[TowerTalk] OCFD

[Jim Thomson]

Date: Fri, 24 Mar 2017 14:55:44 -0700
From: Jim Brown <jim at audiosystemsgroup.com>
To: towertalk at contesting.com
Subject: Re: [TowerTalk] OCFD

On Fri,3/24/2017 2:28 PM, Wilson Lamb wrote:
> All your problems will go away if you build some open wire line and 
> feed with a balanced tuner...HB or Matchbox. 

Those who believe this seem not to realize that any imbalance in the 
antenna SYSTEM causes the feedline to carry common mode current both on 
RX and TX. That may be no big deal on TX, but on RX, the common mode 
current created by noise pickup couples to the antenna if the system is 
less than perfectly balanced. As noted in an earlier post, most wire 
antennas that we mere mortals can install are unbalanced by their 
surroundings or by asymmetry in their construction or rigging.

73, Jim K9YC

##  so what do high powered SW  AM broadcast stations do ?
They all TX only, no RX, and some use LPDAs...and some use curtain
arrays. 

##   I looked up what the typ 50-500 kw stations use, and most will
use 300 ohm balanced feeders, and either 20 ft or 26 ft tall  aluminum 
supports for the 300 ohm line, spaced XXX feet apart.   Took a while
to see what they use for a balun, but it appears they start  with 50 ohm
coax from the TX, then into whats called a tapered line section balun,
where it changes from 50 ohms unbalanced.... up to 300 ohms  balanced.  
I still cant fathom how that concept works exactly. 

## BTW, when you mentioned dipoles etc, having all their voltage at the tips,as
a result of capacitance,  capacitance between what and what ?   Is it capacitance
between the tips, like in an arc ?    I cant see it being C to ground below.  What
if the dipole is in outer space ? 

Jim  VE7RF