[TowerTalk] End Fed Half Wave Noise Remedy

[Jim Brown]

On 10/21/2018 12:31 PM, Tony wrote:
> I use an end-fed half wave for portable work and was wondering what 
> the most effective way would be to reduce the ingress of feed-line noise? 

There are MANY ways of end-feeding a dipole. Mine is shown in this set 
of slides for a talk about the mounting height of vertical antennas. 
It's a long talk with a lot of slides. Skip ahead to slide #71 to see a 
detailed discription of the antenna and some analysis.

http://k9yc.com/VerticalHeight.pdf

In this implementation, a quarter wave of wire is connected to the end 
of the coax, and 1/4 wave down the coax (VF=.98) is a serious ferrite 
choke that acts as an "egg insulator" applied to the coax shield. This 
makes that first quarter wave half of the dipole. The electrical 
performance of this antenna will be exactly the same as any efficiently 
fed end-fed half wave with the same rigging.

It's no trick to wind a choke that yields 5,000 ohms on a #31 2.4-in 
toroid, and NEC says that provides about 15-20 dB of isolation from 
feedline common mode current, depending on feedline length. 13 turns of 
a #12 THHN pair will yield 5,000 ohms. Doubling the choking Z (a second 
choke) adds 6 dB of isolation.

If the dipole is vertical, it's feedpoint Z will be 75 ohms, so 75 ohm 
coax will provide lowest SWR. Feedpoint Z will be closer to 50 ohms if 
it's rigging is horizontal or sloping. If your rig has a tuner, the 
relatively low SWR of a 50 - 75 ohm doesn't matter because the coax run 
is too short.

RG58 or RG8X can be used for the choke in place of THHN IF the center 
conductor is stranded. The coax should be wound tightly to the core. A 
solid center is not recommended, because the tight bend radius could 
cause it to short to the shield.

73, Jim K9YC