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Author Topic: Ur suggestions for dual and triband wire antenna  (Read 1059 times)
K1PEK
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Posts: 51




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« on: July 09, 2010, 06:31:19 AM »

Hi, I'm looking for suggestions as to the best configuration/design for the following antennas with the intention of being broadside beamed, both sides, and other than a random wire fed center or end, unless you have experience that a certain such configuraion is better than any other choice.  Also, any web site or books ref. these.     Thanks !  STeve:
 
3 separate ant's,   Each ant to perform on :
 
80 m  and 40 m

80  40  and 20

20 and 15     
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AA4PB
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Posts: 12641




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« Reply #1 on: July 09, 2010, 06:36:04 AM »

1. A FAN dipole. Separate dipoles fed with coax at a common center point. Inexpensive.

2. A trapped dipole. Direct coax fed, more expensive, narrower bandwidth.
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KD4COX
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Posts: 24




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« Reply #2 on: July 09, 2010, 06:45:14 AM »

A lot of people love the g5rv (search, and you can easily find a description of it). Carolina Windom's are well liked too. I have a fan dipole myself, they work alright. There's also the mystery antenna, which I know someone that has scrapped his g5rv for this:
http://www.w5gi.com/mysteryantenna.htm

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K4SAV
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Posts: 1786




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« Reply #3 on: July 09, 2010, 07:24:20 AM »

Hi, I'm looking for suggestions as to the best configuration/design for the following antennas with the intention of being broadside beamed, both sides, and other than a random wire fed center or end, unless you have experience that a certain such configuraion is better than any other choice.   

As you know a single wire fed with ladderline can cover a wide frequency range, but it can't do that and maintain broadside radiation.  Any wire longer than about 1.3 wavelengths will develop multiple lobes and the maximum radiation is likely to go off at other angles.  At the higher end of that 0.5 to 1.3 wavelength range, the beamwidth will get narrow so the coverage is significantly reduced (even though the peak gain may be greater).

So as with most antennas, you have a choice as to what you would like to give up.  That's why there are so many antennas out there.  If there was one that did everything, there would be only one choice.  You also have a choice to make about elevation angle, low angles for DX or high angles for local.  It's very difficult to get good gain at low angles on the low bands because of the height required.  That's why verticals are often used even though their gain is less than a horizontal antenna.

Jerry, K4SAV
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WA3SKN
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Posts: 5420




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« Reply #4 on: July 09, 2010, 08:25:08 AM »

How high can you get your antennas, and what supports are available?

-Mike.
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K2DC
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Posts: 1340


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« Reply #5 on: July 09, 2010, 10:17:53 AM »

Steve,

  I would agree with AA4PB, and the choice would be made by trading ease and space required for installation, ease of tuning to match and efficiency.  The trapped multiband dipole is a single wire that may require the least number of supports, but there are losses in traps that increase with the number of bands at the lower bands.  The fan dipole is probably cheaper and is probably easier to trim, given that you don't have too much interaction between the band-legs.  But you may have to or want to fan the legs out in different directions and may require separate tie points for the ends for each band (although there are designs that use single tie points for all bands).
   I ran a parallel 80/40M inverted vee pair on a single feed point for 20 years with great success and few problems at two QTH's.  I switched to an Alpha Delta twin half-sloper pair, one leg a simple 80M wire and the other a trapped/loaded wire for 40/160M.  But I switched because I wanted something for 160M, not because I was unhappy with the 80/40M pair.
    So decide what most important for your needs and make the choice.  I'm sure that with the proper attention to construction, matching and installation, you can do very well with either choice.


GL & 73,

Don, K2DC

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N6AJR
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Posts: 9879




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« Reply #6 on: July 09, 2010, 01:16:52 PM »

I reccommend some variation of the fab dipole, they will all work , some better than others, The key to understanding a "Fan Dipole" is in the lazy habits of the electron.
 
if you have a  coax ( or twinlead) feed a  dipole with multiple  paths for the current to flow in,  the  path  closest to the  resonant freq of the signal will find the electrons  folwing the easiest there. and example:
 
you have a coax fee to a pair of dipoles , one with 33 feet on each side and one wire with 66 feet on each side.both connected to the end of the coax, one side to the center conductor, one side to the shield.  Lets send a radio frequency of about 7.100 MHZ  up the pipe. the 33 foot on each side dipole will show an impeadance of somewhere between 30 and 75 ohms.  most of the current will flow in this wire.  The other wire pair will have an impeadance of from several hundred to several thousand ohms. only a small portion of the current will flow in the other pair.
 
so you can run the 80 meter dipole north and south and  you can either hang the 40 meter dipole below it or run that one east and west.  different pattern but same antenna.
 
on the several different designs shown in the example, the path is simply down the diagonal and them off on the wire that comes closest to being the right  impedance.  it couls be divided between several if they are similar in length like a log periodic. Hope this helps
 
 and here is several other ideas...
 

