MMO mount on a steel toolbox

[bigcountry78]

Hey guys, new to the forum. I just got back in to the Cb scene after about 10 years. I always had used radios, so I decided to get a new uniden pro 520 xl. I know it's not really fancy but I didn't want to spend a ton of money just to find out that it was dead. Anyway, I'm currently using an old k40 on a mag mount on the roof of my 78 Chevy truck. I'm pretty satisfied with the performance, as my first contact was about 8 miles away. But I don't like the mag mount and I'm not crazy about the appearance of the antenna on the roof. So I'm looking at the Larsen MMO mount antenna. I don't want to drill a hole in my roof. I know that's the preferred way to do it but I'm trying to fix holes on this truck, not add more. So the best thing I can come up with is to add a steel toolbox and mount the antenna dead center. Will this give me an adequate ground plane and good performance? And will I have any SWR issues? Thinking of using a box like this



Any input is welcome. Thanks.

 

[midnight special]

i have my 102 inch steel whip mounted on my aluminum toolbox. it is on a three inch raised platform ball mounted with a six inch military heavy duty spring,,, of course it is on a ranger by the way,, it works for me,, the box is grounded to bed and bed grounded to frame,,,, a nmo mount sounds like a base mount antenna,,, i dont think it would do good on a tool box but would do much better on roof,,,,,my 2 meter antenna is mounted with a nmo on the roof and works excellent with a base load,,, the ideal antenna for a tool box mount is a steel whip with a top loaded whip second and a center loaded whip third,,, and mounted away from the cap at least 1 foot or more,,,,,

 

[bigcountry78]

I've thought about a 102" whip but the last one I had disappeared from the parking lot one day. I like the looks of the Larsen, it's a very clean looking antenna in my opinion. That's what originally caught my eye as I was shopping. Just was sure what kind of performance I could expect from a toolbox mount.

 

[StrangeBrew]

Most Larsens that I have seen are bottom loaded, this would put the coil well below the roofline when mounted on a toolbox.

 

[M0GVZ]

So the best thing I can come up with is to add a steel toolbox and mount the antenna dead center. Will this give me an adequate ground plane and good performance? And will I have any SWR issues? Thinking of using a box like this

No it won't be an adequate ground plane, not even close. An ideal ground plane would be a circular disc with a diameter of at least 18ft, how big is the box compared to that? Your performance will be worse because the antenna is lower and shadowed by the cab and you may have SWR issues depending on how close to the cab the tool box is because the antenna will be shadowed by the vertical metal of the cab so it'll capacitively couple to it making the antenna appear electrically longer than it actually is.

More reading here: http://k0bg.com/antmount.html

You will need to bond the toolbox to the pickup bed at the very least and ideally, the pickup bed to the chassis and the cab. You're going to end up drilling more holes if you want it to work anywhere near its best.

http://k0bg.com/bonding.html

 

[bigcountry78]

The box is maybe 20" square in the middle where the antenna would mount. I don't have the box yet, didn't want to buy one just to find out that it wouldn't work anyway. Sounds like my current set up is as good as it's going to get unless I do drill a hole in the roof. I'm not even sure how that would work, the truck has a double wall roof, with no dome light or cargo light to access between the layers to place the mount and run cable.

 

[undertaker]

I have a 102 inch whip on my tool box. With 6 inch spring best decision I ever made!!!

 

[w9cll]

Personally I would not use and NMO mount for 11 meters. You would better off with a standard 3/8-24 mount.

 

[bigcountry78]

Well I guess the next question I need to ask is will a 102" whip, 6" spring, ball mounted on a properly grounded tool box out perform my mag mount k40? SWR on the k40 came in at 1.1:1 on channel one and about 1.3:1 on channel 40.

 

[binrat]

Well I guess the next question I need to ask is will a 102" whip, 6" spring, ball mounted on a properly grounded tool box out perform my mag mount k40? SWR on the k40 came in at 1.1:1 on channel one and about 1.3:1 on channel 40.

Your K40 has a coil in it compared to no coil of a whip. Just think about that for a minute.
Coils are used to make the antenna electrical length needed, but are nothing compared to the real thing.

 

[midnight special]

yes it will if properly mounted and grounded

 

[w9cll]

It all depends where the K40 was mounted. If it was on top of the cab in the center then the 102 mounted lower and partially shielded might not perform as good.

 

[undertaker]

my swr is 1.0 to 1.1 across the band...

 

[bigcountry78]

It all depends where the K40 was mounted. If it was on top of the cab in the center then the 102 mounted lower and partially shielded might not perform as good.

K40 is dead center of the roof, but it's on a mag mount. The 102 would be on the back of a toolbox. Centered of course.

 

[w9cll]

K40 is dead center of the roof, but it's on a mag mount. The 102 would be on the back of a toolbox. Centered of course.

The mag mount is not a big issue, its more the antenna placement. Dead center on the roof is the best possible place for an antenna. Mounting the 102 to the tool box brings up bonding issues as has been mentioned before, it also will be partly shielding by the cab. Reconsider the NMO mount as those were designed for smaller higher frequency antennas, as I said before go with the 3/8-24 and look at Breedlove's mounts (good him).