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Mobile swr went from 2.0 to 1.0 with ice on car

c316buckeye

Active Member
Jul 2, 2023
51
79
28
-small sedan
-trimag mount
-tram bigcat antenna
-lmr400 straight down through roof to radio
-best swr i could get out of 3 antennas was 2.0
-2 inches of snow/ice on roof and swr is now flat.
This goes against what i am researching, as snow, ice, salt, etc usually raise swr. several of us are stumped as to exactly why, or what i need to do to achieve that 1.0swr without snow and ice. the hole is drilled, the mount will be staying, no leaks whatsoever. if reflectivity is a problem, would rubber undercoating paint on the roof help? there is no regard for the looks of the vehicle compared to the desired low swr. many thanks for ANY responses
 
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I do not believe adding a rubber coating to the roof would be of any benefit. It would only increase the distance between the metal in the roof and the foil under the magnets that capacitively couples the vehicle body to antenna ground. Since you already have a hole drilled, adding a ground strap from the mag mount to the roof might help, but it should be as close to the base of the antenna as possible with as short of a strap as possible.

Although I do not have enough info to say that your problem is with the grounding, it is always good to make sure that your grounding method is effective before trying to make adjustments elsewhere. Not all mag mounts are created equal. Sometimes the foils under the magnet are not connected to the antenna ground and the little the foil edges overlap with the magnet housings is not enough for the RF to capacitively couple to the vehicle body. I've used copper foil tape on the roof before, but since you are not against holes, just add a strap.

Was the ice on the antenna too, or just the roof? I would assume that ice on an antenna would be like adding a dielectric coating to the antenna, which should make it electrically longer due to the slightly slower velocity of propagation. I am going to assume that you previously tried making the antenna longer and the dip wasn't getting the SWR that low, so the answer probably lies with making the antenna electrically longer while somehow compensating for the effect of the increased distributed capacitance to ground.

In my experience, running a short antenna on a mag mount usually involves series and shunt inductance, but your antenna only has series inductance. If you had an antenna analyzer or nanoVNA. it could tell us exactly what is happening.

I would recommend getting a cheap nanoVNA and an SO-239 female to SMA male adapter. Then, using a pair of PL-259's from old scrap radios (or buy them), make up some calibration loads for the other end of the coax ~ one as a short circuit and the other as a 50Ω load (these will be used to calibrate out the effects of the coax cable so the VNA reads on the antenna end of the cable rather than at its port). The calibration procedure also uses an open load, but at 27MHz, its not worth attaching, just leave the end open and hit the button. You can calibrate at the VNA port with the included SMA calibration loads if your coax is a half wave long at 27MHz (or you measure it and take that into account), but its easier to make some junk box calibration loads. Once you have done that, connect the coax to the antenna and adjust the antenna length so that the impedance at 27MHz is on the constant conductance circle that goes up to the center of the chart (its the line that starts at the center and goes down and curves to the left). Once it is on that line somewhere, the distance that must be traveled along that line to the center represents the value of shunt inductor that you would solder between the feed point and ground to get your match.
 
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that's some very valuable information. i was considering applying a coating without removing the trimag, as to not interrupt it's connection with the car. my line of thinking was if the roof wasn't reflective, it would help. the snow and ice covered the roof and mount, but the antenna was clean and clear. the power supply is grounded with 8ga back to the battery, i had considered making a closer ground to the subframe. not sure if that would help anything, but i also want to stay simply on the topic. the snow and ice were the only new introduction. i can add a ground strap to the roof and trimag. would it be better to run a solid ground to the chassis? the paint will be ground off whichever is better, but i would go with what you suggested about keeping it as short as possible. i couldn't identify a nanoVNA in a lineup, but can borrow an analyzer. thank you for your time and information.
 
