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Author Topic: High SWR when antenna mount is grounded  (Read 5372 times)
WB6BYU
Member

Posts: 13574




Ignore
« Reply #15 on: April 28, 2010, 09:03:47 AM »

Quote from: KJ4OBR

I tested the new antenna first thing on the old mount and ran it there for about a month... It was between 1:1.5 and 1:1.8 across 2m, 70cm, and 6m ....

I did try my original 1/4 wave dual band whip. It tested 1:1.2-1:1.5 on the lip mount (2m and 70cm only obviously). On the new mount, it pegged the SWR when the mount was grounded like the new antenna....

....As I mentioned in a previous post I think I will try to change the length of the coax and see if I have some unlucky divisor of waveform happening.


Changing the length of the coax shouldn't affect the behavior when the mount is properly
grounded, but it could account for a low SWR when it isn't.  The first step is to figure out
why the SWR is high on the new mount.

If there were just a shift in resonance I'd expect that the SWR wouldn't be worse than
3 : 1 or so.  If it really is off scale, there is some other problem, and we need to address
that first.  From your description it sounds like a fault in the mount and/or cable assembly.

So, let me see if I have the facts straight:  you have two antennas, both of have
reasonably low SWR on the old mount, and also on the new mount when it isn't
grounded.  But both of them show high SWR when the new mount is grounded.  (Well,
actually you didn't say that the quarter wave gave a low SWR when the mount is
not grounded.)

Question:  what are you using to measure SWR?  How high does it read?  Do you have
any sense of how high the SWR must be to be "pegged"?  What does it read with NO
antenna mounted, both when the mount is grounded and floating?

I'm presuming that the coax is permanently attached to the mount and came with a
plug on the other end?

If you are running, say, 15 feet of RG-58 coax to the mount, the worst SWR you can
see with a fault at the mount is about 9 : 1 on 2m and 5 : 1 on 440.  If it is higher
than this the fault must be closer to the coax connector, otherwise the loss of the
cable will reduce it.

My guess would be an open (or intermittent) connection between the coax shield and
the shell of the coax connector.  That would mean that the whole length of coax
(both shield and center conductor) plus the mount becomes the "antenna", fed against
the ground of the SWR meter and the coax attached from it back to the rig.  When you
ground the antenna mount you are making a significant change to that antenna.

One way to test this is to install a 50 ohm resistor across the mount, either on the top
or on the bottom.  Run the lowest power available that still gives a reasonable SWR
indication.  You should see a low SWR.  (A 47 or 68 ohm resistor will work, or two 100 ohm
resistors in parallel.)  Try flexing the cable at various locations, especially at the
connector:  it isn't uncommon to find a connection that works when bent one way (to
reach the meter) and has a fault when bent a different way (to  plug directly into the
rig).  If the SWR reads high with the resistor across it, or low with no antenna on the
mount, the problem likely is in the coax (presuming that your grounding straps really
connect to the ground side of the mount, not the center pin.)
Logged
KJ4OBR
Member

Posts: 104




Ignore
« Reply #16 on: April 28, 2010, 06:57:02 PM »

My meter is an older Calrad that I have comparison tested to be accurate against a new MFJ of a friend. middle reading on the right meter is 3 that is where the markings end the top of the SWR meter is infinity.

I'll have some time over the weekend to delve into your theories.. Will let you know what I find.

Thaks for the ideas!
 73
Dave


Quote from: KJ4OBR

I tested the new antenna first thing on the old mount and ran it there for about a month... It was between 1:1.5 and 1:1.8 across 2m, 70cm, and 6m ....

I did try my original 1/4 wave dual band whip. It tested 1:1.2-1:1.5 on the lip mount (2m and 70cm only obviously). On the new mount, it pegged the SWR when the mount was grounded like the new antenna....

....As I mentioned in a previous post I think I will try to change the length of the coax and see if I have some unlucky divisor of waveform happening.


