[Antennas] Dipole Matching problema

[Dennis L. Wade]

Hello everyone,

            I've finally put up a dipole after a 15 years of QRT.  
However, under certain conditions, I have alot of RF around the shack 
getting into the FT-101ZD to the point where it disrupts the VOX break 
in circuit and it drops out of transmit.  I'll start with a description 
of the antenna/tuner combo I'm using and then some observations about 
the conditions under which I'm having problems.

            The antenna is 119 feet in length center fed with about 40 
feet of homebrew 600 ohm open wire feedline.  It is strung between two 
trees and is about 40 feet above the ground.  Below the antenna is 
mostly house (woodframe/stucco construction), some grass and some 
concrete.  The feedline runs off for about 45-50 feet at an angle  to a 
PVC post at the eves of the garage.  Here is where I make a transition 
to doubled coax lines to get it through the garage wall.   The length of 
the doubled coax (RG-8) is about 20 feet.  Once in the garage, it goes 
into a wall mounted 1:1 torridal core balun mounted on the wall right 
above the operating position.  Here a short length of coax runs to the 
tuner.  The tuner is an SPC design out of the 1984 Handbook.  The  
roller inductor is about 30 uh, and the caps are about 200pf with good 
spacing between the plates.


             What seems to be happening is when I load on 40, 20 and 15, 
I can get a matched condition, but using only about 5 turns of the 
inductor on 40, 2 or 3 on 20, and right up against the stop on 15 
meters.  The transmitter side cap is at about 10 to 30 percent of its 
max value, and the antenna side cap is between 70 and 90 percent of max 
value.  When I get close to a match at full power (about 100 watts), the 
radio goes kinda nuts..dropping out of transmit, and the sidetone 
obviously sounding like a lot of RF is running around.  I'm presuming 
that under these matching conditions the circulating currents in the 
tuner are quite large.  The inductor seems to be getting warm as well on 
40. 

                My intuition tells me that either the antenna length, 
feed length or some values in the tuner have to change.  I will say that 
I need to pay more attention to an RF ground.  Plans are to sink at 
least 1 8ft rod, and possibly 2 just outside the garage wall.  I will be 
able to run a short length of wide braid to the rod(s).  In the 
meantime, what do you all think needs to be done?  Hopefully I won't 
have to rebuild the tuner, but I do have some components around.  Would 
moving the balun out to the eves improve things?  If I did that, I could 
probably eliminate the doubled coax lines altogether.  Any and all 
constructive suggestions are welcome. 

                Since I get this list in the digest form, feel free to 
reply direct and copy the list. 

                                    Thanks...Dennis


Dennis Wade
KG6ZI
Carmichael, CA