Strange Problem Loading 160m Dipole.

[Ed/KB1HYS]

Ok,  I built and hung a 160m Dipole around the yard, a big zig on one side, but about 80 ft across the top, nearly flat, with the remainder drooped down from about 65-70ft (Love those white pine antenna supports).  It is fed with twinlead (300 ohm).

I also build a Big Tuna, 3/8th inch copper tubing wrapped around an 18 inch long 6 inch diameter PVC pipe.  Link is ~5 or 6 turns about the center.  There is 300pF available to tune. It's not just enough PF for 160, but I'll fix that later.

How much coupling is best??  I could wire up more turns and tapp, but how many is a good starting point?  SHould I ground the center of the big coil or not, I've found examples of both.

Now the weird thing,  when I tune up on 80m, I can find resonance easiy. but if I transmit for more than a minute, the Plate current rises slowly to max. Looks like something slowly cooking. I can retune for resonance, and chase it around.  When things "cool off"  the settings are back to where they were... 

 I can't find anything smoking in the tunner or feedline, I would think a short in the Antenna system would stay shorted. The rig (A Valiant) runs great into a 50 ohm dummy load.  So...

I'm just looking for Ideas since I'm fresh out...

Looking for

[K3ZS]

That happens to me when its raining, probably changes in the feedline and/or the center insulator.

[The Slab Bacon]

Ed,
     If you are chasing it around and it returns to the original settings afterward, it is most likely something heating up. (which also means wasting power) Make a long test transmission, then after you unkey, feel all of the partz in the tuna and see if something is getting warm. If so, there is your culprit. Also if you are using 300 ohm TV twinlead, it may not be up for the current load and be getting warm as you transmit.  Just my $.02 worth.

                                                   The Slab Bacon

[WA1GFZ]

My guess is your loading caps in the rig are heating up because the tuner input isn't 50 ohms. Tune the rig up into 50 ohm dummy load then connect it to the tuner. Do not change the rig settings just adjust the tuner. The Q could be too high. Try changing the link tap and see what happens. You might try tuning the link. The old pi networks worked over a wide range but the loading caps could see a lot of current with the wrong reactive load on them.

[KB5MD]

Had a similar problem with an RF deck on 75meters.  It turned out to be the copper tubing used in the tank coil was expanding with the heating.  I changed out the copper tubing for a solid wire coil and stopped the problem.  Just something you might consider.

[WA1GFZ]

If you are heating up the tank coil then the Q is too high and a good chance the caps are heating also. A 100 watt TX should not heat 3/8 inch tubing.

[Steve - WB3HUZ]

If you are tapping down on the coil with the feedline, start at outer ends of the coil and work your way in to find a match. The further you tap down, generally the higher the Q of the tank and the more likely you will get heating.

[WA1GFZ]

I would think the same is true on the primary side. That is why it best to set the rig up for 50 ohm load then play with the tuner to get back to the same plate current.

[flintstone mop]

If the TX power is stable in a dummy load for a typical old buzzard, then it might be the coils(loading coils?? Is this a loaded dipole??)  or the cap heating up in your tuner. That's why I love vacuum variables.
Your next best helper for antennas and tunas is one of those Mighty Fine Junk ant analyzers.
Is someone nearby that would let you borrow theirs or come to your station to assist?
I had my K1JJ tuner tapped out for the vac variable and the taps for the feedline in a half hour for 160, 80, and 40M with the MFJ 259. I use my Yagi for 20M-10M.
Start tapping from the outside as Steve mentioned earlier.
The 50 ohm input coil is about 7 turns on a 1 1/2-2 inch form stuck inside the main outer coil OR wrapped in between the main coil windings dead center. Nothing at all fancy about the K1JJ tuner. There are several pics aorund this site. Mine are in there too. That tuner saved me $700, as my Bliss tuner went bizerk and burned out.
Fred

[Ed/KB1HYS]

Thanks Guys!!  I have added several turns to my center link, so I can adjust coupling. I will make some tests and see if I can find a good place after tuning into the dummy load.

Thanks again for all the advice!

[Ed/KB1HYS]

Well, with all the advice, I got the tuna working. 
Seems that chasing things around got me. 


I tuned into a dummy load, and then adjusted the tuna for the same "STABLE" plate current. Works FB. 

I've had QSO's on 75 and 160 so far and no problems!!

Now to fish around and maybe try 40m

Thanks again to all.