Ive never been successful at building an open wire fed antenna that
didnt have feedline radiation bwith resultant other problems. In the
real world its darn near impossible at this QTH. I finally gave up and
turned it into an SWL band antenna.
My old tech AM amps ( 250TH's, 813's, HK-454's) are now happily feeding
coax & antennas that is shared with the modern station. The switching
matrix has become a bit complicated but I havent blown up anything yet.
Carl
KM1H
----- Original Message -----
From: "Jim Brown" <jim@audiosystemsgroup.com>
To: <amps@contesting.com>
Sent: Thursday, April 23, 2009 12:54 PM
Subject: Re: [Amps] High SWR
> On Thu, 23 Apr 2009 12:26:14 -0400, Gary Schafer wrote:
>
>>If the line is well balanced it will not radiate
>>either forward or reflected power that may be on it.
>
> Correct. BUT -- an important clarification about balance. It is
> not the LINE that determines balance, it is what is connected to
> the line -- the ANTENNA and the matching network. Virtually all
> ham antennas are unbalanced by surroundings, even when fed by 2-
> wire line.
>
> By the principle of superposition, transmission line current can
> be analyzed as differential mode current and common mode current.
> The current due to unbalance is common mode current. Differential
> mode current does NOT radiate. Common mode current DOES radiate.
>
> Common mode current can be killed on coaxial line by a serious
> common mode choke. The "coil of coax" and the traditional "string
> of beads" so-called "current balun" do this in a small way (that
> is, they reduce the common mode current a bit. The big ferrite
> coaxial chokes I've designed do it a lot better -- that is, they
> reduce it a lot more. A lot less "RF in the shack," a lot less RX
> noise picked up on the line.
>
> 2-wire line is far more susceptible to RF in the shack and noise
> pickup on the line ONLY because there's no practical way to choke
> it to kill the common mode current. If there's no common mode
> current on 2-wire line, it won't radiate any more than coax.
>
> 73,
>
> Jim Brown K9YC
>
>
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