CB performance questions

[Bografan]

Ok, CB and radio noob here. Please bear with me. During my first Lone Star Jamboree this year, I noticed how crappily my handheld walkie was working out for trail communications, so I bumped a CB install higher up my list. A friend bought me a Cobra unit for my birthday in May, and I ended up purchasing a Comet RS-840 lip mount to go along with the 3 foot fiberglass whip I received from a truck driver I know.

The mount came with 16' 6" of coax, which was *just* enough for a clean install on the side of the passenger footwell. The mount I stuck high on the lift gate, to where a good 2 feet of the whip sticks out above the line of the roof and stock roof rack. Additionally, I added an HD firestik spring to releive some of the stresses from our 85mph speed limits. :tongue: I purchased an SWR meter, and ran some tests. I'm getting a range of values across all 40 channels of 1.4-1 to 1.7-1, which looking a little deeper indicates my antenna is too short. I looked at the whip, and it's not a tunable firestik, it's a 'cheapy' truck stop Road Pro whip.... Which brings me to my questions:

Is it possible that getting a better, tunable whip of the same size would be enough to drop my SWR to more acceptable levels? I don't want to go higher, the spring is already slightly deformed by the wind.
--OR--
I am reading in more than one place that 'short antenna' issues can also be caused by not matching the impedance of the antenna with the proper amount of wire, which the magic length seems to be 18' by consensus. Would adding a little extension to the cable be the better idea?
--OR--
Would adding a QD (4-5") be enough to help?
--OR--
With the SWR numbers I'm getting, is it really even an issue?

Forgive my ignorance on this, I'm just learning. I'm trying to see if there is a correlation between the amount of distance added and how much SWR is adjusted by those distances, and whether it is better to add distance on the cable side or the aerial.

Any help is appreciated!

 

[RonJR]

You're probably OK, given the antenna you have. Here's a bit more background on this stuff.

CB works at 27MHz, which is a wavelength of about 37 feet. The ideal whip antenna is 1/4 wavelength long, or about 9 feet. You occasionally see someone with a long whip like that attached to their bumper, but it's a bit impractical especially for off-roading.

When the antenna is shorter, its impedance doesn't match the cable (or free space), and a portion of the power transmitted by the radio gets reflected back instead going out over the air. An SWR of 1.7 means about 25% of your power is being reflected, and 75% is being transmitted, so your transmitter is about 75% efficient. The same thing happens to reception.

The other negative thing about a high SWR is that it creates larger voltages at the transmitter, which can in extreme cases damage the electronics in the output stage. That's more a problem with high power transmitters, and is not an issue with a CB at an SWR of 1.7

Better antennas are designed to electrically "look" longer to the transmitter to give you a better SWR, and may give you a bit better performance, but you are probably OK the way you are. Tinkering with the cable length may improve SWR (18 feet is about 1/2 wavelength, so that's why 18' is a magic number), but keep in mind that a 25% improvement in efficiency only represents about 2 dB of improvement, which in these kinds of situations is barely significant in terms of observed performance. I wouldn't worry about it.

 

[Tooltime19]

I went with a Wilson magnetic mount antenna called the Little Will.I like the look of wire whips. I ran the cable thru the passenger carpet near the door edge molding, with keeping enough cable free as to be able to pull the mag mount inside the cab. I set my SWR, which required raising my whip about 1/4"up in the loaded coil. When not in use, I unscrew the antenna whip mount from the loaded coil and put it in the back. Im not a big fan of the fiberglass whips for asthetics. However, a true Firestick fiberglass antenna is adjustable by pulling the plastic cap off the top, where you will find enough wire to tune your radio.

 

[EdmundGTP]

I'd say those SWR numbers are on the high side.

I'd get a new tunable antenna. Recommend a K-40 superflex in 3 foot. Adding any type of extra base mount, or quick detach mount is not going to help significantly.

The length of your coax cable should be a multiple of 3 and not excessively long to reach the mounting point. You don't want extra feet of it doubled up on itself or bundled in a coil.

Not sure where your antenna is mounted, but the placement of it may be negatively affecting your SWR.

Another big thing that people have a tendency to overlook is checking the grounds. There needs to be GOOD continuity between the shielding of your coax and your vehicle ground. Use a multi-meter to check. Don't just assume the grounding is good because everything is "bolted up tight". I had to do a pretty good amount of sanding between my antenna mount and the mounting point of the truck to get good electrical contact for the antenna system.

Running through all the tech docs on the firestik website should help you out.
Tech-Docs Index

Even though I'm not running a firestik antenna, they helped me out a ton when setting mine up.

