Antenna Tuning ?

Randy,
Uhhhh...if I understand you correctly, the tech said that 1.7 was the best to expect, and that trimming it any more might make your SWR start to go back higher. If I further understand, you went ahead and trimmed it a little and made it worse. My suggestion is to stop cutting. At a 2.0:1 SWR, you're only losing around half a watt of 5 watts out when you modulate.
And it's not surprising that you can hear stuff better than you can talk out. If everyone were running stock radios, then the TX and RX ranges should be about the same usually.
When skip is rolling, some of the folks you hear might very well be running thousand of watts from across the country (ever listen to "the Superbowl", ch 6?) and your radio will never be heard in the clamor of all those millions of watts being sprayed into the ether at the same time on the same channel (plus the adjacent channels because of badly adjusted modulators and dirty amps....)
If you've got unlimited funds, buy another antenna and cut it to the same length the tech had it at. *Then* go ahead and start trimming the one you've been cutting on and see what happens to your SWR. At the point where it gets up to around 2.7 or 3.0:1, take it off, and either put a spring under it to make up the length you've trimmed and start cutting again and stop at 1.7, or just put on the other whip
Depending upon how well you can hide a sheepish look, you can buy your new whip at the same place where the tech was and ask him to trim it for you -- but you'll have to put up with his "Oh, man!" look.
Not every vehicle installation will allow for that perfect 1.0:1 SWR, but it's not necessary, either. I've got a 102" whip mounted to the aluminum flashing at my house. I don't have a spring or extension on it cuz it's perfect on 10 meters, and only a 1.7:1 on the legal 40 CB channels. I can talk reliably about 25 miles with a stock CB with it, and I'm not hurting anything.
Hope that helps,
-- Handlebar --

 

Randy,
Thanks for clarifying. If I had a nickel for every time a piece of my test gear has gone south (usually one of my sacred, special, silvered, double-shielded test cables), I'd be able to....ummm... buy a new cable But we expect stuff off the shelf to be working right <fizzle>
If you made a crude graph of the SWRs you've got right now, with SWRs on the vertical axis, and the channel numbers across the bottom, you'd have a wide, shallow smile. At channel 1 it would be about 2.0:1, and would dip to about 1.7:1 at channel 20, and then rise again to around 2.0:1 at 40.
It would look a lot the way a Motown bassist (like Yours Truly would set the EQ on his bass amp.....called "mid-scoop"....but I digress.
If, in the unlikely event that you had a way of going lower than CB channel 1, or higher than ch 40, you'd probably see the corners of that smile extend into a bigger happy face. I say "unlikely" because I'm sure *none* of us has a way of going outside the band.......(Pssst, hey kid! Interested in some swampland?)
If you trim any more, you're going to move the lowest point on the smile, but not by moving the whole smile down -- you're just going to shift it to the side, with the SWR at one end of the channels getting higher and higher.
For the learning experience, and so you'll be more comfortable with the changes, there's nothing wrong with charting the SWR on all 40 channels, as carefully as you can. The more points you can put on the graph, the more accurate it will be. It may turn out that it's actually ch 16 where the lowest point is, and lengthening the whip a little bit will change it to ch 14. If it's lowest on ch 22, you're a millimeter or two long.
However, as I'm fond of pointing out: even though you can measure it up close with an instrument, neither you nor anyone on the air will ever hear the difference. And as long as you're consistently under 2.0:1, you're not going to hurt any equipment.
I wouldn't move it any more than it is. Depending upon how long the whip is now, you *may* see a little difference if you loosen the setscrew and pull it out just a teensy bit. But remember the mechanical arrangement; if you only have 1/8-inch of the whip in the adapter, you're more likely to snap it off or have it fall off going down the road than you are if there's 3/4-inch in there.
Dunno if I'm helping here or just making things more confusing. I'd say document everything, and play with it in tiny increments, but be ready to call it quits.
For myself, I'd be happy with the results you have now. I drive a small van with NMO mounts for land mobile antennas down the roof's midline. For CB, I choose between a 2-ft Barjan fiberglass with an NMO adapter (works up to about 15 miles just fine), a Maxrad/PCTel MLB2700 with spring (about 4-ft long, base loaded NMO cop antenna) or a 102-in w/spring on the rear quarter panel. The Barjan is the smallest and has the worst SWR at 1.7:1 to 2.1:1 across 40 channels, but works fine because everything else is correct. I use the Barjan when I'm going in & out of the garage a lot and staying mostly in town.
Hope that helps,
-- Handlebar --

