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Author Topic: 80m Rotatable Dipole Ideas  (Read 4830 times)
AF6D
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« on: February 17, 2011, 04:34:37 PM »

I would like to purchase or build a decent 80m rotatable dipole that can go above my SteppIR. I'd thought of taking two 6-BTV's and trussing them, but that's overkill for the limited bandwidth. I am OK building or buying a 50-70 foot rotatable dipole with decent bandwidth. I am normally down at 3700 but will sometimes go up to 3800 for DX.

Any ideas?
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K0OD
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« Reply #1 on: February 17, 2011, 06:12:54 PM »

I can't think of a single advantage to using two trap verticals as a rotary dipole solely for 80 meters. Yes, very narrow bandwidth and questionable strength.

How high is your tower?  A rotary dipole won't be very directional on 80 unless it's quite high. I'd use the tower to support one or two inverted Vs or maybe a sloper or two. Very common configurations, cheap and easy.  Or shunt feed the tower if you can put in radials. 
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WA8FOZ
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« Reply #2 on: February 17, 2011, 08:11:54 PM »

Four slopers in a vertically-polarized configuration:
http://www.w5gzt.us/9408036.pdf

If you have the room, you could do this:
http://www.oe5.oevsv.at/export/sites/oe5/technik/antennen_dl/sloperen.pdf
You would then have a directional horizontally-polarized array.

If you still want to do a rotatable 80m dipole, Force12 sells a couple for ~$2k. It would require a VERY heavy-duty tower. And, as noted above, it would not be worth the effort at much less than 120 feet.
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WB2WIK
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« Reply #3 on: February 18, 2011, 01:39:47 PM »

50-70' is about half sized for an 80m dipole but it's still really long for something rotatable.

That can be murder on rotators and towers.  Picture holding a 70' long pipe in the middle and balancing it, then twist your body to rotate it.  Now, try to stop it.  It will take a lot more braking force than stopping the SteppIR.  If the rotator can handle it, this still places enormous twisting forces on the tower, which might call for torsion bars and extra guying (unless you already have that).

This is one of the reasons 80m rotary antennas aren't all that common.

How tall is the tower?

Switched slopers supported by a tall free-standing (unguyed, or guyed with Phillystran and not metal cables) are a great alternative to a rotary antenna, and often easier to implement.

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K9MRD
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« Reply #4 on: February 18, 2011, 01:59:04 PM »

Take a look here:

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

Under HF Monobanders look at 80M1LLA

M^2 also has 2 and 3 el 80 meter beams.
« Last Edit: February 18, 2011, 02:02:06 PM by K9MRD » Logged
W4VR
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« Reply #5 on: February 18, 2011, 02:16:12 PM »

If you want a rotary dipole, get one for 40 meters.  Optibeam makes a nice one.  For 75 just hang an inverted vee off your tower.
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AF6D
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« Reply #6 on: February 19, 2011, 12:00:36 AM »

I don't have horizontal ground area for slopers. Too many trees and squirrels... I looked at some of the commercial rotatables and they ARE long. It seems that my desires are not matched by real estate Sad
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G3TXQ
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« Reply #7 on: February 19, 2011, 03:49:50 AM »

Would this be manageable?

Take 4 fibreglass fishing poles each 23ft long and support them in the form of a cross off a central hub. Run 128ft of wire in the form of a square around the tips of the spreaders; feed it in the centre of the wire with a 1:4 current balun (the feedpoint impedance will be about 12 Ohms); keep a small gap in the "square" opposite the feedpoint.

It's 32ft square and 23ft from the centre to one corner. It's lightweight. No need to rotate it - the pattern is close to omnidirectional.

It's just a single-band 80m version of this:
http://www.karinya.net/g3txq/cobweb/

73,
Steve G3TXQ
« Last Edit: February 19, 2011, 03:51:49 AM by G3TXQ » Logged
HB9DDS
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« Reply #8 on: February 20, 2011, 10:31:46 AM »

look here
http://www.optibeam.info/index.php?article_id=65&clang=0
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--
73 de Daniel, HB9DDS
WX7G
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« Reply #9 on: February 20, 2011, 11:03:23 AM »

The radiation efficiency of the 6BTV configured as an 80 meter dipole is 23% using the RM-80S resonator.

Using the RM-40S resonator topped with a DX Engineering top hat (48" spokes) it is 60%. The input impedance at resonance is 37 ohms and the 2:1 VSWR bandwidth is 55 kHz.

To strengthen the BTV antennas a 6' 1.375" aluminum tube can be slipped over the bottom 6' section.

But you want more bandwidth and probably higher efficiency. Let's see what can be done using the DX Engineering top hat can be configured with 72" spokes. The loading inductor is 20 uH and that can be homebrewed on standard PVC pipe and have a Q of 300. Now the radiation efficiency is 90% and the input impedance is 26 ohms. With a hairpin match the 2:1 VSWR bandwidth is 50 kHz.

To obtain 100 kHz bandwidth the top hat capacitance must be doubled. A top hat consisting of six 9' x 0.25" spokes can be resonated at 3.75 MHz with 11 uH. Now the radiation efficiency is 92% and the 2:1 VSWR bandwidth is 80 kHz. We're getting close. Longer spokes or a longer element length will increase the bandwidth.

For the element DX Engineering stocks the aluminum needed along with other bits and pieces. I can design this for you and post the info here. If you'd like me to do this please post the wind survival you need or the highest wind speed recorded for your city.

Note that a loaded 80 meter dipole is the best thing to use close to the StepIRR. The loaded dipole can be designed to not be resonant in any higher amateur band and therefore will not interact too much. It is preferred over a full-size resonant dipole anywhere near the SteppIRR (or near any other Yagi-Uda).

 



 

 
« Last Edit: February 20, 2011, 11:07:30 AM by WX7G » Logged
AF6D
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« Reply #10 on: February 23, 2011, 01:15:08 PM »

I actually thought of adding a second 6BTV forming a dipole. I know that it is still large, but I like to experiment without covering ground already covered by others. The Opti-Beam, in my opinion, is grossly overpriced.
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