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Author Topic: Cheap 80 Meter Antenna  (Read 2792 times)
KI4JGT
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Posts: 114




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« on: January 09, 2008, 09:13:18 AM »

Need a cheap 80 meter antenna to build for the Warbler. PSK31 transceiver
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W8ZNX
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Posts: 1




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« Reply #1 on: January 09, 2008, 11:08:01 AM »

dipole
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ONAIR
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Posts: 1741




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« Reply #2 on: January 09, 2008, 10:34:15 PM »

Dipole, loop, double bazooka coaxial.
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NA0AA
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Posts: 1042




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« Reply #3 on: January 10, 2008, 05:00:08 PM »

For QRP anything less than a resonant dipole is probably a waste - you need as efficient an antenna as possible.
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WB6BYU
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Posts: 13253




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« Reply #4 on: January 12, 2008, 09:04:27 AM »

Do you have room for an 80m dipole, or do you need something shorter?
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KI4JGT
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Posts: 114




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« Reply #5 on: January 13, 2008, 03:04:25 PM »

Want it short, but want really high gain. so don't know
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N3OX
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Posts: 8847


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« Reply #6 on: January 14, 2008, 10:51:09 AM »

"Want it short, but want really high gain"

No such thing.

The best most people can hope for is 90%+ efficiency on 80m... a dipole up more than 30-40 feet will tend to do that.  You put 100W in it'll radiate 90+ W.

To actually get *gain*, say over a dipole, on 80m requires an enormous antenna.  

Don't get taken for a ride by antenna manufacturers and websites that claim "gain" for shortened 80m antennas.

If you use a Hamstick antenna for your reference, for example, let's say a simple dipole, even a low one probably has 10,12, or 15dB "gain" over a Hamstick, but the Hamstick only radiates a tiny fraction of the power supplied to it.

The best you can hope for with a short, small antenna is to make it have a *small loss* with respect to a dipole or full size vertical.

Shortened and loaded antennas always have *negative* gain compared to full size antennas of the same type.  

Sometimes the difference can be very small, but don't fall into the trap of thinking that you can get *gain over a dipole* on 80m without building a multi-element array like an 80m yagi or array of verticals.  You can't.

An 80m gain antenna requires probably at least a hundred foot x sixty foot space or so.

If I were you, I would try to get up a full size dipole as high as you can, especially if your objection is that it's just difficult to put up a 120 foot dipole.  It's really the easiest plan, and worth the work.

If you can't fit it, there are excellent shortened antennas that will work *almost as well* as a dipole, even down to half size, but they'll require some coils and much more careful building.  They'll have *negative gain* with respect to a dipole, but not badly so.

73,
Dan

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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WB6BYU
Member

Posts: 13253




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« Reply #7 on: January 14, 2008, 08:04:56 PM »

Dan gave you a good assessment:  when talking about small 80m
antennas the point is to minimize loss rather than maximizing gain.
A short antenna can be almost as good as a full half wave dipole if it
is well built, but won't be any better than a dipole.

There was a design published in QST that someone came up with to go
with the warbler that was simply a short dipole fed with twinlead to a
tuner.  The efficiency wasn't all that good, but his point was that PSK
is an efficient mode so low power worked quite well on 80m.

The simplest approach is to string up a half wave dipole with the center
running as straight as possible for as far as possible across your lot,
then bend the ends around to make it fit.  One way to do this is to run
the wires out as far as they can go, then fold the leftover back towards
the center mast to form a stub.  Any scheme that folds the wire will
require more wire than a straight dipole and the input impedance will
be low, but you can make it work.   You can also make a dipole of any
length resonant by inserting loading coils, either at the center or at
some point in each side of the wire.  For best efficiency the coils should
be fairly large in diameter with the turns spaced apart rather than close
wound.  Perhaps the simplest method to adjust (but not the most
efficient) is to use a single loading coil in the center, then wrap some
turns of hookup around it to make a link coil for the coax.  With this
approach the main coil sets the resonant frequency and the number of
turns on the link coil sets the inpedance at resonance.

Otherwise, give us a description of the space you have available and we
can see what will fit.  I have about 50 designs for reduced-space 160m
antennas that I've been working on, and I'm sure that some of those
approaches would work here.

