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Author Topic: dipole for 40/30/20 meters  (Read 4451 times)
AE5EK
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Posts: 53




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« on: June 11, 2012, 01:49:25 PM »

How can I get a dipole for 40/30/20 meters?  I would like just one antenna connection and have a VSWR that is < 3.
I want it no longer than ~70 feet. Can I combine a 40 meter OCF dipole (with 4:1 balun)  with a standard 30 meter dipole that is a little too long so that it presents a ~200 ohm Z at 30 meters?
What about a fan OCF dipole?
Can I get 40/30/20 and some shorter wavelengths as well?
If this is practical someone should have this figured out. I really do not have time to get EZNEC up and running and simulate this.

tks

Dennis
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WB6BYU
Member

Posts: 13027




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« Reply #1 on: June 11, 2012, 02:24:02 PM »

There are several ways you can do it.

The first is to put a set of dipole wires for each band on a common feedpoint.  Spread the
ends of the wires out as much as you can, but even if the wires are parallel and only a
few inches apart it should still work - it will just require a bit more care in adjustment
due to interaction between the wires.

You can make a trap dipole, which will cover the three bands in a length somewhat
shorter than a half wave dipole on 40m due to the loading effect of the traps.

Either of these methods can be used to extend the antenna to more bands.  And you
can use a combination - perhaps one trap dipole for  40 / 20 / 10m and another
for 30 / 17 / 12m connected to a common feedpoint.

Or, if you could tolerate a bit longer length, a true FAN dipole:  start with a 4 : 1 balun
and on each side connect 5 wires, each 40' long, arranged in a fan.  The optimum angle
is around 25 degrees I think, though I'll have to confirm that.  Such a combination
can give you an SWR close to 3 : 1 or better across the whole frequency range from
7 to 30 MHz.

There are a few other methods, but these are the simplest.
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VE3FMC
Member

Posts: 983


WWW

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« Reply #2 on: June 11, 2012, 03:26:43 PM »

There are several ways you can do it.

The first is to put a set of dipole wires for each band on a common feedpoint.  Spread the
ends of the wires out as much as you can, but even if the wires are parallel and only a
few inches apart it should still work - it will just require a bit more care in adjustment
due to interaction between the wires.

You can make a trap dipole, which will cover the three bands in a length somewhat
shorter than a half wave dipole on 40m due to the loading effect of the traps.

Either of these methods can be used to extend the antenna to more bands.  And you
can use a combination - perhaps one trap dipole for  40 / 20 / 10m and another
for 30 / 17 / 12m connected to a common feedpoint.

Or, if you could tolerate a bit longer length, a true FAN dipole:  start with a 4 : 1 balun
and on each side connect 5 wires, each 40' long, arranged in a fan.  The optimum angle
is around 25 degrees I think, though I'll have to confirm that.  Such a combination
can give you an SWR close to 3 : 1 or better across the whole frequency range from
7 to 30 MHz.

There are a few other methods, but these are the simplest.

A 4:1 balun? I used a 1:1 on my fan dipole. Plus why not trim the legs for each band?
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WB6BYU
Member

Posts: 13027




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« Reply #3 on: June 11, 2012, 05:04:11 PM »

Quote from: VE3FMC

A 4:1 balun? I used a 1:1 on my fan dipole. Plus why not trim the legs for each band?



Because we are talking about two very different antennas here.  The original FAN dipole
is a WIDEBAND antenna - in this case it covers 7 to 30 MHz continuously, with the SWR
below about 3 : 1 across the whole range.  This is a flattened version of the biconical
dipole.  If you make the wires longer it will work on lower frequencies, but the 40' wires
seem about right to cover 40m and up.

What is often called a "fan dipole" in ham radio usage, that is, multiple half wave
dipoles on a common feedpoint, is a MULTIBAND antenna:  it is resonant on specific
bands, but has a high SWR in between those bands.

Since hams use specific bands, multiple dipoles on a common feedpoint are a reasonable
approach, though it requires adjustment to get the SWR low on the desired frequencies.
For commercial or military users who may need to operate on a variety of frequencies,
the wideband version is more convenient, as it doesn't require any tuning but still gives
a reasonably low SWR.  Here is an example of a commercial version:

http://ascsignal.com/files/hf/omin_directional/1765.pdf
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VE3FMC
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Posts: 983


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« Reply #4 on: June 12, 2012, 05:45:15 AM »

Quote from: VE3FMC

A 4:1 balun? I used a 1:1 on my fan dipole. Plus why not trim the legs for each band?



Because we are talking about two very different antennas here.  The original FAN dipole
is a WIDEBAND antenna - in this case it covers 7 to 30 MHz continuously, with the SWR
below about 3 : 1 across the whole range.  This is a flattened version of the biconical
dipole.  If you make the wires longer it will work on lower frequencies, but the 40' wires
seem about right to cover 40m and up.

What is often called a "fan dipole" in ham radio usage, that is, multiple half wave
dipoles on a common feedpoint, is a MULTIBAND antenna:  it is resonant on specific
bands, but has a high SWR in between those bands.

Since hams use specific bands, multiple dipoles on a common feedpoint are a reasonable
approach, though it requires adjustment to get the SWR low on the desired frequencies.
For commercial or military users who may need to operate on a variety of frequencies,
the wideband version is more convenient, as it doesn't require any tuning but still gives
a reasonably low SWR.  Here is an example of a commercial version:

http://ascsignal.com/files/hf/omin_directional/1765.pdf

OK, got it.

