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Author Topic: 20 and 40 m dipoles  (Read 3166 times)
KJ6ZOL
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Posts: 387




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« on: September 17, 2012, 03:15:40 PM »

I'm new to antenna theory, so bear with me. Once I get my General ticket I plan to put up a dipole for 40m. A 20m dipole may follow later. By my calculations a 40m dipole would be about 30 feet, right? I plan to hang it on a east facing wall from the highest peak of the roof, and feed that into a coax line. I live in city limits but there are no antenna restrictions. What wire should I use? I have a bunch of old speaker wire I used for SWL random antennas, would that work? Can somebody explain how to get the SWR down to 1:1?
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K3VAT
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Posts: 730




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« Reply #1 on: September 17, 2012, 03:54:15 PM »

I'm new to antenna theory, so bear with me. Once I get my General ticket I plan to put up a dipole for 40m. A 20m dipole may follow later. By my calculations a 40m dipole would be about 30 feet, right? I plan to hang it on a east facing wall from the highest peak of the roof, and feed that into a coax line. I live in city limits but there are no antenna restrictions. What wire should I use? I have a bunch of old speaker wire I used for SWL random antennas, would that work? Can somebody explain how to get the SWR down to 1:1?

40M dipole is about 65' feet total (32.5' per side) length.
You probably are referring to an inverted vee, if the center portion is higher than the ends.
#14 wire is fine; I use insulated wire.   You can do a search of this forum for recommendations on wire.
You'll need to purchase or borrow an SWR analyzer like the MFJ or RigExpert models.  Just do a search for these units.  A 1:1 SWR isn't a requirement.  Anything less than about 1.5 or so is fine. 

GL, 73, Rich, K3VAT
« Last Edit: September 17, 2012, 03:56:48 PM by K3VAT » Logged
WB6BYU
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Posts: 13335




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« Reply #2 on: September 17, 2012, 03:59:49 PM »

A 40m dipole would be about 32' on each side, for a total of 64' overall.

A 20m dipole is about half that.

You can use just about any sort of wire:  the main limitations are mechanical
rather than electrical.  You can use speaker wire to start with; I use a lot of
ordinary stranded, insulated hookup wire because it is cheap and easy to
work with.  The insulation only lasts a couple of years outdoors, but in that
time you'll have a much better idea of what antenna you want to put up next,
or how you want to improve it.  #14 or #12 house wire is common because
it is a high volume item at home improvement stores.

My suggestion is to build one on the cheap and see how it works, then once
you are happy with it you can rebuild it with more permanent materials.


You can't always get the SWR down to 1 : 1, but you can usually get it low
enough that you don't need a tuner.  Basically you cut it a bit longer than
formula length (which is only an estimate to start with).  I always hang
it using ropes and pulleys if possible, so I can lower it for adjustment
without having to shinny up a pole.  If you have metal trim or rain gutters,
they will affect the antenna.

To adjust it, check the SWR at a few points across the band.  An SWR
analyzer is handy for this, but your transmitter and a cheap SWR meter
is sufficient.  If the SWR is best at the low end of the band, you need
to shorten the antenna.  If it is best at the high end of the band, you
need to lengthen it.  I leave about 1' of wire hanging down at the ends
and trim the ends, or you can fold back the excess wire.  (That's why
you start on the long side, as it is easier to trim the wire than to
add to it.)  Basically you trim the length until you get the SWR dip
in the desired frequency range, and use it.
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W9GB
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Posts: 2626




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« Reply #3 on: September 17, 2012, 05:29:15 PM »

Quote from: KJ6ZOL
I'm new to Antenna Theory, so bear with me.
Once I get my General ticket I plan to put up a dipole for 40 meters.
A 20 meter dipole may follow later.
By my calculations a 40 meter dipole would be about 30 feet, right?
Byan --

While newcomers "pick-up the terms and jargon", the meanings are often missed or not understood.
You have the answer .... Let's Do a Quick Review:

1.  You desire a dipole antenna for the 40 meter (7.0 - 7.3 MHz) amateur radio allocation [Region 2, Americas].
SO, a HALF-WAVELENGTH antenna (dipole) would be ~ 20 meters in total length
(1/2 the wavelength of 40 meters !! )
The 40 meter dipole, with a total length of about 65 feet, is a good first antenna build.  
At 1/4-wavelength above the ground (~ 35 feet) you will get the figure 8 pattern
that is broadside to the antenna's axis.


