[TowerTalk] Best wire antenna for 75m/80m DX coverage

[Barry Kutner]

On 20 Oct 98, Pete Smith <n4zr@contesting.com> wrote:

> 
> At 11:15 AM 10/20/98 +0000, Barry Kutner wrote:
> >
> >I use 3 "vertical slopers" around the tower. I believe it was 
> >originally written up by K8UR, and is in the ARRL Antenna Handbook 
> >(mine's about 10 years old, don't know if it's still there) as a 40m 
> >antenna, using 5 slopers.
> >Start at the top of the tower with one end, then the lower end is 
> >pulled back in to the tower, so the center insulator is half way up, 
> >and pulled out by a rope and staked to the ground.  It looks like a 
> >diamond configuration.  Pulling the lower end back in cancels out the 
> >horiz component and lowers the andle of radiation.  The wire is fed 
> >with a 3/8 wave of coax (I used RG8X) to a relay switch box.  This 
> >length of coax is supposed to act like a stub, making the non-selcted 
> >wires act like reflectors.
> >To switch from 80 to 75, I go outside and fold back about 10 ft at the 
> >bottom.  In the near future, I plan on adding some relays to do this 
> >from in the shack, and if anyone has any suggestions on physical 
> >construction details (what to use to keep water out, how to support 
> >it, will the control wires bring RF into the shack, etc.), I'd apprec. 
> >it.
> >I'm very pleased with performance, and at times see up to 20 db, F/B or 
> >F/S depending on the path.
> 
> This is a great antenna -- depending on who you talk to, credit goes to
> W9LT, K3LR or K8UR.  Whatever....
> 
> Just a couple of comments -- according to modeling and my practical
> experience, it works much better above the resonant frequency of the
> elements than below, quite independent of the length of the coax feedlines.
>  F/B goes away quite quickly below resonance, and actually peaks a couple
> of hundred kHz above.  I cut mine for the very bottom of the CW band, and
> run it on 75 without change (good ol' SB-220 will still match it up there).
> 
> Also, the shield of the feedlines must also be floated -- I used a
> weatherproof plastic electrical junction box for the relays, so that I
> could use SO-239s.  I think the K8UR design used terminal strips to
> accomplish the same thing.
> 
> The very best way to do this is to make the upper half of the dipoles part
> of your guywires.  If you do it separately with normal materials (wire,
> nylon cord, etc.), you'll probably discover, as I did, that there's a big
> tradeoff between tension on the antenna wire and the space required around
> the base of the tower.  I wound up using almost a 250-foot circle for my
> 4-dipole version.  
> 
> 

My guy wires are Phillystran, so no problems with interactions. I use 
copperweld wires for the elements, and use an Ameritron RCS-8V for 
switching (also have a 160 m antenna on this switchbox).
>From personal experience, I found that when the wires are tuned for CW, 
they still load on 3800, but much of the directivity is lost. When 
tuned for 3800, the mismatch is too great on CW to load up the amp.
73 Barry
--
Barry Kutner, W2UP                            Internet: w2up@mindspring.com
Newtown, PA                                   Frankford Radio Club

                                           

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