Dual band antenna mounting on roof rack.

[AKRover]

So I currently have my 2m/70cm antenna mounted to one of my Thule cross bars. I'm getting a 1.5:1 SWR on 2 meters with full power and 3:1 SWR on 70cm with the radio cutting the power back to half.

If I grounded the antenna to the roof would that help improve the SWR on the high frequency? Or is there something else going on?

I have seen a few people here with antennas on their roof racks so I know it can be done. The antenna I'm using is a Diamond NR770HA 144/440 dual band mobil antenna.

 

[Pskhaat]

Does your roof rack have a nice flat conductive/metal floor?

I would personally advise getting the mounting as physically close to the flat counterpoise of the vehicles body as possible. I have been able to find numerous good mounting places on nearly every vehicle, roof rack would not necessarily be choice.

 

[gary in ohio]

So I currently have my 2m/70cm antenna mounted to one of my Thule cross bars. I'm getting a 1.5:1 SWR on 2 meters with full power and 3:1 SWR on 70cm with the radio cutting the power back to half.

Did you tune the antenna when you installed it. What does the installation direction say about tuning. What antennas you using? Sounds like the coil is shorted basicly giving you a mono band vhf antennas.

 

[AKRover]

The current roof rack is Thule cross bars so no there isn't much metal there. I will be getting a ARB rack eventually so that might be a little better since it is all metal tubing. I know this isn't the best way to mount an antenna but I was in a hurry to get on the air. It works fine in the 2m band, just not so much on 70cm. Here's what I'm working with.

Since I never use the 70cm band I never really paid much attention to tuning on it but I would at least like to be able to use it if I wanted to. The antenna I'm using is a Diamond NR770HA 144/440 dual band mobil antenna. The instructions from Diamond are poorly translated from Japanese and give no real tuning information. The instructions do say that due to it's radialles design it can be mounted virtually anyplace on the car. Maybe that doesn't mean 6" above the roof?

 

[tdesanto]

I'm using the same antenna. It does not need to be grounded to the vehicle. I'm using it with a Diamond Antenna K9000LRM Motorized Mobile Antenna Mount and Diamond Antenna C213 cabling.

I will say that my first one was DOA and I had to get a replacement. I was able to go to a local shop and try another one to determine that my installation, cable, and radio were operating normally.

I didn't need to tune mine at all and the SWR readings were very, very low.

Mine is more towards the center of the vehicle (front to back), but is almost to the side on the passenger side.

 

[AKRover]

I suppose I could move the antenna to the center of one of the rack bars since the tent isn't on there and see what happens to the SWR.

I had a chance to take some readings across the bands before heading off to work.

2 Meter band

144.2 - a little less than 1.5:1
146 - a little above 1.5:1
148 - 2:1

70 cm band

420 - 2.5:1
435 - 2:1
450 - a little less than 2:1

The readings on 2 meters are acceptable but on 70 cm the radio starts cutting back the power. Since it puts out a maximum of 20 watts on that band it would be nice to keep it at full power.

 

[BigJimCruising]

Your mounting and location should be ok. Not the best but should work. I think you need to go through your entire antenna system and double check everything to find your problem. It's hard to tell from some pics. I'd also try another antenna and see if you get a better result. Those antennas are notorious for failures especialy on off road vehicles. Trust me, I found out the hard way and a lot of bucks later! Look close around the base of the antenna rod where it enters the coil section. That's where most of mine would fail. Good luck!

 

[xpdishn]

Your mounting and location should be ok. Not the best but should work. I think you need to go through your entire antenna system and double check everything to find your problem. It's hard to tell from some pics. I'd also try another antenna and see if you get a better result. Those antennas are notorious for failures especialy on off road vehicles. Trust me, I found out the hard way and a lot of bucks later! Look close around the base of the antenna rod where it enters the coil section. That's where most of mine would fail. Good luck!

So what make of antenna did you end up with? Just getting started and just gathering info for now. Thanks.