Since I have been licensed must articles I have read have called the "Multiband Dipole" a fan dipole. Legs for each band with the common feed point.

Pretty sure I would not want that wideband version up in my yard  Grin
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W5DXP
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Posts: 3546


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« Reply #5 on: June 12, 2012, 08:49:42 AM »

How can I get a dipole for 40/30/20 meters?  I want it no longer than ~70 feet.

Here's what I did to get a no-tuner all-HF band dipole.

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

In my article, I state that the same thing can be done for 40m-10m operation using a 66' (40m) dipole fed with ladder-line through a 1:1 choke. The basic 40m design is a 1/2WL dipole fed with 1/2WL of ladder-line. The 50 ohm SWR will be below 1.5:1. So for a 66' dipole, the ladder-line should be ~62' for 40m operation.

To make the full-wave dipole work on 20m, it needs to be fed with an odd 1/4WL of ladder-line. That would be either 47' or 78'.

To make the 40m dipole work on 30m, make the ladder-line ~73' long. That makes the 78' for 20m the most logical choice.

Switching ladder-line lengths may sound like a pain, but it eliminates fiddling with the tuner knobs and it takes only seconds. If one wants to go to relay switching, it takes less than a second.

Here's another idea based on EZNEC. Use a 54' dipole with ~81' of ladder-line for 30m and 20m operation. Remove 7' for 40m operation. Or use ~78' of ladder-line with a tuner for all three bands.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
G4AON
Member

Posts: 516




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« Reply #6 on: June 12, 2012, 12:28:37 PM »

How can I get a dipole for 40/30/20 meters?
It is fairly easy, make a trap dipole for 40/30 and add a parallel dipole for 20m. Details of how to make 10 MHz traps are on my web site at www.astromag.co.uk/vertical/  Those particular traps will take 1KW or maybe more. The capacitors came from the Ukraine on eBay.

For the parallel dipole, I use 1/4" fibreglass tube spacers held in place with self amalgamating tape. About 8" spacing should suffice. The fibreglass tube is sold by kite suppliers, I got mine from an eBay trader.

Feed the dipole in the centre with a 1:1 choke balun.

I have an 80/40 trap dipole with a 20m dipole in parallel. Works great.

73 Dave
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K3VAT
Member

Posts: 701




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« Reply #7 on: June 12, 2012, 02:32:46 PM »

Quote from:  link=topic=83545.msg602179#msg602179 date=1339447765
How can I get a dipole for 40/30/20 meters?  I would like just one antenna connection and have a VSWR that is < 3.  I want it no longer than ~70 feet. Can I combine a 40 meter OCF dipole (with 4:1 balun)  with a standard 30 meter dipole that is a little too long so that it presents a ~200 ohm Z at 30 meters?
What about a fan OCF dipole?  Can I get 40/30/20 and some shorter wavelengths as well? If this is practical someone should have this figured out. I really do not have time to get EZNEC up and running and simulate this. tks Dennis

Dennis,
You can make your own triband dipole by using the high-quality traps from Unadilla (see their website). You'll need two traps on each dipole leg, but you can tailor the dimension to suit the portion of the band that you like to operate.  I use one of these for the WARC bands (12M, 17M, and 30M) which provides decent SWR so no need to fiddle with the tuner.  If you want to operate on both the CW portion of 40M (say around 7.025) AND the SSB portion (say 7.175) you can even install two wires (the CW wire will be about 9 inches longer) as the final leg of your triband trap dipole.  Unadilla has a dimension sheet that is very helpful getting started and you can do one final trim to really get the resonant frequency exactly where you want it.  For DX work I recommend that you strive for a mounting height of 65 feet - this will be a 'killer' on 20M!  GL, 73, Rich, K3VAT
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W9KDX
Member

Posts: 770




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« Reply #8 on: June 12, 2012, 03:20:18 PM »

I made my own 3 band fan dipole this spring and after a little tuning the length, got all the SWR close to 1:1.  Since then, all three antenna have worked the world, subject to propagation.  It was very easy to build and very inexpensive.  Just wire and a balun and a little plastic tubing for spacers.

General question, is one (the multiband fan or the multiband trap) better than the other or are they just two different ways to do the same thing?

Thanks
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Sam
W9KDX
WB6BYU
Member

Posts: 13027




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« Reply #9 on: June 12, 2012, 03:29:47 PM »

Quote from: W9KDX

General question, is one (the multiband fan or the multiband trap) better than the other or are they just two different ways to do the same thing?



In terms of performance, they are pretty much the same thing.  Using multiple dipoles on a common
feedpoint or a traps still gives you effectively a half wavelength on each band (though a bit shorter
on the lower bands for the trap version.)  The trap version may have a narrower bandwidth than
a non-trap dipoles, but with multiple wires close together you sometimes the bandwidth will be
narrower due to interactions.

The "doublet" approach that W5DXP is suggesting is a bit different in that you may end up
with a radiator that is longer than a half wave on some bands, and that may cause the
pattern to be sharper with a bit more gain, depending on the actual wire lengths.  The
wideband true FAN dipole will have a radiation pattern similar to a doublet of the same
length, so again the pattern may be sharper on 20m when the base antenna is designed
for 40m, and the pattern will break up into multiple lobes on 15m.

By contrast, a 40m OCFD will have a 4-lobed pattern on 20m as opposed to two sharper
lobes of a 40m doublet, even though both are the same length.  That's because the
current distribution is different due to the shift in feedpoint location.
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