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Author Topic: High SWR when antenna mount is grounded  (Read 4963 times)
KJ4OBR
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Posts: 104




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« on: April 26, 2010, 11:42:38 AM »

So I got some new toys at the local hamfest.  Stainless steel NMO bracket for the trunk  seam to replace the lipmount NMO, and an NMO connector/coax to go in it. SWR on the lip mount for the Maldol EX-510 antenna is acceptable between 1:1.7 and about 1:1.9 depending on the frequency being checked.

I mount everything up on the new bracket and bolt it to the side of the car trunk (checking for ground, star washers etc) and check the SWR and it is off scale on 2 meters and 70cm and hovering around 1:3 at 6 meters. I checked all the connections and tested for shorts etc everything looks fine. I unbolt the bracket to remount the lip mount to think things through and for grins do a quick check of SWR in the new bracket but NOT bolted to ground and my SWR is back down below 1:2 across all bands.

I figured I need to do some more bonding, but the lower SWR without the bracket being grounded has thrown me. What am I missing?  Huh
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K0BG
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« Reply #1 on: April 26, 2010, 11:55:03 AM »

The input impedance of a mobile antenna is made up of the ground losses, the resistive losses, and the radiation resistance. What's more, they're in series; change one, you essentially change them all. I suspect the problem is a change in ground losses as the antenna remained the same. In any respect, mag, clip, and bracket mounts aren't the stuff of champions. If the antenna's mount is a true NMO (new motorola), and not one of the Pacific Rim knockoffs, do yourself a favor; drill a hole in the trunk!
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KJ4OBR
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« Reply #2 on: April 26, 2010, 12:16:45 PM »

I'm trying to get the antenna off the trunk lid as it causes issues with opening and closing the trunk (I open and close it multiple times every day) and the roof is taken up by the power sunroof. I'm not an anti drill zealot. If I COULD drill I would.. I cannot. The next best compromise option for MY situation is a tab bracket similar to what you see on police and other emergency vehicles. I appreciate and understand your description of the problem, but I still lack a direction to move in to solve the problem.

73


The input impedance of a mobile antenna is made up of the ground losses, the resistive losses, and the radiation resistance. What's more, they're in series; change one, you essentially change them all. I suspect the problem is a change in ground losses as the antenna remained the same. In any respect, mag, clip, and bracket mounts aren't the stuff of champions. If the antenna's mount is a true NMO (new motorola), and not one of the Pacific Rim knockoffs, do yourself a favor; drill a hole in the trunk!
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K0BG
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« Reply #3 on: April 26, 2010, 04:51:08 PM »

It's not. If you properly place the antenna on the trunk lid, it will clear when you open the trunk lid. In some cases, GM particular, the whip portion may indeed hit the roof. This shouldn't be a problem unless you "snap open" open the trunk every time.

By the way, I'd like to see an example of a clip or 'tab' mount on a public service vehicle. If you can come up with one, I promise to write the entity a nastygram!
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WB2WIK
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« Reply #4 on: April 26, 2010, 05:31:30 PM »

I'm trying to get the antenna off the trunk lid as it causes issues with opening and closing the trunk (I open and close it multiple times every day) and the roof is taken up by the power sunroof. I'm not an anti drill zealot. If I COULD drill I would.. I cannot. The next best compromise option for MY situation is a tab bracket similar to what you see on police and other emergency vehicles. I appreciate and understand your description of the problem, but I still lack a direction to move in to solve the problem.

73


The input impedance of a mobile antenna is made up of the ground losses, the resistive losses, and the radiation resistance. What's more, they're in series; change one, you essentially change them all. I suspect the problem is a change in ground losses as the antenna remained the same. In any respect, mag, clip, and bracket mounts aren't the stuff of champions. If the antenna's mount is a true NMO (new motorola), and not one of the Pacific Rim knockoffs, do yourself a favor; drill a hole in the trunk!

What model vehicle is that where the sunroof takes up the whole roof?

