Hello,

Just joined the group and I have a question about constructing a multi-
band dipole.

I plan to construct a 80/40/20/15 band multi-element dipole using
separate dipole wires for each band,
but a single feedpoint. My question is: how important is the spacing
BETWEEN the
individual band wires. I understand that they can't/shouldn't touch,
but is there any distance
that is optimal? or is just keeping them from touching enough?

Thanks,

Mike
KD7VRG

On 19 Apr 2007 12:17:00 -0700, wrote:

Hello,

Just joined the group and I have a question about constructing a multi-
band dipole.

I plan to construct a 80/40/20/15 band multi-element dipole using
separate dipole wires for each band,
but a single feedpoint. My question is: how important is the spacing
BETWEEN the
individual band wires. I understand that they can't/shouldn't touch,
but is there any distance
that is optimal? or is just keeping them from touching enough?

Thanks,

Mike
KD7VRG

I have used those antennas most of my life. I call mine a
parallel-fan dipole. I usually use 4" spacing for convenience. I use
PVC pipes with holes drilled in them the same diameter of the wire for
spacers.
However, there is a little secret to the parallel-fan. If the spacing
is pretty close to 8 inches apart, then you can setup the 75/80 meter
element and reverse engineer the length to the frequency. THen
substitute the new formula for the 468/f formula and you won't do so
much trimming on the other elements.

The advantage to the 4" spacing is that on 80 & 40 meters, you get a
broader bandwidth.

I really don't know of any better dipole arrangement when there is
room enough for this one.

Buck
N4PGW

--
73 for now
Buck, N4PGW
www.lumpuckeroo.com

wrote:
Hello,

Just joined the group and I have a question about constructing a multi-
band dipole.

I plan to construct a 80/40/20/15 band multi-element dipole using
separate dipole wires for each band,
but a single feedpoint. My question is: how important is the spacing
BETWEEN the
individual band wires. I understand that they can't/shouldn't touch,
but is there any distance
that is optimal? or is just keeping them from touching enough?

That's a good question, and here's the answer:

If you fan them apart at a fairly large angle, they don't interact much,
so on each band the antenna acts pretty much like the other elements
aren't there. But as you decrease the spacing, the interaction
increases, with several noticeable consequences. First, the resonant
length of each element is affected by all others. Mostly, the long
elements affect shorter ones. So tuning the antenna requires adjusting
the longest element first, then the next shorter one, and so forth. The
lengths you end up with are somewhat different than you would if the
elements were independent. Second, the antenna becomes more narrowbanded
on each band, except perhaps the lowest frequency band which uses the
longest element. Third, the feedpoint impedance at resonant frequencies
will be somewhat different than it would for an independent dipole. All
these effects increase as the spacing between elements decreases. I
haven't done a study, but believe that coupling will alter the patterns
of the higher frequency antennas somewhat when spacing is close.
Efficiency should be just fine for any spacing, though, so you'll be
radiating essentially all power you feed it.

Having 15 and 40 meter elements in the same antenna might make things
more difficult, because on 15 meters, the 40 meter element will be near
resonance and will get a fair amount of the power. It'll still radiate,
but you might find an unusual feedpoint impedance. You might consider
using the 40 meter element for both 40 and 15 -- you can probably tune
it to be reasonably close to resonance on both bands. On 15 you'll get a
cloverleaf pattern, but if you bend the two wires each 30 degrees in the
same direction to make a vee with 120 degree included angle, you'll get
a fairly narrow bidirectional pattern.

Roy Lewallen, W7EL

Wow! Thanks for so many replies so quickly. I think I will go with a
"fan" design rather than parallel.
Another question though. What balun should I use? 1:1? 2:1? 4:1?
How do I know which?
Or can I just do the "coil the coax" thing...?

Thanks

Mike
KD7VRG

wrote:
I plan to construct a 80/40/20/15 band multi-element dipole using
separate dipole wires for each band,

Side issue - you can get by using the 40m dipole for 15m.
40m dipoles and 15m dipoles interfere with each other
on 15m.
--
73, Cecil, w5dxp.com

So would a 80/40/20 be the thing to make? For whats it's worth, I
don't have a tuner right now.
I just want to cut each for the middle of the band for General phone,
and accept whatever bandwidth
I get.

