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Author Topic: Half sloper on a crank-up tower  (Read 1834 times)
EA2BSN
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Posts: 49




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« on: March 16, 2010, 09:02:58 AM »

All,

Due to limited space I am going to install an Alpha Delta DX-B (quarter wave single wire sloper for 160, 80, 40 and 30 meters, 60 ft. overall length)  on my 10 meters (33 fee crank-up tower).

I was worry about the tower conductivity as it is an steel crank-up model and it is painted so I was thinking to add copper shunts between the tower sections. However I have read in the ARRL antenna handbook that “detailed modelling indicates that a sufficiently large mass of metal (that is, a large, “Plumber’s Delight” Yagi) connected to the top of the tower acts like enough of a “top counterpoise” that the tower may be removed from the model with little change in the essential characteristics of the half-sloper system”. In my case I will have in the tower:

1.   A 6 meter (18 feet) mast.
2.   A Force 12 Delta 240/230 40/30 meter 4 elements shorted yagi. Boom is 20.5 feet, the 4 elements are 138 feet in total although 2 of them will be isolated from boom.
3.   A Titanex TY5, 7 elements HF beam quite similar to Force 12 C3. Boom length is 16 feet and total not isolated elements add 90 feet
4.   An small Cushcraft VHF/UHF yagi, 3 feet boom
5.   The last tower section itself which is 11 feet

I would like to know if that “top counterpoise” would be enough to avoid the copper shunts between the tower sections in order to get a decent performance (I know the kind of lottery these antennas are)

Thanks a lot for your support.

73, EA2BS
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K4SAV
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Posts: 2362




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« Reply #1 on: March 16, 2010, 09:25:52 AM »

Your tower is not nearly tall enough to allow the top Yagis to act as the other half of the antenna on 160 and 80.  As a matter of fact, it is virtually impossible to make that happen on all bands with a multi-band sloper.  For most efficient operation the top of the tower above the attachment point should appear as a low impedance at the operating frequency and the bottom part of the tower should simultaneously appear as a high impedance.  Solving that equation for all bands is virtually impossible.  So you will end up with a situation where on some bands there will be large currents flowing in the bottom part of the tower.  That will produce large ground losses.  

You can lower those losses by adding radials to the tower.  All feedline shields should be attached at the bottom of the tower and the guy wires should be insulated.  Still it won't be as good as a half wave sloper, because when the bottom part of the tower receives large currents the antenna will look like an inverted vee with an extremely small included angle and one leg attached to ground.

Jerry, K4SAV
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K2DC
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Posts: 1551


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« Reply #2 on: March 16, 2010, 12:04:14 PM »

The simple answer is to follow Alpha Delta's instructions for the antenna.  From the Installation Notes on their web site:

 "If the tower does not offer a good ground return, then we suggest attaching a large diameter (No. 14 or larger) wire to the aluminum DX-B mounting bracket and running this wire to a good RF ground connection (ground rod) at the base of the support."

I have an Alpha Delta DX-A mounted at 60' on a tower with a hazer, which also does not provide a low impedance ground return.  I have the mounting bracket  of the DX-A connected to a large diameter wire which is bonded to a ground rod at the base of the tower, and the antenna works just fine.

GL & 73,

Don, K2DC
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EA2BSN
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Posts: 49




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« Reply #3 on: March 16, 2010, 01:44:53 PM »

Thanks a lot for the advices.

I just expect to use the sloper for 80 meters (160 would be a plus!)as I have a beam for 30 and 40 meters.

The main issue is the grounding of the tower.

Please have a look to these pictures:

http://www.qsl.net/ea2bsn/tower.jpg

The tower would be installed in a concrete base at the roof of a 8 story building in city downtown so there is not a choice to install a ground rod. I do not know how good is this "concrete ground".

Best 73
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N6AJR
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Posts: 9930




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« Reply #4 on: March 16, 2010, 05:37:26 PM »

it will work
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KL7IPV
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Posts: 984




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« Reply #5 on: March 21, 2010, 08:43:58 PM »

When I had a tower I ran a long wire from the top of the lower tower out about 120+ feet and fed it at the top of the tower. It worked okay and gave me another antenna to see when I needed something simple. I used an insulator at the top and at the far end. Fed it with coax thru a tuner
Frank
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