 

[Bografan]

RonJR-- I had a feeling that was the explanation on the 18'= magic number, just didn't want to talk out of my ass. Thanks for helping me out.

Tooltime19-- Yeah, I'm thinking about getting something with a tunable tip and see how much I can tune down my SWR. I've got a gift card I wouldn't mind burning up. I like the steel whips as well, but by and large they're usually base loaded if I'm not mistaken, and base loaded is primarily magnet mount antennas.. I usually have one or two kayaks up on top of the 4Runner so I can't run a magnet mount.

EdmundGTP- I agree they're on the high side. The mount didn't come with its own grounding strap, but I got out my dremel and cleaned the paint off the set screws for the lip mount, and cleaned off the areas where the set screws hit my paint on the inside of the door. Everything else has pretty secure grounds, as checked with my V/OM. (See attached pic for my mounting location) For some weird reason instead of 18 feet of coax, my mount came from the Antenna Shack from 16 feet 6 inches. To save myself a little trouble, I think I'll go with your suggestion and buy a tunable tip antenna and see what I came come up with while fiddling with that. I'll also see what else the tech docs say-- Appreciate that.

 

[Bografan]

Another interesting tidbit: On my way home, I got a radio check back from a guy who called out his closest cross streets. I remembered where I was during my drive home when he hit me back, and checked it out on Google Earth-- He was 5.5 miles away, with hundreds (if not thousands) of office buildings, houses, and cars in the way. Not too horrible! I don't know how well (distance wise) that it will perform while out in the sticks but I'm somewhat reassured.

 

[having fun]

Another method for grounding is, take a separate wire and ground it to body/frame and to your mount. Always with low mounting areas( other than roof) 2/3 of the stick/wire above the roof line. Your strongest signal area/direction for your mounting will be in a egg shape circle towards the driver area.

 

[flyinhawaiian01]

Some good info about antennas and swr here, but I gotta chime in about the whole 3 foot/18 feet/specific wavelength required length for coax myth!

It is exactly that, a myth! I've been a ham for about 25 years now, and before that, I was REALLY into cb's! I believed that same myth back then until I was schooled by a radio tech back home who forgot more about commercial rf than I knew about any rf theory!!!

Make your coax whatever length you want, and if the antenna is not tuning correctly, you can literally use the coax to tune it by trimming short sections at a time. Its a painstaking process when tuning by coax, and you always want to start with a bit more coax than you need (say an extra foot or so). Believe me when I say that not every stainless or fiberglass ¼ wave whip tunes right out of the gate!

Of course, there will be a bunch of guys who think they know better, but if you really wanna understand how rf theory works when it comes to impedance matching, get your google-fu on and search for cb coax length myth, you will have a hundred plus sites that debunk this theory of specific lengths!

Here's one that I find amusing:

Exposing the 18' CB Coax Myth

Others are also out there, and will go into great detail about what exactly happens with coax and rf running through it, and why some people will get high swr and others a flat reading.

I could go on and on and get all geeky about the subject, but 99% of you will be bored and just want to know how to get your swr lower. I offer this simple adjustment guide...

If swr is:

high on channel 1 and low on channel 40, your antenna is too short

low on channel 1 and high on channel 40, your antenna is too long

Hopefully, you fall into the second scenario, as its always easier to trim than it is to lengthen.

As for the OP's swr readings, they aren't horrible, and the radio will be able to handle that mismatch just fine. If its 1.7 on 40 and 1.4 on 1, and you don't mind cutting into the antenna's tip, some antennas you can unravel a little of the copper wire, trim it, put the cap back over it and test again. This would work for the wire-wound 5/8th whip antennas that look just like a firestik.

You could also extend it slightly too if you are good at soldering (remove the enamel coating off the wire with sandpaper before soldering, it won't stick to the painted wire til you do!). In either case, adjust in ¼" increments to see how much the swr shifts!

 

[Bografan]

Thanks FlyinHawaiian! I had heard that it was a myth as well but didn't have the resources to do more homework...I appreciate the links. Love to learn!

And again, I appreciate everyone's input. My free adjustable-tip 3' Firestik 2 showed up today (the place I had a gift card with did not have K-40 but they had Firestiks, so...)

Put it on, grabbed my meter and tuned it. I made the antenna longer as indicated by my reading across the channels, and ended up being able to balance it (at one and one-quarter-turn out) to 1.4 to 1 across all channels, which is a sight better than 1.7 at channel 1 and 1.4 at 40...

I'm happy at the increase of efficiency! Thanks again to everyone who lent their knowledge, opinions, links and experience!