Comet SBB-5NMO for Jeep

[donmontalvo]

I am fairly new to thiss space. I am running a 2M/70cm radio in my 2021 Jeep (JLUR). I have a Comet SBB-5NMO ground independent antenna mounted it to my driver side cowl.

SWR on 2M is good…not so good on 70cm. I mostly use 2M, but I wanted to see if there is a way to adjust the 70cm part of the antenna to get better SWR on that frequency?

2M:
146.52 = 1.01
144.00 = 1.69
148.00 = 1.24

70cm:
446.000 = 1.61
420.000 = 3.56 (?!)
450.000 = 2.14

Happy New Year!!!

73
KC3UNA

 

[popnokick]

I am fairly new to thiss space. I am running a 2M/70cm radio in my 2021 Jeep (JLUR). I have a Comet SBB-5NMO ground independent antenna mounted it to my driver side cowl.

SWR on 2M is good…not so good on 70cm. I mostly use 2M, but I wanted to see if there is a way to adjust the 70cm part of the antenna to get better SWR on that frequency?

2M:
146.52 = 1.01
144.00 = 1.69
148.00 = 1.24

70cm:
446.000 = 1.61
420.000 = 3.56 (?!)
450.000 = 2.14

Happy New Year!!!

73
KC3UNA

What are you using to measure SWR on 70cm? Depending on the instrument your actual SWR may be better than the 1.61 you're seeing on 446 mHz... which is only slightly above the mfr spec of 1.5. If you want a better SWR at 420 mHz you're probably going to have to lengthen some portion of the antenna... and I don't see anything in the mfr info that allows you to do that. Most likely your SWR on 70cm is being affected by the proximity of the vehicle body & window pillar to the main coil at the bottom of the antenna, and the lower portion of the antenna to the body / pillar. You could test this by moving the antenna (temporarily... maybe).
446.000 is the 70cm National FM Simplex Frequency. What is your need / usage for a lower SWR at 420.000? Not much in the way of FM voice happening down there.

 

[mmckenna]

420MHz is out side the 70cm amateur band, and that antenna is not designed to work there. Was that a typo?
Plus, as popnokick said, the mounting location is probably part of the problem. Not good to have that much of the antenna that close to a body panel.

 

[donmontalvo]

146.550What are you using to measure SWR on 70cm? Depending on the instrument your actual SWR may be better than the 1.61 you're seeing on 446 mHz... which is only slightly above the mfr spec of 1.5. If you want a better SWR at 420 mHz you're probably going to have to lengthen some portion of the antenna... and I don't see anything in the mfr info that allows you to do that. Most likely your SWR on 70cm is being affected by the proximity of the vehicle body & window pillar to the main coil at the bottom of the antenna, and the lower portion of the antenna to the body / pillar. You could test this by moving the antenna (temporarily... maybe).
446.000 is the 70cm National FM Simplex Frequency. What is your need / usage for a lower SWR at 420.000? Not much in the way of FM voice happening down there.

Thanks! I’m using a Surecom SW-102. I googled for what frequencies to test.

What frequencies should I test for 2M and 70cm (center and edges)?

I use mostly 146.52 (National Simplex Calling Frequency), 146.550 (the usual simplex at Quartzsite, AZ), and 145.3100/147.3600 repeaters also at Quartzsite, AZ.

TBH, I rarely use 70cm.

 

[donmontalvo]

UPDATE: I got the range from ARRL...

 

[donmontalvo]

420MHz is out side the 70cm amateur band, and that antenna is not designed to work there. Was that a typo?
Plus, as popnokick said, the mounting location is probably part of the problem. Not good to have that much of the antenna that close to a body panel.

Not my typo, see my last post, got it from ARRL.

On the proximity, most of the Jeep owners who run this antenna also mount it on the driver side cowl...I can mount it on the spare wheel bracket, but I was afraid the 35" tire might cause issues.

 

[mmckenna]

Not my typo, see my last post, got it from ARRL.

Yep, you are right. My bad.

However, the 'gentleman's' band plan for 70cm has slow scan TV down at the bottom of the band there. Really not much that you'd want to do down that far. Most of the 70cm voice activity is going to be in the 440-450 range. Your SWR looks acceptable there.

Many of these mobile antennas are not designed to do much that low on the 70cm band, so not surprising the SWR would be high. But I do think some of that is because of the mounting location.

On the proximity, most of the Jeep owners who run this antenna also mount it on the driver side cowl...I can mount it on the spare wheel bracket, but I was afraid the 35" tire might cause issues.

Jeeps are a pain the butt to install antennas on. You can search back through posts and find lots...