I've had sunroofs on most of my vehicles and none of them occupy the entire roof, including the retraction area.  There's always space either fore or aft of the sunroof.
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KL2GN
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« Reply #5 on: April 26, 2010, 06:14:02 PM »


When you say that you checked the SWR on the new mount after unbolting it - How did you check it?  Were you holding it?  was it sitting on the vehicle? ground?

And having worked on a number of emergency vehicles and installed countless mobile systems - we always drill a hole - usually multiples!

KL2GN
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KJ4OBR
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Posts: 104




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« Reply #6 on: April 26, 2010, 06:49:28 PM »

KOBG,

You need to write your letters to
1. Raleigh NC PD
2. Durham NC PD
3. Garner NC PD
4. North Carolina State Highway Patrol
5. Wake County NC SO
6. Colorado Springs Colorado PD
7. Denver Colorado PD
8. Colorado State Patrol
9. Reno NV PD
10. Minneapolis PD

Those are the agencies I can personally attest to having seen the trunk tabs being used for antennas. I was a TV News photographer for many years and have interacted with LEO's a lot. If you still do not believe me I'll be happy to go take pics of my wife's cousin's patrol car. He's a trooper for the NCSHP. They also have drilled trunk mounted NMO and I'm not saying at all that that is a bad option at all I would do that if it solved my problem. Like I said earlier I understand and accept this is a compromise location.

WB2WIK:
 On the 99 Audi A6 the sunroof buries back in the roof almost all the way to the rear window with wiring for the rear defrosters and stereo antenna run through there as well. The rear seat has a separate dome/ reading light console in what little space is left. Audi also has a very short trunk that opens past 90 degrees. BELIEVE me when I say that the side tab is the only option that will work. Again I WISH I COULD DRILL!  Now the question remains how do I start to figure this SWR issue out given the situation I have to work with.

KL2GN:
It was sitting in the trunk crease in the location I wanted it, I confirmed there was no ground with a meter (really thick paint)  and the coax disconnected. BTW yes there are often many holes and tabs on the emergency vehicles I am talking about.

73


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KL2GN
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« Reply #7 on: April 26, 2010, 08:58:21 PM »


David-

Thanks for the quick reply - Just trying to get a visual of how you were testing/checking?
I assume from what you said you left the antenna attached to the mount (and this is one of the small L bracket style seam/crease mounts?), unscrewed or unbolted the mount from the vehicle but left it sitting in the crease next to the trunk where you had it mounted and want to have it mounted when you checked the SWR and everything looked great?
But when you had it bolted in the SWR was high in your freq. range of interest?

You mentioned that you disconnected the coax?  Are you using an SWR meter inline, the radio, or antenna analyzer to check the setup?  want to make sure I have the variables accurate.

Thanks
KL2GN
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KJ4OBR
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« Reply #8 on: April 27, 2010, 06:19:16 AM »

Testing methodology:

The NMO was in the mount, the mount was NOT connected to the car, just set in place. I disconnected the coax to check for electrical gound (none). tested the coax center and shield for shorts (none) and continuity center to center and shield to outer threads of NMO (ok). Reconnected the SWR meter inline and did readings as you would expect.

At this point I'm half tempted to mill some Derlin insulators and isolate the NMO from the bracket and see what that does for me.




David-

Thanks for the quick reply - Just trying to get a visual of how you were testing/checking?
I assume from what you said you left the antenna attached to the mount (and this is one of the small L bracket style seam/crease mounts?), unscrewed or unbolted the mount from the vehicle but left it sitting in the crease next to the trunk where you had it mounted and want to have it mounted when you checked the SWR and everything looked great?
But when you had it bolted in the SWR was high in your freq. range of interest?

You mentioned that you disconnected the coax?  Are you using an SWR meter inline, the radio, or antenna analyzer to check the setup?  want to make sure I have the variables accurate.

Thanks
KL2GN

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K0BG
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« Reply #9 on: April 27, 2010, 06:29:32 AM »

David, everyone has their own justification for not installing antennas correctly, mobile or otherwise. You can find examples of poor mounting techniques everywhere, but that fact along doesn't justify the act. It just means others don't understand how antennas behave.

