Grid input coil - self resonant??

[Ed/KB1HYS]

I was working on the grid input circuit for my HB transmitter.  It consists of a coil 0.75" (18mm) in diameter, 59 turns of #24 wire full coil tune 160, with taps for 80, 40, 20 and 10 meters.  It's in parallel with a 200pf air variable. The coil form is about 1.5 inches away from the switch deck, and about the same from the variable cap.

I was checking this out with my Grid dipper, and it works Ok on 40 and below.  There seems to be a resonance at 16MHz that is uneffected by the variable capacitor setting. I can't find any resonances (not consitently anyway) above that. (well there is something going on at 40 Mhz).

Calculated inductance 36uh.  IF there is 2-3pF capacitance available this will resonate at 16MHz but will that cause the loss of resonance above that frequency? 

I could wind two or more separate coils and make it a turret type input, but thats a bit of work, and one coil seemed like it should have worked.

Any ideas?

[W3GMS]

Ed,
I am not sure if this will help your situation or not but when winding a resonant coil that I want to tune over a wide frequency range separating the turns really seems to help especially up on the higher bands above 40 meters.  I am sure your shorting out the un-used coil as your going up in frequency.  The other thing is to try and keep the coil at right angles to the sheet metal closest to the coil.  By doing this you will have less capacitive coupling into the coil. 
Joe, W3GMS       

[N2DTS]

I usualy build seperate coils for each band I want, and switch them in.
That takes more space, but seems to work better.
The 2X813 rig does 160 through 10, and seems to without problems.

On all the rigs with one coil I have looked at, they did seperate the bands, and the higher frequencies are wide spaced......

Brett

[KM1H]

On all the rigs with one coil I have looked at, they did seperate the bands, and the higher frequencies are wide spaced......

The reason for that is twofold. One is to maintain a useable Q with a lower inductance and the other is to be able to resonate with the minimum Tune, tube,  and stray C available. On 10M its also easier to have a wide spaced 3-4 turn coil than try and find the exact tap on 1-2 close turns. The Heath SB-200/220 series are well known examples which barely make 10M and some need the coil stretched a bit for the high end of the band.

Carl
KM1H