[CRTech] Christian Radio Tech [MSG 52214]
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1. I am quite sure an RF wattmeter cannot read 60Hz AC power.

2. In a closed-delta secondary, ALL of the generated current must return to
the original secondaries of the 3-phase transformers, where the energy was
created.

3. There are TWO ground paths:
      [a] A direct wire from your breaker panel "ground," and...
      [b] a current path thru the earth, back to the pole transformers.
      Current in a parallel circuit will divide proportionately with the
inverse ratio of the path resistances.
4. Unless your site has extremely poor conductivity, I'd guess your station
ground is better than the utility ground conductor. Therefore, a portion of
the current flows back to the transformers thru the "ground" Can you get a
O-scope on the ground current measurement? IS the ground current a pure sine
wave, or is it "modified" bwecause of being 3-Phase.

Be sure to let us know what happens.
John S.





On Mon, May 16, 2011 at 2:40 PM, Pete Allen <pallen@wrbs.com> wrote:

> Folks,
>
>
>
> Here's our situation:
>
>
>
> Got a call at 10:25 PM that our NV-20 FM xmtr was on, but not making power.
>  A quick check of the last hour's logs showed about 20 minutes of gradually
> increasing VSWR, leading to a couple of VSWR foldbacks and eventually a VSWR
> shutdown.  After reset I did a low power startup and saw over 650 watts of
> reflected before I made 10 kW of forward.  The high reflected power also was
> also indicated on an independent Bird Wattmeter as confirmation. A quick
> switch to our Aux tower allowed me to put the legal limit of 11 kW from the
> same xmtr into the Aux antenna with less than 30 watts reflected.
>
>
>
> The night had been warm (low 70's) and calm.  No wind, rain, or lightning.
>  Line pressure was normal with the dehydrator cycling as usual, indicating
> no new leaks.
>
>
>
> All this indicated the problem was likely in the 500' run of 3 1/8" Heliax
> or the Jampro JADP-3/3 3-bay antenna.
>
>
>
> 1 ~  Next morning did a visual on the coax and antennas using binoculars
> and a telescope.  Nothing unusual was found, except a slight bend of the
> coax at the tower base ground kit.
>
>
>
>
>
> 2 ~  Swept the line, hooking up a SiteMaster into the N connector on our
> manual antenna patch bay.  It indicated similar results to what was
> discovered during an August 2009 sweep:  A small anomaly at approximately 17
> feet out.   This anomaly, however,  was not big enough to cause such a large
> reflection.  A second SiteMaster was calibrated produced identical results.
>  Pictures were stored for each of the readings.
>
>
>
>
>
> 3 ~  We opened ground kit at the base of the tower and found had been
> installed properly, with no significant dents, holes, or leaks.  Adjusted
> the coax clamps to remove the slight bend at the ground kit and added a coax
> standoff to keep the line from coming in contact with the bottom flange.
>
>
>
>
>
> 4 ~  Opened up the ground kit just before the coax enters the building, but
> found no irregularities.  Both ground kits were reinstalled and taped.
>
>
>
>
>
> 5 ~  A slight improvement was seen at the 17' spike.  It was attributed to
> removing the slight bend at the tower ground kit.
>
>
>
>
>
> 6 ~  At just after 1 PM we attempted to place the main antenna back on the
> air.  Again VSWR spiked to 650+ watts on a forward power of about 10 Kw.
>
>
>
>
>
> 7 ~  Climbed the tower and installed a load at the top of the coax line.  A
> sweep of line showed it clean and flat to the top, with the exception of the
> anomaly @ 17'.
>
>
>
>
>
> 8 ~  As pressure was released from the line, there was no odor of burning
> or soot found in the line at the base of the antenna.
>
>
>
>
>
> 9 ~  Reconnected the antenna, and did a visual of antennas, jumpers and
> power divider.  All showed no visible physical damage.
>
>
>
>
>
> 10 ~  Currently looking at antenna specs with the manufacturer with the
> goal of sweeping the power divider, jumpers and antennas to look for
> possible problems.
>
>
>
>
>
> Now it may be a total coincidence, but the one thing that was changed about
> 8 hours before the failure was the connection of a new ground system to the
> tower.  The new system consists of a 18' diameter 'halo' around the tower
> base with 3 eight foot ground rods equally spaced, and a ground line that
> runs from that circle around our building to our 200' Aux tower, with 8'
> ground rods driven every 18'.  This line adds to our existing grounding
> system and completes a loop around our building. The system was
> professionally installed to industry specs.
>
>
>
> We took current readings (using an clamp on Amprobe) of the tower ground
> before and after making these connections an found about 4 amps of current
> on one of the existing ground wires.  That current then moved to one of the
> new ground wires once they were bonded to the tower.  Further testing shows
> small amounts of current on other grounding points (.25 amps to 3.5 amps).
>  We also found that readings would change as grounds were lifted and
> reattached, as in #'s 3 & 4 above.  A check of the voltage differential on
> both ends of an open ground or from ground to ground shows nothings, as does
> a similar ohmmeter test. A second clamp on ammeter test was identical.
>
>
>
> Also noteworthy is the fact that we have a Lightning Masters lightning
> deterrent system mounted on the tower. That system has proved effective in
> eliminating or minimizing damage to our office and studio equipment that is
> located at the base of the tower.  One other factor in this chapter is to
> note that we "corner grounded" closed delta power, also known as 240 3phase
> center ground.
>
>
>
> Sorry for the long diatribe, but I wanted to list as many details as
> possible in what seems to be a pretty unique situation.
>
>
>
> Questions:
>
>
>
> Have you ever heard or seen such a problem?
>
>
>
> Could the current found on the tower ground line and the coax ground line
> cause a high (and maybe false) reflected VSWR back into the xmtr?
>
>
>
> If the antenna proves to be the problem, is this current detrimental in any
> other way?
>
>
>
>
>
> At the moment I'm working on a diagram to log current readings and then
> break the new ground attachments to check for improvement before going up
> the tower.
>
>
>
> Any comments or insights would be appreciated.
>
>
>
> Thanks!
>
> Peter J. Allen
> Chief Engineer
> WRBS 95.1 Shine FM /WRBS AM1230
> 3500 Commerce Drive
> Baltimore, MD  21227
> 410.247.4100    (f) 410.247.4075
> pallen@951ShineFM.com
> web: 951shinefm.com
>
>
>
>
>
>
>
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>
>
>


-- 
Are U "FAT?" *F*eelings / *A*ction / *T*hink
Lord, Help us be Feeling / THINK / Action, instead.
-- From Saul Pritchen

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