[TowerTalk] TH7 SWR trouble + TH6 to TH "2.5" conversion

[Bill Coleman AA4LR]

On 5/3/00 4:07 PM, K7GCO@aol.com at K7GCO@aol.com wrote:

>     In regard to the high SWR problem I'd change the balun first--if you 
>can reach it.  Just having a 6 turn coaxial balun in the high current area on 
>the feedline--IS NOT AN EFFECTIVE BALUN in particular without some donut 
>toroids. Who ever started the wrap a few turns of coax for a balun in the 
Hi-Current 
>area should be hung with it.

I beg to differ. A solenoidal coax balun can be very effective and 
inexpensive to build. Ed Gilbert wrote an article to TowerTalk several 
years ago which shows that a properly designed balun of this type can be 
effective. See: 
<http://www.contesting.com/_cq-contest/199507/0648.html>

The key elements of Ed's article are to make sure these baluns are 
constructed correctly, otherwise you'll induce the "Dreaded RF 
Spillover." Key points are: 1) Use the right number of and size of turns, 
2) Don't bunch turns, use a single uniform layer, preferably on a form. 

The only problem I could envision with this type of balun is the 
potential for magnetic coupling to other baluns or conductive loops. For 
a single tribander in the clear, there's not much chance of that.

>   Why is it that hams will add a few turns of 
>wire at a verticals base to lower the resonant frequency like from 3.8 to 
>3.6 
>MHz and not get a choke affect they expect in a coaxial cable coil before a 
>balanced feed point?? 

Because a "few turns" doesn't make an effective choke at 3.6 MHz. You'd 
need lots of turns, like about 12 on an 6.5" form, or 20 on a 4" form.

>The donut toroids only work in the high current area as they are RF 
>resistors.  You need a bunch of them also and they get heavy.  They don't 
>work in the Hi-voltage area--there is no current.  Toroids are a lossy 
>balun. 

This is misleading. The act of putting toroids on the wire reduces the 
current flowing in the wire. Bead or Toroidial coax chokes don't just 
dissapate the RF that would ordinarily be flowing on the outside of the 
coax -- they "choke" it off. Current that would be flowing on the outside 
of the coax without the choke balun just doesn't flow that way any more 
-- the reactance of the choke prevents that flow. Instead, that energy 
goes elsewhere -- which ought to be into the antenna. The same is true of 
solenoidal coax choke baluns. They all work under the same principle. 

Bead baluns are more lossy than the coax baluns (see Ed's article). But 
the same isn't necessarily true of baluns made from coax wrapped around 
large toroids. 



Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr@radio.org
Quote: "Boot, you transistorized tormentor! Boot!"
            -- Archibald Asparagus, VeggieTales


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