High VSWR with Dual Band 2m/7cm Antenna

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johnc1949

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Hello,
I just installed a Comet CA-2X4SR antenna on my pickup. I installed it using a ball mount by attaching it to the truck bed side of my tool box just behind the driver seat. Since the antenna has a PL-259 connector, I used a stud to SO-239 adapter to mount the antenna to the ball mount. I have also run a 1" ground braid from my trucks frame to the ground plate on the back of the ball mount (inside the tool box). To stiffen the installation I used a 1/8" steel backing plate to sandwich between the ball mount's backing plate and the wall of my aluminum toolbox. When I checked the VSWR I get a 1.8:1 in the 2m band and a whopping 3.2:1 in the 70cm band. Neither is really what I was looking for, but the 70cm VSWR is unusable. BTW, The top of the antenna is about 1 1/2 feet above my trucks cab, but most of the 40 inch antenna is below the cab.

Does anyone have any suggestions for improving the VSWR? I was thinking that raising the antenna a foot or so would help, but I could not find a extension pole that had a PL-259 connector on one end and a SO-239 on the other. I am not sure how I could improve the ground plane without moving the antenna and leaving a big hole in my weather-tech toolbox. According to the manufacturer, the antenna is factory matched to deliver a max 1.1:5 VSWR in both bands and is not adjustable. Also, I was wondering what impedance I should expect to see with the antenna installed on the ball mount and measuring between the center pin of SO-239 inside my toolbox and the backing ground plate. I was thinking that perhaps I have a bad or shorted connection in the ball mount.

Any help would be greatly appreciated.
 

prcguy

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What you did changing a ball mount to an SO-239 might work for an HF antenna but not for a VHF/UHF. The Comet antenna expects to be mounted to a flat sheet metal ground plane and you have raised the feedpoint at least several inches above any ground plane. The ball mount by itself is almost big enough to be a 1/4 wave radiator on UHF without any antenna attached and you probably have some of the radiating antenna starting well below where the Comet starts.

Forget using the ball mount and put in a proper SO-239 mount in the center of your tool box, making sure there is a couple of square feet of flat metal around the base of the antenna.
prcguy
 

Tracker44

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Hello,I have several of those antennas, and I really like them alot , they do me a great job with no adjustment to the antennas at all.
I also have it mounted on the inside of the bed of my pick up truck.off to side of one of the bed rails.
These antennas are as with most mobil antennas sensitive to impediance issues being too close to metal .
The rule is 19 inches or further , away from any metal parts of the truck or car. Such as back of the cab or inside of truck bed. This distance is not set in concrete but close . If you are too close to the back of the cab or any other part of the truck it will run the VSWR up especially on UHF and VHF .
Play with this distance alittle , if you can come close you should see a swing in the SWR issues.
I do use a mag mount on both of the antennas. Try something different than a Ball mount.
Prcguy is right about that.
I also had a SWR issue with both until I started moving it around and trying to get as close the 19 inches as possible. When I did this the SWR went way down , almost flat.
Top of the cab would be great , but I am an old Ham, and the truck is garage kept . So doing all that climbing in and out of the truck bed is out of the question..
You are right, UHF is .alot more affected by this than VHF.
Hope this may help.
Take Care
 

johnc1949

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Thanks for the advice. I really don't want to use a mag mount on the top of my cab. It will surely hit something. I am going to have to just move it around and check as I go. Now I have to figure out a clean way to cover up the mounting holes I drilled for the ball mount :)
 

mmckenna

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Keeping the antenna away from the cab is really important, but the issue was as PRC said.

You can try one of these if you are really stuck on that location for your antenna:

SO-239 antenna ballmount for mobile Station | QRZ Now – Ham Radio News!

Check about half way down the page here:
Tower Electronics Online Wholesale Catalog - Page Five

NMO, center of the cab roof is really your best option. Done right with a reasonably sized antenna you won't have an issue. Much better performance, but I understand your desire to keep it off the roof.
 

prcguy

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If the ball mount was centered front to back on the tool box and in from the edge a foot or more to give the antenna adequate ground plane then the original location might be just fine. In this case you could get or make an aluminum sheet metal disc to cover the ball mount holes, then put an SO-239 mount in the middle of that and have the antenna in the same spot as you intended.