In the case of verticals, there must be an adequate ground plane under them, and in this case you're using the trunk lid. At VHF and above, the trunk lid is the second choice location (the roof being number one). But... you have to assure a decent connection between the coax shield, and the trunk lid which is acting as the ground plane for the antenna. Please note, we're not talking about DC ground. We're talking about ground plane. Similar name, different animal. When this connection is inadequate, all sorts of maladies occur. While clip mounts can be used effectively, they require a lot of maintenance to keep them working correctly.

On the other hand, a properly installed NMO mount is virtually maintenance-free, but does require the drilling of a hole. Apparently, you've convinced yourself this is a bad idea, without explaining why. If that reason is vehicle devaluation, then here's a suggestion. Go to my web site, and bring up the Antenna Mounts article, and look at the first photo.
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KJ4OBR
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Posts: 104




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« Reply #10 on: April 27, 2010, 08:29:43 AM »

I respect your knowledge and all along have acknowledged that this is a compromise location. Why is that not good enough?

I have been to your web site, it helped me solve some RFI problems. I also understand the difference between DC ground and ground plane. I really wish you would read my message again. I would LOVE to drill.. in my case I cannot because of the way that that Audi over-engineered the car.  Why do I have to keep explaining?

One more time!

I cannot put it on the trunk because mechanically it will beat the antenna and the roof as the trunk is opened and closed. the temporary trunk lip mount has shown that in spades. Perhaps I should load the trunk by removing the car seats and folding the seat down 4+ times a day? There is NOT a good location on the roof for a hole! I cannot make it clearer!

The car has 166,000 miles, paid for and I'm planning to drive it into the dirt. Devaluation because of a hole is the LEAST of my worries. You can harp about it all day long and it will not change my options! I'm not looking to DX 99% of what I do is local repeater.

I asked the question hoping for help, not to get berated for trying to work with the situation I have. Your suggestion is the best, but IT WILL NOT WORK ON A 99 AUDI A6!  Do you have any further suggestions for me to actually help solve the problem or are you going to remain hung up on criticizing?

It seems my mistake was asking for help here. I'll go away now. You can rest soundly in the fact that you have left a sour taste in a new HAM's mouth about asking for help.

Dave


Apparently, you've convinced yourself this is a bad idea, without explaining why. If that reason is vehicle devaluation, then here's a suggestion. Go to my web site, and bring up the Antenna Mounts article, and look at the first photo.
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WB6BYU
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« Reply #11 on: April 27, 2010, 12:53:49 PM »

I'm going to make a guess here:  did you have to adjust the antenna to resonance when
you first put it on the existing mount?

If the mount isn't grounded then the coax braid is acting as the ground plane for your
antenna.  The actual load impedance and resonant frequency depends on the length
of the coax.  The effect on the antenna can be quite different when the base of the
antenna mount is grounded.    Whether this would account for the large difference in
SWR is difficult to tell.

(Are you grounding the mount to the trunk lid or to the body of the car - that is, does
the antenna move when you open the trunk, or does it stay in place? That can make a
big difference, too, as the trunk lid makes a better ground plane.)

My approach would be to try a simple quarter wave whip on the mount and see what
the resulting SWR is.  The bandwidth should be quite broad, and you should be able
to get the SWR below 2 : 1 quite easily.  I've just stuck a piece of brazing rod or stiff
copper wire in place of the stock antenna in an NMO base to do this.  You can always
start with the antenna a bit long and trim it half an inch at a time to see if it tunes
up as you would expect.  Unfortunately I can't tell whether you can do this with the
base from the EX-510 or not.

Remember that simply being connected to a piece of metal does not guarantee a good
RF ground plane for an VHF antenna.  The shape of the metal makes a big difference,
both in SWR and antenna performance.

If you can tune the grounded mount with a simple quarter wave 2m antenna, I'd look
at the instructions for your EX-510 to see how to adjust the tuning.  If you can't get
the base to work with a quarter wave whip, there likely is something wrong with your
base and/or the coax connection.  But that would be a simple way to narrow down the
source of the difference.
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K0BG
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« Reply #12 on: April 27, 2010, 02:24:15 PM »

Okay, David, I read it again, as well as your other responses. Here's something to consider.