2.  The easy to remember Metric measurement formula for the Frequency/Wavelength relationship.
300 / Frequency (in MHz) = Wavelength (in Meters)
gets you the specific wavelength (meters) for a specific frequency (MHz).  
A tuning fork has ONE fundamental (primary) Frequency.  
You TUNE (desired frequency) a guitar or piano by adjusting the LENGTH (wavelength) of the strings !!


You then take that wavelength for various type of antenna builds.
http://www.radio-electronics.com/info/antennas/dipole/length-calculation-formula.php

The English measure version is 468 / Frequency (in MHz) = Wavelength (in Feet)

BTW, Antenna theory is universal.  
Let's look at the 2-meter band (144-148 MHz), which a Technican class licensee can use.
SO, you want to build a Quick and Easy 1/4-WAVELENGTH Vertical.  That is 0.5 meters OR 19.685 inches.

Quote from: KJ6ZOL
What wire should I use? I have a bunch of old speaker wire I used for SWL random antennas, would that work?
You can use the speaker wire (22 - 18 AWG) that you have, especially for your first experiments.

IF you are buying new -- 14 AWG STRANDED THHN wire (choose you insulation color if you desire to camouflage the wire).  This electrical wire for conduit runs can be found in 100 and 500 foot roles at your local hardware store (Home Depot, Lowes, ACE, True-Value) OR Electrical supply house -- this is the easiest retail wire source throughout the USA for radio amateurs to find and purchase on weekends !!
« Last Edit: September 17, 2012, 06:27:31 PM by W9GB » Logged
WB2WIK
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Posts: 20611




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« Reply #4 on: September 17, 2012, 06:11:21 PM »

As has been said, the wire doesn't matter as long as it's strong enough to not break.  Even very thin wire, properly supported, can handle a kilowatt just fine: Remember the goal of the antenna wire isn't to dissipate any power, and radiate all of it.

But the deployment of any antenna, including simple wire dipoles, is way more important than the wire or almost anything else.  A dipole staped to the house won't work anywhere near as well as the same wire dipole strung between 75' tall treetops.

It's the old issue: People talk about "G5RV" multiband dipoles not working well, and I reply, "String one between the tops of two 150' tall towers and see how that works," because in reality, if you could do that, it would work very well.
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KJ6ZOL
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Posts: 387




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« Reply #5 on: September 17, 2012, 10:42:12 PM »

Clarification: (bear with me, I'm autistic and stink at communication sometimes)

I wanted to make a 1/4 WAVELENGTH dipole for 40 meters. Thus the "30 feet" reference. I'm not sure I can put up a 65 ft long antenna, space limitations. I have a line of trees near my back window, and plan to string the wire through them.
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LA3AKA
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Posts: 30




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« Reply #6 on: September 18, 2012, 02:38:15 AM »

Hi Bryan. 
If you want to make a dipole for 40m it has to be approximately 65 feet long.  A resonant dipole is  1/2 wavelength long.  If you want to use a 1/4 wavelength antenna it has to be a vertical with radials on the ground or a Groundplane antenna eith 1/4 length radials.  If you don't have space for a 65 feet long antenna, its's possible to shorten it by using loading coils.  See this page on how you can make a shortened Dipole:

http://www.k7mem.com/Electronic_Notebook/antennas/shortant.html
 
If you want to learn more about antennas, I would recommend you to get a copy of the "ARRL Antenna Handbook". It has a lot of theory and also a lot of practical info on how to design and build antennas

Good look with your General ticket.

73 de LA3AKA
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K5KNE
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Posts: 65




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« Reply #7 on: September 18, 2012, 04:06:36 AM »

I have used #12 house wire with insulation on it.  I have run it through trees and tied it off where ever I could.  If you get it off the ground it will work. I have used a hammer to throw a nylon line over limbs and pull the antenna wire up.  Just try something and then you can improve on it.

