Mobile install, Trunk open: good SWR, trunk closed: bad SWR

[CanoeCanuck]

Hello! First time poster here. thought I'd seek advice for this issue I am troubleshooting. I am in the process of tuning a CB antenna to my car and I'm looking for any advice.

Vehicle: 2015 Honda Civic
Mounting Location: Center Trunk (Drilled 1/2'' hole)
Mount: Firestik K-154A Cowl
Coax: RG-8X 18'
Antenna: Wilson Silver Load FGT 3' Fiberglass
(Also tried Firestik Firefly 3' Fiberglass)
Radio: President Thomas

The issue is that the SWR is about 1.3 when the trunk is open, but 2.3+ when it's closed. I guess this is a ground plane issue?

I confirmed the trunk latch is not the problem, I completely removed the trunk latch, closed the trunk and still had 2.3+ SWR. I also completely disconnected the car battery to check if there was something electronic causing the issue. I tried pushing against the antenna with a spring installed while the was trunk closed to see if the angle was an issue, but it made no noticable difference.

I went inside the trunk and closed it to see if there is any kinks in the coax or anything obvious; not as far as I could tell. I do have two ground straps installed on each trunk hinge to the vehicle chassis, and confirmed good continuity. I also confirmed no continuity between the mount and center pin.

Note: I am using a NanoVNA to check the SWR, I find it a lot easier because I don't have to transmit every time to check. I do have a generic SWR meter and it is about the same readings. Some of the pictures I attached are in my garage but I am tuning in a wide open area.

So my question is, is there anything I can do to help the SWR when the trunk is closed without having to install a tuner? I find it strange that I get a good reading when it's open. The trunk is sitting on the rubber gasket when it's closed, only the trunk hinge has continuity with the rest of the car. I have a spring, but for now it is off because the SWR appears to be lower without the spring. Thanks for any advice.

 

[prcguy]

Most of the trunk lid perimeter is floating in rubber an insulated from the rest of the vehicle leaving a fairly small ground plane area. On opening the trunk lid and improving the match, it could be the whip is getting closer to the roof increasing capacitance between whip and ground simulating more ground plane.

I would try placing a 50pf disc ceramic capacitor across the hot and ground side of the mount to see what happens. If the problem is lack of ground plane and resulting lack of capacitance between whip and ground plane the capacitor will provide some missing capacitance and probably improve VSWR. If it improves things you can zero in on a value that will give you a perfect match. This will not improve performance, just VSWR.

As an easier test you can attach a 9ft length of wire to the top side, ground side of the mount and stretch it out horizontally. If that improves things it points to lack of ground plane.

 

[garys]

I would think that a ground strap from one of the trunk bolts to some solid metal in the body in the general area of the package shelf would help as well.

Most of the trunk lid perimeter is floating in rubber an insulated from the rest of the vehicle leaving a fairly small ground plane area. On opening the trunk lid and improving the match, it could be the whip is getting closer to the roof increasing capacitance between whip and ground simulating more ground plane.

I would try placing a 50pf disc ceramic capacitor across the hot and ground side of the mount to see what happens. If the problem is lack of ground plane and resulting lack of capacitance between whip and ground plane the capacitor will provide some missing capacitance and probably improve VSWR. If it improves things you can zero in on a value that will give you a perfect match. This will not improve performance, just VSWR.

As an easier test you can attach a 9ft length of wire to the top side, ground side of the mount and stretch it out horizontally. If that improves things it points to lack of ground plane.

 

[CanoeCanuck]

Most of the trunk lid perimeter is floating in rubber an insulated from the rest of the vehicle leaving a fairly small ground plane area. On opening the trunk lid and improving the match, it could be the whip is getting closer to the roof increasing capacitance between whip and ground simulating more ground plane.