Mike
KD7VRG

You won't need much of an impedance transformation, so a 1:1 "current"
balun (common mode choke) is appropriate. A coil of coax makes a broadly
resonant circuit for currents on the outside, so acts as a current balun
over a fairly wide range of frequencies. My experiments show a
reasonable impedance over about a 3:1 or so frequency range. To be
effective over a wider bandwidth, you'll need a couple of different
sized coils, or a balun using ferrite, which is inherently broad banded.

Roy Lewallen, W7EL

wrote:
Wow! Thanks for so many replies so quickly. I think I will go with a
"fan" design rather than parallel.
Another question though. What balun should I use? 1:1? 2:1? 4:1?
How do I know which?
Or can I just do the "coil the coax" thing...?

Thanks

Mike
KD7VRG

wrote:
So would a 80/40/20 be the thing to make? For whats it's worth, I
don't have a tuner right now.
I just want to cut each for the middle of the band for General phone,
and accept whatever bandwidth
I get.

That'll get you on those three bands and, as I mentioned, you can
probably get a reasonable impedance on 15 as well by fiddling the 40
meter antenna a bit. It won't be quite resonant on 40, but probably
close enough so your rig won't shut down.

Roy Lewallen, W7EL

I use a fan type dipole. The wires are 66' 33' and 16' each side so double
that for total length. (befor I tuned them but thats close) with an air
wound choke at the feed point.
Fan is not exactly right for my antenna ...here is what I did...

I used pvc as a spacer between each wire, the first spacer is 12 inches
from the feed point (I wanted to get the wires apart from each other as
close to the feed point as possible to minimise interaction between them)

They are separated 12 inches apart over the whole length and hang under
each other, the longest being on top.
The top wire (the 66') is the one that has rope connected to it (via
insulator) to tie the ends of the dipole to whatever. in my case, It is an
inverted V so I used 2 20' poles to anchor the ends of the dipole to.

Since the ends of the 33 and the 16 footer were just hanging from the pvc
spacers attached to the 66 footer, I placed a weight at the ends of them to
keep it all from twisting up in high winds.

I also ran a line from the ends of the 33 footer down to my poles (a few
feet below where the ends of the 66 is tied off, for the same reason. ( I
find that weed eater string holds up really well for this and it is cheap)

I can use it on all bands from 10 to 80 meters and even right around 1.8
mhz on 160 with a cheap mfj tuner. (not sure why it will work at 1.8 mhz, I
never worked out the math)

I get great signal reports with it on my ts-440s (bare bone at 100 watts)
and as a general rule, if I can hear them, I can work them.
"Roy Lewallen" wrote in message
...
wrote:
So would a 80/40/20 be the thing to make? For whats it's worth, I
don't have a tuner right now.
I just want to cut each for the middle of the band for General phone,
and accept whatever bandwidth
I get.

That'll get you on those three bands and, as I mentioned, you can
probably get a reasonable impedance on 15 as well by fiddling the 40
meter antenna a bit. It won't be quite resonant on 40, but probably
close enough so your rig won't shut down.

Roy Lewallen, W7EL

On 19 Apr 2007 12:17:00 -0700, wrote:

Hello,

Just joined the group and I have a question about constructing a multi-
band dipole.

I plan to construct a 80/40/20/15 band multi-element dipole using
separate dipole wires for each band,
but a single feedpoint. My question is: how important is the spacing
BETWEEN the
individual band wires. I understand that they can't/shouldn't touch,
but is there any distance
that is optimal? or is just keeping them from touching enough?

Thanks,

Mike
KD7VRG
There is a picture in most ARRL antenna books that shows a fan dipole
made from twin lead that just don't work!

Other than that, my experience with fans has been good. They are easy
to adapt to any environment. The same tactics adapt to a vertical
quarter wave as well.

John Ferrell W8CCW
"Life is easier if you learn to
plow around the stumps"