John,
In what I would call the "classic" version of the G5RV the ladderline
section needs to be an electrical half-wave on 20m - that usually works
out at just over 30ft, not the 20ft you are using. The combination of a
102ft dipole with the 30ft of feedline then presents a moderate
impedance on several bands.
Once you've achieved that, you certainly don't want to alter the
impedance with a 4:1 balun; what you need is a good 1:1 current balun at
the ladderline/coax junction. One of the major benefits of the balun -
apart from reducing feedline radiation and pattern skew - is the
potential to reduce Rx noise. This was G8JNJ's experience:
"As an example, I built a G5RV antenna and used 10 turns of coax on a 4"
former as the 1:1 balun at the base of the open wire feeder section. I
thought that the antenna was working perfectly well, until I replaced
the 10 turns of coax balun with two ring cores. This reduced the noise
level on 80m from S6 to S0 and on 160m from S8 to S0. The difference in
the reception of weak signals was staggering, and I have now added
additional ferrite sleeves along the coax in order to further reduce the
background noise level. This experience suggested to me that many
amateurs who have tried adding a balun to a G5RV and have not noticed a
difference, may in fact have not have been using a suitable design."
More information on his web site:
http://g8jnj.webs.com/baluns.htm
Steve G3TXQ
On 31/01/2011 17:31, John Geiger wrote:
> I have a G5RV type homebrew dipole that has been up for the past 11 years
> and I am now planning on doing some maintainance and replacing the coax and
> balun on it, and cleaning and redoing all connections on it. It has been my
> main low band antenna over this time and is currently being used on the
> higher bands as well until I get a beam back up. I have a 20 foot long run
> of 300ohm twin lead going to a 4:1 W2AU balun, then to 75ohm coax. I have
> read some reviews of this balun, and many of them are less than
> complementary. I am wondering if a better balun would improve performance.
> Does anyone have good balun recommendations for this type of antenna? Also,
> would a 1:1 work just fine also, as the impedance on the twin lead might not
> be 300 ohms at the feed end anyways.
>
> 73s John AA5JG
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