 

Here's a little hint that I use when setting short, loaded whips. Take the regular whip out and straighten out a hanger. Measure it to the length of the existing whip. Get it as straight as possible. You may also have to file the bottom of the wire to fit the coil. Insert the hanger wire, measure the SWR and trim the hangar wire as needed. This way, you don't destroy your antenna and can tell if the antenna needs trimming, will be helped or harmed by cutting. Once the thing is at best SWR you can get, you can trim the real whip AFTER you are sure! Saves a lot of expensive whips that way. Hangers are lots cheaper than replacement whips!

Just as an aside, there is ONE "SWR" in ONE feedline! The popular CB thing seems to be "swr'zzzzzzzzzzzzzzzzzzz" (swr's). Doesn't hurt anything, but it is inaccurate. One VSWR (Voltage Standing Wave Ratio).
Basically, it is simply the ratio of power transmitted thru the line versus the amount of power reflected BACK to the transmitter. For those who want to know what is going on with their antenna, it is just a bit of useful beginner info.

GF

 

When you cannot lower the SWR reading, I would check out the antenna ground plane.
.
The first thing I would check is the resistance between the antenna mount and the door. It should be zero. If not, ground the antenna mount to the door.
.
Next, check the resistance between the antenna mount and the door frame. It should also be zero. If not, run a ground strap from the door, to the top of the top door hinge located on the door frame. Recheck the resistance to insure that it is zero.
.
Next, check the resistance between the antenna mount and the frame. It should also be zero. If not and your door has two separate hinges on the door frame then do the following. Run a ground strap from the bottom of the top hinge on the door frame to the top of the bottom hinge on the door frame.
.
Finally, run a ground strap from the bottom of the bottom door frame to the chassis ground. And recheck your SWR.

 

I would just check the cab to chassis ground. Pull the ground and clean the connections and reinstall. If the cab to chassis ground is bad then you will run into other electrical issues anyway.

Also make sure the hood is well grounded if it is not a fiberglass hood or plastic hood. I know this is an extreme but I have seem people take long sections of ground strapping and make a grid on the underside of their plastic/fiberglass hoods. They attach it with a high quality adhesive. Then they ground this to the chassis.

One thing that makes me think is the placment of his antenna. He has it right behind the cab on the box rail on the right side.

He needs to get it centered on the vehicle and get the loaded/coil section or the ball section of the antenna above the roof line as much as possible. This will give him a better radiation pattern and eliminate any reflect issues that he migh have with the antenna being right next to the back of the cab. This could also bring is swr down a bit because he is probly getting some reflect off the back of the cab.

 

When I referred to grounding the antenna mount to the door, door frame and etc , the terminology I used was not correct. The correct term that I should have used is BONDING. The purpose of bonding is to interconnect the large metal components of the truck to create a large antenna ground plane. It has nothing to do with electrical issues with the truck.
.
If you have a faulty antenna ground plane your SWR reading will be 3 or higher and this can cause damage to the power transistors. If the antenna ground plane is not sufficient, the symptom is a SWR reading that cannot be lowered by tuning the antenna.
.
With the exception of special no-ground plane antenna, the cb radio antenna needs an adequate ground plane to work correctly. Unfortunately in todays modern trucks, the large metal components may be connected physically but not interconnected for RF [radio frequency] purposes and are therefore not an adequate antenna ground plane.
.
I hope this clarification clears up any misunderstanding that I inadvertantly created by not using clear and correct wording.

 

Really? All those 50-ohm antennas have different SWRs? Assuming they're installed over an adequate ground plane, and there's no capacitive loading effects by nearby metal body parts, please explain this point.
I'll grant that they may have different useful bandwidths, but even the 2-ft Barjan I have mounted on my service van is less than 1.5:1 across the entire CB freq range.
But I'm all ears -- I love learning new stuff
-- Handlebar --