Mag-mount antenna SWR is very sensitive to position

Mag-mount antenna SWR is very sensitive to position

Post by Dave Martinda » Mon, 08 Sep 1997 04:00:00

I don't have a mobile radio in my car, but sometimes use my handheld
there.  When I do, I usually mount a "mini mag mount" antenna on the
roof.  This is certainly an improvement over using the HT's ***
antenna.

However, an SWR meter indicates that the mag mount sometimes has a
terrible SWR (often 3:1, I think I've seen as bad as 5:1) on VHF.  The
SWR varies all over the place as I move the magnet base around on the
car roof.  On UHF, SWR is low and more stable.  (The antenna is a
dual-band design).

My guess is that the problem is one of not having a "ground return" for
the RF.  The car roof is steel, but it has no metallic electrical
connection at all to the RF ground.  The antenna, feedline, and
handheld are all electrically isolated from the car frame ground.  (The
handheld is operating on battery power).  So the only coupling between
the car roof "ground plane" for the antenna and the transceiver RF
ground is whatever capacitive coupling exists between the two.

The magnet base on the antenna is only about an inch in diameter and is
covered with plastic on the bottom, presumably to avoid scratching the
paint.  I'd guess that the capacitance between anything metallic inside
the antenna base and the car roof is too small, and the result is a
high-impedance path between the two at VHF frequencies.  I'd guess that
the high ground plane impedance is giving a high antenna feedpoint
impedance, and thus a high SWR.  But I don't have any means of directly
measuring the impedance, only the SWR.

On UHF, the impedance of the capacitive coupling between antenna base
and car roof is reduced by a factor of 3, and it seems that it becomes
low enough for the antenna to behave reasonably well on UHF.

So, some questions:

        - Is this a plausible explanation for the VHF SWR problem?

        - I know that permanently installed antennas are grounded to
          the car frame at the mount point - this is ideal.  But how do
          other people using magnet mounts get a decent RF ground?  Do
          they depend on grounding the transceiver?  Is the DC power
          ground adequate, or does the transceiver have an additional
          low-impedance ground strap?

        - What can I do in my situation?  Should I try grounding the
          feedline coax shield to the car frame at some point?  Does it
          matter much whether the ground is at the transceiver end, or
          about midway along the feedline?

I intend to do more experiments, but thought I'd check to see if I
was on the right track.  Thanks for any suggestions,

        Dave

 
 
 

Mag-mount antenna SWR is very sensitive to position

Post by Arrow1 » Mon, 08 Sep 1997 04:00:00


I intend to do more experiments, but thought I'd check to see if I
was on the right track.  Thanks for any suggestions,

Hello,  Dave

I think you are making this way to hard, a mag mount should work
just fine with a HT.

Bet you a Dollar to a Donut you have a bad connection in
the antenna, or in the coax.

73    Allen Lowe   N0IMW

 
 
 

Mag-mount antenna SWR is very sensitive to position

Post by Cecil Moor » Tue, 09 Sep 1997 04:00:00


>         - What can I do in my situation?

Hi Dave, I had the same problem and a much larger mag-mount solved
the problem giving adequate 2m capacitance to chassis ground.
--
73, Cecil, W6RCA, OOTC  (not speaking for my employer)
 
 
 

Mag-mount antenna SWR is very sensitive to position

Post by Dave Martinda » Tue, 09 Sep 1997 04:00:00


>Bet you a Dollar to a Donut you have a bad connection in
>the antenna, or in the coax.

If I had an open or short in the antenna or coax, I'd expect the SWR
to be high on both VHF and UHF.  That's not the case.

On UHF, the antenna has a low SWR which changes little no matter
where I stick it on the car roof.  Also, running my hand along
the feedline has almost no effect on SWR.  If I lift the antenna off
the roof and hold it in mid-air, the SWR *does* rise because it doesn't
have a decent ground plane any more.  All of this is in accordance with
how I think a properly-operating unbalanced vertical should behave.

