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" for my vhf/uhf antenna in the woods this spring. Guess that explains
the high SWR, not that I use it that much.
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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