Mounting there comes with compromises, you just gotta do the best you can. Having the base of the antenna down that low and so close to the body is going to impact SWR and radiation pattern. Maybe not a big deal for most users that are only looking for short range.
Tire would probably have some impact due to steel cords, and the lack of ground plane isn't going to help things, even if it is a 'no ground plane' antenna.

There are a few posts where people have worked through this, and often what you'll see is either the antenna mounted on top of the cowling so the base is clear of the sheet metal, or more it farther forward along the hood to get it away from metal.

But like most things, do the best you can with what you have. Honestly, your SWR doesn't look that bad on VHF and the upper portion of the 70cm band.

 

[donmontalvo]

Yep, you are right. My bad.

However, the 'gentleman's' band plan for 70cm has slow scan TV down at the bottom of the band there. Really not much that you'd want to do down that far. Most of the 70cm voice activity is going to be in the 440-450 range. Your SWR looks acceptable there.

Many of these mobile antennas are not designed to do much that low on the 70cm band, so not surprising the SWR would be high. But I do think some of that is because of the mounting location.



Jeeps are a pain the butt to install antennas on. You can search back through posts and find lots...

Mounting there comes with compromises, you just gotta do the best you can. Having the base of the antenna down that low and so close to the body is going to impact SWR and radiation pattern. Maybe not a big deal for most users that are only looking for short range.
Tire would probably have some impact due to steel cords, and the lack of ground plane isn't going to help things, even if it is a 'no ground plane' antenna.

There are a few posts where people have worked through this, and often what you'll see is either the antenna mounted on top of the cowling so the base is clear of the sheet metal, or more it farther forward along the hood to get it away from metal.

But like most things, do the best you can with what you have. Honestly, your SWR doesn't look that bad on VHF and the upper portion of the 70cm band.

It’s raining here in Quartzsite, AZ right now. Once the rain stops I’ll go out and redo the tests.

Thanks for the heads up and feedback!

 

[AK9R]

Most of the 70cm voice activity is going to be in the 440-450 range. Your SWR looks acceptable there.

This.

In most parts of the US, 70cm FM and digital voice repeaters operate in the 440-450 MHz segment of the band. There may be links or hot spots or data in the 433-440 MHz segment and weak signal (CW, SSB, and data) can be found around 432 MHz. But, for the average mobile user, 440-450 MHz is where you want to be.

 

[K6GBW]

Those numbers are actually not bad at all. That type of antenna will be pretty narrow so it's to be expected. For your uses it should be fine and as has been said, Jeeps are a pain to mount antennas, but the way you did it is probably about the best you can do without taking extreme measures. I'm assuming it will be used mostly for repeaters and relatively short range simplex and for that it should work well. An advantage of your mounting location is you can keep an eye on it when off road. If it's going to hit something you'll know about it.

 

[donmontalvo]

Retested today:

2M:
144.000 = 1.68
146.520 = 1.02
148.000 = 1.24
70cm:
420.000 = 3.57
430.000 = 2.02
440.000 = 1.62
446.000 = 1.63
450.000 = 2.15

Tomorrow I need to test a new 3’ CB antenna for my Cobra 75 All Road.

 

[prcguy]

If you need a wider BW you could try the Comet CA-2X4SR series which advertises 140-160MHz and 435-465Mhz. I run one occasionally on my Jeep JT hood mount.

 

[mmckenna]

The design of the antenna isn't likely to allow you to just tune the UHF portion, as the entire whip is active on both bands.

2 meters looks good. You won't be transmitting down at 144, even though 1.68 isn't bad. Your simplex and repeater stuff is going to be higher in the band. 1.02 at 520 is perfect….

70cm looks fine. You won't be transmitting up at 450, even though 2.15 isn't going to hurt the radio. That higher SWR is likely in part due to the antenna base being so close to body steel and part due to bandwidth.

The radiation pattern is probably what is going to suffer due to the location, and not much you can do about that.

 

[donmontalvo]

If you need a wider BW you could try the Comet CA-2X4SR series which advertises 140-160MHz and 435-465Mhz. I run one occasionally on my Jeep JT hood mount.

I mentioned in my original post why I can’t use that antenna

 

[prcguy]

I mentioned in my original post why I can’t use that antenna

The Comet CA-2X4 should work ok on the mount you have, lots of people use them there. But any antenna will work better on top of the hood using an NMO trunk lip mount on the hood next to a hood hinge. It gets the antenna up higher with a better ground plane, I run two of them and all my antennas have a great SWR. Plus right below where this antenna is mounted is a hole in the firewall just waiting for you to stick the coax through.