SWR Question

[WSDT708]

I have a raddioddity DB25-G unit in my truck. I’m using an NMO mount that attaches directly to the fender using the existing bolt inside the engine compartment. I have a Tram 1126-B antenna. I’m using a surecom meter.

My first test showed SWR of 1.5:1 on channels 1-7, 2:1 on channels 15-21, and greater than 3:1 on repeater channels. I took the mount off and removed the star washer that came with it and reattached the mount then tested again.

My second test showed SWR of 1.01:1 on channels 1-7, 1.45:1 on channels 15-22. I’m good with that. However, on repeater channels the SWR was 2.84:1. Any suggestions on how to get that SWR down?

[SteveShannon]

12 minutes ago, WSDT708 said:

I’ll take a look but from what I’ve seen with people trying that with this antenna, it seems to get worse. I may go back to the UHF stealth antenna I have and see if the SWR is where it should be across all GMRS freqs.

An SWR of 3.0:1 isn’t as bad as you might think unless you’re using lossy cable. I would go by performance rather than SWR. 
If your antenna has a low SWR at a lower frequency and higher SWR at a higher frequency, shortening it is the correct adjustment. There’s really no other way to reduce the actual SWR of the antenna. Just don’t shorten it too much. Or consult the instructions for the antenna. 

[SteveShannon]

8 minutes ago, WSDT708 said:

I have a raddioddity DB25-G unit in my truck. I’m using an NMO mount that attaches directly to the fender using the existing bolt inside the engine compartment. I have a Tram 1126-B antenna. I’m using a surecom meter.

My first test showed SWR of 1.5:1 on channels 1-7, 2:1 on channels 15-21, and greater than 3:1 on repeater channels. I took the mount off and removed the star washer that came with it and reattached the mount then tested again.

My second test showed SWR of 1.01:1 on channels 1-7, 1.45:1 on channels 15-22. I’m good with that. However, on repeater channels the SWR was 2.84:1. Any suggestions on how to get that SWR down?

Is there an adjustment on the antenna to shorten the element?  

[WSDT708]

13 minutes ago, Socalgmrs said:

The fender is not the place for an gmrs antenna.  Get it up in the middle of the roof.   

My issue has not been transmitting or receiving. Both are very clear. The bracket I have has a good ground plane. I have no intention of drilling or trying to run coax externally. Just trying to find a solution to high SWR on repeater channels with the antenna set up I have.

[WSDT708]

6 hours ago, SteveShannon said:

Is there an adjustment on the antenna to shorten the element?  

I’ll take a look but from what I’ve seen with people trying that with this antenna, it seems to get worse. I may go back to the UHF stealth antenna I have and see if the SWR is where it should be across all GMRS freqs.

[WRQL370]

It’s already answered for you with shortening the antenna.  Your repeater channels are transmitting at 467.  That’s at the edge of the band.  Antenna is tuned way lower currently. 
 

 

[WSDT708]

19 hours ago, SteveShannon said:

An SWR of 3.0:1 isn’t as bad as you might think unless you’re using lossy cable. I would go by performance rather than SWR. 
If your antenna has a low SWR at a lower frequency and higher SWR at a higher frequency, shortening it is the correct adjustment. There’s really no other way to reduce the actual SWR of the antenna. Just don’t shorten it too much. Or consult the instructions for the antenna. 

I shortened the antenna a couple of mm at a time until I got the SWR to settle between 1.4:1 and 1.6:1 on simplex channels, and around 2:1 on repeater channels. My last couple of cuts didn’t improve the readings beyond that so I figured that’s the best I’d get with that basic antenna. I may add an NMO ground cable and rely on the non-continuous 1 sq ft ground plane concept for this setup or try another antenna.
 

Clear transmission confirmed on a local repeater 15 miles from my location so I’m just going to call it good with SWR readings that aren’t perfect but are at least not going to damage my radio.
 

FYI the UHF stealth antenna from Rugged Radios that I had originally planned to use was showing SWR of 3.5 on simplex and greater than 5 on repeater freqs. So that’s basically just a paperweight at this point.

[SteveShannon]

2 minutes ago, WSDT708 said:

I shortened the antenna a couple of mm at a time until I got the SWR to settle between 1.4:1 and 1.6:1 on simplex channels, and around 2:1 on repeater channels. My last couple of cuts didn’t improve the readings beyond that so I figured that’s the best I’d get with that basic antenna. I may add an NMO ground cable and rely on the non-continuous 1 sq ft ground plane concept for this setup or try another antenna.
 

Clear transmission confirmed on a local repeater 15 miles from my location so I’m just going to call it good with SWR readings that aren’t perfect but are at least not going to damage my radio.
 

FYI the UHF stealth antenna from Rugged Radios that I had originally planned to use was showing SWR of 3.5 on simplex and greater than 5 on repeater freqs. So that’s basically just a paperweight at this point.

Good job!

What kind of cable are you using?

[WSDT708]

On 10/20/2024 at 2:12 PM, SteveShannon said:

Good job!

What kind of cable are you using?

HYS RG-58

I actually changed to a Browning 5/8 wave whip antenna today and cut it to tune to around the 465MHz range based on their guide. I’m now getting 1.3:1 on repeater channels, 1.4:1 on 467MHz and 1.5:1 on 462MHz frequencies. 

Perfect set-up? No but it will work for now. If I intended to keep this truck long-term I’d be more inclined to make permanent modifications to the vehicle.

Thanks for helping me walk through this.

[SteveShannon]

1 hour ago, WSDT708 said:

HYS RG-58

I actually changed to a Browning 5/8 wave whip antenna today and cut it to tune to around the 465MHz range based on their guide. I’m now getting 1.3:1 on repeater channels, 1.4:1 on 467MHz and 1.5:1 on 462MHz frequencies. 

Perfect set-up? No but it will work for now. If I intended to keep this truck long-term I’d be more inclined to make permanent modifications to the vehicle.

Thanks for helping me walk through this.

Perfect is the enemy of good enough! Nice job tuning the antenna!

[WSBV762]

Good day 708! 

Please have a look at the Midland MXTA26 antenna and the NMO mount.  I have it on the center of my roof on my Tuscon.  It has 6 meters of coax- please do not shorten it.  This antenna is tuned to GMRS and I am using a Midland MXT500 mobile radio.  Whether on my car or on a steel sheet in my back yard, I am hitting my local repeater that's 14 miles away- (houses, trees, the moon, etc. are all in the way) and my SWR is less than 1.2:1 across the board.  Transmitting and receiving beautifully!  

All the best!

Dan

WSBV762

https://midlandusa.com/products/micromobile-mxta26-6db-gain-whip-antenna

https://midlandusa.com/products/mxta12-antenna-mag-mount?_pos=2&_psq=mount&_ss=e&_v=1.0

[Guest]

The fender is not the place for an gmrs antenna.  Get it up in the middle of the roof.