High SWR

[prcguy]

When your talking VHF or UHF I would agree there is generally no difference in performance between an NMO style antenna on a 5" dia NMO mag mount and a permanent NMO mount in the exact same location. I've done antenna range type testing between the two and have not seen enough difference to favor one over the other.

At lower frequencies like CB, which the OP is having trouble with and especially the lower HF bands, there can be a huge difference in performance between a mag mount and a well grounded permanent mount. At some point in frequency, the mag mount doesn't have enough capacitance to the vehicle body to couple the ground side of the antenna and the coax will become the dominant counterpoise for the antenna.

When that happens ground losses go up, the feedpoint of the antenna will usually go up and antenna efficiency goes down. Not to mention your coax can be hot with RF on transmit and the antenna match will change as you move the coax around.

I've done some other testing where I've taken a 40m HF antenna tuned on a grounded mount, placed it on a mag mount in the same location and the antenna tuning was completely off the charts. I then made a large surface area capacitor from an 8 1/2 X 11" sheet of copper glued to a refrigerator magnet the same size and connected that to the ground side of the mag mount. The antenna then matched and performed almost exactly like it did on the permanent grounded mount in the same location.

So, you can supplement a magnetic mount with additional capacitance to ground where needed and cure a lot of problems and generally improve performance. I'm not sure if this pertains to the OPs problem, as others mentioned having good results with the same antenna. This was more of a rebuttal to MisterLongwire's comment that mag mount antennas "are just as good as roof mounted antennas", which is not always the case and I would be surprised if anyone concurred when discussing an HF mag mount.
prcguy

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.

 

[mmckenna]

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.

If your (or someone else's) permanent mount NMO's are leaking, then they are not being installed correctly. That isn't the fault of the mount or the manufacturer, but of the installer.

Antenna range tests will show a slight difference between a permanent NMO and a mag mount in the same spot. It's often on the order of 0.1db less for the mag mount, which most people will never notice. Manufacturer documentation will confirm this. And it's not a trick to get you to buy a different product, the major manufacturer permanent NMO mounts are a lot cheaper than their NMO mag mounts.

Mag mounts do have their place. I understand that some just can't drill holes in their vehicles. However, you won't see a police car or fire truck running around with mag mount antennas, and for a very good reason.
For amateur/CB use, use a mag mount if you want.

 

[rfradioconsult]

I recall a paper published by Motorola many years ago when mag mount antennas first became available. They did basically the same test took a sedan with a center mount permanent antenna, both a VHF and a UHF quarter wave, placed the vehicle on a turntable and plotted the field strength. Then ran the test with mag mount antennas in same location. Their results was <.2dB in all directions, the difference being insignificant.

 

[jim202]

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.

To give you some solid feedback on an NMO mount, Like has been said already, if they are installed correctly, they will not leak and cause rust to the roof of a vehicle. I have installed 4 of them on my truck way back in 2004 and they are still going strong with no leaks or operational problem. I use them with 100 watt transmitters.

Yes I have changed some of the antennas as the tree branches on some trails I have been on have broken a few of the antennas. But the NMO mounts keep right on trucking and don't leak.

 

[RockinMiner]

What would yous recommend in a NMO antenna then?

 

[cmdrwill]

For the 27 m CB: Comtelco CB1800.

 

[mmckenna]

Good choice. Also, Larsen NMO-27. I've got one that's 20+ years old.

 

[prcguy]

My favorite NMO CB type is the Antenex/Laird CWB27, its a special wide band antenna that covers 26.8 to 29.7Mhz with no tuning. Its also 67" long and very efficient. They are expensive but sometimes come up on Ebay for reasonable prices.
prcguy

 

[FiveFilter]

I use my CBs with magnet mount antennas on the roof for long highway trips. After I arrive where I'm going, I place the antenna in the trunk to keep it from getting weathered or from encouraging a low-life to break in to get at the radio. I haven't noticed any scratches, bubbling or anything else happening to the paint where the antennas are placed, but I do keep a coat of wax there to help things out.

For me, the magnetic mount antenna is the answer to my CBing needs.

If I had to punch holes in my cars and trucks to use a CB, I would do without. Simple as that.

 

[Rred]

I keep a couple of 3M (top quality) microfiber wipe cloths in the car. Bag of two dozen is cheap enough on Amazon or even at the local rip-off big box store. They're great for cleaning sunglasses and windshields...but also magmount bases. Wipe the roof, and the antenna base, and they will pick up and trap any grit that might scratch the paint.

Felt is just a way to trap grit and guarantee scratches. There's a thin plastic sheet on my store-bought bases, and just a layer of enamel paint on my home built. Which was made with radar magnetron magnets, back before all these fancy rare earth magnets were available. (After it sets up for a while, it needs two hands to get it back off again.(G)

 

[rescue161]

Here is the reason that I don't like magnet mounts. We have units that bring use vehicles for installs and will say that they don't want holes because of damage. After 3 years of magnet use, the damage speaks for itself. It doesn't matter how careful you are, a hole and a stationary antenna will never do as much damage as a long-term "permanent" magnet-mount.

Amber light that was on the vehicle for about a year.

LSAG camera system on a police vehicle for 3 years. It consists of three cameras on the trunk, each having three magnets.

Another angle after the LSAG cameras were removed. This is after rubbing compound to remove the rust and scratches.

Not only do the magnets cause problems, but the cables for the equipment (coax) usually rubs against the paint, causing scratches and puts extra wear on rubber gaskets are where the cables enter the vehicle. Usually after we show the units what damage a mag-mount can do in a permanent install, they will opt for a permanent install, holes and all.

 

[FiveFilter]

That type of effect is probably the result of never removing the magnet mounts.

I suspect that if the magnet mounts were lifted periodically and a good application of wax applied on the mounting surface, that the rust would not be a problem.

Any object sitting on a metal surface long term getting wet and retaining moisture is bound to cause some damage unless some maintenance is performed to prevent it.

In any case, my car shows more damage on the doors and fenders from parking in a city environment than those surface rust spots from three years of magnet mount usage.