[Elecraft] 270 foot (ish) Doublet & K3 ATU

[Rick Bates, WA6NHC]

I use a 104 meter (340') dipole (double extended Zepp for 75/80 meters); fed in the middle with 100' of 450 ohm window line on all bands.  In it's original configuration, I fed that with a 1:1 common mode choke (a DX Engineering balun, converting the feed to coax) into the K3.  The K3 tuner was happy almost anywhere except 160M (6:1 was the best it could do there, at the higher end).  It loaded on 6M but I don't recommend it (expensive lesson).

I found that the length of the window line was not critical; adding 30' had zero effect (started with 70').

The coax portion of the feed should be as SHORT as possible, in my case it is currently about 10' (2.8 meters).  The losses are highest there and the extreme SWR makes it MUCH worse, keep it short; use the best stuff, not the cheap stuff.  Changing the length of the coax portion has a HUGE impact on where (or if) the system tunes; the window line, none.

The wires are strung over/through oak trees that vary in height from 30-40' (oaks just don't grow taller here).  Tuning changes with the seasons (fuel moisture content).  Although cut for the lower portion of 75 meters, because of the low height, it doesn't like the low end of the band so I'm going to add another 12 meters of wire to bring the resonant point down on the bottom of 80 (should help with 160 too) since DX is more available on CW than phone.

Since I added the KAT500 (to use the KPA500 et al remotely), I found that a 4:1 CMC gives me all bands, including 160 (3:1 so I don't run a lot of power).  That one change alone has given me 5 new band mode countries, on 160.  I currently stand at 229 countries worked (all bands/modes) so it all seems to work pretty well.  Since I live in an HOA/CC&R infested neighborhood (a mistake of the most grievous nature), this stealth antenna is the best I can manage.  It's hidden in plain sight in a common area that no one uses (and no one looks UP anyway, human nature).

Expect to play with the dimensions and feed systems to find your sweet spot(s).

73,
Rick, WA6NHC

iPad = small keypad = typos = sorry ;-)

> On May 15, 2014, at 12:22 PM, Iain G4SGX <g4sgx at justfans.co.uk> wrote:
> 
> Hello,
> 
> Does anyone use a 270 foot doublet with the K3 ATU?
> 
> Is there a combination of feeder type/length and Balun type & coax length that tunes OK on 160-10?
> 
> I am currently thinking a 1:1 balun and 33 foot of 300 ohm feeder.
> 
> Obviously the coax length should be short but too short may cause rfi probs, but then I only run 100W.
> 
> Iain G4SGX