testing an NMO mount (1 Viewer)

[lovetoski]

Hi all,

I am in the process of installing a 2M Yeasu 2900r into my 60. I decided to do a dual antenna system. 1/2 wave off the back drivers side for when there's a lot of overhead "stuff" & a 5/8 wave in the center of the roof for open areas where distance is desirable. I have an a/b splitter to select which ever antenna I wish to use. The 1/2 wave works fine. The 5/8 wave have only the faintest reception, with lots of background noise. Note that as I don't have my license yet, I'm only using the weather channels to test the antenna function. Both antenna's are NMO mount, where I installed the PL259 connector. Presumably it's the connector that's the problem. Before I cut it off and install a new one, I'd like to test it to make sure that's the problem. But I'm not sure how to do that. I have a multi-meter. I get good continuity between the PL259 ground and the NMO mount ground for the roof mounted 5/8 antenna. I get infinite resistance/no continuity between the center pin of the PL259 and the center of the NMO mount. However, the 1/2 wave (which receives fine) tests the same.
I'm new to this, and would appreciate any input as to next steps please.
Thankyou

 

[1911]

You really need an antenna analyzer that will at the very least measure SWR for the frequencies that 2M resides in. The NOAA weather channels are 162.400 to 162.550 MHz. 2 Meters in the U.S. goes from 144.000 to 148.000 MHz. So for that radio, you need to trim both antennas to get optimum SWR between 144 and 148 Mhz. You will still be able to receive the weather channels fine, but you don't want to tune or make any decisions based on the weather channels performance. If you don't want to buy an antenna analyzer, a ham operator near you probably has one you could borrow.

BTW, you could accomplish both of your goals with a single NMO mount in the center of the roof - use the 5/8-wave antenna when clearance is not an issue, and swap it out for a Sti-Co Flexi-whip 1/4-wave antenna when it is an issue. As a matter of fact, my 1/4-wave Flexi-whip works so well that I just leave it on all the time as my default antenna now.

Flexi-Whip

 

[lovetoski]

Thanks for the info, and especially the Sti-co flexi whip antenna.

So, from what you're saying, my PL259 connector could be installed correctly, but the antenna needs to be tuned? This makes sense I guess. Though I'd read that the Larson whip antenna's typically worked fine right out of the box. I will track down a local Ham w/SWR meter to check.

Is there a way to test the PL259 once installed onto the NMO coax and mount?

Thankyou

 

[1911]

Thanks for the info, and especially the Sti-co flexi whip antenna.

So, from what you're saying, my PL259 connector could be installed correctly, but the antenna needs to be tuned? This makes sense I guess. Though I'd read that the Larson whip antenna's typically worked fine right out of the box. I will track down a local Ham w/SWR meter to check.

Is there a way to test the PL259 once installed onto the NMO coax and mount?

Thankyou

An antenna analyzer (like the venerable MFJ 259B) will tell you if there is a short or an open somewhere in your feed line, including the PL-259 connectors. Another possibility is your A/B switch. Another possibility is stray signals from your alternator and/or ignition system being picked up by the 5/8-wave antenna and its coax cable.

Some antennas are better than others for being close out of the box, but personally I have seen very few (like one or two) that couldn't be improved by trimming the length of the whip, using an antenna analyzer to measure SWR.

 

[TrickyT]

...I get good continuity between the PL259 ground and the NMO mount ground for the roof mounted 5/8 antenna. I get infinite resistance/no continuity between the center pin of the PL259 and the center of the NMO mount...

An antenna analyzer is not necessary for troubleshooting this problem. You should measure low resistance between the center contact on the PL259 and the center contact on the NMO mount. I just measured the resistance on my setup and it was 1.7 ohms.

EDIT: Attached photos.

Note this is with the PL259 disconnected from my radio and the center pin shorted to the connector barrel.

 

[lovetoski]

Ok, I learned a lot from this post, thanks for the informative responses. Now that I know how to measure correctly I find that my roof mount NMO has very high resistance, where the rear mount one has 2.4 ohms. So clearly I botched up the PL259 install on the roof mount NMO. I'll redo that today. I also need to start looking for an SWR meter, so I can tune the antenna's.

 

[LandCruiserPhil]

If you have a Ham Radio Outlet (HRO) in your area they can help you. I have found Larson to be very close out of the box and they also have friendly tech support.

 

[TrickyT]

Agree. Most 2m antennas are tuned very well right out of the box. It's a pretty safe bet that you can just run them as they are (provided there's not a problem with the coax wiring or connectors). No real need to buy a SWR meter. As Phil says visit a HRO store and ask if they can measure your SWR. Or talk to the ham you take your test from. Most are eager to help out a fellow ham and the ones that administer the license tests have typically been hams for years and have more test equipment then you'll ever need.

 

[terrx]

The SWR meter won't help if you're only listening. Depending on which Larsen it is, the whip may have a cut chart. If not look on their website. If you can find the cut chart it's easy to double check the whip length. You didn't say how you're mounting on the roof. Mag-mount? If so, take that into account. e.g. if it calls for X inches of length, that's from tip to ground plane so subtract the thickness of the mag-mount from X. If the Larsen came as a package (whip and mag-mount combined) it may already be cut to correct length. After that check with SWR meter when you can transmit. Set the radio on low or mid power when checking SWR. That way something way out of whack won't damage the radio transmitter. Avoid high till everything is tuned up.

If you're traveling in brush and take the roof antenna off, don't forget and switch back to the roof and start transmitting!

 

[ThreeEyedBandit]

Both antenna's are NMO mount, where I installed the PL259 connector.

You didn't say how you're mounting on the roof.

Answer above.

 

[ThreeEyedBandit]

I get good continuity between the PL259 ground and the NMO mount ground for the roof mounted 5/8 antenna. I get infinite resistance/no continuity between the center pin of the PL259 and the center of the NMO mount.

There is a problem, you should have continuity from NMO ground -> PL259 Ground and NMO Center -> PL259 Center(and infinite resistance from any center to ground, this is where most people screw up connectors). Ditch the selector first and see if things improve. I bet it is something stupid like the selector is in the wrong position when testing, or A is wired to B sort of thing.

If you test good, try swapping antennas, that will tell you if it is the antenna or wiring. Depending on where you are NOAA frequencies will be very strong and you might not even need an antenna for testing.

 

[lovetoski]

There is a problem, you should have continuity from NMO ground -> PL259 Ground and NMO Center -> PL259 Center(and infinite resistance from any center to ground, this is where most people screw up connectors). .

I found another PL259 connector type at a Marine store that only requires soldering at the tip - the center contact. I was able to get low resistance using these. Step in the right direction. But still poor reception. I don't have my license yet, so I can't transmit (I take the test this weekend). But I bought an SWR meter and a dummy load so I can test different configurations and hopefully find where/what's the issue.

Really glad that I can learn from others on this forum. I am psyched to have better comm capability wheeling.

 

[terrx]

Answer above.

Awesome.