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Antenna Mounting Question

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steve108

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I am trying to lower the SWR on my antenna setup and I am trying to narrow down the problem.

Ground is good throughout the setup at DC, I do not know how to accurately check RF ground.

I have attached a cross section, basically I removed the AM/FM antenna and mounted the 3' CB antenna in it's place.

SWR is about 2.7 on 1 and 40 with a dip to 2.2 on 19. When I had a 5' antenna with the same mount I had about 1.8 across the band.

I am wondering if there is an issue with having a portion of the antenna beneath and in close proximity to the fender? I can make up a mount that puts the entire antenna above the fender, bur I'm not sure if it will make enough of a difference to be worth the effort.
 

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WA0CBW

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You have the ground plane (the fender) above the base of the antenna. The ground plane goes under the base of the antenna. The transmitting part of the antenna begins at the point where the coax braid (shield) quits covering the center coax conductor. On the other hand some antenna brackets put the antenna too far above the ground plane. The base of the antenna mount needs to be connected to the fender and not below it.
BB
 

steve108

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Thanks, that has been my suspicion. I think with the 5' antenna since a much greater proportion of the antenna was above the ground plane I had better results. I guess I will make a different bracket.

Interesting what you said about the transmitting part beginning where the coax braid ends. I would have thought it began where the center conductor is no longer surrounded by any shield, be that the coax braid or the body of the connector. This is not the case?
 

Oldme

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Your setup looks a lot like my front fender mount on the E450 chassis
of my RV.
I use a 4' Everardt SST.
Before adding a spring I had a 1.4:1 to 1.6:1
Adding a standard spring and cleaning mounting
area (after a year) on the mount now = 1.1:1 to 1.3:1.
However ALL the antenna and mount are above the fender.

From my reading I find they say that a reading between 2.5 and 3 usually indicates
that the aerial is being affected by something around it, or that the groundplane is present
but maybe not big enough

Your older taller antenna did better, from what you wrote.
How close was that mounted to where this one is now?

Mounting beneath the fender will be a problem, I believe,
as the goundplane (fender) is now up to the radiating element
and reflecting it back affecting the antenna.
 

steve108

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My other antenna screwed in to the same stud you can see in the picture. I am going to try moving it up and I will report back.
 

steve108

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Looking at my mount I think it will be difficult to move the base of the antenna higher. Forgive me if this is completely ignorant but if I were to surround the portion of the antenna that is below my fender with the proper size metal tube (to create Z=50 ohms), and ground that tube, could I effectively turn the portion of the antenna below the fender in to a shielded cable?
 

WA0CBW

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As I mentioned before the antenna starts where the braid/shield/adapter/etc. ends and the center conductor or "antenna" begins. Putting a metal tube over the antenna would maintain the 50 ohms of the feed line but remember you now just shortened the antenna changing its length which in turn changes the resonant frequency and its impedance. The "base" of the antenna should be mounted in the hole in the fender. The fender is the ground plane and the base of the antenna needs to be above the ground plane.
BB
 

Oldme

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The answer does not change.
The antenna needs to be ABOVE the ground plane.
You have it below.
 

sloop

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A lot of factors can affect an antennas swr. I have a firestick mounted on the inside of my truck bed and my swr runs around 1.4 to 1. But that will vary depending on if the door is open, someone standing next to it, etc. I would be interested in what the swr was of the antenna in a different mount/location, was the hood up or down when the measurement was taken, how was the swr taken? I would find a HAM radio operator (or CBer) with an antenna analyzer to check out your antenna. You will find out a lot more about the efficiency of the antenna than you could with a basic swr meter (esp. one built into the radio).
 

Oldme

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There are still answers coming in for you.

If you are still trying to mount the antenna with the radiating
shaft below ground-plane as several of us have stated you will have problems.

This his truck is has limited choices.

A front fender mount will work well.
Nothing is perfect that is why I had to use that mount on the E450 based Motor Home.

The answer does not change.
The antenna needs to be ABOVE the ground plane.
You have it below.
What did you end up doing?
 

ipfd320

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The Fender is Radiating the Signal Back into the Antenna thats why the swr,s are so high--You Got to Get that Antenna Up Higher--Maybe You Can Try a Magnet Mount to Suit the 3 footer??
 

steve108

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Thanks for the help everyone. I haven't had a chance to change anything yet. I have taken some measurements and I think I can make a bracket that will get the bottom of my stud mount level with the top of the fender.

Relocating the antenna from the factory antenna location is not an option for me. I will live with the best I can get there. May go to a 4' but it needs to fit in my garage.

On a side note those 4runners are neat because they come with an antenna on the fender and in the rear glass. When I put my CB antenna on the fender I left the rear window glass antenna hooked up to my am/fm radio. Reception on am and fm did not change at all. Cool!
 

steve108

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Update

Finally got around to making and installing my new bracket. SWR is now about 2.5 on 1 and 40 and a dip to 2.0 on 19, better, but still not great. I have attached a quick drawing of the new mount.

I guess that is the best I can get with that location and a 3' antenna. Must be too close to the windshield...
 

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Oldme

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Interesting.
Like I posted before I am using a standard fender mount on the RV which
is an E450 chassis. It is a 4ft antenna with the top being 2 ft from the camper
bunk overhang. Not above it, but it is a fiberglass top section.
1.1:1 to 1.2:1 across the board.

I do not think the glass windshield will matter.
The metal roof edge will.
Is it close to the metal roof and does it go above it?

You may also want to look at bonding straps for the fender and hood to
help give it a better ground plane. Often they are not really bonded well
just assembled.

I assume that the shield ins making good contact with the bracket through the 90* you added.
Maybe a standard antenna mount and wire ends where you can ground the shielding separately
will be better.

It still seems like a Antenna ground / ground plane issue.

Hope you get if settled soon.
 

steve108

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I'd say the tip of the antenna is about 1' above the roof. Horizontally the antenna is about 5" from the winshield frame at the base and about 15" at the roof. I have a nice wide ground strap from the hood to the chassis.
 

Oldme

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Sounds like you are getting the bases covered.

That leaves the coax grounding for the antenna.
Can you run a temp ground attached to the outside
of the Pl259 at the antenna base to a chassis ground?
A wrap or tow of wire will work temporarily.
Then check the SWR.
IF it goes down, then you have an answer.

Most other mounts take the shielding wire and put that on a lug at the
antenna mounting base. That is insulated from the center by the nylon washer you have.
You used the 90* Pl259 and it is possible that the PL259 is not making enough contact
with bracket for the antenna ground properly.
 

steve108

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Ok, I will give that a try. I did check for, and do have continuity between the connector housing and ground at the radio end (at DC) but I do understand this does not indicate proper RF ground. Also checked for and I do have an open between the housing and center of the connector at the radio end.

Perhaps related, the meter on my radio will not go to full power on lower channels (below 11), but will on all other channels. Channel 10 indicates about 3.5W and it goes down to maybe 1.5W on channel 1. Does that mean anything significant?

Talking does raise the meter a bit (as expected) but not all the way. The meter only goes up to 4W so you can't see the modulation on the higher channels, but it is there. On channel 19 I have made some contacts at about a mile or so, but even at that I'm told the audio is weak.
 

Oldme

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The power problem sounds like you need a Tech to check it.
There should be very little difference in power between any channels.

Also are you checking the SWR with the built-in meter or a
standalone meter. Built in meters are notorious for not being
accurate.
 
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