Trimming antenna..how much is too much?

[Matego]

First off, thanks to you guys for the wealth of information on this site. I could not find a lot about this particular subject.
I have a Jeep with a Bearcat 980 radio, 18’ of coax and a Everhardt TSM 4’ antenna.
I made sure to run the coax so it wasn’t looped or coiled, cleaned the grounds from the antenna base, tire mount, frame and body, verified low resistance with a DMM.
I am using the built in SWR meter in the radio for tuning. The general consensus from the internet says the meter in the 980 is pretty reliable, I don’t have another meter to confirm.
I started out 2.5 SWR or so CH1 and 4.x CH40. I have trimmed the antenna to where I have 1.2 CH1 and 2.1 CH40. Every time I cut 1/4in from the wire, it does get better.
Now for the question, I have cut 6”, maybe more to get to this point. I will soon have to cut down the fiberglass rod because I can’t tuck the wire end into the slot without stretching the coiled end.
Is this an indication of some other issue and I am ruining this antenna?

 

[Hurricane145]

6 inches would seem to be a lot to trim off from my experiences with CB antennas used on CB.

What you want is an acceptable SWR curve when measured throughout the 40 channels.Not high at one end and low at the other. Sometimes in some settings it is not possible to get the SWR really low in the middle channels but there still is a good curve to it or as much a good curve as you can get.
I have at times quit trimming when I got close enough not wanting to have it get worse all of a sudden or knowing I am never likely to use the lower channels or more likely the higher channels since I don't have SSB in the car anyway.
If your SWR is 1.2 on CH1 and 2.1 on CH40 it would seem that the antenna is too long yet but probably really close to what you would want. As you get closer, trim less and less as smaller bits will help keep you from going too much the other way with the SWR curve.
If you added a spring to the bottom of the antenna (certainly not uncommon) it will add length and effect the resonance of the antenna making it longer than usual. A GOOD spring has a piece of braid inside it from the bottom to the top so that the only length added is the length of the braided wire itself. Otherwise as the spring flexes and coils separate a bit, the SWR can go all over the place.

Most antennas have extra length to them when purchased. The manufacturers can't be sure just what you are doing with it or just where you want to tune it. Better to start out too long than too short!!

 

[Hurricane145]

There is some discussion about that antenna and trimming it here, a couple posts down.
https://www.rubiconownersforum.com/...-electronics/26239-everhardt-cb-antennas.html

 

[loosecannon]

can you post a picture of your antenna installed on the vehicle?

Im wondering if you might have it running adjacent to some body metal and causing SWR issues.

Using a DMM will only tell you that there is a DC connection between the two measuring points.

you are looking for a good RF ground, and if you mounted the antenna to the tire rack, depending on the type, you may need to bond it to the rest of the vehicle with some 1" braid. on some tire mounts the hinges are the only connection to the vehicle, and im sure you can see how that wouldn't be a very good RF connection.

also, just to dispel what i think is a myth you were told; coiling up coax in a mobile install is not a bad thing, and as long as you dont try to coil it too tightly (definitely no less than 4" or so) then it is fine to do. coiling your coax can actually be helpful in some situations, and mobiles are a good example.
if you had coax you wanted to coil up, do it right at the base of the antenna.
this will help keep common mode currents from coming back down the coax.
LC

 

[Matego]

I have an aftermarket tire carrier that pivots on the back bumper instead of the tailgate. There is a bracket on top that’s supposed to be for the 3rd brake light, I could drill the hole out and use for an antenna mount if it would help.
The soft top frame is only a few inches from the antenna and the roll cage is about 1 foot away. The antenna does extend 2-3 feet over the roof.
I do have some flexible ground straps that I could use. I am very much a novice when it comes to radios. I did clean all the ground points and I load tested the grounding of the mount by using a 55w halogen bulb I use to load test circuits. It draws a little over 5 amps at 12v. It’s bright when it has a good circuit and dim if poor.

 

[nomadradio]

An antenna ground needed to make that kind of antenna function like it should is not quite the same as needed for a DC-current load like a tungsten bulb.

For that kind of antenna to function properly, it needs a ground connection with surface area.

The ideal "ground" for a fiberglass whip is the roof of the vehicle. This provides metal surface area in all directions leading away from the base of the whip.

The farther you get from having metal in all directions leading away from that mounting point, the more trouble you'll see trying to tune the antenna.

A ground wire won't behave the same, since it has almost no surface area. Using a wire to ground a whip antenna causes the wire to become part of the antenna, making it seem to be longer than it really is. Might explain why trimming it made the SWR come down a little.

A headache rack is a little better, since it has more surface area than a piece of wire. The spare-tire rack also has that going for it, although we have seen that some antennas will seem to tune okay mounted there, while other antennas just refuse to.

The wire you removed from this whip has probably ruined it for use on CB frequencies. If you took this antenna and screwed it into a mount that's already on somebody's cab roof, you would need an antenna analyzer that goes way above channel 40 to see the frequency that it's now tuned to. Probably somewhere around 28 or 28.5 MHz in the 10-meter ham band.