On VHF, the SWR is also high when the antenna is held in mid-air, and
it does drop somewhat when the antenna is stuck to the car roof, but
not as low as I would like.  I can change the amount of reflected power
indicated on the SWR meter by a factor of 3 by wrapping my hand around
the feedline and moving it a few feet along the feedline length.  SWR
also changes markedly when I move the antenna from one place to another
on the roof.

All of this suggests to me that the car roof acts as a good ground plane
at UHF but an inadequate one at VHF.  And I suspect that *that* is due
to insufficient capacitance between the antenna base and the steel roof.

What is the feedline shield grounded to on a glass-mount antenna?  There
must be a similar problem there.

        Dave

 
 
 

Mag-mount antenna SWR is very sensitive to position

Post by Gary Coffm » Tue, 09 Sep 1997 04:00:00


[snip]

Quote:>So, some questions:

>    - Is this a plausible explanation for the VHF SWR problem?

The mag base having too little capacitance to the roof is a plausible
explanation. Most mag bases have about 4 to 6 square inches of
contact, and either have no insulating coating or a very thin one.
This gives a very low impedance coupling to the roof (which is
what you want for this type of antenna to operate properly).

Quote:>    - I know that permanently installed antennas are grounded to
>      the car frame at the mount point - this is ideal.  But how do
>      other people using magnet mounts get a decent RF ground?  Do
>      they depend on grounding the transceiver?  Is the DC power
>      ground adequate, or does the transceiver have an additional
>      low-impedance ground strap?

Whether the radio is RF grounded or not is irrelevant. The coax provides
the two wires needed to transfer RF to the antenna, IE the inner conductor
and the *inside* of the outer conductor. These two conductors carry opposite
and equal currents, no other path is needed. But the vertical monopole *must*
have a counterpoise to function correctly, and the vehicle roof supplies that.
The coax outer must be coupled to the roof by a low impedance at RF, the
capacitance of the magnetic base serves this function.

Quote:>    - What can I do in my situation?  Should I try grounding the
>      feedline coax shield to the car frame at some point?  Does it
>      matter much whether the ground is at the transceiver end, or
>      about midway along the feedline?

It matters extremely. The connection to the counterpoise must be at
the antenna base. Connecting anywhere else will cause the feedline
to radiate due to currents on the *outside* of the coax shield. This
is not a good thing, and will seriously degrade antenna performance.

Quote:>I intend to do more experiments, but thought I'd check to see if I
>was on the right track.  Thanks for any suggestions,

You may be on the right track, and a larger magmount may solve
your problem. (I've had very good luck with Larsen magmounts.)

Or it could be that you have a bad piece of coax. If one of the
conductors is broken near the antenna, it would give the symptoms
you are experiencing. The coax is a better dummy load at UHF,
which would explain the lower *apparent* SWR reading on 70cm.

And of course wiggling the broken cable would make a difference.
Magmount cables typically take a beating because they are exposed
to flapping in the wind (and get pinched in doors), so this wouldn't
be unusual.

Gary
Gary Coffman KE4ZV          | You Make It  | Email:

534 Shannon Way             | Guaranteed   |         or

 
 
 

Mag-mount antenna SWR is very sensitive to position

Post by Arrow1 » Wed, 10 Sep 1997 04:00:00


>>Bet you a Dollar to a Donut you have a bad connection in
>>the antenna, or in the coax.
>If I had an open or short in the antenna or coax, I'd expect the SWR
>to be high on both VHF and UHF.  That's not the case.
>On UHF, the antenna has a low SWR which changes little no matter
>where I stick it on the car roof.  Also, running my hand along
>the feedline has almost no effect on SWR.  If I lift the antenna off
>the roof and hold it in mid-air, the SWR *does* rise because it doesn't
>have a decent ground plane any more.  All of this is in accordance with
>how I think a properly-operating unbalanced vertical should behave.
>On VHF, the SWR is also high when the antenna is held in mid-air, and
>it does drop somewhat when the antenna is stuck to the car roof, but
>not as low as I would like.  I can change the amount of reflected power
>indicated on the SWR meter by a factor of 3 by wrapping my hand around
>the feedline and moving it a few feet along the feedline length.  SWR
>also changes markedly when I move the antenna from one place to another
>on the roof.
>All of this suggests to me that the car roof acts as a good ground plane
>at UHF but an inadequate one at VHF.  And I suspect that *that* is due
>to insufficient capacitance between the antenna base and the steel roof.
>What is the feedline shield grounded to on a glass-mount antenna?  There
>must be a similar problem there.

