[Antennas] Re: Antennas digest, Vol 4 #150 - 2 msgs

2003年6月13日 18:28:44 -0700

Pat and Robert, the reason I wanted to use a balun also extended to 
matching the impedance for the folded dipole. From what I have read 
they are typical designed for a 600 Ohm or 900 Ohm impedance using a 
terminator (resistor) that equals or is near that impedance, possibly 
470 and 650 Ohms depending upon the balun ratio either 9:1 (9*50=450) 
or 12:1 (12*50=600). The articles weren't specific about why the 
termination should be higher than the mathematical calculation. The 
length of the folded elements, typically about 90 ft, 45 ft after being 
folded, is supposed to produce an antenna that is fairly broad banded 
with an SWR at 2:1 or less for 80 meters through 10 or 6 meters. While 
this wouldn't be as efficient as a dipole cut for any particular band 
it would fit my property better than a dipole cut with legs for 40 and 
80 meters.
> Message: 2
> From: "Robert Lay" <[email protected]>
> To: <[email protected]>
> Subject: Re: [Antennas] Home Brew Balun Construction
> Date: 2003年6月12日 11:29:05 -0400
>>> Why do you want a balun with a 'balanced' antenna? Or did you mean 
>> you
> want
>> a transformer ?
>>>> Pat W0OPW
>> Dear Pat,
>>> From your question, I would infer that you do not believe a balanced 
>> antenna
> should need a balun.
>> That would seem reasonable until you discover that a balanced antenna 
> fed
> with a coax (rather than balanced line) does need a balun in order to
> eliminate or reduce the effects of RF current flowing on the outer 
> surface
> of the shield.
>> The approach that Don Williams plans to take in his experiments is 
> quite
> valid, in that a balun should be used with a coax feedline. I have 
> inferred
> from his posting that he is planning to use coax, otherwise he would 
> not
> even consider a balun.
>> Bob Lay in Dahlgren, VA
> http://www.qsl.net/w9dmk
>