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Author Topic: 80 meter rotatable dipole  (Read 5265 times)
AB2MA
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Posts: 45




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« on: March 21, 2007, 05:29:15 PM »

Anyone have any ideas about homebrewwing a 80 meter rigid dipole,so it can be mounted on your tower and rotated.I see MFJ just came out with one(MFJ-1785)TNX!
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WB6BYU
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Posts: 13335




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« Reply #1 on: March 21, 2007, 05:46:55 PM »

I've seen this done using aluminum tubing for the center of the element
and fiberglass fishing rod sections for the outer portions, with a wire
running inside the fiberglass.  There was another one that I think used
just aluminum for the elements - and showed the author standing
on one of them (it was about 4" diameter at the boom end.)  Actually
that one was a full-sized 3-element yagi, so it can be done!

In most cases you'll want some sort of truss system to keep the element
from sagging too much.  And perhaps two wires/ropes coming down
from a crosswise spreader above it to minimize fore/aft motion as well.

Personally  I might manage 10 feet of aluminun tubing with a fishing
rod attached at the end, and with enough loading to bring it to
resonance, either through linear loading or with large diameter loading
coils at the ends of the tubing.  That is probably about the limit of my
personal abilities with construction, and if I had a tower high enough
to make it worthwhile to mount such an antenna (a half wave or more
high) then I'd live with the slight reduction in gain due to loading.
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WB2WIK
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Posts: 20611




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« Reply #2 on: March 22, 2007, 08:39:30 AM »

The MFJ product is very heavily loaded by a pair of big coils and capacitance "hats" on the element ends, and is thus only one-eighth wavelength long.

Whether you achieve any directionality from it depends on whether you can get it high enough above ground...looks like at 65' above ground it's very slightly directional (lobes about 3 dB stronger than nulls at max signal elevation) but at 130' a more dipole-like pattern is achieved.

Still, it's about 6 dB below a 1/2-wave dipole and a pretty narrowband antenna (BW = 30 kHz at 2:1 points).

With a tower tall enough to make a rotary dipole for 80m work well, I'd probably just hang four wire half slopers off it and use a relay to switch between them to get the 3 dB boost in each direction.  Cheaper, broader bandwidth, higher efficiency, and no rotator required...

WB2WIK/6
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WW5AA
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Posts: 2086




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« Reply #3 on: March 22, 2007, 09:31:02 AM »

The first experiment I did in modification of my Cushcraft A3S beem was to make a dipole out of two Huster 5BTVs. It worked so well that I used this as the driven element on the beam, used the original driven element and 40 meter add on kit as the reflector. The original reflector then replaced the director. Works OK on 80, but since the beam is at only 35', my doublet at 60' works much better. I have wondered what it would do on 80 if up over 100'. Two 5BTVs are cheaper than the MFJ and also covers more bands. Have fun!

73, de Lindy
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AB2MA
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Posts: 45




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« Reply #4 on: March 22, 2007, 10:28:05 AM »

Thanks for the responses,the only reason I'm considering this is that I plan to replace my current tower with 140' of rohn 45 that I picked up last summer.WW5AAs idea of making a dipole out of 2 5-BTVs is interesting.You could brace it against drooping,but side wind pressure may cause damage.WB2WIKs idea was my back up if I couldn't come up with some sort of dipole.Still open to ideas.73s Stan
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N3OX
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« Reply #5 on: March 22, 2007, 10:47:16 AM »

You could just build the driven element of VE6WZ's antenna:

http://www.qsl.net/ve6wz/intro.htm

Total element length is 66' ... elements are loaded in the middle... I bet it's not more than a dB or two down from full size with the massive coils VE6WZ uses.

SK3W (I think) used two 40 foot Spiderbeam telescoping fiberglass poles and an aluminum center section to do a full size 80m rotatable dipole.