Changing the length of the coax shouldn't affect the behavior when the mount is properly
grounded, but it could account for a low SWR when it isn't.  The first step is to figure out
why the SWR is high on the new mount.

If there were just a shift in resonance I'd expect that the SWR wouldn't be worse than
3 : 1 or so.  If it really is off scale, there is some other problem, and we need to address
that first.  From your description it sounds like a fault in the mount and/or cable assembly.

So, let me see if I have the facts straight:  you have two antennas, both of have
reasonably low SWR on the old mount, and also on the new mount when it isn't
grounded.  But both of them show high SWR when the new mount is grounded.  (Well,
actually you didn't say that the quarter wave gave a low SWR when the mount is
not grounded.)

Question:  what are you using to measure SWR?  How high does it read?  Do you have
any sense of how high the SWR must be to be "pegged"?  What does it read with NO
antenna mounted, both when the mount is grounded and floating?

I'm presuming that the coax is permanently attached to the mount and came with a
plug on the other end?

If you are running, say, 15 feet of RG-58 coax to the mount, the worst SWR you can
see with a fault at the mount is about 9 : 1 on 2m and 5 : 1 on 440.  If it is higher
than this the fault must be closer to the coax connector, otherwise the loss of the
cable will reduce it.

My guess would be an open (or intermittent) connection between the coax shield and
the shell of the coax connector.  That would mean that the whole length of coax
(both shield and center conductor) plus the mount becomes the "antenna", fed against
the ground of the SWR meter and the coax attached from it back to the rig.  When you
ground the antenna mount you are making a significant change to that antenna.

One way to test this is to install a 50 ohm resistor across the mount, either on the top
or on the bottom.  Run the lowest power available that still gives a reasonable SWR
indication.  You should see a low SWR.  (A 47 or 68 ohm resistor will work, or two 100 ohm
resistors in parallel.)  Try flexing the cable at various locations, especially at the
connector:  it isn't uncommon to find a connection that works when bent one way (to
reach the meter) and has a fault when bent a different way (to  plug directly into the
rig).  If the SWR reads high with the resistor across it, or low with no antenna on the
mount, the problem likely is in the coax (presuming that your grounding straps really
connect to the ground side of the mount, not the center pin.)
Logged
WX7G
Member

Posts: 6327




Ignore
« Reply #17 on: May 01, 2010, 12:39:36 PM »

Solution: Do not ground it.
Logged
KJ4OBR
Member

Posts: 104




Ignore
« Reply #18 on: May 04, 2010, 11:43:06 AM »

LOL.. I thought about that, but I'm stubborn.

An update: I replaced the connectors and re-soldered  the coax to the mount and saw quite a bit of improvement, I'm going to replace the coax next and if that doesn't get me anywhere I'm going to try a different NMO mount. If THAT doesn't work I'm gonna isolate the mount from the bracket and go sit in the hot tub with my XYL.  Grin

Thanks for all the help!

Dave


Solution: Do not ground it.
Logged
KE5PPH
Member

Posts: 73




Ignore
« Reply #19 on: May 19, 2010, 04:15:06 PM »

What was the final outcome/ solution?  And why didn't you consider the Hood?  Opposite the Stereo antenna.
Logged
KJ4OBR
Member

Posts: 104




Ignore
« Reply #20 on: May 26, 2010, 06:21:23 PM »


Well, I ended up replacing the nmo mount and the coax/connectors. It helped quite a bit, but swr was still too high. I hate to admit it, but I milled some derlin insulators and went to the hot tub. ;-) insulated, the swr is 1:1.2- 1:1.9. Performance is good so I called it a day.

I did consider the hood, I didn't have a wide enough seam for the mount (damn German over-engineering). As it was I ended up loosening the trunk bolts and adjusting the lid slightly to make that work.

I'm sure this will bite me at phase 2 when I add the 10m antenna and duplexer to the mix, but I'll deal with that when that happens.

Thanks all for the suggestions!
73
Dave




What was the final outcome/ solution?  And why didn't you consider the Hood?  Opposite the Stereo antenna.
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