Hello,  Dave
A glass-mount antenna is a 1/2 wave, it don't need a ground plane.
Where is the lowest SWR 144, 146 or 148 ?
If its 144 you need to shorten the antenna.
If its 148 you need to lengthen the antenna

What brand & model # antenna is it ?

Does it have a wire coil about in the center like cell phone
antenna?  If so can you replace it with a 19 1/4" piece of wire?
I don't think the problem is the Mag Mount, unless the magnet is
just to small.

Be interesting to see what you finely come up with.
73    Allen Lowe   N0IMW

 
 
 

Mag-mount antenna SWR is very sensitive to position

Post by Anthony R. Gol » Wed, 10 Sep 1997 04:00:00



> >Bet you a Dollar to a Donut you have a bad connection in
> >the antenna, or in the coax.

> If I had an open or short in the antenna or coax, I'd expect the SWR
> to be high on both VHF and UHF.  That's not the case.

> On UHF, the antenna has a low SWR which changes little no matter
> where I stick it on the car roof.

The impact of greater losses at UHF alone will create a lower feed
point VSWR.  A lower VSWR at the feed point at a far higher frequency
does not prove that you have a better coax to antenna match.

Regards,
--

 
 
 

Mag-mount antenna SWR is very sensitive to position

Post by Dave Martinda » Wed, 10 Sep 1997 04:00:00


>The impact of greater losses at UHF alone will create a lower feed
>point VSWR.  A lower VSWR at the feed point at a far higher frequency
>does not prove that you have a better coax to antenna match.

I do realize that.  But if I had a poor match at the antenna on UHF,
the SWR would still fluctuate with position on the roof and when I move
my hand along the feedline.  The reflected power would be less at UHF
than at VHF because of the losses, but the variation in SWR would still
be apparent.  And I do see UHF reflected power rise substantially when
I lift the antenna off the ground plane.  All this tells me that the
antenna is well matched to the feedline on UHF as long as it is mounted
on the ground plane.

On the other hand, on VHF the reflected power is high even when the
antenna is stuck to the car roof, and it is sensitive to mounting
position and the position of my hand along the feedline.

My point is not just that reflected power is higher at VHF, it is that
the behaviour of the antenna is different between UHF and VHF.

        Dave

 
 
 

Mag-mount antenna SWR is very sensitive to position

Post by Tom Rau » Thu, 11 Sep 1997 04:00:00

Very good answers Gary, and I'd like to add one more potential problem
with mag mounts.....


Wrote:

Quote:>Or it could be that you have a bad piece of coax. If one of the
>conductors is broken near the antenna, it would give the symptoms
>you are experiencing. The coax is a better dummy load at UHF,
>which would explain the lower *apparent* SWR reading on 70cm.

>And of course wiggling the broken cable would make a difference.
>Magmount cables typically take a beating because they are exposed
>to flapping in the wind (and get pinched in doors), so this wouldn't
>be unusual.

And have a peep inside a typical mag mount for a look at the poor
construction of most mounts. Most of them ground the shield of the
cable to the metal top of the mount, while having an adhesive backed
foil stuck on the bottom over the magnet.

Neither the adhesive backed foil OR the magnet are grounded very well,
if at all, metal base, which itself is often grounded poorly to the
shield. All the poor grounds cause the capacitance to ground problem
much worse than the square inches of foil area would ever lead you to
believe.

It's more amazing to NOT have common mode problems on the feedline
with a mag mount than it is to have them.

73 Tom

 
 
 

Mag-mount antenna SWR is very sensitive to position

Post by Dave Martindal » Fri, 12 Sep 1997 04:00:00


> The mag base having too little capacitance to the roof is a plausible
> explanation. Most mag bases have about 4 to 6 square inches of
> contact, and either have no insulating coating or a very thin one.
> This gives a very low impedance coupling to the roof (which is
> what you want for this type of antenna to operate properly).

