I have a C.B. coax question.

[Metalhead396]

I have the dreaded C.B. coax question. So hear it goes. First off I'm not new to this but I'm not a HAM by any means. Some of this technical stuff I'm still in the learning phase. Anyway here goes I am using an MFJ antenna analyzer to work on my mobile CB. All testing was done on 27.205 Mhz. I have a 102" whip with a spring for an antenna. I am using LMR400 coax. I and running a linear in my setup also. I made two 1/4 wave jumpers and tuned them to "0" or less resonance. I put one jumper from the radio to the linear and the second from the linear to the antenna. With everything installed my antenna analyzer reads R-43, X-4, SWR 1.2. From trying different random jumpers I have seen different wattage out put from my linear. I'm looking for what is correct. I'm happy with the SWR of 1.2 but I would like to see the "R" closer to 50 and the "X" closer to 0 on 27.205 Mhz with everything hooked up. Now my question is, is running two 1/4 wave jumpers OK for my system or should it be two 1/2 wave jumpers or does it really make a difference? Am I just nit picking or can it be better than it is? Thanks for the input.

 

[mmckenna]

If your antenna system is good, properly tuned, proper ground plane, etc. the length of coax does not matter. You use the amount of coax you need to get from your radio to your amp, and from your amp to the antenna.

 

[Metalhead396]

It's a whole can of worms touching on this subject. But different lengths do give different wattage readings from the amp.

 

[mmckenna]

It's a whole can of worms touching on this subject. But different lengths do give different wattage readings from the amp.

Yep, they will. But are you reading the wattage AT the antenna, or somewhere in line? The "cut to specific length" thing has been getting circulated in the CB world for years. If you cut the coax to specific lengths, including taking into account the velocity of propagation for the specific cable, you can do all kinds of tricks, but it doesn't fix an improperly tuned antenna, it just hides it.

I have a slight grasp of the impedance and reactance stuff, but not enough to really explain it to someone else. You can tune for lowest SWR or you can tune for lowest reactance, they are often pretty close together. You can chase this stuff all day long, but unlikely you'll notice a huge difference in performance. Most CB'ers don't even check SWR, so you are quite a ways ahead. 1.2:1 is pretty good, and from my understanding a reactance of 4 isn't shabby.

I do know there there are some sharp folks on this site that can give better advice on this. But if you get a couple of different hams replying, you'll no doubt get a couple of different 'correct' answers. Trick will be picking which 'correct' answer you want to use.

 

[Metalhead396]

Thanks for the input. I might be chasing a dream too. LOL I just want it to be correct.

 

[mmckenna]

Thanks for the input. I might be chasing a dream too. LOL I just want it to be correct.

Hang tight, I'm sure someone will come along and give you some good directions.

Your CB/amp is going to be happy with low reflected power. Sounds like you are using decent coaxial cable, better than many use.
Is this a mobile or base install?

 

[Metalhead396]

Mobile,

 

[prcguy]

Consider your wattmeter is designed to work in a 50 ohm system and it was factory calibrated into a 50 ohm resistive load. You antenna is not a 50 ohm resistive load so the wattmeter will not read the correct power and since the antenna or "load" in this case is not 50 ohms, the coax to the antenna and its jumper cable and the internal line inside the wattmeter all add up as an impedance transformer, further changing things.

Tune your antenna the best you can and use it. When you want to know exactly how much power your putting out put a 50 ohm resistive load on your wattmeter, assuming your wattmeter is not a POS.

BTW, if you can't get the antenna tuned any better its probably due to insufficient ground plane. You can play with small values of capacitance at the antenna feedpoint like 25pf or 50pf, which can fool the system into thinking the whip has more capacitance to ground meaning a larger ground plane. That can sometimes push things over the edge into a near perfect match where an inline wattmeter might read right. You can even connect a foot or two of good coax to the feedpoint, center conductor to the antenna and shield to ground and leave the other end open. 50 ohm coax usually has about 25 to 30pf per foot and LMR400 is about 23pf per foot. You could attach 2ft of LMR 400 to the feedpoint for about 50pf and trim it several inches at a time to see if it improves the match, which I think it will. If it doesn't, I don't know where you heard that and I will deny any knowledge.

 

[Metalhead396]

I'm not sure how you're going to tune a 102" whip.

