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Author Topic: 80 meter dipole antenna...  (Read 3927 times)
KC0OHP
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Posts: 66




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« on: September 11, 2004, 12:24:55 AM »

Question....Wanting to build a 80 meter
dipole antenna.  So what confiuration
would be the best to use?

  T-shape, Inverted vee, or Sloper shape.
And how high should it be placed to be
a very affective and resonant type antenna
on the 80 meter phone band?  Or maybe
use trap coils for the 80 meter band?

  Also can the 80 meter dipole antenna
be cut and resonant for the operating
phone band only from 3.850 too 4.000?

  What is the length for this freq range.
I have checked arrl ant book.  But
really can't find any related info
about it.  Any help greatly appreciated...

  Dwight...KC0OHP..
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K6XR
Member

Posts: 63




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« Reply #1 on: September 11, 2004, 12:46:52 AM »

suggest the antenna hand book by arrl or the hand book by arrl. both have good references and constructions articles
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N4ZOU
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Posts: 340




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« Reply #2 on: September 11, 2004, 01:05:09 AM »

Last 80 meter dipole I had up was an inverted V with the ends just close enought to reach them from the ground in order to clip on extra lengths of wire. This allowded full band operation without an antenna tuner. Simply trim the antenna so that the maximum SWR of 1.5:1 is at 4.0 and drops as you go down the band. When you get to the point where the SWR is just above 1.5:1 simply clip on an extra length of wire to each end of the antenna and trim both for a 1:1 match or as low as you can get it and then mark that frequency on them. Continue the process untill you get to the bottom of the band. Now when you need to move from one part of the band to another simply grab the pair of tuning wires you need and clip them on the antenna. Easy and cheap!
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N4ZOU
Member

Posts: 340




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« Reply #3 on: September 11, 2004, 01:13:11 AM »

I forgot to give you the lengths! Just cut two lengths of wire 60 feet long and add center feed point and insulators. This should be a little long so you can trim it for a low SWR at the top of the band. 1/4-wave lengths of wire for a dipole are found by dividing the frequency in MHz by 234, which will equal the length in feet. IE: 234/frequency=feet.  
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KC8VWM
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Posts: 3121




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« Reply #4 on: September 11, 2004, 02:27:04 AM »

You want it to work only on 80 and nothing but 80 meters?

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KC0OHP
Member

Posts: 66




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« Reply #5 on: September 11, 2004, 03:26:53 AM »

Yes....On the 80 meter band for 3.850 to 4.00 MHz.
For the phone part of the band.  So divide 234
by the operating freq.  Correct??

This will give the correct length? Thanks..

Dwight..KC0OHP...
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K0CWO
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Posts: 419




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« Reply #6 on: September 11, 2004, 11:04:47 AM »

Dwight,

You are correct.  Using 3.925 Mhz as your center frequency between 3.850 and 4.000 Mhz your formula would be 234 : 3.925 = 59.62 feet for each half of your dipole.  I would follow a previous poster's recommendation and make the dipole 120 feet long.  I doubt that you will have a 2:1 SWR bandwith from 3850 to 4000, but if you have an antenna tuner, that can be tuned to where the output power of your radio wont fold back.  Put the dipole as high as you can.  50 feet high would be great, but if you can't get the antenna that high don't let this stop you.  I have heard dipoles work great at 20 feet in the air.  I run one at 35 to 40 feet and can work coast to coast on 80 (75) meters.  As far as what kind you want to put up (sloper, inverted V, or flat top (T)), slopers and inverted V's are directional in respect to the antenna ends.  The T or flat top is more omni directional when you put it relativly close to the ground. My advice, put up what fits the best in your situation.  They all work out to be about the same.

Another very afordable version of a dipole you can build for little money is called a "doublet".  This antenna is the same as a dipole except for the fact that it is fed with balanced feedline also referred to as ribbon cable or ladder line.  It comes in 300, 450, and 600 ohm impedances.  It is a non- resonant antenna when put up like a dipole, but can be put to use with an antenna tuner.  It is a very efficient antenna that will work from 80-10 meters if used with the above mentioned dipole length.  I have used this type of dipole for years, and is the only antenna I use on all bands anymore.  The only drawback of this antenna is that you need to get handy with a manual antenna tuner, especially when you do a lot of band hopping.  If you would like instructions on how to build one, let me know, I'll email it to you.