This particular antenna (A Valor/Pro-Am MM270B) has less than one square
inch of mag base area.  The base is fully enclosed in ABS plastic, so I
can't tell how thick the base bottom (which forms the capacitor
dielectric)
is.  Also, the magnet or whatever forms the upper plate of the capacitor
must be a bit smaller than the base itself.  Whatever the details, it
must have a much smaller capacitance that something with 4 square inches
of metal area and only the car's paint as dielectric.

Quote:> The coax outer must be coupled to the roof by a low impedance at RF, the
> capacitance of the magnetic base serves this function.

That's what I thought.

Quote:> >       - What can I do in my situation?  Should I try grounding the
> >         feedline coax shield to the car frame at some point?  Does it
> >         matter much whether the ground is at the transceiver end, or
> >         about midway along the feedline?

> It matters extremely. The connection to the counterpoise must be at
> the antenna base. Connecting anywhere else will cause the feedline
> to radiate due to currents on the *outside* of the coax shield. This
> is not a good thing, and will seriously degrade antenna performance.

Unfortunately, I can't provide a better counterpoise connection at the
antenna end unless I do something drastic like cutting open the mag
base, or using a secondary larger magnet and connecting it to the coax
braid by cutting open the coax.  Neither is likely to be good for the
water resistance of the antenna.

So I had hoped that grounding the coax shield to the car frame somewhere
else would be better than leaving it completely isolated (except for the
minimal coupling provided by the mount capacitance).  You seem to be
saying that that is pointless - if I can't get a better counterpoise
ground *at the antenna*, nothing else will do any good.

Quote:> Or it could be that you have a bad piece of coax. If one of the
> conductors is broken near the antenna, it would give the symptoms
> you are experiencing. The coax is a better dummy load at UHF,
> which would explain the lower *apparent* SWR reading on 70cm.

But the UHF SWR is low and stable when the antenna is on the roof,
and rises when the antenna is lifted off the roof.  This suggests to
me that (a) the roof is working as a counterpoise at UHF, and (b)
when the antenna SWR is bad, I *can* still see it on the SWR meter.

The antenna is nearly new, so it hasn't developed a broken coax from
wear.  That doesn't mean it doesn't have a manufacturing defect, but
again I'd expect a coax failure to show up at UHF as well as VHF.

        Dave

 
 
 

Mag-mount antenna SWR is very sensitive to position

Post by Gary Coffm » Sat, 13 Sep 1997 04:00:00



>> The mag base having too little capacitance to the roof is a plausible
>> explanation. Most mag bases have about 4 to 6 square inches of
>> contact, and either have no insulating coating or a very thin one.
>> This gives a very low impedance coupling to the roof (which is
>> what you want for this type of antenna to operate properly).

>This particular antenna (A Valor/Pro-Am MM270B) has less than one square
>inch of mag base area.  The base is fully enclosed in ABS plastic, so I
>can't tell how thick the base bottom (which forms the capacitor
>dielectric)
>is.  Also, the magnet or whatever forms the upper plate of the capacitor
>must be a bit smaller than the base itself.  Whatever the details, it
>must have a much smaller capacitance that something with 4 square inches
>of metal area and only the car's paint as dielectric.

Yep, that's pretty small. You might want to change to a Larsen mount.
I *know* that supplies adequate capacitance.

- Show quoted text -

Quote:>> >       - What can I do in my situation?  Should I try grounding the
>> >         feedline coax shield to the car frame at some point?  Does it
>> >         matter much whether the ground is at the transceiver end, or
>> >         about midway along the feedline?

>> It matters extremely. The connection to the counterpoise must be at
>> the antenna base. Connecting anywhere else will cause the feedline
>> to radiate due to currents on the *outside* of the coax shield. This
>> is not a good thing, and will seriously degrade antenna performance.

>Unfortunately, I can't provide a better counterpoise connection at the
>antenna end unless I do something drastic like cutting open the mag
>base, or using a secondary larger magnet and connecting it to the coax
>braid by cutting open the coax.  Neither is likely to be good for the
>water resistance of the antenna.

