Once again, we're back to the power supply. I guess I have to agree with
Jerry that before taking the antenna down you should try a power supply with
more current capacity or a big cap on the SteppIR supply output.
Question for Bill that I should have asked before: Does your SteppIR have
the 30/40 driven element?
73, Dick WC1M
> -----Original Message-----
> From: K4SAV [mailto:RadioIR@charter.net]
> Sent: Monday, June 18, 2007 7:33 PM
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] SteppIR problem-update from the front lines
>
> I don't think I would take the antenna down unless you are at the point
> of shipping the whole thing back to FM for a refund. I can't think of
> any possible problem with the antenna that could cause this, that
> hasn't
> already been eliminated.
>
> A motor winding with a short or any relatively low impedance to ground
> was eliminated with the ohmmeter measurements. A motor short, or
> partial short, should not be a problem because the motors are current
> driven. Excessive friction, tape jams or anything like that should not
> cause the motor to draw more current for the same reason.
>
> One concern I had was that maybe the DC ground was connected at the
> tower and a ground loop was causing a problem. After looking at the
> cable info in the manual, I see there is no ground wire in the cable,
> so
> that is not possible. There is a shield wire and that should be
> connected to tower ground.
>
> My best guess as to the cause of this is that you are right up to the
> limit of what the system can do. The long cable length combined with
> the
> excessive current pulse presented to the power supply by the controller
> is causing the supply to drop too low. Remember the power supply
> voltage transient is present whether the cable is connected to the
> antenna or not, so a large part of this transient problem is not due to
> load, but due to some "feature" in the controller.
>
> I opened my power supply to see if I could determine how much
> capacitance was on the output of this supply. I wanted to know if the
> supply was capable of handling a 6 amp transient without dropping the
> voltage. It is a switch-mode supply with a 150 uf 400 V cap on the
> primary side of the regulator, and there are 3 - 230 uf 63 V caps on
> the
> output side. I am guessing these are all in parallel since I can't see
> the bottom of the circuit board without a lot of disassembly. So with
> 690 uf of capacitance, and a 6 amp current pulse, the voltage will drop
> 8.7 volts in 1 ms. So the cap is not capable of supplying the load
> current for the transient and the regulator must do it. Since the
> supply is only rated for 2 amps, it may be that the regulator is going
> into current limit. I can't be sure of that without the schematic, but
> I think that is a good guess.
>
> A band-aid fix might be to add large cap on the power supply line. It
> would probably take about 5000 uf at 40 volts. Another possibility is
> a
> different supply capable of more current. The real fix would require a
> controller mod. Remember this is only my best guess based on limited
> information.
>
> I shouldn't take FM more than a couple of hours to verify this
> information. (Of course, then there will be a big pause to determine
> what to do about it.)
>
> Jerry, K4SAV
>
> Dick Green wrote:
>
> >I'd like to know the answers to Jerry's questions #1 and #2 below,
> too.
> >Also, what make and model DVM was used to make the measurements? Was
> the
> >resistance of the leads checked and subtracted from the readings?
> >
> >As I see it, there are three possibilities:
> >
> >1. The SteppIR controller doesn't work reliably with cables greater
> than a
> >certain length. If I remember correctly, Bill has a total of about 520
> feet
> >of cable between the controller and the antenna (440' in the conduit
> and 80'
> >on the tower.) It would seem a simple matter for SteppIR to run a
> series of
> >tests with a 4-el and lengths of cable ranging from 400-600 feet. I
> wouldn't
> >rely on anecdotal evidence from customers.
> >
> >2. One of the motors has a borderline defect, such that the combined
> motor
> >load doesn't overload the controller with shorter lengths of cable,
> but
> >somehow the load is increased beyond the capacity of the power supply
> by the
> >greater capacitance and/or resistance of the longer cable.
> >
> >3. There is a marginal defect in the cable or wiring on the tower. Not
> too
> >long ago, I moved my SteppIR from a temporary cable to a permanent
> buried
> >cable (yeah, I know it's not rated for that.) I stupidly forgot to
> power
> >down the controller and unplug the cable before making the switch.
> Luckily,
> >I didn't damage the controller. However, when I got to the shack, I
> found
> >that the controller was periodically tuning the antenna -- I was
> alerted by
> >the N8LP Tuning Relay cycling on and off on a regular basis. I didn't
> >measure the cycle time, but it was under a minute -- might have been a
> few
> >seconds. Anyway, I powered down the controller and checked the cable
> from
> >the shack. Sure enough, one of the leads was shorted to ground. The
> >resistance was not zero -- it was a few ohms. I checked wiring at one
> of the
> >suppressor panels and found that a tiny tendril of shield braid was
> touching
> >one of the suppressor terminals. Snipping that piece of braid fixed
> the
> >problem. My point is that it doesn't take much of a short to confuse
> the
> >controller and, depending on the type of DVM you have, and how it does
> >continuity tests, a partial short may go undetected. Further, it's
> >particularly easy for tendrils of shield braid to touch the motor wire
> screw
> >terminals at the antenna end. That's one reason I switched to telco
> >waterproof connectors. I'm not saying this is the exact problem Bill
> has. I
> >would expect something like that to show up with a long or short
> cable. But
> >a marginal condition, where a motor wire is only partly shorted to
> ground or
> >another motor wire, might be interacting with the longer cable. A
> good,
> >sensitive DVM should be able to reveal a problem like that.
