Posted by
Don Wilhelm-4 on
Jan 11, 2012; 10:02pm
URL: http://elecraft.85.s1.nabble.com/ladderline-and-balun-question-tp7179004p7179088.html
Don,
According to L B Cebik's modeling, a 2 wavelength horizontal loop will
have a feedpoint impedance of 180 - j125 ohms, and a 4 wavelength loop
(the same loop used on 80 meters will have a feedpoint impedance of 235
- j135.
Both are good candidates for the use of a 4:1 balun - but that only
applies if you mount the balun right at the antenna feedpoint. The fact
that you have 60 feet of 450 ohm feedline changes the situation
drastically - that is about 1/8 wavelength on 160 and close to 1/4
wavelength on 80 - so the feedoint impedance at the end of that 60 feet
of feedline is quite a bit lower - that is most likely why your 1:1
balun worked so well. You can work with L B Cebik's antenna feedpoint
impedances and the TLW application available from the ARRL to find the
feedpoint impedance at the end of the 60 foot feedline, but I think you
will find it closer to 50 ohms than to 200 ohms.
The other question is what type of balun is your 4:1 balun - if it is a
voltage type, it has no chance of stopping common mode current.
My guess for why your current choke did not work is either that you did
not place it at the right point on the coax, or it was at a point where
the RF voltage was high, and in light of a high RF voltage (think
impedance equal to 2000 to 4000 ohms, a choke of 5000 ohms impedance
does not have much of a chance to stop any current.
So bottom line - go back to what you had before, or put the 4:1 balun at
the antenna feedpoint where it should work if your antenna is anything
like L B Cebik's simulations.
73,
Don W3FPR
On 1/11/2012 4:12 PM,
[hidden email] wrote:
> Gents,
>
> I have a ladder line and balun question that I hope someone can guide me
> thru.
>
> I had a good working antenna system with two antennas. First being an
> Alpha Delta Dipole and the second a 2WL 160m loop antenna at 40 ft in
> the air strung in trees. So the wire does touch the trees.
>
> Dipole is fed with coax and choked at the feed point and the loop was
> fed with 60 ft of ladder line to a 1:1 current balun then 25ft coax to
> the shack tuner. Then the amp and radio. This combination was working
> real well no RF problems in the shack.
>
> I was advised that I would obtain better results for better SWR at
> resonance and easier matching if I removed the 1:1 balun and went to a
> 4:1 balun. I made this change with everything else being the same. I
> now had RF in the shack again.
>
> I then added an isolation balun to the system by adding 6ft coax from
> the output of the 4:1 to a balun designs 116du which is supposed to have
> 4500 ohms of choking impedance on 160m and over 5000 ohms choking
> impedance on 80 and 40 meters. This did not eliminate the RF and I am
> still having problems.
>
> So here is the question, is it possible that with the combination of
> length of the ladder line at 60ft plus the 6 ft plus 25 ft of coax I am
> at a feed line length that will be a problem no matter what I do. I
> keep looking at the internet to try and figure this out but I am
> confused as some information I find says it only applies to 1:1 baluns
> or other factors.
>
> So anyone who can help me understand this feed line question I will be
> greatly appreciative.
>
>
______________________________________________________________
Elecraft mailing list
Home:
http://mailman.qth.net/mailman/listinfo/elecraftHelp:
http://mailman.qth.net/mmfaq.htmPost: mailto:
[hidden email]
This list hosted by:
http://www.qsl.netPlease help support this email list:
http://www.qsl.net/donate.html