I'm trying to understand this.. you have a mag mount but drilled a hole in the roof for the coax? If drilling your vehicle is of no concern why not ditch the mag mount entirely. Nothing against "Tram" but if it were me I would run a quality permanent mount..(breedlove, sirio, daimond/comet) make sure the metal on the underside of the roof is roughed up but clean. The mount should see this as a decent counter poise. Get a sirio flavor antenna for new mount...tune and enjoy.
 
that's some very valuable information. i was considering applying a coating without removing the trimag, as to not interrupt it's connection with the car. my line of thinking was if the roof wasn't reflective, it would help. the snow and ice covered the roof and mount, but the antenna was clean and clear. the power supply is grounded with 8ga back to the battery, i had considered making a closer ground to the subframe. not sure if that would help anything, but i also want to stay simply on the topic. the snow and ice were the only new introduction. i can add a ground strap to the roof and trimag. would it be better to run a solid ground to the chassis? the paint will be ground off whichever is better, but i would go with what you suggested about keeping it as short as possible. i couldn't identify a nanoVNA in a lineup, but can borrow an analyzer. thank you for your time and information.
Think of grounding the antenna as giving it a counterpoise, not a path back to the battery. Where the battery is has nothing to do with the RF that comes out of the coax. Where that coax shield connects to the base of the antenna is where the counterpoise begins, so running a ground wire anywhere else is just adding an inductor in line with the actual counterpoise.

The car body is essentially the other half of a dipole, and you wouldn't detour the connection to one side of a dipole with a random chunk of wire, so why detour it in a mobile setup? Running long ground wires to the frame instead of the closest contact with the counterpoise conductor is begging for common-mode currents that make your dash cluster blink.

The roof being metal (reflective) is a good thing and coating it with rubber will not change anything. The fact that ice on the roof alone (and not on the antenna) made the SWR better would make me think there is a grounding or connectivity issue. The vehicle may have had a coating of salt from the highway and in between the mag mount and roof was salty water that made the ground connection better.

Add a short wide copper strap between the mag mount and roof metal, making sure to remove paint and oxides. A little conductive grease (or a star washer and regular silicone dielectric grease) will make sure the connection stays good for a while.
 
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I'm trying to understand this.. you have a mag mount but drilled a hole in the roof for the coax? If drilling your vehicle is of no concern why not ditch the mag mount entirely. Nothing against "Tram" but if it were me I would run a quality permanent mount..(breedlove, sirio, daimond/comet) make sure the metal on the underside of the roof is roughed up but clean. The mount should see this as a decent counter poise. Get a sirio flavor antenna for new mount...tune and enjoy.
Many of the later model vehicles that still have metal roofs are so flimsy that the magnets provide the surface area needed to not immediately bend the sheet metal. If one has to go find a metal plate or buy a new mount of equal surface area to achieve the same goal, why not let the magnets do the work? You also need to remove the head liner unless you want to ground externally and bolt through it.
 
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I'm trying to understand this.. you have a mag mount but drilled a hole in the roof for the coax? If drilling your vehicle is of no concern why not ditch the mag mount entirely. Nothing against "Tram" but if it were me I would run a quality permanent mount..(breedlove, sirio, daimond/comet) make sure the metal on the underside of the roof is roughed up but clean. The mount should see this as a decent counter poise. Get a sirio flavor antenna for new mount...tune and enjoy.
i never considered or thought about a roof mounted antenna. used to having a mirror bracket or mag mount for open coil antenna to withstand 70mph + wind speed. thanks for the input.
 
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Many of the later model vehicles that still have metal roofs are so flimsy that the magnets provide the surface area needed to not immediately bend the sheet metal. If one has to go find a metal plate or buy a new mount of equal surface area to achieve the same goal, why not let the magnets do the work? You also need to remove the head liner unless you want to ground externally and bolt through it.
I have installed a few perm mounts in roofs and ball mounts in quarter panels back in the day. I ran into the flimsy roof issue as well. This was sloved with a large washer on the underside of the roof and in one case we actually used a circular saw blade & washer to distribute flexing strain over much more surface area of the roof. In all installations the permanent mount (when done correctly) outperformed a mag mount. It tends get out slightly better, receive cleaner and not give near the swr issues that mag mounts exhibit when running higher power.

