On Jun 25, 8:04*am, Michael Coslo wrote:
Nick wrote:
* *Thanks for all the great responses guys! *That is exactly what I
had thought, that since the outer coax braid is grounded at the
antenna, and at the shack, and (in my case) at the center with a
lightening protector,

Wad is dis? Is that something to protect your coax from becoming too light?

Jes kiddin'!

* then I just couldn't figure out why some antenna

companies, such as reliable DX engineering with their DXE-VFCC H05-A
Vertical Feedline Current Choke, strongly recommend such a line
isolator for verticals.

Since they make them, they're happy to sell them to you.

* *A direct quote from DX Engineering's Website:

"2/5 kW Vertical Feedline Current Choke
If your antenna SWR is already low and you wish to reduce feedline
radiation and improve reception, a Feedline Current Choke is
recommended.

I am really at a loss as to how this isolator improves reception.

Adding a DX Engineering Feedline Current Choke at the
base of a vertical antenna will substantially reduce unwanted feedline
radiation (RFI), reducing the need for improved station grounding.

* Am I a hopeless noob, or are there a lot of things wrong here?
Feedline radiation = RFI. and almost a lure toward poor station grounding?

When quarter-wave antennas are constructed over a good radial system,
they have a feedpoint impedance of about 36 ohms. When they are
constructed over less than a good radial-system there is a loss
introduced into the feed system that adds to the 36-ohm figure. This
improves the SWR but there is a loss in the efficiency of the antenna,
signals transmitted and received have a higher take-off angle and
often there is current introduced on to the feedline.

Are there tests showing this somewhere?

Anyway, you are probably best served by placing as many radials as you
can handle - but more than 4. I just went till my knees and back said
"no more", then came back the next day and laid more. Have as good a
grounding system as you can get, and good lightning protection. IMO, if
that isolator is helping any, it is just trying to mitigate a problem
that should be taken care of elsewhere.

* * * * - 73 de Mike N3LI -

Thanks again guys. I'll have to study common versus diff mode
currents on coax cable again to gain a better understanding of the
situation! And as was stated by many, a good radial field should
mitigate the situation anyway.

73,

-Nick