ohms
resistance and a few J impedance. It's a 1.2:1 to 1.3:1 match for my rig at
7050 mHz; absolutely no need for a tuner there! The antenna has a fairly narrow
band width, and I need the tuner for 40 SSB. I think the 2:1 SWR bandwidth is
about 150 kHz. The Zepp loads easily on all higher frequency bands with my
TenTec 238 tuner (had to get the TecTec plug in for this newsgroup
somewhere...).
The performance of this antenna on 40 has been spectacular; I got lots of
glowing reports in the Virginia QSO Party and felt very LOUD. It's not oriented
well for working Europe, but that will be a changed soon when I put up another
one.
BTW, Eznec shows that this antenna also has a ~ 1.3:1 ohm impedance when fed
with 6" ladder line of a similar length.
As always YMMV...
73 - John, W4AU
At 10:28 4/5/01 -0400, you wrote:
>Several months ago, I was contacted by several people on this board who were
>interested in learning more about the E.F. Johnson Antenna tuners. Please be
>advised that the March, 2001 (latest issue) of Electric Radio Magazine has a
>very interesting article describing both the 250 watt and kilowatt models.
>This is the only source I have ever seen that has actual side by side
>photographs of both units showing their rather massive internal components
>along with schematic diagrams of each. The article also explains how to
>modify them to operate on 160 meters. The author is using them to center-feed
>a 180 foot long hertz (dipole) antenna for use on his favorite bands of 160,
>80 and 40 meters. I have not verified his math, but the author states that on
>40 meters, a 180 foot long dipole will act a double extended zepp and has 3 db
>gain and can be used as a dx antenna. This seems reasonable. (Note: the
>terminal impedance will not be anywhere near 50 ohms; probably closer to 1000 !
>ohms and will have a large unknown reactive component so a tuner or matching
>stub is needed.) The author is using common 450 ohm low loss ladder line to
>connect from the tuner to antenna. The author's comments about using the
>Johnson tuners exactly duplicate my experience; except I used home made,
>unknown impedance - 5" spaced, open wire line with excellent results. The
>tuners really work great for what they were designed to do, provide tremendous
>bandwidth ( 80 -10 meters operation) with very high efficiency using balanced
>feed to a dipole, hertz or any balanced type antenna. They were not really
>intended to be used with coaxial cable, but can be modified to work with coax.
> (I really discourage modifying them or the use of coax in this manner.) I
>think the author said he is operating with about 300 to 500 watts of 100%
>plate modulated a.m. (his favorite mode) through them with no evidence of
>arcing or heating of the components. I do not remember his name or call but !
>this info. is in the article. For anyone wanting to understand exactl
>excellent book entitled Reflections, Transmission Lines and Antennas by Mr.
>Walter Maxwell or his series of six articles entitled "Another Look at
>Reflections" published in QST in the mid 1970's. - 73's- Corn - k4own.
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