[TowerTalk] Insights on 30m EDZ - nowhere near what I thought

[jimlux]

On 2/5/20 5:31 PM, john at 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.