I'm at Fort Riley right now, if you need "on air" reports about your dipole let me know.  We'll set up a schedule.

Good luck, and get it in the air any way you can!!

BJ
k0cwo
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K8AC
Member

Posts: 1477




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« Reply #7 on: September 11, 2004, 01:57:24 PM »

I suggest you consider using a half wave dipole fed at the center with ladder line.  A simple tuner (even an L network) with a balun at the output will allow you to use the antenna across the 80/50 meter band and it will also work just fine on all the bands that are higher in frequency.  For reasonably close-in work on 75 SSB, the configuration of the antenna won't be of much concern - just use the supports you have to get the antenna as high as possible.  Every ARRL antenna book since the beginning has had the half-wave antenna formula: 468/freq in MHz = length in feet.  Cut the antenna for the center of the frequency range you'll be operating in on 75.
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X-WB1AUW
Member

Posts: 559




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« Reply #8 on: September 11, 2004, 08:59:01 PM »

Try building a dipole for 20 meters.

You can then experiment with it as a dipole, inverted vee, and maybe even a sloper.

Then you will have enough experience to decide what you want to do for 75 meters.

Have fun
Bob
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N6AJR
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Posts: 9921




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« Reply #9 on: September 12, 2004, 12:29:05 AM »

it will work as a flat top, sloper or inverted V depending on the supports available.. chec this out for a kick

http://www.hamuniverse.com/multidipole.html
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W8MW
Member

Posts: 329




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« Reply #10 on: September 12, 2004, 02:26:21 PM »

Dwight there's a lot of ways to go about it and the right way is best defined as the way that's right for you.  I love 80 & 75 and spend most of my operating time somewhere between 3.5-4.0 MHz.  What is right for me is a coax-fed 80 meter antenna tuned to resonance at a target frequency and supported in the center by my tower.  So the result is an inverted vee.

Phone operation between 3850-4000?  Another way I might phrase the question, is it practical to expect the 2to1 SWR curve on an 80 meter dipole to cover 150 kHz?  Yes.  I would tune the antenna for the half way point between those frequencies (3925).  Use the formula 468/3.925 and the resulting 119 feet is the total length of a half wave dipole.  Divide that by 2 for the length of each "leg" from the center insulator.
It is a good idea to cut the wire intentionally long for final fine-tuning to hit the target frequency.

How high does it need to be?  If you have support structures that let you get a half wave of elevation (120 feet above ground) you would be a rare and lucky fellow.  Most of us don't have that luxury and over the years I have used 80 meter antennas as low to the ground as 20 feet.  At that height, the antenna will work, but efficiency is reduced because of ground loss.  Just from practical experience I have found 35-40 feet okay, but getting it up 50 to 80 feet makes it play noticeably better on receive and transmit.  

Most of us using horizontal wire antennas on 80/75 are dealing with whatever support structures are available and we end up with antennas low to the ground in terms of electrical height.  This means substantial high angle radiation. If you are interested in regional ragchewing or nets within say 800 miles, you'll come to appreciate the combination of a high angle radiator and the high angle propagation that supports nice strong signals on 80/75.  It is possible to work DX on a typical dipole but people primarily interested in DX will usually opt for antennas with a low angle of radiation.

Good luck with your project.  The fall and winter are prime time on 80/75.

73 Mike  
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KT8K
Member

Posts: 1490




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« Reply #11 on: September 14, 2004, 10:52:34 AM »

Note that ground losses will lower the resonant point of the antenna, so you may have to cut off quite a bit if it is low to the ground.  In one case I was involved in we had to cut an 80 meter dipole at 20-25 feet (crossing over a house, too) to only 110 feet to get it to resonate in the middle of the band.  At greater heights the length won't be reduced as much, however.  Higher is always better!
Good luck & 73 de kt8k - Tim
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