Kinda clumsy to put that wire back on the antenna.

But I'm pretty sure that your SWR meter made it appear you needed to trim this antenna because of an insufficient "RF" ground.

73

 

[Road Squawker]

I have an aftermarket tire carrier that pivots on the back bumper ....
I do have some flexible ground straps that I could use....

The problem with tire racks is that they pivot, the hinge pieces roll against each other and do not make a good solid connection.
in fact, most of the actual contact areas are either painted or ride on plastic bushings.

A ground strap across the hinge will work.

as a side note.
If you have ever bonded a vehicle , this applies to all sliding contact hinges, doors, hoods, lift gates, ect.

Even most engine blocks and exhaust systems ride on rubber insulators and are poorly grounded to the frame.

good luck

 

[Shadetree Mechanic]

Tons of information here:

http://www.k0bg.com/

 

[Matego]

I will try and see if I can hook some straps from the swinging arm to the frame and then run a strap from the frame to the Jeep tub. It can’t hurt anything.
I did try using a jumper cable from the mount to the body at a cleaned ground and made no difference before I started to trim the antenna.
I heard the NGP antenna systems were not necessary but a lot of guys like them on the Jeep forums. I wanted to get this pretty good so I have an option on antennas and not be limited to NGP antennas.

 

[Hurricane145]

But I'm pretty sure that your SWR meter made it appear you needed to trim this antenna because of an insufficient "RF" ground.

That could be it. It could be giving you misleading readings that never get better as you trim so you keep trimming.
At the point when you have trimmed a little and don't see any change in SWR it is time to stop and look at things to figure out why things are not working as they should.

I agree it is not the best place to mount an antenna. The most convenient or easiest place is not ever the best!

 

[RT431]

Maybe this will help?

https://www.extremeterrain.com/wran...aWmghUMQn8PzL5LyjEYbqud2N-BH8VIhoC1gIQAvD_BwE

 

[M0GVZ]

My bet is the antenna is capacitively coupling to the adjacent vertical metal bits of the soft top frame which will make it appear electrically longer to RF. I bet if you take that soft top off or fold it right down that the SWR changes.

And to add, that mount provides no RF ground and even straps across the hinges won't do much. It is about the metal directly underneath the antenna, not what is alongside it, and only horizontal metal counts, not vertical. The idea of the RF ground on a vehicle is that the horizontal metal provides a much more efficient surface for RF currents to flow over than the ground (as in the surface you're driving on) below the vehicle hence only horizontal metal counting. Currently as far as your antenna is concerned, even with all the ground straps in the world, the only thing below it is the road.

 

[Hurricane145]

Sometimes people mount antennas like that and although performance suffers, it doesn't matter much to them as they are only talking car to car (or jeep to jeep) at close proximity to each other. Not trying to "really get out" as they say. They do not have big demands on communications and don't need much range or performance.

In the end you see a lot of antennas mounted like that and figure it must work pretty well. It probably does in a more caravan like situation or at a job site but not so much across any distance.

 

[Matego]

I have seen some people mount their antenna beside the hood on a fender mount. I don’t really like it there, kinda inconvenient off-roading.
I am mainly worried about the high SWR hurting my radio. For the most part, we might need to talk 1 mile apart. 75% of the guys I off-road with have those handheld Midland radios with the rubber duckie antenna plugged into a cigar lighter.
For whatever reason, I am usually the one out front blazing the trail. I have had difficulty talking to the other guys 1/2 mile away through very dense woods and mountains. I believe it’s mostly their handhelds and not my rig. I would just like my radio to be right and working to the best of its ability.

 

[fourstringburn]

I always tune an antenna to the lowest SWR point at the center of the the band or any part of a band I normally operate at and rely on the antennas bandwidth to cover the other frequencies. A decent 4 or 5ft. mobile CB antenna should have at least 500 KHz bandwidth if not more.

This means if you can get a near flat SWR on Channel 20, you should be just below a 2.0 SWR at 27.435 and 26.935 MHz which is perfectly OK to operate SWR wise. They won't always be evenly balanced between the low and high ends of the band though so there maybe a bit more range on one side or the other.

Some antennas have up to 800 KHz bandwidth which will increase the operating range before retuning.

The common method of balancing out channel 1 and channel 40 is CB shop folklore. All this does is give you a compromise across the band which usually isn't the lowest SWR.

If your mobile antenna shows a flat SWR across the whole band, you either have a leaking dummy load on a stick or your test equipment isn't accurate. Even a 102 whip will have a SWR curve, it’s just a wider one compared to a shortened coil loaded antenna.

With a good analyzer such as the Rigexpert, you can quickly scan a band, find the frequency with lowest SWR, and see the antennas bandwidth. From there you can easily see if your antennas is too long or short and adjust accordingly.

If your antenna reqires cutting, I would start with a 1/4 inch and check how much the SWR moved. You should be able to determine how much more is needed from there.