[Antennas] Baluns and tuners

2002年6月07日 10:41:40 -0400

Jan,
Comments below.
At 03:07 PM 6/6/2002 -0700, Jan Reimers wrote:
>Thanks again to everyone who helped out with my balanced tuner question. The
>Richard Measures article was very informative.
>> I have managed to build a simple Pi network tuner (unbalanced) with
>a roller inductor, and would like to use it to feed parallel line going to
>some sort of wire antenna (I suppose I'll start with a 40M dipole). So I
>need a balun between the tuner and the feedline. I have purchased a ferrite
>core for this purpose (man these traditional parts are hard to find!). I
>also have some 300 Ohm "radio shack special" flat line, but I could also
>make a higher impedance parallel line with thicker wire and Plexiglas
>spreaders. I plan to run max 100W but with high SWR between the tuner and
>antenna. I have never done any of this before so here are my novice level
>questions:
>>1) Is the 300Ohm flat line suitable, or would I be much better off making my
>own (more manly) feedline?

300 ohm ladder line is better than TV line and 450 ohm ladder line is even 
better.
>2) Should I wind the balun in a 4:1 arrangement, or do I need to closely
>match the resistance ratio of the antenna feed line and tuner output (even
>though the antenna Z will be jumping around depending on what band I am on).
>The only reason I prefer 4:1 is that the ARRL antenna book tells me exactly
>how many turns to do for 4:1.

4:1 is the best choice for feeding open wire line. Current baluns have 
wider bandwidths than voltage baluns, but a voltage balun is fine on a 
limited bandwidth.
>3) Once I wind the balun how do I know it works? i.e. How do I know I have
>the right number of winds and that the current on the feedline is really
>balanced. (I am the proud owner of 1 voltmeter and 1 SWR meter, not much
>more!)

Connect the high impedance side to a 200 ohm non inductive resistor and 
make SWR readings on it on all bands you will be operating using your SWR 
meter and low power so you don't burn up the resistor. A good SWR for a 
4:1 is 1.1:1 and 1.5:1 is acceptable. Hook up to the open wire line and 
measure SWR. A good SWR for an open wire feed line is 5:1 or less and even 
10:1 is acceptable under certain conditions. Avoid high impedance points 
on your feed line as the high voltage at these points may burn up your 
balun. Apply low power and see if the balun gets warm on any frequency you 
plan to use the balun on first while connected to a dummy antenna then 
connected to the open wire line. Increase power and repeat the test. My 
experience is that a good balun will get only barely warm at an SWR of 5:1 
or less at 100 watts after an extended operating period. Don't hold your 
key down at 100 watts or you may burn up your final, but you should be able 
to key CW at 100 watts for 10 minutes or use SSB voice, preferable while in 
QSO with another station. If the balun gets more than perceptibly warm on 
the dummy it probably is not working. If it is ok on the dummy and not on 
the antenna, the impedance point on the open wire line may be too high or 
too low, in which case change the length of your transmission line slightly 
and repeat the test.
As for the balance, I wouldn't worry about that. It's hard to measure and 
if the other tests pass, it probably is close enough not to make any 
difference anyway.
GL
73, Chas, W1CG