antenna Tuna

[Blaine N1GTU]

well, I got the tuner all setup, the new antenna is up and a high flat top at around 60'
fed with open wire feedline.
the coil is 40 turns of 3/16" tubing with 6 turns on the input.
Bob and I putzed around with it and got it to tune on 160 but not on 80.
The big bread slicer 300pf was very touchy, would drift as you put your hand near it.
is this normal?
We removed the 300 and put the 500pf from the classe rig (the cap in the pics)
it tuned a little better and was not touchy at all, which is odd being a higher value.
still could not get it to tune on 75-80 then i had a brainstorm.
I disconnected the  cap from the circuit and it tuned right up 1:1 swr anywhere on the 75/80 mtr band.
I am assuming this is due to the fact that the antenna is 64' per leg and fairly resonant on 75.
basically acting like a balun for 75-80
anyone have any input? suggestions?
I really only need 160 and 80, not too worried about other bands for now.
I was thinking of removing the variable and maybe just switching in doorknobs for 160.
Its very easy to move the taps on the coil.

[W1VD]

Nice tuna!

Guessing that the coil has an inductance of around 24 uH...it would take about 85 pF to resonate at 3.5 MHz. You probably have that much in the variable capacitor minimum C + strays + distributed capacitance of the coil...which is why it works with no tuning capacitance.

Try shorting out an equal number of turns on each end of the coil to reduce the L and get back into range with the tuning capacitor. Or, put alligator clips on the tuning capacitor and tap inboard of the coil ends an equal distance.

You may find that more than one open wire tap placement makes a match. Choose the open wire tap locations that are nearest the outside of the coil - this will provide the lowest Q arrangement and lower losses.

Can't tell from the pix, but make sure to ground the center of the coil since you're not using a split stator cap. 

[Blaine N1GTU]

yep the center is grounded.
I will play with it more tonight.
hope to make some contacts
I think i will try moving the 300pf cap up and down the coil.

[kf6pqt]

Ohhh... you gotta ground the center of the big coil when running a non-split cap? I've been running a 1/2 scale version FB for a while now without the center grounded... what have I been missing out on?

Thanks,
Jason kf6pqt

[Blaine N1GTU]

Jason,
it provides a DC ground for safety factor.
It will work without it but when the cold comes and the wind blows and the light fluffy snow falls, the sparks start snapping.
I just feel better having it.

[N3DRB The Derb]

might want to use copper battery clips too. Sometimes da juice is rather high and you'll know it when you reach for the coil taps and it's 300 degrees hot. 

[k4kyv]

More than likely the antenna/feedline combination is not exactly at resonance, and you lucked out with just the right amount of capacitive reactance at the feedpoint to resonate the coil without an additional capacitor, or with just the stray capacitance in the circuit.

You can avoid the hand-capacity effect by using a split-stator capacitor with the dual sections in series, each having double the capacitance of the present single-section, but you need only half the spacing between the plates.  If you don't have a dual section variable available, just use a long insulated shaft made of bakelite, glass, ceramic, or even a varnished wooden dowel rod to isolate the capacitor from the tuning knob.  Besides, while adjusting the capacitor at higher power, the metal shaft just might arc through the insulated tuning knob and give you a very nasty rf burn.  I speak with experience, since I still have many scars on my hands and fingers from rf burns of years past.

Ground the midpoint of the coil through an rf choke, not directly.  If you  ground it directly, you are setting yourself up for common mode radiation.  The entire antenna + coil may act as a grounded Marconi antenna, independently from the action as a balanced feed system.  This is called the Marconi Effect, and can result in unbalance in the currents in the open wire feeders, radiation from the feedline, and cause unnecessary pick up of local electrical noise on receive.

Also, if you use a balanced dual section variable capacitor, never strap the midtap of the coil to the frame of the capacitor.  This may result in high circulating current and even two separate resonant points, and nearly always increases losses in the tuner.  If the antenna, feedline and the link to the coil are properly balanced, the tuned circuit in the antenna tuner will self-balance.  You  can never force-balance a tuner/feedline without causing a Marconi Effect, and it will inevitable reduce the efficiency.  Better to tolerate a slight unbalance in the feeders than to correct it by force-balancing it with respect to ground.  The  current and voltage in the transmission  line at the feed point may appear balanced, but 99% of the time the balance where the feedline attaches to the antenna will actually be made worse by force-balancing the feeders at the tuner.