I have a 130' dipole with the center at 32', drooping to about 15' at each
end. It is fed with a little over 100' of 450 ohm twin lead to an AH-4
Icom autotuner, thence with RG-8X into the shack to the transmitter.
The AH-4 matches it on all bands to a 50 ohm load. Due to the rather low
height, it is pretty much a cloud warmer on 40M and below, but on the
higher ones it seems to be doing a surprisingly good job considering how
modest it is. It's not a monobander at 120' of course.
I had a couple of experts model this antenna. I have a complete set of
reports on 80-10. The patterns are pretty strange starting at 30 meters
and getting stranger up to 10M with lots of lobes and dead spots. Below
30M it shows to be essentially omnidirectional.
As I understand it, resonance is desirable only for purposes of matching
the transmitter to the load and has no implications for radiation
effectiveness.
73 de W6OGC Jim Allen
On Fri, May 23, 2014 at 7:51 PM, Dan Presley <dpresley1@comcast.net> wrote:
> I set up a random length 'balanced' equal side length antenna (hate to
> call this a dipole!) fed with ladder line and a johnson matchbox for a
> friend who always struggled in a bad and space limited urban location. He
> couldn't believe the difference on most bands. now he is actually
> competitive on qrp. Yes it's not perfect and I'm sure has some lobes and
> directionality but it's a good compromise antenna for many situations. Much
> better than a bad vertical.
>
> Dan Presley 503-701-3871 N7CQR
> Dpresley1@Comcast.net
>
>
> > On May 23, 2014, at 15:43, "Rick - DJ0IP / NJ0IP" <Rick@DJ0IP.de> wrote:
> >
> > Hi Neil,
> >
> > Yes, absolutely computer modeling. I don't think I ever would have found
> > that point by experimenting.
> > It's not exactly 85/15... I think 83/17 or something like that. I don't
> > recall if the exact measurements are on my web site or not. If not I can
> > send them to you. Pse write me directly: Rick@dj0ip.de
> >
> > The modeling was done for me by DJ1AT. He does all of my modeling for
> me.
> > I'm a bit of a novice at modeling, he's very good. When I get wild
> ideas, I
> > ask him to model them and he gives me the results in short order. Then
> I go
> > to the field, build it, then first adjust the overall length and then
> begin
> > moving the feedpoint one inch at a time. I found the 80m quite quickly.
> > Took a bit longer with my 40m OCFD.
> >
> > Tip: If you want to build multiband antennas, don't full around with an
> > MFJ-259B or similar. Get yourself a scanning analyzer that can scan the
> > entire hf spectrum in on go, such as the RigExpert AA-54. You will save
> > light-years of time that way.
> >
> > I ran an extensive test on common mode current last summer.
> > I found that the farther away from the center you move the feedpoint, the
> > worse problems you get with common mode current. (duh).
> > When you use such a radical split as I did, it is imperative that you
> use a
> > good balun AND also a good 1:1 RF choke directly attached to the balun.
> The
> > balun should be dual-core with each transmission line wrapped on its own
> > separate core. In my case, my power level (600w) and with the antenna in
> > the clear, a 1:1 Maxwell was good enough. If more power or antenna
> located
> > near other objects, it is better to use a 1:1 Guanella.
> >
> > If you plan to run very high power, you should use 4 cores (piggy back
> two
> > pairs) for your balun.
> > Beware of people selling baluns. Very few have any clue what they are
> doing
> > and pass out bad advice.
> > Best is to build your own. (cheapest too).
> >
> > If you have never built any baluns, I suggest you begin by reading a
> couple
> > of documents (in this order).
> > 1) "Baluns: what they do and how they do it." By Roy, W7EL
> > 2) "RFI Ham" by Jim, K9YC. (read it 2 or 3 times, unless you're a lot
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