On 2/5/20 5:31 PM, john@kk9a.com wrote:
If AB7E's model looks anything like the one in figure 2 of L B Cebik's
article http://www.antentop.org/w4rnl.001/mu3a.html I would also be
concerned about the pattern. A simple dipole will beat it in most
directions.
John KK9A
David Gilbert AB7E wrote:
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 is one of those "good match on lots of frequencies, but give up
pattern performance" antennas. They come about because until modeling
programs became widely available, folks focused on "is the SWR low
enough", and "do I make QSOs" (no doubt in the late 50s, early 60s, when
the sunspots were spectacularly high, and environmental RFI was a lot
lower than it is today)
It's also how the T2FD (and similar lossy doublets) became popular -
good match, works in all directions, and you don't notice the 6dB hit
because either there's propagation or there's not. (or you're a
government or commercial licensee, and you just crank up the amplifier a
bit)
There are tons of antennas in the various ARRL books from over the years
that in an objective sense are worse than a dipole and a tuner, or even,
a dipole and attenuator. They may be fun to build, easy to deploy, or
have some unique characteristic that is useful. It's fun to build a new
antenna design and try it, and if propagation is good, it doesn't really
matter.
Designing a simple multiband antenna with decent pattern on all bands
and good match on all bands is exceedingly hard. The only thing that
makes it possible at all is that we don't need continuous coverage - we
have bands, and if we happen to have a terrible match for the 39 meter
shortwave band, that's not a problem.
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