The SWR is mainly influenced by the length of the antenna and the frequency you are using compared to the resonant frequency of the antenna.  So if you get an antenna near perfect on 7200 mhz. and then operate within 100 khz of that frequency - it should be pretty good.
Getting an antenna to be 1:1 may not be as important as you think, but try to get it as low as possible on the frequency you like to operate most.
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W5DXP
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« Reply #8 on: September 18, 2012, 04:31:10 AM »

I wanted to make a 1/4 WAVELENGTH dipole for 40 meters.

You can indeed make a 1/4WL dipole for 40m but it will be non-resonant with a feedpoint impedance of 15-j700 ohms with an SWR of 650:1 on coax. It is hard to match, i.e. very difficult to deliver much power to a 1/4WL dipole. An autotuner at the feedpoint is one compromise solution.

A Walter Maxwell minimum length rule-of-thumb is to make the single-wire dipole 3/8WL long on 40m. That would be the G5RVjr length of 51 feet. With a G5RVjr, you can work 40m, 20m, and 10m.

Or you could stick with the 30 foot dipole and install a loading coil in the middle of each dipole element. A center loaded 30 foot dipole would work fairly well on 40m. It would look something like this:

----------//////----------FP----------//////----------

where FP is the feedpoint and ////// is the loading coil.

Or you could use the 51 foot G5RVjr design and let 10 feet of wire hang down vertically at each end.
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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.
AA5WG
Member

Posts: 498




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« Reply #9 on: September 18, 2012, 08:15:42 AM »

KJ6ZOL:

If you have time look at AA5WG comments at these two links:

http://www.eham.net/ehamforum/smf/index.php/topic,85288.30.html

http://www.eham.net/ehamforum/smf/index.php/topic,85317.0.html

The comment made be Cecil in regards to Walter Maxwell is a good one.
This is for minimum antenna lengths.  Longer would be better in terms of
ease of tuning.

Chuck
« Last Edit: September 18, 2012, 08:22:17 AM by AA5WG » Logged
KJ6ZOL
Member

Posts: 387




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« Reply #10 on: September 18, 2012, 01:11:05 PM »

I wanted to make a 1/4 WAVELENGTH dipole for 40 meters.

You can indeed make a 1/4WL dipole for 40m but it will be non-resonant with a feedpoint impedance of 15-j700 ohms with an SWR of 650:1 on coax. It is hard to match, i.e. very difficult to deliver much power to a 1/4WL dipole. An autotuner at the feedpoint is one compromise solution.

A Walter Maxwell minimum length rule-of-thumb is to make the single-wire dipole 3/8WL long on 40m. That would be the G5RVjr length of 51 feet. With a G5RVjr, you can work 40m, 20m, and 10m.

Or you could stick with the 30 foot dipole and install a loading coil in the middle of each dipole element. A center loaded 30 foot dipole would work fairly well on 40m. It would look something like this:

----------//////----------FP----------//////----------

where FP is the feedpoint and ////// is the loading coil.

Or you could use the 51 foot G5RVjr design and let 10 feet of wire hang down vertically at each end.

What's a G5RV antenna? I see a lot of references to it. A single wire dipole sounds better than a double wire.
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WB6BYU
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« Reply #11 on: September 18, 2012, 01:26:20 PM »

A G5RV is a common multi-band antenna that is basically a 102' doublet (single wire fed in
the center) with about 30' of twinlead between the center and the coax feedpoint.  The
basic antenna covers 80, 40, 20 and 12m with reasonably low SWR (though you may still
need a tuner.)  The half-sized version (51' overall) covers 40m, 20m and 10m.

Here is more information:

http://www.vk1od.net/antenna/G5RV/index.htm
http://www.vk1od.net/antenna/G5RV/optimising.htm
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W5DXP
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« Reply #12 on: September 18, 2012, 06:57:14 PM »

What's a G5RV antenna?

The G5RVjr that we are talking about is a 51 foot dipole fed with about 15 feet of 300 ohm twinlead through a 1:1 choke and then 50 ohm coax all the way to the tuner.
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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.
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