 

[prcguy]

You first map out the VSWR on ch 1 and 40, or a wider range if the radio can do it to find where the best match is and to see if the antenna is too long or too short. Fix that if it needs fixing, then if the VSWR is still a little high use the capacitor trick mentioned above to further reduce the VSWR to hopefully perfect. I have a half dozen or so SS 102" whips in my garage and they all have three set screws in the threaded adapter at the base. Take the whip out of the base, trim it and stick it back in the base to shorten. You can lengthen them about 1/2" before running out of whip for the lower set screw to grab.

I'm not sure how you're going to tune a 102" whip.

 

[Metalhead396]

The antenna SWR reads 1.2 for 1 to 40.

 

[prcguy]

Sounds like the length is fine but looking at where you mounted it the ground plane is probably not sufficient and the impedance is a bit high. Capacitance at the feedpoint can help that.

The antenna SWR reads 1.2 from 26.480 through 28.136. So below channel 1 and above 40.

 

[Metalhead396]

I've ordered a roll of bonding straps to bond the hood, bed, cab. etc. I have a 4ga. wire for a ground at the moment from the antenna bracket to the frame. Do you think that will do the trick?

 

[Metalhead396]

I'm not sure how I would go about adding that capacitor you mentioned.

 

[RichardKramer]

I don't know how much wattage your linear is running, but 400LMR in a mobile installation is a bit much. If you're not using more than 75' of cable and running more than a 100 watts, I would just use RG58 cable as most mobile installs are usually not more than 18' from the cb to the antenna. I would just use a 2' RG58 cable from the cb to the linear and whatever length of RG58 to the antenna. If your swr is 1.2, you're not going to notice much difference by trying to get it any lower to a 1.1; even at a 1.5 reading there won't be much of a change in tx/rx until you approach a reading of about 1.8 or higher. To tune any cb antenna if the swr is higher on channel 40 than ch1 you just start trimming about a 1/8" off of the antenna until the swr is more uniform across the band. On a base or top loaded antenna if the swr is lower on ch40 than ch1 you slide the antenna up a bit in the base to make it "longer" and hopefully you will have a lower swr on ch 1 before you run out of space to move it up out of the base. I have never had a 102" whip with spring having a higher swr on ch1 than on ch 40 as you actually have a 108" whip which if anything could be shortened a bit. You're tx/rx pattern will be diagonal across the length of the vehicle to which the antenna is mounted. My dad had a pickup truck and he wanted the best tx/rx possible, so I mounted a 102" whip with spring on the roof - what a sight that was!, but he sure did get his signal out running barefoot.

Rich - N3VMY - KAG0096

 

[prcguy]

Does your ball mount have an SO-239 or screws for coax attachment? If its screws attach a 25pf to 50pf disc ceramic with maybe 500 volt rating where the coax center conductor and shield attach and measure the match. If its better but not perfect try more or less value capacitance.

If the ball mount has a coax connector then stick a PL-259/SO-239 "T" adapter on it and make up about 2ft of coax with a PL-259 connector and stick that in the extra socket you got from adding the T adapter. That will leave you with 2ft of coax going nowhere and flopping around. Measure the match. Cut a few inches off and measure again. If the match is better when you started and keeps getting better as you cut the flopping coax shorter it will reach a low point and go back up to your usual 1.2:1 match. If you cut it too short and passed by the best match make up another floppy coax a little longer.

If you were in my end of town I would connect a small 200pf variable capacitor and tweak that to find the best match, then measure the capacitance and install a fixed value.

I'm not sure how I would go about adding that capacitor you mentioned.

 

[Metalhead396]

I run LMR400 because i run 2K watts. I'm not trying to get the SWR down. It's not bad. I was messing with the system ohms and resonance reading of the coax.

 

[Metalhead396]

No ball. Just a stud.

 

[prcguy]

Ooh, that is going to fall apart while you are on the road and the whip will be laying in the street some day. Those little mounts fall apart over time with 3ft whips. Get a real ball mount on that. And for 2kW use a 1kV or higher rated capacitor or you can use the coax T adapter and floppy coax capacitor for now since you have an SO-239 to play with.

No ball. Just a stud.

 

[Metalhead396]

I've had that type of mount for twenty years. Never fail yet.