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Daytime Impedance high?

Posted: Mon May 15, 2017 7:56 pm
by mbrockm
Appears our center tower impedance is high. Non directional center tower should have an impedance of 433 ohms with 3.4 base amps to make 5KW. In order to make 3.4 amps at the base, the SX5 is putting out 5.6KW (nearly maxed out). The inline OIB and amp meter at the transmitter end confirms the power output to the atu. Transmitter is seeing 50 ohms and +4J. Non directional has a single T network between transmiter and tower. The directional is not showing any problems with the center tower. The directional network is separate from the non directional network. Directional tuning may be such that it has tuned out the higher center tower impedance change?? We have had problems in the past getting consistent base impedance measurements with an inline OIB. We have a delta inline oib with the extended range but readings have not been consistent or match other OIBs we have tried. Looking for any insights there to get an accurate base impedance or at least consistent.
The three towers use isolation coils for the sample lines to cross the base insulator. The center tower (both da and non da) has an added variable vacuum cap between input and output ground of the isolation coil. I assume to cancel out the coils inductance on the center tower. I would think the cap would open and cause a drop in impedance? Not sure how to tune this vacuum cap. By measuring base impedance or should there be a peak or dip in readings somewhere?

Re: Daytime Impedance high?

Posted: Mon May 15, 2017 9:08 pm
by Deep Thought
Hi Mark. This is not an uncommon problem on high impedance towers. What is the measured ND reactance? That's what is really in play here.

The capacitor across the isolation coil is (usually) to parallel resonate it at the carrier frequency. Depending on the tower reactance and the coil reactance, having the capacitor fail or change value could send the tower R up or down. The change is only about 12% according to your figures, which isn't that much, so I don't think you have a major failure here.

One of the nice things about sample loops is they pretty much ignore any changes in tower impedance. You could have no change in the DA readings (or have had them happen slowly enough that they were just "adjusted out" along the way) but the ND impedance change would not have that luxury. Your 70° tower spacing will drive the nighttime operating impedance of that tower well below 100 ohms so whatever is bothering the daytime Z may not even be an issue at night.

Or maybe the base metering has developed an issue. Do you have another meter to check the main one with? A thermocouple meter plugged into the output jack would work.

If you want to try to tune the isolation coil, you'll want to adjust it so you get the original "R" value of 433 ohms. The "X" may not come back to where it was but you can compensate for that with the output coil of the tee network(s). This is only treating the symptom though, and if it doesn't come back to close to the original "X" something else happened on the tower. It could be as simple as a sample line bond coming loose. However, if you do this make sure you can get back to where it is tuned now.

OIBs can have issues with high Z towers, even if they are extended range, due to the lead configuration. You might get better results if you insert a coil or capacitor in series with the tower to cancel out most of the tower reactance and then add it back to the reading later. It will help reduce the effect of the OIB stray reactance.

Re: Daytime Impedance high?

Posted: Tue May 16, 2017 7:32 am
by mbrockm
I have tried a couple of different delta base meters and got similar results. Yesterday I put in one I got tuned by delta electronics. Current readings stayed about the same. Several years back the cap across the sample loop coil was shorted and the affect was obvious. Since this cap is essentially across the base insulator I am thinking this cap could easily get knocked by a lightning strike.
I still wondering about proving if the cap is adjusted correctly. Even though it was years ago we replaced it. At the time we just adjusted till the readings came back to previous readings. The center tower has been replaced since then in 2008. No telling if that knob got turned when we retuned the da after the tower change. I am going to look thru the knob setting records.
The problem is non severe as you say so I am looking for a minor change somewhere. Like a cap value change or something which has worked it way lose and we tuned out over time.

Re: Daytime Impedance high?

Posted: Tue May 16, 2017 1:04 pm
by Lee_Wheeler
Sounds like you need to get an actual OIB or some other bridge on the ATU output and see what the impedance really is. With a 433 Ohm tower you aren't going to get anything accurate with a common point bridge since it is designed to monitor something at or near 50 Ohms. If the ND tower impedance is substantially different than the license (+/- 2% real or imaginary) you need to temporarily determine power by the indirect method (Transmitter Power meter) and file a Form 302 AM for direct measurement. One question on the 302 asks for an explanation of why the impedance changed and that can be tough to answer. I normally attribute it to repairs to the ground system or something else that is plausible. So long as it is stable and this has occurred over a long period of time then you will just wind up with a new set of operating parameters and problem solved.

While you have the bridge at the tower it is a dandy time to touch up the ATU tuning so as to get a perfect 50 J0 match at the transmitter.

A direct measurement application has no fee but most of the time my clients run the application through their lawyers so there does wind up being a cost, it just depends on how expensive the lawyers are.


Re: Daytime Impedance high?

Posted: Thu May 18, 2017 7:51 am
by mbrockm
I have a delta electornics OIB and potomac sd31. Which I dusted the cob webs off yesterday and bench tested. I talked to ed trombly at Munn Reese. When I can get clearance to take KMA down again I am going to isolate the cap and monitor loop coil to see if they are nulled out at our frequncy. Then verify the monitor loop vacuum cap has not been hit by lightning. also going to take a closer look at the sample loop cable where it is bonded to the tower base. Yesterday the base meter readings changed and are closer to where they should be. It had been very stable until yesterday. With all the wind lately wonder about something being loose. Hopefully near the ground and not in the air.