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Author Topic: 80m Dipoles?  (Read 1757 times)
KA3VEZ
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Posts: 92




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« on: May 08, 2012, 06:28:51 PM »

Or even 160m(I know it is 250 feet) Here is a question for a 80m dipole antenna. Can you cut the dipole for 125 +/-. Put the center of the dipole on a mast, put the center half on a tree/mast but just leave the other half just hang down and maybe even lay on the ground?  Think it is called a inverted L but correct me if I am wrong.   Thanks for any input.

~Kirk, KA3VEZ
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W5DXP
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« Reply #1 on: May 08, 2012, 06:33:04 PM »

Where is the transmission line with respect to the two dipole elements?
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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.
KA3VEZ
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Posts: 92




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« Reply #2 on: May 08, 2012, 06:38:06 PM »

The coax feeding the dipole will go straight up and the "hot" part of the dipole will go in one direction and the other will more or less dangle. I may have to find a way so it does not touch.
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WB6BYU
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« Reply #3 on: May 08, 2012, 08:31:24 PM »

Well, you CAN, but that doesn't mean it is going to work well, or have a low SWR.

An "Inverted L" is an end-fed wire typically fed at ground level that goes up vertically
and then horizontally out to another support.  At the feedpoint there should be a
reasonable number of ground radials to provide the "other half" of the antenna:  these
don't radiate themselves, but they collect the return currents which would otherwise
flow through the high-resistance dirt, thereby improving the efficiency.

When the feedpoint is at least 1/4 wavelength above the ground you can install a
dipole with one wire horizontal and one running vertically backdown the mast.  One
significant problem is the feedline:  you don't want it running parallel to the vertical
wire for any distance if you can help it, or you end up with a lot of RF on the coax
even if you use a balun at the feedpoint.  This arrangement probably works best
when the coax runs horizontally to the feedpoint - for example, if it is hung off a
corner of a house, with the coax coming in along the roofline.

When the antenna is lower than 1/4 wavelength, as will usually be the case on
80m and 160m, it becomes more problematic what to do with the wire.  The closer
it is to the ground, the more it will shift the tuning and increase losses.  Not that
you might not be able to make some contacts, but it just won't work as well.
You also want to try to keep from running it back under the other wire, as that
will tend to cancel some of the radiation.

Generally the most effective way to install a dipole in an area that isn't long
enough for it is to put the feedpoint in the middle and run the wire as far as
possible to the two furthest corners.  That gets the center of the antenna up
in the air where it can do the most good.  The ends that don't contribute as
much to radiation can then be allowed to hang down or run off at various
angles as necessary to use up the remaining wire length.  (But be aware
that this will actually require a longer antenna wire than if the antenna were
straight.)

There are a number of reasonably efficient methods of loading a dipole so it
fits in half the normal length.  As you shrink it beyond that, the losses increase
faster, but sometimes that's still the best you can do.
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K0ZN
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« Reply #4 on: May 08, 2012, 09:06:23 PM »

Short version:  "No."  You can't have part of an antenna laying on the ground. A Dipole must be insulated and *reasonably* above ground or all kinds of not good
stuff starts happening.  Antennas, such as a real Inverted L work against a "real" ground system. They are an entirely different breed of cat. The ground is the
other HALF of those antennas.

Sounds like you have an interest in Antenna building... it is a lot of fun and satisfaction.  Seriously and respectfully:  Pick up a copy of the ARRL Antenna Book
and put in some study time. It will be the single best thing you can do at this point; arm yourself with CORRECT information. Antennas are not complicated, but they
have some specific parameters that MUST be met in order for them to work well or even be able to feed energy to them. Antennas do not lend themselves to
guesses and assumptions. The antenna is literally what connects your station to the world, so it is a subject you really need to understand. There is a TON of myth and misinformation out there, consequently accurate knowledge will reward itself many times over in a better signal and reduced frustrations. i.e. more fun in the hobby. You can pick up an older copy of the ARRL Antenna Book off of Ebay, etc. quite cheaply and the technical basic info is as accurate as the new book. WELL worth your time
and effort.