>So I had hoped that grounding the coax shield to the car frame somewhere
>else would be better than leaving it completely isolated (except for the
>minimal coupling provided by the mount capacitance).  You seem to be
>saying that that is pointless - if I can't get a better counterpoise
>ground *at the antenna*, nothing else will do any good.

Nothing else will *be* good, IE you'll have a skewed pattern. You
could try grounding the coax a halfwave back from the mount, but
you *will* have feedline radiation. The better solution is to use a
magmount with adequate capacitance.

Quote:>> Or it could be that you have a bad piece of coax. If one of the
>> conductors is broken near the antenna, it would give the symptoms
>> you are experiencing. The coax is a better dummy load at UHF,
>> which would explain the lower *apparent* SWR reading on 70cm.

>But the UHF SWR is low and stable when the antenna is on the roof,
>and rises when the antenna is lifted off the roof.  This suggests to
>me that (a) the roof is working as a counterpoise at UHF, and (b)
>when the antenna SWR is bad, I *can* still see it on the SWR meter.

>The antenna is nearly new, so it hasn't developed a broken coax from
>wear.  That doesn't mean it doesn't have a manufacturing defect, but
>again I'd expect a coax failure to show up at UHF as well as VHF.

Remember that the coax itself will have capacitance to the roof. So
lifting it could still have an effect at UHF even if the shield is broken
in the mount. But of course the capacitance  isn't concentrated in the
right place, and it wouldn't be enough to make 2m work.

Gary
Gary Coffman KE4ZV          | You Make It  | Email:

534 Shannon Way             | Guaranteed   |         or

 
 
 

Mag-mount antenna SWR is very sensitive to position

Post by Cecil Moor » Sat, 13 Sep 1997 04:00:00


> So I had hoped that grounding the coax shield to the car frame somewhere
> else would be better than leaving it completely isolated (except for the
> minimal coupling provided by the mount capacitance).  You seem to be
> saying that that is pointless - if I can't get a better counterpoise
> ground *at the antenna*, nothing else will do any good.

Hi Dave, If Murphy has his way, grounding the coax "somewhere" will
result in a near-open-circuit at the base of the antenna which is
what happens with odd multiples of quarter wavelengths.
--
73, Cecil, W6RCA, OOTC  (not speaking for my employer)
 
 
 

Mag-mount antenna SWR is very sensitive to position

Post by Dave Martinda » Sat, 13 Sep 1997 04:00:00


>Hi Dave, If Murphy has his way, grounding the coax "somewhere" will
>result in a near-open-circuit at the base of the antenna which is
>what happens with odd multiples of quarter wavelengths.

I suppose I *could* get a decent capacitive ground like this:

        - Take a relatively large flat permanent magnet - the rotor
          of the motor from an old 8 inch floppy drive for example

        - Glue some sheet brass or copper to the underside of the
          magnet.  Glue some iron to the upper side of the magnet,
          if there isn't already some there.  Wrap a tab from the
          end of the brass or copper sheet up to the top of the
          magnet, above the iron.

        - Glue another piece of brass or copper to the base of
          the existing antenna mag mount.  Extend it out so it
          parallels the feedline.  Remove the outer jacket from
          the coax at that point, and solder the coax to the
          sheet metal.  Waterproof the joint.

        - Stick the large magnet to the car top, possibly with
          a thin sheet of plastic underneath it to prevent scratching.
          Then stick the small mag mount to the large magnet.

This way, the large magnet with its outer layer of brass or copper
would get good capacitive coupling to the car top, because of its
larger area.  The small mag mount ground would no longer depend on
capacitive coupling; it would have a metallic connection from the
feedline shield to the large magnet's conductive jacket.

But then, I'd still have to worry about corrosion of the copper
or brass.  Better make the sheet metal stainless steel.

I expect this would work, but it's probably a lot easier to just find
a larger commercial mag mount.  One real advantage is that a larger
mag mount will support a larger antenna, as well as (I hope) providing
decent coupling to ground.

        Dave