I've installed a number of UHF co linear antennas on large tool boxes behind cabs in pickup trucks at the request of the owners and they have worked out just fine and the radios got great range. Mounting the antenna on the roof is better but much of your antenna would be above the roof and in the clear, so it will probably work fine.
prcguy



Thanks for the advice. I really don't want to use a mag mount on the top of my cab. It will surely hit something. I am going to have to just move it around and check as I go. Now I have to figure out a clean way to cover up the mounting holes I drilled for the ball mount :)
 

K5MPH

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Also think about getting rid of the grounding braid for sure if your truck box is already grounded i had a friend install an antenna on the trunk of an car and he used an ground braid and could not figure out why he was having an high swr we removed the grounding braid and the swr went way down,just something to think about.......
 

johnc1949

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The antenna saga continues. Well after reading all the great comments I decided to go with a different mount. I purchased a Comet CP-5M lip mount for my Comet CA-2x4SR dual band antenna. I mounted it on the rear passenger door of my 2016 GMC Sierra Pickup. Initial measurements were pretty good as follows:
144.1 MHz - 1.5:1
148.0 MHz - 1.1:1
430.0 MHz - 1.1:1
450.0 MHz - 1.5:1

I thought I as all set but decided to check at high power and mid-band. Using high power produced slightly poorer results all around but really not something I could not live with. Then I checked mid band and found the VSWR was horrible.
146.0 MHz - 2.5:1
444.0 MHz - 3.4:1

I am pretty confused at this point. I have never had an antenna react in this manner. VSWR usually either climbs or dips slowly and predictably as you tune from low to high band. I am at a lose what to do now to correct this. I have not added any ground bonding or trimmed the coax to a minimum length but could do either or both if I had any hope things would improve. I have moved additional coax around to see if the VSWR changes and did not see anything. Does anyone have any idea what may be causing this behavior? BTW, I have tried both my MFR 3-band VSWR meter and a BIRD power meter and see almost identical VSWR. Thanks
 

AI7PM

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I am pretty confused at this point.
Nothing to be confused about. The antenna needs to be mounted into, and over a decent piece of metal.

Over the years I've mounted antennas in any number of configurations, both for myself, and helping other hams. I recently fought a battle similar to your's with a quarter wave UHF install on a fiberglass truck topper. No ground planes we tried would suffice. A collinear UHF would get 1.6:1. Half wave VHF 1.3:1. However, the user wanted quarter wave to get into the parking garage.

I finally convinced him to drill a hole in the truck cab and mount the quarter wave UHF.
Results? 1.0:1 VSWR, it clears the garage, and outperforms his buddies dual band gain antenna mouted on the bed rail.

I and others have mounted antennas similar to your's on an L bracket in the hood seam (fender) with good (less than 1.5:1) results. That type of mount is still a bit of a compromise, but is well grounded via the screws that mount the bracket to the fender. I'd recommend a braid between the hood and fender as well.

$0.02
 

mmckenna

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I'd concur.

Those lip mount brackets don't provide a good ground connection. Also, trying to mount off the edge of the door isn't a good location for an antenna needing a ground plane.

I understand the desire to find a bracket that will let you install an antenna quickly, easily and without drilling holes, but that's where your issue is.

Most mobile antennas require a ground plane under them. Not providing a proper ground plane is going to result in all kinds of issues. Immediate issues are high SWR, poor performance. Long term issues are vehicle damage, failed mounts, antennas getting knocked off, damaged coax, etc.

There's a reason that police cars, ambulances, fire trucks, public works, etc. always permanently mount their antennas to the vehicle. When your life or job depends on the radio, you don't take shortcuts. Installing the antenna correctly and providing a proper ground plane really pays off. Sure, you have to drill a hole, but the benefit is much better performance. In addition, it looks professional, last longer and works better.