Good luck!
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AD5VJ
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Posts: 10


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« Reply #8 on: April 20, 2009, 06:30:09 PM »

The most efficient antenna you can have and still be a bit shorter than a dipole, with some gain due to an increase in dynamic range, with a lot less noise for digital modes, etc would be a Double Bazooka Antenna.

You might want to look at these reviews if in doubt:
http://www.eham.net/reviews/detail/8159

I have many people who are really liking their experiences after I have started making them.

BTW this is not an ad, just giving some input, you can also make one (rather easily) they have the formulas, etc are all over the web, so you don't have to get one from me, again this is not an ad.

I just believe in design very much.

Bob AD5VJ
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N3OX
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Posts: 8847


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« Reply #9 on: April 21, 2009, 09:38:05 PM »

"The most efficient antenna you can have and still be a bit shorter than a dipole, with some gain due to an increase in dynamic range, with a lot less noise for digital modes, etc would be a Double Bazooka Antenna. "

False.

A dipole can't have gain over a dipole, and a Double Bazooka is, at best, a dipole.



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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
AD5VJ
Member

Posts: 10


WWW

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« Reply #10 on: April 21, 2009, 09:57:43 PM »

Yeah your right in most cases (today at least).

Since in most cases what is being pawned off as a "Double Bazooka Antenna" is nothing more than a "Bazooka Coaxial Dipole".

But then, what is being passed off as a 'dipole' is not a dipole either, since nowadays, they are usually fed with unbalanced line and even then not a quarter wavelength long.

Bob AD5VJ
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WS4E
Member

Posts: 223




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« Reply #11 on: April 26, 2009, 06:46:47 PM »

If you don't have the room for a full length 80M dipole, consider a LOOP.

For instance on my suburban lot, I don't have the length that would be needed for a full length 80M dipole, but I actually do have the length needed for a 80M horiz loop just between 2 trees and a roof peak on the garage in a delta shape.  I even have room for a full 160M loop if I stretched one end over the top of the house to use the whole lot from side to side, that's my next project.  You would be surprised just how big a loop you can fit in even on a small lot.

 

Its easier to fit the length of one of the many sides for a horizontal loop down a part of the yard then a whole 1/2-1 wavelength.


http://www.donkeith.com/n4kc/Skywire.htm
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KT4WO
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Posts: 150


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« Reply #12 on: April 30, 2009, 05:50:41 AM »

Having used a "Warbler" and PSK-40 for many years now..
Trust me ..YOU DONT WANT ANYTHING LESS THAN A HALF-WAVE DIPOLE!... With only 3 watts out..you will really need the dipole or as others said.. a loop.
I have had my warbler and psk-40 for about 4 years now and LOVE them,,, for the cost and the fun building them, you cant beat it. DO NOT OVERDRIVE !!
If you need more than 2-3 watts PEP.. goto a full size radio.

Have fun !
Trip - KT4WO
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N5LRZ
Member

Posts: 0




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« Reply #13 on: May 04, 2009, 05:33:11 AM »

Re OX...

What you are stating is true IF IF IF the actual real physical dipole itself were used as the basis for comparison...which seldom happens today.  Today they use computer generated perfect models aka DBI to determing antenna gain for the vast majority of antennas.

sorry to have to tell ya but a dipole does have a gain per the computer generated perfect quarter wave in perfect free space.

It, the dipole, has a gain of 2.1 I seem to remember compared to a theoretical mathmetical calulated antenna in perfect free space.

IF an antenna has a gain number stated in DBI terms then SUBTRACT 2 db to convert the antenna gain to that of a comparison to an actual real dipole aka DBD.

Antenna people love to site DBI terms because it makes their advertiseing sound much better.

Thus in math theory and computer caluclated theoretical models a dipole does have a gain over a perfect math theoretical DBI a antenna.
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N5LRZ
Member

Posts: 0




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« Reply #14 on: May 04, 2009, 05:36:51 AM »

Re to WS4E comments...

Loops are definately good.  BUT you need trees and or other tall things in the right places to suppor the 3/4/or whatever number of corners of the antenna.

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