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Author Topic: 2m/440 vertical - coupling and static question  (Read 1411 times)

Posts: 2

« on: October 09, 2012, 11:19:54 AM »

Hi there,

I'm just getting back in to 2m/440 ops, and I have a FT-897D that I'm running into a Diamond X50 dual band antenna. The antenna is on my balcony and I'm having some issues with SWR and static, so I have a few questions.

First, how much will a metal balcony railing affect VSWR on these types of antennas? When I tune up, it seems to be absolutely all over the place with no rhyme nor reason.

Second, I am not sure if I'm getting a great connection with my coax, but am having a hard time diagnosing the issue. It doesn't appear to have a short. I have two pieces of LMR240 joined with a coupler, and have done continuity testing across each piece - no issues. But merely touching the cable jacket close to the ends sometimes produces static or drops the S-level. Not sure if this is RFI or what, but perhaps this is adding to my SWR issues.

I would welcome thoughts and opinions...coming from HF, I can see how these shorter wavelengths might be picky, but this is a bit frustrating.


Posts: 353

« Reply #1 on: October 09, 2012, 12:15:47 PM »

If it were my antenna, I would use my MFJ analyzer and a dummy load on the coax to see if there are any problems there, then I would look at the antenna.
Not knowing what is inside the housing, I don't know what or if any rf decoupling is required.
Perhaps 6 turns of coax at the base might decouple the coax, or you may need ferrite cores?
From what you describe tho, it sounds like rf flowing on the outside of the coax.
Maybe the shield isn't connected to the connector at one end very well?

Posts: 376

« Reply #2 on: October 09, 2012, 12:26:10 PM »

Note that although you have DC continuity on the coax joint, you might have a signal reflection point there (if the coupling is not of the same impedance (reactance and resistance) as the coax, it will cause some reflection)). Your length of coax might be "just right" for a problem to occur as well.

As well as the suggestions above, try one coax at a time, and try moving the antenna.

Posts: 1825

« Reply #3 on: October 09, 2012, 12:59:50 PM »

But merely touching the cable jacket close to the ends sometimes produces static or drops the S-level. Not sure if this is RFI or what, but perhaps this is adding to my SWR issues.

That's a sure indication of feedline common mode currents.  That can be caused by an insufficient radial system, or a bad connection in the feedline or connectors.

Posts: 13029

« Reply #4 on: October 09, 2012, 02:57:31 PM »

While such symptoms could be due to common mode problems, they sound much more like
a bad coax connection to me.  Can you pull out both lengths of coax and test them as one
piece, including the connector between them?  Does wiggling the coax at any specific point
(especially near a connector) make it particularly noisy?

I presume you've already examined the coax for obvious physical damage - getting squished
in a door, perhaps, or chewed by a squirrel. 

A good next step is to arrange some way that you can hear the difference: crackling as the
coax is wiggled is one way, but also listening to a weak signal with the squelch open may make
it clear by the audio when the antenna is connected and when it isn't.  That way you can be
looking at what you are fussing with and tell by ear how the signal is responding.

Then go along the coax, especially near the connectors, and prod, twist, bend, flex, wiggle it,
etc. to see where the problem is most common.  Most commonly the problem is an intermittent
connection:  in general the shield is perhaps more problematic than the center conductor, but
either (or both) can be at fault.  With more flexible cables, often the shield breaks off at the
back of the connector from bending.  Sometimes the outer conductor is stripped back too
far before the connector is installed, or the shield isn't crimped or soldered properly to the
barrel of the connector.  For the inner conductor, a common problem is that it isn't soldered
(or crimped) properly to the center pin of the plug.

Then the connectors and adaptors themselves can have problems:  the female connectors
rely on spring pins to make contact to the male part, and these can be sprung out (often
from mating with a plug that has too much solder on the center pin) or dirty, leading to
intermittent connections.  With BNC and SMA connectors I always check that the center
pin extends out to the end of the connector:  both can be too short (due to the contact
slipping inwards in the BNC and wear in the SMA) to make reliable contact.

I had a particular failure mode recently that caused me a lot of grief (and needlessly replaced
connectors):   the center pins of the socket on one of my SWR meters were splayed out
somewhat from years of use.  A standard solder-type PL-259 went deep enough into the socket
that it made good contact, but crimp-type plugs have a smaller diameter tip, and the full
thickness portion didn't extend far enough into the socket to be reliable.  This wasn't a fault
of the crimp plug, but of the socket I was using to test them.

Sometimes you can have an antenna with a poor connection internally that will show similar
symptoms, and I've also seen issues with some barrel connectors that were poorly made
or damaged.  But a methodical approach of checking each connection, and a convenient
way to detect intermittent failures (not just when you are looking at a meter) will help
to track it down.

Posts: 2

« Reply #5 on: October 09, 2012, 07:58:21 PM »

This is all great advice - thanks everyone! I don't have too much time to sort this out this week, but I will report back once I've figured it out.
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