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

Monkey/silicon cyborg, beeping at rocks since 1995.
WB2WIK
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Posts: 20611




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« Reply #6 on: March 22, 2007, 11:28:58 AM »

With 140' of Rohn 45, I'd put up this for 80m:

http://www.m2inc.com/index2.html

Click on model 80M1LL.  It's a linear loaded rotary dipole for 80m made by M2.  It's 85' long and should only be down about 1 dB from a full-sized 1/2-wave, but is rotatable and strong.

I wouldn't even consider other approaches to an 80m rotary dipole, especially one up over 140 feet where servicing will be difficult.

WB2WIK/6
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WA1RNE
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Posts: 825




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« Reply #7 on: March 22, 2007, 12:30:59 PM »


  With a 140' tower available I wouldn't waste my time with a rotatable dipole. At that height an  electrically steerable sloper array would be a much better performer, providing useful gain, F/B ratio and better bandwidth.

   The sloper array is 4 or 5 dipoles spaced around the tower each fed with a 3/8 wavelength line from a relay box. The relays select 1 dipole as a driver and the others are left open as refelctors, with the 3/8 wave line electrically lengthening the other dipoles so they will act as reflectors.

 The 5BTV as a horizontal dipole sounds good but whether it will hold up mounted horizontally in wind and ice is questionable, as I doubt the traps and tubing are sized for the stress.  You may want to call Hustler and confirm, especially to see if there any warranty implications, unless you don't care.


  ...WA1RNE
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WW5AA
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Posts: 2086




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« Reply #8 on: March 22, 2007, 01:05:07 PM »

By the way...The two 5BTVs are supported by the same high strength dacron and add on mast that Cushcraft uses to support the 40 meter add on kit for the A3S. I also left the existing dacron support on the driver turned reflector. The aim was not really to use it on 80 meters, but to obtain front to back on 40 since I am limited to a roof mounted tri-pod. Yes! one day a 199' rotating tower and full size 80 meter yagi (:-)

73, de Lindy
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AB2MA
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Posts: 45




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« Reply #9 on: March 22, 2007, 08:07:16 PM »

I checked out the M2 80M1LL and it seems to be what I'm looking for.Does anyone know where to download a manual? I built a Hy-gain av-18ht from the manual with great results.The almost $1400.00 price tag was a little more than I wanted to spend. 73s Stan
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WA1RNE
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Posts: 825




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« Reply #10 on: March 23, 2007, 11:01:17 AM »


 Great antenna as long as you are willing to give up 70-85% of your towers wind load capacity for a single band rotatable dipole.

  The M2 antenna weighs about 75 pounds and has a 8 Sq Ft wind load rating - which will likely turn into 10-11 sq ft with ice, eating up 85% of your 12.7 sq ft capacity - something you need to consider living in NY.  BTW, the 12.7 sq ft wind load rating assumes a 45G tower guyed in 3 places with 0.1875 and 0.25 in guys.

 If this is the only antenna on the tower - besides a 6 or 2 meter yagi , then you are all set.


  ...WA1RNE
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K4WYS
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Posts: 7




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« Reply #11 on: February 14, 2008, 08:44:17 AM »

I am wondering if the MFJ-1785 antenna would be worth installing.  I presently have a G5RV that is 30 feet directly above a chain link fence, the top pole of the fence makes for a metal reflector to make this antenna an NVIS.  Local service is very good as expected, up to 300 miles.  Another antenna available is an OCF dipole "windom" cut for 160 meters.  This antenna is basically 75 feet east of the G5RV and parallel to it.
   The G5RV is not a full half wave length dipole so it's radiation factor is less than the 2.15 dBi that a true dipole would yield.  The 160 windom at 75 meters become a full wave radiator and should produce slightly more than the 2.15 dBi, say more like 3.0 +/-.  However the pattern of both antennas is or should be 90 degrees to the axis of the wire.  So in my case signals to the North and South will be in the range of 10 to 18 dB below the East and West.  The windom will have lobes  about 45 degrees on either side of North and South, but still no good signal due North and South.
   Therefore will the rotatable loaded (shortened) dipole, the "MFJ-1785" produce a strong signal to the desired direction?  Similar to the G5RV, the radiator compression of the shortened dipole will not yield the full 2.15 dBi; however we now have a lobe directed to the desired direction.
   The transmitted signal now has the area intended and increasing the power will increase the signal to compensate for the lower radiation efficiency.  Now comes the real negative of the shortened dipole, capture area or length of the element to be induced by the incoming signal wavelength is only a small percentage of the wavelength.  This is where I need the answer as to the strength of the compact antenna verus a half wave dipole.  What is the loss expressed in negative dB?  Would it be 33/132  or 25% of the available signal or -3 dB gain or WHAT?     
   This is where the decision to use this antenna rests.  Will I hear better from the area that I point the antenna or be better off with the two wire antennas in the 75 meter band.
   Comments and opinions requested.  Is it worth trying this antenna?
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W4NU
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Posts: 1