 

[BigJimCruising]

I wound up with a Larsen dual band antenna. I think the model was 2/70 or something like that. It covers 2m and 70cm. Works great. Base coil, very flexable metal rod with an open coil in the middle. I've only lost the rod once in years of off roading with it. I've never had one just fail on me. Amazing antenna! Only problem you'll have is you will have to change your base to an NMO type mount. Another plus to the NMO mount is if you know you'll be in very heavy brush and trees you can change the antenna to a short rod that you cut yourself to your band. Like the short ones you see on police cars. They're cheap and nearly indestructible in the brush. Wish I could remember the model numbers but you'll see them at almost any Ham Radio store. I seem to remember cutting mine to either 19 or 19.5 inches and getting a good enough match to use on both bands. No gain like the Larsen but very good for use in heavy brush or low trees. Happy Haming!

 

[gary in ohio]

Only problem you'll have is you will have to change your base to an NMO type mount. Another plus to the NMO mount is if you know you'll be in very heavy brush and trees you can change the antenna to a short rod that you cut yourself to your band.

Larsen sells a base with a spring on it that will solve your brush/tree issue.

 

[bj70_guy]

Larsen sells a base with a spring on it that will solve your brush/tree issue.

Or if you already have the base/whip you can just buy the spring and cut the length of the spring off the whip.

I have the NMO 2/70 with springs on my 70 and my 74 and no trouble with brush etc.

 

[mk4]

I'm wondering why you're getting advice to tune your dual band antenna? I also would like to know from these members, how they'd go about such tuning measures with the antenna?

I've never heard of any commercial VHF/UHF (excluding 6m) needing tuning or even a manufacturer calling for a tune of any sort. You should get 1.5 or less on the desired frequencies.

In regards to another response that your antenna does not require grounding, I'd ask them to explain their reasons and where they got this nugget of wisdom from.

As far as I know, and it's not very much, all antennas require an RF ground. In the case of a balanced antenna such as a dipole, the RF ground (or counterpoise) is just the other half of the dipole. In the case of unbalanced antennas such as the vertical in question, the RF ground is provided through the mount.

I'd try to make sure you get some metal-metal contact on your mount/antenna and to the crossbar. If that doesn't help, then I'd run a strap from the antenna mount to the gutter mount feet.

If you've got a spare coax, I'd give that a try as well.

 

[tdesanto]

In regards to another response that your antenna does not require grounding, I'd ask them to explain their reasons and where they got this nugget of wisdom from.

As far as I know, and it's not very much, all antennas require an RF ground. In the case of a balanced antenna such as a dipole, the RF ground (or counterpoise) is just the other half of the dipole. In the case of unbalanced antennas such as the vertical in question, the RF ground is provided through the mount.

I'd try to make sure you get some metal-metal contact on your mount/antenna and to the crossbar. If that doesn't help, then I'd run a strap from the antenna mount to the gutter mount feet.

If you've got a spare coax, I'd give that a try as well.

Here's where I got my nugget of wisdom:

"The Diamond Antenna NR770HB Dualband VHF/UHF Mobile Antenna is resonant on the 2m and 70cm amateur bands. The NR770HB is a 1/2-wavelength design providing 3.0 dB of gain on the 2m band, and a 2 x 5/8-wavelength design providing 5.5 dB gain on the 70cm band. The NR770HB is 39" tall and weighs 200 grams. The NR770HB features fold-over ability for entry into low overhead clearance areas. Power rating is 200/200 watts. The connector type is a gold-plated UHF/PL-259. Black finish. Grounding is not required."

Also, I'm using the Diamond Antenna motorized mount. I can assure you that this mount is not grounded to the vehicle. The only connection the antenna has to the vehicle is via the coax to the radio. My Yaesu 857D has an SWR meter and the SWR on both 2m and 70cm is very low.

 

[tdesanto]

Another nugget of wisdom:

K9000LRM Motorized Luggage Rack Mount

Notice the first bullet point:
"• Accepts "Ground Independent" antennas up to 45" in length"

Also, "Recommended Diamond antennas include NR73BNMO, NR770HNMO, NR770HBNMO, NR770HA, NR770HB, SG7500A & SG7500NMO."

 

[AKRover]

Unfortunately I haven't had the time to mess with that antenna. I got distracted with putting a j-pole on my roof and getting a spot ready in my garage for a base station. I want to get that done before it snows which could be anytime now. I bought a VHF antenna analyzer to tune the j-pole I just wish it covered the UHF band too so I could use it on the mobil antenna.

I'm trying to get my hands on a new roof rack so I think I'm going to have to return to this subject after I get that and re-mount the antenna in it's new location.