High SWR

[RockinMiner]

I have a Cobra LX 29 with a Little Wil magnet base antenna. I recently bought an SWR meter to adjust my antenna to get peak performance, but when I was doing the SWR readings I was above 3 every time I keyed the mic. I tried adjusting the antenna but i cant get it to come down. What should I do? Buy a new antenna? I'm just lost on what I should do.

 

[jonwienke]

Magnet mounts won't do as well as other options. If there is a loose connection in the base, you'll never get a good reading. Another possible problem is where it is located. Center of the highest part of the vehicle is best. If the antenna is on a fender or behind the cab the RF reflecting off the cab will raise SWR.

Post a photo of your setup.

 

[MBill]

I like the big display on the LX29.
Did you use the internal radio check menu steps to confirm the high reflected? Press the channel
knob and scroll down to select the radio check. You can check DC voltage, Forward power and
Antenna system as I recall. Will give you a pass/fail indication.
Also if you have ohm meter check that your antenna cable is not shorted or open. Disconnect the
antenna connector from the radio and put meter leads from center pin to outer shell of the
connector. You should not read a short. Also check continuity from center pin to antenna. You
may not be able to check the continuity between the connector shell to antenna ground and/or chassis
due to this being a mag mount with internal connections. If may also be you have a bad antenna.
Check You Tube for a video on this antenna for some issues it has due to internal parts failure.
Most antennas need tuning, so that also may be an issue. Check manufacturer for procedure,
if any.

 

[QwKiE]

If the swr meter has a field strength antenna on it make sure you don't use it checking the swr it will give you a false reading always high. Just take the antenna off till you are done checking the swr.

 

[RockinMiner]

Today I went out and checked my SWR again and it is now reading lower. Could this be a result of having felt on the bottom of the magnet? Here are channel 1 and 40 SWR readings.

 

[MBill]

Looking at a YouTube video on how to disassemble this mag mount I see that it has a loading
coil with apparently a resistor and capacitor. If the capacitor (and not the resistor) is in the center
conductor circuit then you won't get a reading on your DC ohm meter. Just so you are aware of
this.

 

[kf4eyr]

yes felt will cause this problem,,, if you are worried about scratches mag mounts is not for you,,,,, i have seen on vhf and up the use of a balloon over the magnet but still wouldnt recommend it for hf and trying to have high wattage,,,, the shinny metal on the bottom of the mag mount antenna is a great big part of the antenna matching system,,,

 

[JayMojave]

Hello RM: I believe this is as good as it gets with that small antenna. It will have a good SWR on one end of the band and higher SWR on the other end of the band. Do some radio checks with it, see if your getting out somewhat.

If the felt came with the antenna its not a problem.

Jay in the Great Mojave Desert

 

[FiveFilter]

My 35-inch K30 magnet mount antenna gives similar readings: 1.1 on 1 and 2.7 on 40. I stopped shortening it at that point because I'm getting an excellent 1.4 on 19 where I use the radio.

If I ever use any channel but 19 on the road, I'll select 18 and below to keep the SWRs in great territory.

The shorties aren't as flexible as their taller kin, but they are invaluable when height is restricted by low-ceiling garages.

You could try a single layer of plastic sandwich bag under the magnet to see what the SWRs do. I've tried it and the SWRs didn't change. For convenience, I usually just leave a good layer of wax where I place the magnet to keep the scratch threat controlled, but the original padding is still in place on my antenna.

By the way, when you say you are "adjusting" the antenna, does that imply you are simply lowering and raising it in the housing? If so, I'd push it down as far as it will go and begin cutting it a quarter-inch at a time. That's how I got the SWRs down.

 

[QwKiE]

If 40 is higher than 1 you shorten the antenna, If 1 is higher than 40 you lengthen the antenna. again i will say do no't test swr if you have an rf antenna on the meter and do not test where you are close to buildings and trees get out in the open area.

 

[K5MPH]

Today I went out and checked my SWR again and it is now reading lower. Could this be a result of having felt on the bottom of the magnet? Here are channel 1 and 40 SWR readings.

I have had two of these antennas over the years and never had and issues at all new they come pre-tuned for the C.B. band and yes the Felt will cause you mucho problems dont use it at all the metal looking tape on the bottom of the mag-mount need to make full conntact to a metal surface.....

 

[RockinMiner]

What would be the best way to protect the paint underneath the magnet mount?

 

[FiveFilter]

I have had two of these antennas over the years and never had and issues at all new they come pre-tuned for the C.B. band and yes the Felt will cause you mucho problems dont use it at all the metal looking tape on the bottom of the mag-mount need to make full conntact to a metal surface.....

Are you saying the factory's magnetic base should be stripped of the material it came with? I've never done that with any of my magnet antennas, assuming the factory tuned it with the material it camel with. Not true?

 

[KC4RAF]

Your CB antenna came from the factory with "felt" pad on it? All the mag mounts I've seen had tough teflon type of skin on it.

 

[lmrtek]

magnet mounts couple rf via capacitance

do not add or subtract ANYTHING from what the antenna came with

putting anything under the mount will radically change the performance

 

[rescue161]

What would be the best way to protect the paint underneath the magnet mount?

Don't use magnet mount antennas!

If you must use a mag-mount, I'd just get some screen protector material, like that for cell-phones, and place a piece on your cleaned / waxed car where you want the antenna. Then place the antenna on that screen protector. This works, but you are still going to experience damage from where dirt gathers at the edges of the screen protector. You'll notice it in a few years when you remove the screen protector. You'll also see damage where the coax rubs back and forth on the paint.

In the long run, you'll experience less damage to your vehicle by drilling a 3/4" hole for an NMO mount. When you sell the car, just place a 3/4" rubber plug (not many people will even notice), or if you want to go all out, you can get a small GPS style antenna to fill the hole when you sell the vehicle. Those are more common and people don't seem to care about those.

 

[KC4RAF]

100% what rescue161 posted. Less damage, thou it may raise the swr up a might bit. NMOs is the ONLY way to go.

 

[MisterLongwire]

What is all this HATE regarding mag mount antennas? I have a Wilson 5000 and it works fine on my Toyota truck(11 meters). Slight discoloration on the roof is no problem, versus leaks eventually a few years down the line when the gasket putters out, causing rust. Electrically speaking, and you should "know" being in amateur radio, that mag mounts are just as good as a roof mounted antennas. I speak from experience and I am sure there are a lot of others that would concur. In my opinion to each their own, however don't give false information, especially about capacitance, that mag mounts do not work. I operate 40, 10, 11, and 2 meters with a mag mount with various whips having made more contacts than people at the shack.

 

[rescue161]

I have never had an NMO leak - ever. If it is installed correctly, it will out-perform a magnet-mount every time. There is a time and a place for magnet-mount antennas, but they should never be used as a permanent solution.

Edit:
I install antennas and public safety equipment as part of my job. I've installed so many that I've lost track. We typically de-install equipment after about 3 years. None of the NMO mounts that we install have leaked. On the other hand, we do install some equipment that utilizes magnets. One is a license-plate reader with three cameras, all of which use magnets on their bases. Every single car that gets de-installed that has the LSAG system has rust, bubbled paint and scratches where the magnets were. I'd much rather have a small hole vs. bubbled paint and rust the size of the magnet base.

 

[KC4RAF]

Who said anything about "HATE"? Just stating the plain truth. I've had NMOs for long periods of time and none ever leaked. Plus the vswr was always better in my opinion. To each their own.
I started in the communications field back in '68. Found mag mounts good for temp use ONLY.