[Antennas] questions on Windom

Tue Jan 4 06:31:42 EST 2005

G'Morn "RB" and the group,
I have a 135' Off-Center-Fed antenna with a Radio Works 4:1 current balun 
at the feed point at 45'-90' 35 ' high, which is somewhat similar to the 
Carolina Windom. It is fed with of RG8X and then I have a 1:1 air core 
choke balun at about 40' from the top and then the coax shield is connected 
to earth ground on the shack side of the choke balun. The balun at the top 
forces nearly equal currents on both legs of the antenna, transforms the 
antenna impedance of approximately 200 ohms to 50 ohms, and also reduces 
the current on the outside of the coax. The off-center point is a feed 
point on most bands. On my antenna, 45' and 90', it has a feed point on 
80, 40, 20, 17, 12 and 10 meters, as verified by both measured SWR and by 
modelling using EZNEC. The antenna with the feed point at 14% is 
different, and I don't know on which bands it has a feed point. By feed 
point, I mean the impedance is on the order of 200 ohms and it will take 
power. On 60, 30 and 15 meters on my antenna, the impedance is very high, 
and it will not take much power using the coax and balun and its impedance, 
several thousand ohms, is too much for any balun to handle. You can get an 
off-center fed antenna to take power using an open wire, tuned transmission 
line with no balun, like an end fed zepp antenna, but that is a different 
story.
Back to the Windom fed with coax and a balun at the top. While the balun 
at the top reduces current on the shield from the antenna. the coax then 
may pick up additional current on its shield just by being in the RF field 
of the antenna, particularity is the coax is closer to one side of the 
antenna or the other. The second balun reduces this induced current on the 
shield before it goes to the shack. I found that the second balun does in 
fact reduce RF in the shack. While a 1:1 balun does transform from 
unbalanced to balanced, it also functions to reduce the current on the 
outside of the shield. From the output of the 1:1 balun, the coax appears 
to be a balanced line, but that makes no difference in its function as a 
feed line. You can say the coax is floating from earth ground on the 
antenna side of the choke balun. However, as you go up the coax, the 
differential mode voltage is going to change anyway. For example, suppose 
the length of the coax is 1/4 wave at the frequency of operation and it is 
connected to earth ground at the base of the antenna. The differential 
mode voltage on the shield will be at a high with respect at the ground at 
the antenna, and at that point there will be a current minimum and no 
current. If the length of the coax were 1/2 wave at frequency of 
operation, its differential mode voltage at the antenna would be the same 
as at the earth ground and its current would be at a maximum at the 
antenna. This is the physics of the situation.
However in your case, when you operate the antenna at 160 meters you are 
correct in assuming the antenna is functioning primarily as a top loaded 
22' vertical. The 160 meter part of the antenna starts above your 1:1 
choke balun, so the choke balun serves to start the 160 meter antenna and 
to keep current on the coax shield from both 160 meters and induced current 
on the shield on other bands from getting into the shack. If all or most 
of the 22' is vertical and is in the clear, it is a reasonably effective 
antenna. The power rating of 500 watts is probably to keep from exceeding 
the voltage rating of one or the other of the baluns as there will be high 
voltage at the top of the antenna on 160 meters.
Hope the above is helpful.
73 and have a good year,
73, Chas, W1CG
At 12:24 AM 1/4/2005, RB wrote:
>I have a 136' (1/2 wave on 80m) Carolina Windom from Radio Works. It's a 
>good antenna, and works well for me.
>>There is a transformer or balun at the feedpoint (14% off center). Then, 
>a 22' coax downdrop to a choke balun. Coax on to the shack.
>>What function does the choke balun play? There's coax on both sides of 
>it. I thought unbalanced signal goes in one end of a choke balun, and 
>balanced signal comes out. Having coax on the balanced output side of the 
>balun would seem to be counterproductive. But, it works, and works 
>well. How/why?
>>Does the off-center feed shift the current peak over by the amount of 
>offset on some bands? Is the current peak symmetrically centered on some 
>bands? What is the effect of off center feed?
>>The antenna is rated for 160m use, too. What do I have when on 160? Is 
>it sort of a 22' top loaded vertical? It's a shortie, physically, for 
>160, if you consider only the horizontal part. Likely very 
>reactive---which is why power input rating on 160 is only 500 watts.