I would try placing a 50pf disc ceramic capacitor across the hot and ground side of the mount to see what happens. If the problem is lack of ground plane and resulting lack of capacitance between whip and ground plane the capacitor will provide some missing capacitance and probably improve VSWR. If it improves things you can zero in on a value that will give you a perfect match. This will not improve performance, just VSWR.

As an easier test you can attach a 9ft length of wire to the top side, ground side of the mount and stretch it out horizontally. If that improves things it points to lack of ground plane.

Thank you @prcguy. I'll look into the 50pf disc ceramic capacitor. First, I will try the 9ft wire test on the ground side and top side of the mount. This Wilson antenna in particular has a impedance wire which has continuity with the center pin. Should I connect the 9ft wire to that or go straight from the mount and ignore the impedance wire?

 

[prcguy]

Thank you @prcguy. I'll look into the 50pf disc ceramic capacitor. First, I will try the 9ft wire test on the ground side and top side of the mount. This Wilson antenna in particular has a impedance wire which has continuity with the center pin. Should I connect the 9ft wire to that or go straight from the mount and ignore the impedance wire?

The 9ft wire test should go to the mount ground. What is the "impedance wire" and what are you supposed to do with it? Some HF antennas have a shunt coil in the base for matching and you must ground a point on the antenna above the mount. Could that be what the impedance wire is?

 

[wtp]

i would put the ground strap to the trunk and not its bracket.

 

[CanoeCanuck]

The 9ft wire test should go to the mount ground. What is the "impedance wire" and what are you supposed to do with it? Some HF antennas have a shunt coil in the base for matching and you must ground a point on the antenna above the mount. Could that be what the impedance wire is?

I think that is what it is. In the sales listing it was called an impedance wire. The official website calls it a Unique ground lead to insure reduced static ignition interference. Either way I'll get a wire test going from the mount ground and report back with results. Thank you very much for the suggestions.

 

[CanoeCanuck]

i would put the ground strap to the trunk and not its bracket.

I thought about that, but I read that the ground strap should be as short as possible. At one point I did Daisy Chain a couple ground straps directly from the mount to the vehicle chassis but it didn't seem to make any difference to the SWR. Any ideas which part of the trunk other than the bracket would be a good place for the ground strap?

 

[wtp]

i worked at a place that had 800mhz stuff.
it was under the bracket, but i could see where they sanded down to the metal there and on the chassis.
metal to metal means not having paint in between.
and if i recall they had star lock washers under there too.
i just took a closer look at your pic, did you sand the metal ?

 

[CanoeCanuck]

i worked at a place that had 800mhz stuff.
it was under the bracket, but i could see where they sanded down to the metal there and on the chassis.
metal to metal means not having paint in between.
and if i recall they had star lock washers under there too.
i just took a closer look at your pic, did you sand the metal ?

Yes, it is not very visible on my pictures but on every connection I did sand it down to bare metal with my Dremel tool and a wire brush. Though I did not check the stock connections between the trunk hinge and the trunk itself, maybe I need to disconnect it, sand it down and re-connect. I did test continuity and it was good though.

 

[prcguy]

i worked at a place that had 800mhz stuff.
it was under the bracket, but i could see where they sanded down to the metal there and on the chassis.
metal to metal means not having paint in between.
and if i recall they had star lock washers under there too.
i just took a closer look at your pic, did you sand the metal ?

Grounding the underside of the trunk lid probably doesn't do much to improve the ground plane. RF currents flow from the antenna feedpoint out over the ground plane or radials (or mostly insulated trunk lid) and bounce back and forth over the top surface. When they reach a discontinuity or edge they bounce back towards the source or refract off the edge, so they may never know the ground strap is connected on the back side. This is somewhat frequency dependent with more bouncing around the trunk lid at UHF and higher vs CB or HF.

 

[wtp]

and the shop had even put a strap on the hood.
i never did check the doors.
yes that would be over the top, but would be very through.

