[TowerTalk] Antenna shift, VHF/UHF matching unit burning out

Tue Apr 22 20:20:41 EDT 2014

On 4/22/2014 2:33 PM, Bill wrote:
>
>>
>> At 10:29 AM 4/21/2014, Earl Morse wrote:
>>> On the Diamond it's most likely a bad connection at or in the 
>>> antenna.  Or it has been damaged by a strike but not blown apart.  I 
>>> found the top 3' "exploded cigar" 

That's the way the old Comet departed
>>> for my vhf/uhf antenna in the woods this spring.  Guess that 
>>> explains the high SWR, not that I use it that much.

I have two of them. One side mounted at 50 feet on the 45G and one on 
about a 15' mast on the N side of the shop.  As that's LMR-400, it's 
possible the center conductor has migrated down.I don't remember if 
that's an N connector or UHF

The 45G has taken 17 visually verified hits with no damage  since the 
ground system has been finished.  One removed all the water proofing and 
plating of the connectors "up there" before the ground system was 
finished  That one also took out a 2-meter rig and a brand new computer

OTOH I've had two of the diamonds fail on the N side of the shop, and 
they took out the finals in the 897D and the TM-D700

The last failure was in the winter, so I'm pretty sure it wasn't lightening.

Hy-Gain sent me a new, complete matching network for the AV640. I tried 
it with the exciter alone. No difference from the existing network.  I 
had also wound a new set of transformers with the same results.  I had 
been running 600-800 watts SSB on 6-meters which is well above the 
published ratings, but the Hy-Gain Engineer said that the network should 
handle it with ease. Just don't do it with digital or AM.  CW should be 
OK as long as there were no extended tune-ups.  I tune using a VNA with 
milliwatts before even going to either rig.  100 watts with the 756 Pro 
and 897D and 200W for the MK V and 5000MP.

I have eliminated everything I've been able to think of and this had 
happened twice now, as well as since the first failure, I've not been 
able to get either an R8 or AV640 to load properly.

I've even cut a run of LMR400 to run directly from the antenna to the 
rig and tried 3 different rigs.  They work fine on the sloping dipoles 
for 75 and 40.  Maybe it's a coincidence and I'm looking for something 
common where there is none, but I don't think so.

Before the bees built the nest in the AV640 matching network of the 
AV640 the antenna matched perfectly on all bands and it was very easy to 
adjust the bands.  SWR was almost a perfect 1:1 at resonance. I believe 
the highest was 1.1:1

http://www.rogerhalstead.com/ham_files/AV-640/AV640.htm  gives a pretty 
good account with photos although I didn't have the VNA at that time so 
this includes some tests that were not done originally.

The vertical is now mounted on top of a 24'length of 2", 1/4" wall 
6061T5 that is hinged to the NW guy anchor post the hinge is about a bit 
more than a foot off the ground. All guys are Phillystran. Nothing has 
changed at the antenna mount.  The rigs are all powered from an old 50A 
Astron PS

I'm trying to get an eye bolt in the base of the N wall so I can use a 
come-along to move the welding bench about 2 feet to the N for more 
room.  Then I can move the desk out to clean up and organize all the 
cables behind there.

  73

Roger (K8RI)

>>>
>>> On the AV-xxx vertical I don't think it is a bias issue saturating 
>>> the matching transformer/balun.  Where does the DC current go that 
>>> flows through the balun (saturating it) into the antenna?  You don't 
>>> have a top light or preamp on the antenna do you?
>>>
>>> If you have high SWR on all the bands take a look at the balun. 
>>> Maybe it got overheated and smoked the core.  Some of these antennas 
>>> use baluns that are designed right on the edge and work good until 
>>> something else goes bad.  While the baluns are pretty efficient they 
>>> can be overheated if operated outside of the design parameters.  
>>> Once the ferrite core material starts hitting the Curie temperature 
>>> it stops being a core and the transformer stops working.  It can be 
>>> a runaway condition in which the core eventually fails.
>>>
>>> Earl
>>> N8SS
>>>
>>> ------------------------------
>>>
>>> At 10:23 PM 4/20/2014, Roger (K8RI) on TT wrote:
>>> >I think that I've lost a ground.  The SWR has gone sky high on the
>>> >Diamond 144/440 repeater antenna. As the antenna is only rated at
>>> >200 watts the matching network is pretty light duty. Also, the
>>> >AV640 which originally was very easy to match, is now difficult to
>>> >find the resonant points.
>>> >
>>> >I believe this is due to a DC bias through the matching transformers
>>> >putting the cores into, or near saturation.
>>> >
>>> >I keep using, I think and I believe as I really don't know, but it
>>> >sounds logical.
>>> >
>>> >The new AV840 just arrived and I need to work on the Diamond
>>> >antenna, so I need to find the problem before going any farther with
>>> >the antennas.
>>> >
>>> >I had been talking to N8ERF on the repeater when he mentioned I was
>>> >getting noisy.  I immediately stopped transmitting, came in, got the
>>> >SWR bridge, took it to the shop and put it in the line. The SWR had
>>> >gone from 1.2:1, to off scale.  I think the finals are OK. I
>>> >certainly hope so.
>>> >
>>> >So tomorrow, I'm going to add one of those grounding plates that tie
>>> >everything together and to the station ground. The space behind the
>>> >desk out there is a mass of wire, that could be neatened up, so I
>>> >can see at a glance what is connected to what.
>>> >
>>> >That and all the projects need to be finished up so I have an
>>> >organized shop in which to work.
>>> >
>>> >Any thoughts on the grounding would be appreciated
>>> >
>>> >73
>>> >
>>> >Roger (K8RI)
>>> >
>>>
>>>
>>>
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>>>
>>>
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>>