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Author Topic: Best antenna for 80 meters  (Read 5281 times)
KG6UHC
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Posts: 7




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« on: April 02, 2004, 10:15:47 PM »

Hello,

Im new to Ham radio and was wondering what the best antenna for 80 meters would be.  Im on the side of a hill, close to a valley floor, and Im surrounded by quite a few trees.  I have a Cushcraft R5, but I think its getting alot of propogation interference from those trees.  Also, how long is too long for a coax feed to the antenna...and whats the best type of coax for longer runs.  Anyway, thanks in advance.

73,
kg6uhc
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KB8YJU
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Posts: 54


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« Reply #1 on: April 03, 2004, 12:13:49 PM »

Ok.......
First of all, I wouldn't worry too much about those trees affecting your signal on 75/80. Depending on how tall they are, most people USE the trees as antenna supports!
A few suggestions:
1) An inverted-v with the feed point as high as you can get it in a tree.
2) A horizontal full wave loop (if you have the room) will work real well for most short haul work. You will want to use a tuner for this one.... it can be a powerfull antenna on lots of other bands as well.

There are lots of other ideas you can use for more serious dx work, such as elevated wire verticals and half square antennas, but for now, since you are new to the hobby, I'd stick to the inverted-v.

Hope this helps,
Joe K8NMU
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KB8YJU
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« Reply #2 on: April 03, 2004, 12:18:02 PM »

Oh....... and about the coax,
The shorter the run, the better of course. That goes for any coax at any frequency. How long is too long?
I guess when your loss is equal to the gain you achieved by using the long run, lol.
At 75/80 meters, loss is not a huge factor..... not like at 6 and 2 meters.
I personally use full size RG-8 for my coax, but you can use mini-8 all day long and be happy with it.

Joe
K8NMU
 
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WB6BYU
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Posts: 13579




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« Reply #3 on: April 06, 2004, 05:18:30 PM »

"Best" antenna depends on where you want to work and
usually exceeds the resources (time/money/space) you
have available.  But there are a number of good options,
and you will have to evaluate them to see which would
fit best in your situation.

The dipole/inverted vee is probably the simplest to
install and works well.  If you use the dipole
configuration (supported from the ends) then RG-58 coax
is probably a good choice for feedline, since the coax
weight will make the antenna sag in the middle.  If the
feedpoint is supported (as with an inverted vee) then
you can use something larger.  With the inverted vee
tie the ends out as far as possible from the antenna
so the wires are as flat as possible.  The dipole is
long and skinny, and fed in the center - it best fits
a location with trees on roughly opposite sides of
your house, or a single support near the house.

The next step up is a full wave loop:  this will be
about 70' on a side if square (though other shapes work
about as well.)  It can be fed anywhere around the loop
and will have a reasonably low SWR on coax feed when
tuned, depending on the height above ground.  This
would be a good choice if you don't have room for a
dipole, but have trees in all directions.  The easiest
method I've found for installing one is to lay the
wire on the ground about where you want it, throw ropes
over the support trees, tie the ropes around the wire
so the wire can slide back and forth (plastic egg
insulators for electric fences were great for this, but
I haven't seen them recently), then pull the ropes
up one at a time to work the loop into the desired
shape.

The best 80m antenna I put up recently was a big bow-tie
loop using 540' of wire:  imagine two equalateral
triangles point-to-point, with each side 90' long.  In
the middle the wires should cross over each other, so
instead of two separate loops it really is one big loop
with "half a twist".  Break one of the wires where they
cross in the center and insert a 4 : 1 balun.  Like
the dipole, this can be supported from the corners or
from a single center support.  Takes more space than
a dipole and twice the wire of a full-wave loop, but
it was great on 80m and usable on all bands 160 - 10m.

Any of these antennas can be fed with open wire line
to a tuner instead of coax, which allows all-band
operation.  This is particularly attractive if it is
not practical to tune the antenna length to resonance.
For example, sometimes when there are lots of trees I
just toss the end of the wire over one branch at a time
in a big circle until I get back to the starting point.
This makes it hard to take the loop down to adjust the
length, so I use a tuner instead and don't worry about
the exact loop resonant frequency.

All of these are good antennas, especially for local
work when installed less than 80' high.  (Performance
shifts more in favor of DX and less in favor of local
stations as you get the antenna higher.)  Take your pick.

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WB6BYU
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Posts: 13579




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« Reply #4 on: April 06, 2004, 05:27:09 PM »

Coax cable loss isn't a big problem in 80m, so even
RG-58 will work for runs of 100' or more.  RG-8X is
better, and RG-213 will handle the full legal limit
and a few hundred feet.  This presumes the antenna is
resonant and the line is not operated at a high SWR.

For longer lengths you can use open wire line and a
balanced tuner to 1000' or more, but you will find
that you have to readjust the tuner more often as you
change frequencies with long feedlines.
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KB9CRY
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« Reply #5 on: April 07, 2004, 05:44:29 PM »

The dipoles and bow-tie antennas are mostly horizontally polarized and will be good on the low bands for domestic work.  But if you want to work DX, you'll need a vertically polarized signal, most of the time.  My recommendation for this would be the Butternut HF2V, that will give you both 40M & 80M since your R5 stops at 20M and should not be used on 40 or 80 or you'll blow up it's matching system.  I know since I've done that by accidently tuning up on 40 and 80 into mine.  I would never use RG-58 for anything on the low band except QRP power levels; there's just too much loss and possibility of damage in high mismatches.  I have run RG-8X to my HF2V for about 100 ft. with a KW (SSB & CW only) and had no problems for 10 years.  More than 100 ft., then I'd say you'd have to break down and purchase RG-8U or 213.  I like CableXperts brand; they're also local here in the MidWest.  If you really have the room, a 4 square antenna array is the best, besides a full size yagi up one wavelength.  CU in the pileups, Phil  KB9CRY
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K2VCO
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Posts: 9




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« Reply #6 on: April 08, 2004, 02:46:41 PM »

I agree with KB9CRY that the correct answer to your question is "it depends on what you are trying to work."  If you want to work stations out to several hundred miles distant with excellent signals, the best antenna is a low dipole.  I used a dipole at only 25 feet with excellent results on 80 meters when I had a regular sked with fellows nearby and up to 250 miles away.  This antenna was much more effective than a vertical for these contacts.  However, if you want to work DX or even stations 1000 miles away, a vertical will be far superior, and the HF2V is a very good choice.

I am currently using an inverted L for 80 and 160 meters.  If you can hang a wire from a tree about 40 feet high, then it's easy to make one.  You make the antenna about 1/4 wavelength long, with the top part as horizontal as possible.  This has the advantage of providing some high-angle radiation from the top part for close-in QSOs as well as low-angle radiation from the vertical part for DX.

Either an HF2V or similar antenna or an inverted L as described above will require a ground system.  If you have the space to bury 16 radials 25 feet long each, that will work very well, although the optimum is 120 radials 65 feet long.  If you can't bury a radial system, then you can use elevated radials.  My antenna has two 1/4 wavelength radials for each band running around the eaves of my house.  This is not as efficient as a good buried system, but it works surprisingly well.
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