 

[slowmover]

Grounding the underside of the trunk lid probably doesn't do much to improve the ground plane. RF currents flow from the antenna feedpoint out over the ground plane or radials (or mostly insulated trunk lid) and bounce back and forth over the top surface. When they reach a discontinuity or edge they bounce back towards the source or refract off the edge, so they may never know the ground strap is connected on the back side. This is somewhat frequency dependent with more bouncing around the trunk lid at UHF and higher vs CB or HF.

My approach to this with a roof antenna re the engine hood of my pickup was to RF Bond both hinges then to replicate what I’d seen on cars decades back for AM Radio antenna performance:

Coiled some 1” W woven braid back on itself and attached to radiator crossmember adjunct to the hood latch both sides. Then similar above on hood underside.

These loops contacted each other on hood closure. (Does it work? Hellifino. Seemed worth a few minutes work).

— Might be able to replicate without permanent install for testing.

Daisy-chain down to frame is the other. Body panels aren’t necessarily “bonded”. Short runs (a foot or so each; item to item).

K0bg on Bonding

.

 

[prcguy]

My approach to this with a roof antenna re the engine hood of my pickup was to RF Bond both hinges then to replicate what I’d seen on cars decades back for AM Radio antenna performance:

Coiled some 1” W woven braid back on itself and attached to radiator crossmember adjunct to the hood latch both sides. Then similar above on hood underside.

These loops contacted each other on hood closure. (Does it work? Hellifino. Seemed worth a few minutes work).

— Might be able to replicate without permanent install for testing.

Daisy-chain down to frame is the other. Body panels aren’t necessarily “bonded”. Short runs (a foot or so each; item to item).

K0bg on Bonding

.

A ground plane for a CB antenna on a vehicle is purely an outside metal surface thing. If you can bond a hood or a trunk lid at the very edge of the lid to the very edge of the adjacent external sheet metal part then RF currents can flow on the outside surfaces and you might have extended the ground plane. Placing wide braid from the inside of a hood to the inside of the engine compartment or inside of the trunk lid to the inside of the trunk makes a great DC ground but the antenna may never see it and it probably won't add to the ground plane or benefit antenna performance.

If you can't see the grounding braid from outside the vehicle from the antennas point of view then the antenna probably can't either. For reducing engine noise a braid across the hood hinges inside the engine compartment can help keep the hood from re-radiating spark plug noise to the outside because the braid is on the side of the hood where RF currents are flowing from radiated spark plug wires and it will help make the hood a shield instead of a passive radiator.

 

[CanoeCanuck]

The 9ft wire test should go to the mount ground. What is the "impedance wire" and what are you supposed to do with it? Some HF antennas have a shunt coil in the base for matching and you must ground a point on the antenna above the mount. Could that be what the impedance wire is?

Hi @prcguy I went ahead and did the wire test as suggested, thank you. I did confirm lower SWR with the wire installed, but still a .5 difference with trunk open vs closed, not as big of a gap as before! With trunk closed, SWR read about 1.5, with trunk open SWR had a nice 1.068 reading. Attached some pictures here of my testing. I guess this confirms that the ground plane is the issue. Any suggestions for a permanent fix? With the wire attached it's great, but I don't think I can drive around with it unless I make a proper connector. For the test I just added a makeshift connection wedged below the nylon washer.

 

[prcguy]

Hi @prcguy I went ahead and did the wire test as suggested, thank you. I did confirm lower SWR with the wire installed, but still a .5 difference with trunk open vs closed, not as big of a gap as before! With trunk closed, SWR read about 1.5, with trunk open SWR had a nice 1.068 reading. Attached some pictures here of my testing. I guess this confirms that the ground plane is the issue. Any suggestions for a permanent fix? With the wire attached it's great, but I don't think I can drive around with it unless I make a proper connector. For the test I just added a makeshift connection wedged below the nylon washer.

Its lack of ground plane that is the issue. I see a couple of options for you.

