Likley not "saturation", but it is obviously thermal induced problem
(heating). I would say that you have a balun wire heating problem *except*
for your mention of the problem happening more when the weather's
damp/raining. That clue indicates either a bad connection or moisture
inside the coax.
Look for a bad connection first. Bad connections will widen and become
worse when they heat up. Most common point of such connection failures is a
coax plug, so inspect the PL259 or N-M plugs on your coax cable first,
particularly center conductor connection. IF the center conductor is
crimped onto the connector pin, you should try re-crimping and/or soldering
(high-heat gun required to help avoid melting coax dielectric); if already
soldered, re-heat and add a little bit of solder to cure any "cold" or
fractured solder connection inside. Next, inspect the shield connection in
the plugs. If the shield is soldered to the plug, see if you can determine
if the dieletric was melted when the connector was originally soldered
(common problem)...if yes, you'll need to replace the plug (I recommend
using crimp-on connectors, as they are much less risk of damaging the coax
and can be made moisture-proof).
IF your coax uses solid center conductor (typical LMR400), then look for a
tight bend or kink along its length where the center conductor may have
fractured slightly inside. Disconnect the cable at both ends, short center
conductor and shield at one end, then use a VOM (ohmeter) and can measure
less the 1 ohm and check resistance between center conductor and shield at
opposite end while someone shakes or moves the coax at different points
along its length. Changes in resistance indicate a fracture. Upon heating
up, the fracture widens and will increase your SWR during operation.
Checking for moisture inside the coax is a bit harder. Other than
inspecting the plugs, you should disconnect the coax cable on both ends, and
then use a VOM (ohmeter) that can go at least 5 megohms or higher, and
measure across the center conductor and shield on one end. Aside from a
possible initial surge in measuring current, ie a resistance reading, as the
coax "charges" the capacitance over its length, you should have no
measurable resistance reading. If there is a reading, moisture inside the
coax is indicated. Unfortunately, there's not a solid way I know of drying
coax out (debate on this elsewhere on the Inet), but when it happens it's
usually at the antenna end of the coax and for several feet so you may wish
to somehow heat that end up *with the plug removed) for a couple hot, dry
sunny days (treat occasionally with a hair dryer). Then put on a new
If not already done, make sure the outside coax plugs are properly sealed
against moisture by (1) wrapping with cold-weather electrical tape first,
(2) forming "coax seal" liberally as the next layer (available from HRO,
Radio Works, Texas Towers, etc), which is a rubber-like clay/putty that
seals and is reusable, and (3) final layer of cold-weather electrical tape.
PL-259 plugs are by their nature not the best water-resistant outside
connectors, whether solder or crimp, so you need to take special care in
"weatherizing" them; N connetors are superior in this respect, even the
Compression type (read: no soldering), so you may wish to consider changing
the connection on your antenna & coax to N-types.
FWIW, this is where I'd place my initial bet. I, too, have had problems
similar to your description, and it turned out to be bad coax plug
connection on one occasion, and coax moisture in another.
After above, I'd look at (1) internal balun housing, to make sure there's no
moisture inside it; if yes, then dry out and seal any cracks/holes using
silicon caulk; if no, check for cold solder connections or evidence of
over-heated (dis-colored) windings. (2) antenna coax/balun connection, for
bad connections or evidence of moisture; clean connections, re-solder, and
re-seal with silicon caulk as needed.
YMMV --Stephen W9SK
[mailto:firstname.lastname@example.org] On Behalf Of Art Trampler
Sent: Sunday, July 12, 2009 1:49 PM
Subject: [Antennaware] symptoms of balun saturation?
I have an AV640 vertical from Hy-Gain, which is about 18' off the ground at
the feed point. Overall I have been satisfied with its performance, which
has generally met or exceeded (apparently realistic) expectations.
My chief complaint comes with respect to QRO. When running a KW or more
into it (fed with LMR400, about 125'), my SWR will initially stay flat. I
am a CW operator and after two or three ragchew exchanges the SWR will begin
to increase a bit, and then suddenly go high (8:1 or better). It seems that
this happens more frequently if it is damp/has been raining.
Can you help me understand what is likely going on? Is this likely a
symptom of balun core saturation which is somehow related to heat, rather
than simply the power/magnetic flux?
Or is it more likely this has something to do with the RF choke from the
vertical element used to give a discharge path for static electricity? That
choke is 470uh, .3 amp. Is there any reason not to replace it with
something of greater current handling capability, and is the inductance
critical (would 500uh work just as well?).
Thanks for your replies.
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