Trouble tuning an antenna

[fireslayer123]

Well this is driving me nuts, so I will finally break down and ask for assistance.
I bought a Class "C" motor home last fall and it came with a "through the glass" antenna. I found that it only works about 1/4 of a mile so I decided to buy a magnetic antenna for the hood, because I am not ready to drill a hole in my new ride. I found out that I have a fiberglass hood, so I decided to buy an 18' extension and run it to the back. I installed a metal plate at the top of the ladder and stuck it to that. Just to be sure that I was getting a good ground, I ran a separate wire with the coax and bolted it to the plate that the antenna sits on.

I borrowed a SWR meter and I am way off. In fact the needle only moves a little. So with the suggestion of the SWR owner, I began to trim the antenna. It was getting better (down to 3) but then it got worse again. Out of frustration, I stuck the antenna to the rear bumper and lo and behold, I was down to 2. Obviously this is a horrible location for an antenna, but I was actually able to hear people talking. Mostly skip from Bakersfield some 300miles away.

Knowing that this is a horrible mess that I have created, I borrowed a buddies magnetic antenna just to see if things work without an extension. I took an elbow bracket and screwed it to the inside of the front fender in the engine compartment. (it is a 90* and comes out of the fender and goes horizontal over the fender.It gives me a good ground and a flat surface for the antenna to stick to)
When I checked the swr meter, it was pretty bad. Way above 3.

Now a few questions:

1) Why won't a magnetic antenna work when fully grounded and not using an extension? I have used mag antenna's in the past and they worked well enough.

2) Why am I getting a better signal from the back bumper then I am from the top of the ladder with a grounded plate?

3) When I tried to check my ground on the back bumper with an ohm meter, I wasn't even getting a ground until I scraped through the coating on the bumper. The antenna isn't even getting grounded and it works pretty well.

4) 1/4" at a time I ended up taking almost 8 inches of antenna off before it began to work (I was down to a 2 on the swr) The antenna has the 18' of extension in it and it works better from the bumper. Is the extension acting like an antenna? Is that why I needed to remove so much antenna?

I'm stumped
Dave

 

[rfradioconsult]

You are facing a couple of problems; first most factory antennas designed fo a specific band should not require massive cutting the rod to get a better match, you will probably need to start over with the original length. You are indeed suffering a lack of a suitable RF ground issue, there is a difference between a DC ground snd a RF ground. I suspect your "ground wire" is close to the quarter wavelength at 27 MHz roughly 108" from the place to the vehicle frame, a quarterwave wire carrying RF will transpose the impedence so you start at the plate with a low impedence and end up with a high impedence at the far end, BTW mag mount antennas don't depend on making a direct ground connection but are capactively coupled to the body of the vehicle.

If your motorhome has a metal trim strip running around the sides just below the top you can take a small L bracket mount it to the trim strip and use a NMO style antenna suitable for 11 Meters and route the coax along the trim strip using small nylon cable clips to enter the RV. Years ago I had this setup with a CB antenna one one side and a 2 Meter antenna on the other, worked well for both.

 

[methusaleh]

And do you even know for sure that the ladder/rack is grounded to the frame of the vehicle?

 

[fritz73]

rfradioconsult offers good advice, and I will add my own experience for what it's worth. A magnetic mount antenna and it's coax work together as a system, they are kind of in a world of their own as far as resonance goes. The coax is a specific length for a reason. Never cut the coax to make it shorter, and I wouldn't even add to it for that matter. I almost learned my lesson the hard way once: with a SWR meter and 2ft jumper in line the meter said the antenna needed to be shortened, A LOT, but take them out of line and guess what- almost perfect resonance. The meter and jumper were indeed adding to the length of the antenna so to speak. As long as the antenna is stuck to a piece of metal with some decent size area (to have a ground plane to work off of) preferably the center of a metal plate, wide bar, etc. up in the open you should be in good shape.

 

[fireslayer123]

It still doesn't make sense that the damn thing will pull a 2 on the swr meter from the back bumper, with 18' of extension, but won't pick up squat from either the front bumper or the "L" bracket screwed to the body, using zero extension.

To tell you the truth, you guys are talking over my head a little. DC ground vs RF ground doesn't make sense.
I have the ground running to the bracket in the rear that is coming from a frame bolt. It should be a decent ground.
Plus the "L" bracket up front is screwed right to the sheet metal. The meter is way up above 3 in both locations.

I have access to another antenna with an "L" bracket. I did think about screwing it to the trim around the top, but it probably isn't grounded.

Is a NMO antenna something special or a regular antenna?

 

[rfradioconsult]

NMO is an industry standard name for a mobile mount designed to mount through a car body or an L bracket using a 3/4 inch hole; BTW NMO = New Motorola. Look on the internet and you can find serveral vendors that will sell you an NMO antenna for 11 Meters (CB Band) and also sell you the antenna mount with coax and usually the L bracket. Chances are your antenna would work bolted to the hood, if it was returned to it's original length. The term "Grounding" can mean different things at DC vs. RF. Even if the trim isn't grounded to the frame the trim can function as radials and perform quite well.

A SWR meter in the hands of someone without some knowledge of how to work around RF equipment can lead to a lot of issues and keep you from finding the real solution, if your antenna comes untuned start by carefully studying the enclosed cutting chart, don't go crazy with the cutters, it's awfully difficult to put those little pieces back on the antenna.

 

[fireslayer123]

Thank You!

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NMO is an industry standard name for a mobile mount designed to mount through a car body or an L bracket using a 3/4 inch hole; BTW NMO = New Motorola. Look on the internet and you can find serveral vendors that will sell you an NMO antenna for 11 Meters (CB Band) and also sell you the antenna mount with coax and usually the L bracket. Chances are your antenna would work bolted to the hood, if it was returned to it's original length. The term "Grounding" can mean different things at DC vs. RF. Even if the trim isn't grounded to the frame the trim can function as radials and perform quite well.

A SWR meter in the hands of someone without some knowledge of how to work around RF equipment can lead to a lot of issues and keep you from finding the real solution, if your antenna comes untuned start by carefully studying the enclosed cutting chart, don't go crazy with the cutters, it's awfully difficult to put those little pieces back on the antenna.