My .02....your mileage my vary.
 
Think of grounding the antenna as giving it a counterpoise, not a path back to the battery. Where the battery is has nothing to do with the RF that comes out of the coax. Where that coax shield connects to the base of the antenna is where the counterpoise begins, so running a ground wire anywhere else is just adding an inductor in line with the actual counterpoise.

The car body is essentially the other half of a dipole, and you wouldn't detour the connection to one side of a dipole with a random chunk of wire, so why detour it in a mobile setup? Running long ground wires to the frame instead of the closest contact with the counterpoise conductor is begging for common-mode currents that make your dash cluster blink.

The roof being metal (reflective) is a good thing and coating it with rubber will not change anything. The fact that ice on the roof alone (and not on the antenna) made the SWR better would make me think there is a grounding or connectivity issue. The vehicle may have had a coating of salt from the highway and in between the mag mount and roof was salty water that made the ground connection better.

Add a short wide copper strap between the mag mount and roof metal, making sure to remove paint and oxides. A little conductive grease (or a star washer and regular silicone dielectric grease) will make sure the connection stays good for a while.
thankfully, no cluster blinking at this time. when the trimag and coax was all tight, i put 4oz of liquid electric tape around the roof hole and all 3 magnets so it was all sealed. verified with product manufacturer of the tape that no metals were ingredients. swr didn't change when liquid tape was applied. i will be adding a dielectric treated ground strap tomorrow. thank you for your time.
 
I have installed a few perm mounts in roofs and ball mounts in quarter panels back in the day. I ran into the flimsy roof issue as well. This was sloved with a large washer on the underside of the roof and in one case we actually used a circular saw blade & washer to distribute flexing strain over much more surface area of the roof. In all installations the permanent mount (when done correctly) outperformed a mag mount. It tends get out slightly better, receive cleaner and not give near the swr issues that mag mounts exhibit when running higher power.

My .02....your mileage my vary.
yahoo! that would look neat with a 36" sawmill blade on the roof, a few washers, and some clearcoat to prevent rust. the car is all black, so i will write Blackcat360 on it somewhere with your permission. 3 bolts and washers would hold that to the roof real good. I'm for function and low swr. that equates to more smiles per gallon.
 
thank you for your time spend compiling all of those resources on that website. amazing how one can get by successfully for over 20 years with the same simple stuff, having no problems. things get complicated when we desire more. 6 months ago, i had no idea what single sideband was. i been missing all the fun, and have learned from way more than a few of your comments. valuable resources and knowledge is appreciated from all you folks.
 
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yahoo! that would look neat with a 36" sawmill blade on the roof, a few washers, and some clearcoat to prevent rust. the car is all black, so i will write Blackcat360 on it somewhere with your permission. 3 bolts and washers would hold that to the roof real good. I'm for function and low swr. that equates to more smiles per gallon.
The washer and/or saw blade goes on the Underside of the roof mount. This way you will most likely destroy your antenna before pulling your mount through a flimsy roof and splitting the roof metal. After dropping the headliner and drilling a hole I would take a wire brush and/or sand paper to the underside or the roof. This gives your mount and backing plate good metal to metal contact.

Mag mounts are great for convenience and the flexibility of swapping out between vehicles BUT if performance is what you're after the permanent mount is the way to go.
 
The washer and/or saw blade goes on the Underside of the roof mount. This way you will most likely destroy your antenna before pulling your mount through a flimsy roof and splitting the roof metal. After dropping the headliner and drilling a hole I would take a wire brush and/or sand paper to the underside or the roof. This gives your mount and backing plate good metal to metal contact.

Mag mounts are great for convenience and the flexibility of swapping out between vehicles BUT if performance is what you're after the permanent mount is the way to go.
1 sawblade on each side of the body would double the rigidity, i would think. surface preparation is key. mark me down as guilty, for reading for marketing literature than technical.
 
1.5 now, good to go. thanks again for all the help. you folks take care out there, be safe, and God bless. 316 in Ohio
 

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