Dual band mobile antenna test

[Km4jba]

Hi, I'm a newbie,and have a question about testing this antenna I mounted on my truck on side of a fiberglass camper shell. Experienced hi vswr while radio was at highest power setting. Useing ohms meter,only the lowest section(5 sections) had a continuity reading. The upper sections did not read. It's a Super Gain SG7900...The radio is a Yaesu FM transceiver. FT2800M..Wondering if I need to ground it somehow,or is it not needed.Bought radio thinking it was a 2meter 440. Anyway, showing me ignorance.. Thanks for any advise.

 

[mmckenna]

It is a 2 meter/70 centimeter dual band antenna.

It will need a ground plane under the antenna, so simply grounding the base will not suffice. These antennas want to be mounted over a large conductive plane, like on the roof of the truck. Mounting on the side of a fiberglass shell isn't going to result in a good ground plane.

There's a difference between a ground connection, like if you ran a wire down to the metal truck bed, and having an actual ground plane under the antenna.

You are not going to see low SWR with that antenna mounted that way.

As for ohming the antenna out, it's going to depend on it's exact design. I don't own one, so I can't tell you if what you are seeing is correct or not.

If mounting the antenna to the fiberglass camper shell is your only option, then you need to switch to an antenna that is designed to be a "No Ground Plane" or "ground independent" antenna. Since you are using this with a 2 meter single band radio, there's some good options out there for no ground plane 1/2 wave antennas. Larsen and Laird make good ones, but they'll have an NMO style mount.

Better option is to mount the antenna correctly, smack in the center of the truck roof if you can.

 

[mmckenna]

Useing ohms meter,only the lowest section(5 sections) had a continuity reading. The upper sections did not read.

Looking at this:

https://www.diamondantenna.net/pdfdocs/SG7900A%20&%20NMO%20Instructions.pdf

There's a capacitor in the middle that may explain why you are not seeing DC continuity through the antenna.

 

[jwt873]

mmckenna pretty well covered it..

Just out of curiosity..How are you measuring SWR (what are you using) and what readings are you getting? Is there any change if you take a measurement at 144 Mhz and then another one at 148 Mhz?

Looking at the pdf file above, the antenna comes factory-adjusted, but it says that there are a couple of set screws you can loosen in order to change the whip length slightly.

 

[mmckenna]

….the antenna comes factory-adjusted….

"Factory Adjusted" and "Factory Tuned" is just a fancy way of saying "we got it sort of close and we're using this as a sales gimmick". Each antenna installation will be a bit different, and variations in the ground plane will change SWR.
The only antennas I've found that work well right out of the box/bag is 1/4 wave antennas.

 

[K6GBW]

If this is on a pickup with a camper shell then you are going to get frustrated trying to mount any antenna on the shell. You'd be a lot better off just putting a quarter wave with NMO mount either on the roof of the truck or, if it's covered, then on a hood mount.

 

[merlin]

Well, my observation for the antenna itself is no where near the gain claimed,,more like 3Db Vhf, 5.5 Db UHF.
They tune broad enough tuning is more waste of time, but 'WHERE' you mount the antenna affects performance. Center roof is more omni directional. deck lid more gain to the front.
There must be a ground plane about 20% longer than 1/4 wave. Without, or mounted to non metal, the resistive reactance increases causing reflected wave on the feed line.
I have seen people mount to a fiberglass shell but us a piece of sheet metal inside like sandwich the fiberglass with the mount.
As for resistance end to end, it should be zero ohms. A 3rd section has a capacitor so no resistance.
They do make VHF no ground plane antennas, motorcycles, marine etc.
70CM being a good 3rd order (3Xfrequency) over 2M, A vhf antenna is also resonant at UHF.