Pacific Rim NMO mounts (Maldol, Diamond, Comet, etc.) are NOT directly compatible with real, honest-to-john, American made, NMO mounts (Larsen, et. al.). If there was no connection between the center pin as it were, and the antenna radiator itself, the SWR would be off the scale. Yet, if the coax cable was just the right length (!), the SWR on 6 could be rather low.

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KJ4OBR
Member

Posts: 104




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« Reply #13 on: April 27, 2010, 04:06:07 PM »

First, thank you for your help. After reading your website I was aware of the incompatibilities between the mounts. I'm using a Comet Maldol antenna and a Comet Maldol NMO mount. One would think they would be designed to work together.

I have tested for center pin connectivity with a meter and a sprinkle of baby powder as a visual double check. BTW I had 2 2" bonding straps from the car body to trunk lid already. One running from the bracket directly, the other on the opposite side of the trunk.

The coax length may well be my next experiment. If I understand it right it is possible to be lucky enough that the coax is just the correct length to generate high SWR as well as low. The coax of the new mount is about 2 feet shorter that the coax on the lip mount that shows good readings.


Okay, David, I read it again, as well as your other responses. Here's something to consider.

Pacific Rim NMO mounts (Maldol, Diamond, Comet, etc.) are NOT directly compatible with real, honest-to-john, American made, NMO mounts (Larsen, et. al.). If there was no connection between the center pin as it were, and the antenna radiator itself, the SWR would be off the scale. Yet, if the coax cable was just the right length (!), the SWR on 6 could be rather low.


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KJ4OBR
Member

Posts: 104




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« Reply #14 on: April 27, 2010, 04:34:38 PM »

The new mount has the antenna not moving as the trunk lid opens alongside it.

I tested the new antenna first thing on the old mount and ran it there for about a month until I could get to a bending brake to match the angles of the trunk edge on the new bracket. It was between 1:1.5 and 1:1.8 across 2m, 70cm, and 6m depending on what end of each band I was testing so it was pretty much OK out of the wrapping (verified electrical grounding between mount, and trunk lid and chassis).

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. ( I have 2 2" bonding straps from the body to the trunk lid from before and I moved one of them directly to the mount after finding the current problem).

 I like your idea of using brazing wire to see if I can get things heading in the correct direction. I'm sure the coax is acting as the ground plane at the moment. 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.

THANKS!!
73 Dave


I'm going to make a guess here:  did you have to adjust the antenna to resonance when
you first put it on the existing mount?

If the mount isn't grounded then the coax braid is acting as the ground plane for your
antenna.  The actual load impedance and resonant frequency depends on the length
of the coax.  The effect on the antenna can be quite different when the base of the
antenna mount is grounded.    Whether this would account for the large difference in
SWR is difficult to tell.

(Are you grounding the mount to the trunk lid or to the body of the car - that is, does
the antenna move when you open the trunk, or does it stay in place? That can make a
big difference, too, as the trunk lid makes a better ground plane.)

My approach would be to try a simple quarter wave whip on the mount and see what
the resulting SWR is.  The bandwidth should be quite broad, and you should be able
to get the SWR below 2 : 1 quite easily.  I've just stuck a piece of brazing rod or stiff
copper wire in place of the stock antenna in an NMO base to do this.  You can always
start with the antenna a bit long and trim it half an inch at a time to see if it tunes
up as you would expect.  Unfortunately I can't tell whether you can do this with the
base from the EX-510 or not.

Remember that simply being connected to a piece of metal does not guarantee a good
RF ground plane for an VHF antenna.  The shape of the metal makes a big difference,
both in SWR and antenna performance.

If you can tune the grounded mount with a simple quarter wave 2m antenna, I'd look
at the instructions for your EX-510 to see how to adjust the tuning.  If you can't get
the base to work with a quarter wave whip, there likely is something wrong with your
base and/or the coax connection.  But that would be a simple way to narrow down the
source of the difference.
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