> >
> >Bill, at this point you can reasonably conclude that the only thing
> left to
> >do is take the antenna down, especially if you are seriously
> considering
> >another type of antenna. But there's one more test I would do: Take
> the end
> >of the spare 440' cable up to the SteppIR and wire it in place of the
> run
> >going up the tower. Connect the controller at the other end of the
> 440'
> >cable. If it works, there are three possibilities: 1) The 80' cable
> run up
> >the tower has a marginal defect, 2) There was a marginal wiring
> problem with
> >the 80' cable run at the antenna, 3) 440' is below the threshold for a
> 4-el
> >SteppIR cable (again, SteppIR should be able to answer the cable
> length
> >question.)
> >
> >If the 440' run works, I'd cut it at the bottom of the tower and wire
> it to
> >the cable in the conduit.
> >
> >If the 440' run doesn't work, it's time to take the antenna down. If
> it was
> >me, I'd ask SteppIR for one spare passive EHU and one spare driven
> EHU, and
> >substitute to see if there's a marginal motor problem. If that doesn't
> work,
> >I'd pack it all up in a big box and ship it back to SteppIR for a
> refund.
> >
> >73, Dick WC1M
> >
> >
> >
> >>-----Original Message-----
> >>From: K4SAV [mailto:RadioIR@charter.net]
> >>Sent: Sunday, June 17, 2007 11:03 PM
> >>To: towertalk@contesting.com
> >>Subject: Re: [TowerTalk] SteppIR problem-update from the front lines
> >>
> >>Doing engineering analysis from where I am sitting is very difficult,
> >>and you are down to that point. The part substitution method hasn't
> >>worked and there is only one part left that hasn't been substituted.
> >>(And I doubt that will solve the problem either.) Plus you didn't
> give
> >>us many details of your measurements other than you made them and
> they
> >>were OK. So everyone is just taking wild guesses. (I thought the
> water
> >>in the cable guess was pretty good.) Here are some questions. The
> >>answers to these would help a lot.
> >>
> >>1. When you ohmed the lines to the motors what were the readings?
> Were
> >>they all very close to the same?
> >>
> >>2. Did you check every line for resistance between each line and
> every
> >>other line and ground? Were they all open except for the mate for
> the
> >>one being tested?
> >>
> >>3. Is the DC ground for the power supply tied to station ground? Did
> >>you measure it?
> >> Note: This can be a difficult to measure, because there may be some
> >>small voltage on it.
> >> (This DC ground should not be connected to the tower ground at the
> >>antenna.)
> >>
> >> If it is connected, remove the serial cable to the computer and
> >>re-measure it. If it is still connected, this may be a problem.
> >>
> >>4. Are there any other devices connected to this cable such as
> >>transient suppressors or a tuning relay?
> >>
> >>
> >>I have a 4 element SteppIR with 250 feet of cable. I just looked at
> >>the
> >>output of my power supply with a scope to see if I could learn
> >>anything. Seems this is the only way since SteppIR doesn't publish
> >>their schematics. I was a bit surprised at what I saw. The supply
> >>output was close to the nominal 33 volts, but when the antenna was
> >>tuned
> >>there was a 12 volt negative going spike on the DC power supply line.
> >>It then ramped back up the nominal voltage in 30 ms. This did not
> >>continue as the antenna was tuning, but occurred only once at the
> start
> >>of tuning. It also occurred each time the antenna was moved one
> >>frequency step. I also measured the current spike that was causing
> >>this
> >>voltage spike. It was 6 amps.
> >>
> >>If the cable to the antenna is disconnected, this 12 volt spike is
> >>still
> >>present on the power supply, but it only takes 1 ms to recover to
> >>nominal voltage.
> >>
> >>The cause of this spike or its effect is unknown since I don't have a
> >>schematic, but it doesn't look like something that is very good, and
> it
> >>is sure going to complicate the problem of keeping the controller
> from
> >>shutting down when heavily loaded. I could take a guess at the cause
> of
> >>this, but I won't because it would only be a guess. This kind of
> >>engineering stuff belongs back at the factory.
> >>
> >>I also tested the supply by itself to see if it had good regulation.
> I
> >>did transient load tests up to 2 amps which is the rated current for
> >>the
> >>supply. The output showed no sign of dropping voltage. I did not
> test
> >>to see what it would do with a 6 amp transient.
> >>
> >>The resolution of this problem, is it is a problem, belongs at the
> >>factory. A beefier power supply might hold the voltage up better,
> >>however since the controller can cause this spike without the cable
> >>connected, the source of the problem is probably in the controller.
> >>
> >>I hope FM reads this message because I would like to suggest that
> they
> >>set up a test fixture and measure the acceptable limit of cable.
> >>Having
> >>someone report that they have one working that has 500 feet of cable
> >>hardly qualifies as engineering data. The max limit should be
> measured
> >>and the reason for failure above that limit should be determined. If
> >>they haven't done the system analysis for the cable system, that
> should
> >>be done also.
> >>
> >>Jerry, K4SAV
> >>
> >>
> >>
>
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