Noob Questions about antenna and ground plane

[oscarach]

I am new to the whole GMRS radio thing and have been finding some conflicting information on the internet.

My setup: Midland MXT275 with MXTA25 ghost antenna connected with provided coax cable. The antenna is mounted an antenna bracket that attached to a load bar on a aluminum truck bed cap. The bottom of the antenna sits about 2 inches above the roof of the bed cap.

On channels 1 to 7 SWR of 1.46 and 2.1 on channels 16 to 22.

So my questions are

Will that aluminum roof make a good enough ground plane? If so does the antenna need to be mounted directly to it or is sitting an inch or 2 above it ok? Don't really want to drill holes in the roof. 

The mounting bracket is painted and I have had people tell me I need to remove the paint where the antenna attaches and also to run a grounding strap from the mount to the vehicle chassis ground, is this really the case?

About the only thing everyone agreed on is that the MXA25 is not a great antenna, any recommendation for something to replace that with that is not too tall and will fit on the existing NMO mount?

 

[axorlov]

So, what you are saying, that you see SWR 1.46 on channels 2 (462.5875) and 3 (462.6125) and SWR 2.1 on channel 17 (462.6000)? It is indication that your SWR meter is garbage. SW-33, I hazard to guess?

With regards to antenna, any 1/4 wavelength antenna will work better and will not be too tall (about 6"). The 1/2 wavelength antenna does not require ground plane, and it will be around 12" tall. 5/8 antenna will also work without ground plane, but it's not going to be more efficient (if mounted without good ground plane), and will be a bit taller than 1/2.

[Lscott]

1 hour ago, oscarach said:

On channels 1 to 7 SWR of 1.46 and 2.1 on channels 16 to 22.

By any chance were you using the repeater function? I'm not familiar with the exact radio but I believe you can select between what is call simplex operation and using a repeater. Using the repeater function, which should only work on channels 15 through 22, the radio will try to transmit on the 467.xxx frequencies which is noticeably higher than the frequencies on the channels below that, 1 to 7. That might account for the higher SWR reading on those higher numbered channels. The antenna might be optimized for the simplex only 462.xxxx frequencies.

In any case you need to get the antenna up and in the clear to get some decent range out of it. That is critically important at the frequencies used by GMRS.

[oscarach]

3 hours ago, axorlov said:

So, what you are saying, that you see SWR 1.46 on channels 2 (462.5875) and 3 (462.6125) and SWR 2.1 on channel 17 (462.6000)? It is indication that your SWR meter is garbage. SW-33, I hazard to guess?

With regards to antenna, any 1/4 wavelength antenna will work better and will not be too tall (about 6"). The 1/2 wavelength antenna does not require ground plane, and it will be around 12" tall. 5/8 antenna will also work without ground plane, but it's not going to be more efficient (if mounted without good ground plane), and will be a bit taller than 1/2.

Thanks for the information on antenna, I will go look into that. Yep I have the SW-33 as it seemed to be regarded as decent by some of the youtubers. what would be a decent one?

I think most of my confusion is around what is a good ground plane. I have read everything from it must be connected a chassis ground point but can just be a bit of angle iron, or the antenna mount must be attached through a large metal surface, or as long as the antenna sits just above a large metal surface. I the case of my placement in the picture do I have any ground plane or just a bad one?

2 hours ago, Lscott said:

By any chance were you using the repeater function? I'm not familiar with the exact radio but I believe you can select between what is call simplex operation and using a repeater. Using the repeater function, which should only work on channels 15 through 22, the radio will try to transmit on the 467.xxx frequencies which is noticeably higher than the frequencies on the channels below that, 1 to 7. That might account for the higher SWR reading on those higher numbered channels. The antenna might be optimized for the simplex only 462.xxxx frequencies.

In any case you need to get the antenna up and in the clear to get some decent range out of it. That is critically important at the frequencies used by GMRS.

Checked and the repeater channels are off by default on the radio, but if I change the power from hi to low on channel 22 the SWR drop to 1.34. I also tried the small magnetic mount antenna in that also came with the radio in the same location and ran the coax the same route and get a SWR or 1.12 across all channels. 

[axorlov]

The Surecom SW-33 is a garbage, as your experiment shows. Youtubers will youtube, of course. The reason why I guessed it's SW-33, because it's very characteristic of it to react on power level, like 5W vs 40W on UHF. Your regular yuotuber is not going to have a clue. I use Diamond SX-600, which is still cheap, and cannot be used in any serious setting. But the measurements are consistent across power levels and frequency ranges. 

Good ground plane is where your NMO mount contacts the conductive plate, that is at least about 6" in diameter. The picture shows that there is no "good" ground plane but there is "some". I would think, 1/4 wave antenna will work better, and I would think 1/2 antenna will work tons better.

[gortex2]

1.4" wave UHF whip is a better antenna and is cheap and short. 

[oscarach]

Thanks for the advice, I have ordered a 1/4 wave antenna and will see how that works.

I have some aluminum plate laying around and could make a 12"x18" plate bolted to the two load bars and mount the antenna in the middle of that, I assume that would give me a better ground plane. If that would help the antenna work better then a question is does that plate need to be bonded to a vehicle chassis ground? I have seen some of the HAM guys on the off-road forums saying that the ground plane must be connected to a vehicle electrical system ground point and that the antenna must have metal to metal contact with the ground plane, but I have also seen other HAM guys say that is not required so I am confused :-).

[gortex2]

Put the 1/4 wave on and test. Think you'll be surprised. Think of the Ghost antenna as your rubber duck on your portable jammed inside a pepper shaker.

 

[n4gix]

Ground bonding is really benefic  ial for HF mobile antennas.  On my Toyota Camry, I had to run copper braid for each of the trunk hinges, all four doors, and the hood hinges.  

[oscarach]

The 1/4 wave did make a big difference.

Channel 1 & 22 (low power) SWR is now 1.07 car running 1.03 car not running. Channel 22 (high power) SWR is now 1.72 car running 1.65 car not running.