[TowerTalk] KW-80 trap

[Jim Brown]

On Fri, 29 Feb 2008 15:39:44 -0800, Tom Osborne wrote:

>I'm trying to built an 80/160 antenna with a Unidilla KW-80 trap.  I can get 
>the SWR on 80 OK, but my MFJ-259B doesn't like 160 so can't get a good 
>reading there.

>How much wire do I need on the other end of the trap to get it to resonate 
>in the 160 meter band??  Close would help as I can trim it as needed. 

Tom,

I've put up two 160/80 dipoles with a 40M fan element, and an 80/40 dipole, all 
using loading coils from Hypower Antenna Company (a ham in his basement, I 
think). Google to find him on the net. His name is Barry, I don't remember his 
call. I am VERY happy with them. The 80/40 was my only 80/40 in Chicago, where 
it just fit between the available supports on my city lot. 

The concept is a loading coil rather than a trap. When I recently modeled the 
160/80 in NEC based on measurements of the coils, it told me that it should be 
only 0.6 dB down from a full size half wave dipole on 160, even though the 
160/80 version is less than 170 ft long. There is some sacrifice in SWR 
bandwidth, but these antennas still work quite well over the entire 160M and 
80/75M bands with a decent tuner (I use the TenTec 229 and 238). The loading 
coils handle the full output of my Ten Tec Titan (legal power) during contest 
conditions -- I can find no sign of overheating or arcing after several years of 
being very active contest use. 

Of my two dipoles that work 80 and 40, the 160/80/40 is the one that is 
broadside to about 55 degrees.  

Last I looked, Hypower sells the coils for a very reasonable sum. Although they 
would be easy to build, since he's done the engineering, I believe that it's 
only fair to buy them from him. He also sells complete antennas. 

My only criticism of the product is that the method of strain reliefing the wire 
needs to be considerably more robust for the tough conditions of a high dipole 
between trees in hihg winds (and I've got suitable counterweights on them). So I 
modified mine a bit to address that. 

[A high dipole for 80 or 160 has a lot of stress on it to keep tension on it (to 
keep it high) and to support the feedline. Most mfrs of hardware for ham wire 
antennas fail to address this adequately. The old DX Engineering W2DU-style 
dipole balun (discontinued, I suspect, because W8JI wasn't satisfied with its 
electrical performance), is the only dipole center insulator I've ever seen that 
is really sufficiently robust for this kind of use. I wish they would sell it as 
a hardware product, even without the choke beads.]  

73,

Jim Brown K9YC