73,  K0ZN
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W5FYI
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Posts: 1044




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« Reply #5 on: May 09, 2012, 07:50:50 AM »

The antenna you describe is a dipole, if I'm getting the picture right. By "...put the center half on a tree/mast..." I assume you mean the inner half of each leg will be elevated and the outer half hanging down. It will work, probably reasonably well, but not as well as a full dipole up high from tip to tip.  Dipole antennas have most of their radiation coming from their center portions, so you should be in good shape.

The antenna you describe is common for folks who don't have enough room for a full length dipole. Try to make the elevated part as long as possible until you run out of room, then let the ends droop or bend toward another direction. Remember, the ends of a dipole are the high-voltage points, and can be very dangerous to a passerby who might accidentally touch it; and if the ends touch the ground, they may arc and could start a fire in dry grass or other debris--and cause havoc with SWR. If you can bend the ends of the antenna so that it looks like a "Z" in a bird's-eye view, and keep the ends at least eight feet above the ground, you have a valid "bent" dipole.

The suggestion about getting a good antenna book is very worthwhile. Some hams in your situation will add loading coils where appropriate in order to shorten the antenna's overall length. The ARRL Antenna Book has a section on short antennas that can help you determine the placement and value of the loading coils, and efficiency and loading for the antenna.

By the way, you will probably find that you can get broader bandwidth on 80-meters if you use 75Ω coax rather than 50Ω.

(An inverted L antenna is a quarter-wave vertical antenna that is fed at its base and has its top portion bent more-or-less parallel to the ground). GL
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WA3SKN
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Posts: 5458




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« Reply #6 on: May 09, 2012, 11:07:50 AM »

There are many ways to electrically "shorten" or "legthen" an antenna, but having a portion of it laying on the ground is not a good plan... you will want it off the ground.
So how much room do you have and what bands and times do you plan on operating?

-Mike.
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KA3VEZ
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Posts: 92




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« Reply #7 on: May 09, 2012, 05:33:39 PM »

Ok all. I get the idea.  Thanks again for the thoughts.  I know what a dipole antenna is just wish there was a way to have the wire length but condense it to my situation which I now believe is not possible.  Thanks all.


~Kirk, KA3VEZ
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W5DXP
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« Reply #8 on: May 10, 2012, 05:35:09 AM »

Remember that the ground-mounted radials for a vertical antenna are the other half, i.e. one of the dipoles, of the antenna. I've seen field day antennas where one element of the dipole was vertical with the other element was laying on the ground and it worked well. I just modeled a dipole with one vertical element and one horizontal element one foot above ground. The maximum gain is only about 1.2 dB down from the same standard vertical with 8 radials one foot above ground.

Consider a vertical dipole where part of the lower element was laid upon the ground. That would not be such a bad antenna. Now bend the upper element from vertical to horizontal. That is still not such a bad antenna. It's a lot better than no antenna at all.
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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.
W4VR
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« Reply #9 on: May 10, 2012, 02:31:16 PM »

Kirk:  I tried dipole mounted that way (but not touching the ground) many years ago on 40 meters.  I compared it to a horizontal dipole and really could not tell the difference.
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KE6SLS
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Posts: 33




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« Reply #10 on: May 16, 2012, 10:18:14 PM »

Ok all. I get the idea.  Thanks again for the thoughts.  I know what a dipole antenna is just wish there was a way to have the wire length but condense it to my situation which I now believe is not possible.  Thanks all.


~Kirk, KA3VEZ

Kirk:

You can make is smaller.  My dipole is about 55 to 60 feet on each end and I load it from 160m all the way up to 6m regularly.  Granted, it isn't the best but I'm in a tiny lot here.  I use balanced 450ohm line and an MFJ transmatch.  On 80 & 40 m, it works quite well--160 m not so good--but at least the match will handle it.

My mast here is about 20 feet high, and each leg is about 10 feet off the ground.

Coax won't work so well, it will just get hot and lossy.

Anyway, balanced feeders are so often over-lantooked today, I just wanted to chime in and tell you how I have my antenna set up.

gl and 73 om

Jaye
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