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« Reply #12 on: August 13, 2011, 12:27:34 PM »

I have an M2 80/75 rotateable dipole at 100 feet . It has been up since around 2005. It is an awesome antenna. It far outperforms wire antennas such as inverted V's , slopers, and even 4-squares.

The only thing that beats me are 2 el and 3 el rotaries. At 94 feet long it does an awesome job and the sag is very minimal due to the robust construction. It has built-in switching between Fone and CW using a relay system.

73,

John, W4NU
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W4VR
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Posts: 1194


WWW

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« Reply #13 on: August 13, 2011, 06:18:12 PM »

I have an M2 80/75 rotateable dipole at 100 feet . It has been up since around 2005. It is an awesome antenna. It far outperforms wire antennas such as inverted V's , slopers, and even 4-squares.

The only thing that beats me are 2 el and 3 el rotaries. At 94 feet long it does an awesome job and the sag is very minimal due to the robust construction. It has built-in switching between Fone and CW using a relay system.

73,

John, W4NU

I'm not surprised it beats your 4-square or inverted vee.  I'm sure a horizontal dipole at 100 feet would do the same.
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W8JI
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Posts: 9296


WWW

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« Reply #14 on: August 13, 2011, 07:53:33 PM »

Linear loading is exactly like using really low Q coils for loading. Nothing more, nothing less. It is all hyped up to be something great, but in the real world what you have with any antenna is a certain length in space. If that length is too short, we have to cancel the capacitive reactance with some inductance.

The more stretched out we make the inductance, the more wire it takes to get the inductance. The more wire, the lower the Q and higher the loss.

So if we have an 80 foot long antenna, we can dram all we want but we have an 80 foot long antenna. If we use large hats on the end and not much coil, it will be about the same as a full size dipole. If we don't use the hats and use loading coils alone, it will be a little worse. If we linear load it, it will be a little worse than the coil if the loading is at the same point.


If you put a 30-40 foot long 80 meter dipole up, it will be a 30-40 foot dipole. It will be optimum with big end hats and a good loading coil, but as we degenerate from that form it gets worse and worse. The very worse we could make it, other than using a rotten exceptionally lossy coil, is helically wound or linear loaded.

None of this stuff is magic. It is about two things and two things only:

1.) Keeping the current as uniform and spread out over space as possible. This is where end loading, especially with large hats, comes in.

2.) Keeping the additional resistance to compensate missing length as low as possible. This is where capacitance hats at the ends come in, and large conductor conventional loading coils of good form factor.

As for performance, I compared my wire dipoles at 150-160 feet to a good 4 square with large ground system and my dipole would pretty much always win broadside to the dipole. Usually by a few dB. It would also usually beat or tie a linear-loaded 2 element Yagi at 150 feet. I think that antenna has 80 foot or so elements. It is laying on my car trailer now.

Antennas are antennas. There is no magic. When one antenna beats another antenna of similar size, it is more about what the losing antenna has wrong than the winning antenna has right.

A dipole at 150 feet or so, for example, has about 8.5 dBi gain. A four square over the soil we have here, with a very good radial system, has about 4 or 5 dBi gain on 80.

The only band where a four square really comes to life is 160. I don't know why, but my four square on 160 is usually a few dB up on a dipole at 300 feet.   Not so on 80......and definitely not so on 40.

73 Tom





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