So, what you are experiencing is exactly what I'd expect. Considering the mount and location, it sounds like it's working like it should.
If you want low SWR and better performance, you need to consider the benefits of doing the install correctly.
 

prcguy

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VSWR has nothing to do with power and should be the same at any power up to the point of something arching inside the antenna. Did you move the VSWR meter between readings? It could be the ground plane is adequate to allow the antenna to match but small enough where the coax is having an influence on the tuning.

Try running your hand up and down the coax near the mount to see if that changes the match and if so would indicate the antenna needs more flat metal surface beneath it.
prcguy



The antenna saga continues. Well after reading all the great comments I decided to go with a different mount. I purchased a Comet CP-5M lip mount for my Comet CA-2x4SR dual band antenna. I mounted it on the rear passenger door of my 2016 GMC Sierra Pickup. Initial measurements were pretty good as follows:
144.1 MHz - 1.5:1
148.0 MHz - 1.1:1
430.0 MHz - 1.1:1
450.0 MHz - 1.5:1

I thought I as all set but decided to check at high power and mid-band. Using high power produced slightly poorer results all around but really not something I could not live with. Then I checked mid band and found the VSWR was horrible.
146.0 MHz - 2.5:1
444.0 MHz - 3.4:1

I am pretty confused at this point. I have never had an antenna react in this manner. VSWR usually either climbs or dips slowly and predictably as you tune from low to high band. I am at a lose what to do now to correct this. I have not added any ground bonding or trimmed the coax to a minimum length but could do either or both if I had any hope things would improve. I have moved additional coax around to see if the VSWR changes and did not see anything. Does anyone have any idea what may be causing this behavior? BTW, I have tried both my MFR 3-band VSWR meter and a BIRD power meter and see almost identical VSWR. Thanks
 

jbantennaman

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Nothing to be confused about. The antenna needs to be mounted into, and over a decent piece of metal.

Over the years I've mounted antennas in any number of configurations, both for myself, and helping other hams. I recently fought a battle similar to your's with a quarter wave UHF install on a fiberglass truck topper. No ground planes we tried would suffice. A collinear UHF would get 1.6:1. Half wave VHF 1.3:1. However, the user wanted quarter wave to get into the parking garage.

I finally convinced him to drill a hole in the truck cab and mount the quarter wave UHF.
Results? 1.0:1 VSWR, it clears the garage, and outperforms his buddies dual band gain antenna mouted on the bed rail.

I and others have mounted antennas similar to your's on an L bracket in the hood seam (fender) with good (less than 1.5:1) results. That type of mount is still a bit of a compromise, but is well grounded via the screws that mount the bracket to the fender. I'd recommend a braid between the hood and fender as well.

$0.02
Some of the PSP vehicles has a VHF high antenna mounted between the trunk lid and quarter panel on some of the larger FORD vehicles.
No - using sheet metal screws and a bracket mount does not provide an adequate ground plane at that location. But there is 3 other antennas on the roof of that vehicle, and the QP is probably the best place to put the antenna where they don't get cross talk between the antennas.

On a side note, the roofs of the newer ambulances are Fiberglass.
No ground plane at all!
The solution my friend Marty came up with was a metallic tape that he applied to the underside of the roof that was mechanically bonded to the antenna and vehicle.
An artificial ground plane that worked.
But you have to realize, those radios are probably 50 - 100 watts and they only have to talk 5 - 15 miles to the nearest repeater. It's not the same as a ham radio that might have to talk 20 - 50 miles to a repeater that was build by Joe Schmoe and put on what ever tower Joe had available to him.
LMRS and PLMRS radio towers are purposely built in prime locations.
If the county or the state does not own or lease the land, they condemn it and take it.in the name of public safety and progress.

Only a fool would install a antenna wrong and expect good results.
Only a fool would trust a VSWR meter as the last word in an antenna installation.
LOW VSWR does not indicate resonance.
It only indicates that the antenna system is pure resistance, or the coax is lying to you.
A 50 ohm resistor will show 1:1 yet radiates no power.
 
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