Bye the way, none of this is mine, Its just stuff I pulled from here and there. thanks to all the folks with the great sites. this is My take on the multi-band, single feed wire dipole ( but fan dipole is easier to type.  a true fan dipole is also called similar to a cage dipole and has 3 or 4 wires of different lengths on the same band , like a 64 foot , 66 foot and 68 foot  double leg wires on the same feed to give wider bandwidth, but bottom line, they are cheap, easy to build, and they work.. or do a google for home brew  and fan dipole ..
 
 
http://www.ku4ay.net/dipole.html
http://www.qsl.net/kd7rem/antdipole.htm
http://www.hamuniverse.com/multidipole.html
http://www.angelfire.com/nb/ni4l/ni4ldipole.html
http://www.arrl.org/tis/info/pdf/9611073.pdf
http://www.geocities.com/n2uhc/2banddipole.html
 

 
 
by N3JBH on October 12, 2006
  FAN DIPOLE OR MULTIPLE BAND DIPOLE SPECS:
(Each leg is shown in length so you will need two legs.)
 
10 METERS = 8'4"
12 METERS = 9'5"
15 METERS = 11'1"
17 METERS = 12'10"
20 METERS = 16'8"
30 METERS = 23'2"
40 METERS = 32'9"
60 METERS = 43'7"
75 METERS = 60'9"
80 METERS = 65'6"
160 METERS = 123'5"
137 KHZ = 1708'1"
 
 
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WB6BYU
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Posts: 12979




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« Reply #7 on: July 09, 2010, 01:27:08 PM »

Quote from: K1PEK
80 m  and 40 m

80  40  and 20

20 and 15     

There are a number of options, and the choice among them will depend on your individual circumstances.
Some of the more likely ones are:

Multiple dipoles on a common feedpoint.  This is the simplest to set up and tune (as long as the wires
are not too close together.)  Can be set up to cover any combination of bands.  If all the wires share
the same end supports just tie them at different heights or let the shorter ones droop below the longer
ones.  This is what I use for all my portable antenna kits - in some cases the wires were originally tuned
over 30 years ago and I have used them in many different configurations without a tuner.

Center-fed doublet with ladder line or twinlead to a tuner.  To keep the broadside pattern the overall
wire length must be less than about 1.25 wavelengths ("extended double zepp").  Within that range,
the longer the wire the higher the gain or efficiency.  So in each of the cases you give, you can just
calculate 1.25 wavelengths for the highest band and use it on the others.  Note that in the case of
80 / 40 / 20m the wire will be about 0.3 wavelengths - this is getting pretty short, and is approaching
the point where the efficiency drop becomes noticeable.   This approach requires a good antenna tuner
and readjusting it each time you change bands.  Here is how W5DXP made this approach work for a
dual-band 80 / 40m antenna without a tuner:

http://www.w5dxp.com/HEDZ.htm


Trap antennas:  reasonably simple to install once tuned.  Can use traps made from coaxial cable or
coils without discrete capacitors, depending on the design.  Here is an example of a 40 / 80m version:

http://vk1od.net/antenna/coaxtrap/8040.htm

Either of the dual-band antennas can be built this way with a trap frequency between the two bands,
which minimizes losses.  The tri-band version requires more experimentation to do that way (probably
using a modeling program) because there are more interactions, but even using resonant traps the
efficiency is still pretty good.


A combination of the above options:  My current 40 / 80 / 160m antenna system uses two dipoles
on a common feedpoint, one for 80m and one for 40m with loading coils on the end to operate 160m
in about the same length as the 80m wires.  The same approach can be used for 20 / 40 / 80m.
Or you can use a ladder-line fed doublet that is long enough for good 80m efficiency, but with a
pair of 20m traps to prevent the pattern from breaking up on that band.


A TRUE fan dipole: by using multiple wires of the same length spread apart as a fan on each side
of a dipole you can get a low SWR over a very wide bandwidth.  For example, with 5 wires each
40' long over about a 25 degree span (from memory) on each side of a 4 : 1 balun the SWR is
below about 2.5 : 1 from 40 through 10m.  While the match isn't perfect, it should be well within
the range of most built-in tuners.  The pattern breaks up beyond a certain length, just as it does
with a regular antenna, but it should work over a 2 : 1 frequency range.  Such a dipole could
cover, for example, 20 / 17 / 15 / 12 / 10m with reasonably low SWR.

Then there are the more creative approaches, especially allowing the antenna to be 2-dimensional
(or more) rather than just a straight wire.  A vertical full-wave loop on one band can be used as
a bisquare on the second harmonic by putting a quarter wave open stub at the top.  (This will
probably need ladder line feed.)  A folded dipole for the lower band can be twisted into a pair
of phased loops for the upper band - I've done this horizontally for NVIS on 80 / 160m and it
has matches well with a 4 : 1 balun.   One of the more creative methods would be to build a
doublet using 102' of wire, letting 17' hang down at each end and attaching about the same
length of ladder line to the center.  This will load up in the normal manner on 40 and 80m (just
like a G5RV if you switch in a bit of extra twinlead.)  On 20m you can short the ends of the
ladder line and feed it as a bobtail curtain against ground.  (In fact, you could try using that
extra piece of feedline you used on 40 and 80m as a quarter wave zepp feed on 20m...)

The bands are close enough together for the 20 / 15m antenna that something like a lazy-H
or other broadside array could work on both bands.  The simplest would be to build a pair of
1.25 wavelength elements for 15m stacked one above the other (say at 60 and 30' above
ground) and feed them in parallel with equal lengths of ladder line.  That would give you a
reasonable amount of bidirectional gain on both bands.  Even a single (or double) 20m full
wave vertical loop will have a bit more gain on 15m without the pattern breaking up.

I'm sure we can come up with a few options given the chance...
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