[Elecraft] Problem Tuning K3: now off centre fed dipole

[Jim Brown]

On 11/5/2021 4:24 AM, CUTTER DAVID via Elecraft wrote:
> As has been said on many occasions, there is no real difference between a dipole fed in the middle and one fed off centre: as long as you can get the current to run in the dipole by suitable matching and don't waste it along the way, they are indistinguishable to the dx station.

Statements like this ignore three fundamental facts.

1) Off-center fed antennas are noisy on receive, and you can't work who 
you can't hear. This usually doesn't matter in the middle of nowhere, 
but it matters a LOT in most of the developed world, where all of us are 
surrounded by switch-mode power supplies (almost everything that plugs 
into the wall), controllers for variable speed motors (HVAC systems and 
other systems), grow lights, and solar systems.

2) When used on bands other than their half-wave length, horizontal 
patterns are wildly variable, with multiple peaks and nulls at angles 
predictable by antenna fundamentals.

3) It is impractical to EFFECTIVELY choke them at the feedpoint to kill 
RX noise, the feedpoint being at the off-center point on the horizontal 
wire, not down the matching section from the antenna, and it is also 
impractical to EFFECTIVELY choke a feedline where it is severely 
mismatched. Again, this doesn't matter in the middle of nowhere, but it 
matters a LOT where most of us live.

And it DOES matter to the DX station who calls you repeatedly and you 
don't hear him, thanks to your RX noise. This has been happening to me a 
lot on 60M FT8, where I'm licensed for 100W EIRP, and most of the rest 
of the world is licensed for 15-25W. That power gives me a 6-8 dB 
advantage, and stations I decode at -10 dB don't hear me!

73, Jim K9YC

73, Jim K9YC