High SWR- Help!

[smcclellan06]

Hey guys, I seem to be having some issues with my antenna setup on my FJ cruiser. I’m currently using about 6’ of LMR240UF from my mobile radio (Motorola XPR5550e) and at the other end is a Diamond Antenna dual band SG7900ANMO. I also purchased a PCTEL Maxrad MWU4505S 1/4 wave UHF antenna that I plan on mounting on the roof solely for that Motorola radio and only using the Diamond dual band antenna for a kenwood portable radio. The issue I’m having is that I’m getting high SWR readings with both antennas on frequencies around 46MHZ such as an FRS channel. When I test the SWR on an amateur radio band such as 448.301, my SWR goes down to 1.2 or less. I’m kind of at a loss because I feel like I should have a better SWR on the FRS/GMRS/ business frequencies, especially on the PCTEL Maxrad MWU4505S because I picked that antenna because of the frequency range which is 440-480MHz.
The first set of photos are readings taken with the Diamond SG7900ANMO:


And these other 2 photos below are from the PCTEL Maxrad MWU4505S:


Does anyone have any insight on this? Maybe I’m using a cheap/crappy SWR meter?

 

[K4EET]

Hi @smcclellan06, a few observations...

1. Your Surecom SW-102 meter should be "good enough" for these measurements.
2. Your Diamond SG7900ANMO antenna can be adjusted. Have you set the center frequency per these instructions?
3. Your PCTEL Maxrad MWU4505S antenna covers 440-480 MHz per this datasheet with an SWR LT= 2.0:1.

Other than adjusting the center frequency for the Diamond antenna, I don't really see any problems.

 

[mmckenna]

1/4 wave antennas are quite broad banded. On UHF, you should have pretty low SWR from the 70cm band up to GMRS.
2.0:1 isn't 'bad', but you can likely do a bit better.

Your mounting location/method may be playing into this. How and where is the antenna mounted?

 

[bb911]

You can find some good educational info in this SWR by example pdf doc.

 

[prcguy]

Sounds like the antenna(s) are a little long for 467MHz and just about right for 448MHz. I would trim them a little at a time to balance the SWR between the two extreme freqs and you may get around 1.5:1 on both amateur and GMRS when your done. Otherwise I agree a 2:1 match is not terrible and should not hurt anything.

 

[K9DWB]

Generically speaking from my days in hooking up CBs in the trucks I drove, and if I have it right, a high SWR on a higher frequency but low/good SWR on a lower frequency would/should mean the antenna needs to be shorter. If the antenna is adjustable that's one option.

I also think defining a "working" SWR is somewhat dependant on the equipment you use. Specifically the radio. Some radios are fine on a SWR that's a bit higher while radio x y or z may not be OK with it. What I mean is radio x may be fine with an SWR at or around 2:1 but radio y or z won't tolerate it. And I always believed that all parts counted when it comes to the SWR; radio, antenna, mount, cable and location. Final thought, check the antenna mount ground. Make certain it's grounding well. Things change so checking it again never hurts.

 

[mmckenna]

Commercial radios, like the Motorola the OP was referring to, will do just fine with a higher SWR. Commercial radios have a high SWR monitoring circuit that will start reducing transmitted RF power if the SWR goes high to prevent any damage.

You are correct on your length vs. resonate frequency statement. In the case of the OP, the antenna is showing a lower SWR on the lower frequencies (70cm band) and higher on the higher frequencies (GMRS channels). That would indicate the antenna is too long for the given application. The Diamond antenna should be designed to work best on the 70cm band, so as it stands, the higher SWR on the GMRS frequencies would be expected. One could shorten the antenna to 'split the difference' and achieve a lower SWR on GMRS and a bit higher/tolerable SWR on the 70cm band. But it sounds like the OP is going to keep the Diamond antenna for just amateur radio use.

The 1/4 wave antenna, if mounted/installed properly, should show a pretty low SWR across the 70cm and GMRS frequencies. It seems to be doing that, a bit better towards the lower end, which suggests it is either a bit too long, or it's a mounting/ground plane/too close to something reflective situation. I'd expect the 1/4 wave to perform a bit better than it is, but the shown SWR isn't an issue and would be totally acceptable.