1. Pull a trailer with something to hold up the 9ft of wire trailing behind you.
2. If there is an "impedance wire" sticking out of the whip that can reach the ground side of the mount, ground it and see if it improves VSWR.
3. Try the 50pf capacitor across the mount.
4. Find a way to bond the outside of the trunk lid to the surrounding EXTERNAL sheet metal skin of the car, without it being too obvious.

 

[CanoeCanuck]

Its lack of ground plane that is the issue. I see a couple of options for you.

1. Pull a trailer with something to hold up the 9ft of wire trailing behind you.
2. If there is an "impedance wire" sticking out of the whip that can reach the ground side of the mount, ground it and see if it improves VSWR.
3. Try the 50pf capacitor across the mount.
4. Find a way to bond the outside of the trunk lid to the surrounding EXTERNAL sheet metal skin of the car, without it being too obvious.

Understood, thank you very much for the advice.
1. If I ever need need trailer and still have SWR issues perhaps I'll give this a go, why not.
2. Tried attaching the wire sticking out of the whip to various ground locations, it actually made the SWR higher so I'll leave it out for now. I had to extend it with another piece of wire to get it anywhere other than the top side of the mount. Looks like I can rule this one out.
3. I just placed an order on eBay for some ceramic 50pf capacitor. This will be a learning curve for me but I'll give it a shot.
4. I tried to run a ground strap from the top side of the mount on the grounded side, and wrapped it into the trunk on an existing bolt. Didn't do much but it was worth a try. Aside from this I don't really see a way to externally strap the mount to the other external sheet metal, without drilling into the body. I think I'll have to rule this one out too.

That leaves me with #3 being next option. Once I get the ceramic capacitors I'll see what I can put together. I assume the capacitor will be on the top side of the mount, and no connection needs to be made to the ground inside the trunk? I attached a picture just to confirm where it should go. Thanks again for all the help I have learned a lot this weekend.

 

[prcguy]

Understood, thank you very much for the advice.
1. If I ever need need trailer and still have SWR issues perhaps I'll give this a go, why not.
2. Tried attaching the wire sticking out of the whip to various ground locations, it actually made the SWR higher so I'll leave it out for now. I had to extend it with another piece of wire to get it anywhere other than the top side of the mount. Looks like I can rule this one out.
3. I just placed an order on eBay for some ceramic 50pf capacitor. This will be a learning curve for me but I'll give it a shot.
4. I tried to run a ground strap from the top side of the mount on the grounded side, and wrapped it into the trunk on an existing bolt. Didn't do much but it was worth a try. Aside from this I don't really see a way to externally strap the mount to the other external sheet metal, without drilling into the body. I think I'll have to rule this one out too.

That leaves me with #3 being next option. Once I get the ceramic capacitors I'll see what I can put together. I assume the capacitor will be on the top side of the mount, and no connection needs to be made to the ground inside the trunk? I attached a picture just to confirm where it should go. Thanks again for all the help I have learned a lot this weekend.

If you add a capacitor to the mount it can be on the top side or inside the trunk. It just has to be at the very end of the coax at the mount across hot and ground.

 

[slowmover]

A ground plane for a CB antenna on a vehicle is purely an outside metal surface thing. If you can bond a hood or a trunk lid at the very edge of the lid to the very edge of the adjacent external sheet metal part then RF currents can flow on the outside surfaces and you might have extended the ground plane. Placing wide braid from the inside of a hood to the inside of the engine compartment or inside of the trunk lid to the inside of the trunk makes a great DC ground but the antenna may never see it and it probably won't add to the ground plane or benefit antenna performance.

If you can't see the grounding braid from outside the vehicle from the antennas point of view then the antenna probably can't either. For reducing engine noise a braid across the hood hinges inside the engine compartment can help keep the hood from re-radiating spark plug noise to the outside because the braid is on the side of the hood where RF currents are flowing from radiated spark plug wires and it will help make the hood a shield instead of a passive radiator.

Got it. Thx!

 

[n7lrg]

Make sure you are using a star washers to ensure good ground contacts.