CB Tuning help

[hattrik21]

OK I met up with Tony (Tdesanto) so we could tune in my newly mounted CB and we got some really weird readings so we're looking for some ideas on what to check.

I grounded my antenna mount with just a 12 gauge wire, not sure if this has a bearing or not but I thought I'd mention it. I am running a Uniden 520 XL and a 36" firestik antenna. The radio mount is not grounded as the mounting information did not say it was needed. I'll post answers to any questions as best I can as we didn't know what more to try other than below.

On our baseline reading we got the following numbers.

ch1=3.2, ch20=1.7, and ch40=1.3

Extending the tip on the antenna the following numbers.

ch1=1.6, ch20=1.7, ch40=3.1

Lowering the antenna 1/8 of an inch.

ch1=2, ch20=1.3, ch40=2.5

To make sure my ground was sufficient we grounded the antenna to the frame and got the following numbers.

ch1=2.1, ch20=2.2, ch40=2.5

Shortened the antenna another 1.4 turn and removed the heavier ground cable the last set of numbers which is where it is at presently.

ch1=2.4, ch20=1.1, ch40=2.1

Now my receiving range seems pretty good, about 4 miles from where I started getting a clear transmission to where he said he was. Once I got on the highway, I called a radio check and 2 guys said my transmission had alot of squealing. One of them said my mic might be bad. So would a bad mic cause the odd readings we got above? If its not the mic, any ideas on what to look at for the squealing?

Thanks everyone in advance.

 

[DaveInDenver]

Squealing is often a symptom of a bad antenna ground in my experience. I would really look at the ground on the antenna. Make sure paint is scraped away and you are getting good contact, screws are well seated, etc.

 

[gary in ohio]

I am running a Uniden 520 XL and a 36" firestik antenna. The radio mount is not grounded as the mounting information did not say it was needed.

Where is antenna mounted, The radio mount shouldnt need a ground, it will be "grounded" via the power lead and antenna ground.

What quality device was your measuring the SWR with? Often a big difference between a truck stop SWR meter and and precision BIRD meter. You should be able to keep the SWR under 2 across the band.

Where were you when you measured the swr? You should be in the vehicle with all the doors closed and the vehicle out in the open, ou can check SWR in the garage. ANY around you will interact and cause false SWR readins.

[/QUOTE]
Now my receiving range seems pretty good, about 4 miles from where I started getting a clear transmission to where he said he was. Once I got on the highway, I called a radio check and 2 guys said my transmission had alot of squealing. One of them said my mic might be bad. So would a bad mic cause the odd readings we got above? If its not the mic, any ideas on what to look at for the squealing?
[/QUOTE]
Your receive range on CB will not be affected by SWR. squealing might just be road noise to some people, need to get a friend close by to listen, Key the mic and dont talk, what does he hear, then talk and see if he hears a squeal.
Squealing wouldnt affect SWR and if its truly squealing most likely its a bad mic or cable. Under standing what someone is calling squealing is the key.

 

[hattrik21]

The antenna is mounted to my factory roof rack via an ARB bumper mount.

The device we used is Tony's. It's made by Vanco Inc and the only model number I see is a SWR-1.

We did tune it next to his garage so maybe I'll pull it in front of my house tonight and run the tests again just to make sure. We did do all tests with all doors closed with just me inside.

On the squealing, I'll see if I can meet up with Tony again to test the squealing issue so for now I'll leave that as something to not worry about.

Thanks so far for the feedback.

 

[tdesanto]

The radio mount shouldnt need a ground

I asked him to post this up as a question, since my radio does require that it be grounded. My radio is a bit different, though, than the standard dash-mounted style. It's the Cobra 75WXST, and the power/antenna module is what needs to be grounded according to the "corrections". So, it’s probably my radio that is unique here.

http://www.walcottcb.com/product_info.php?cPath=284_286&products_id=520.

What quality device was your measuring the SWR with?

Also, the SWR meter is a cheap one, but the guys at Walcott claim that it's sufficient for basic use.
http://www.walcottcb.com/vanco-swr138x-swr-meter-3ft-coax-p-1031.html

Where were you when you measured the swr?

In my driveway, no other cars or heavy metal around. Antenna was about 8-10' from the garage. I suspected that this might affect performance slightly, but wasn’t sure.

Dan, you might want to re-perform, as you suggested, out in the open with even more room than we had last night.

Also, I found some good articles on Firestick’s website. All of these can be accessed, and a wealth of info, at: http://www.firestik.com/Tech_Docs.htm

The dip at channel 20 is clearly explained here: http://www.firestik.com/Tech_Docs/Off-tune.htm, and is apparently normal. My apologies for not recognizing this sooner.

Standard SWR testing and tuning info found here: http://www.firestik.com/Tech_Docs/Setting_SWR.htm

So, with a little more careful tuning, it appears that you may be able to get channels 1 and 40 down to about 2.2, and channel 19/20 will probably still stay around 1.3.

Let me know if you want to do some more testing soon.

 

[tdesanto]

I still suspect that the small gauge ground cable from the mount to the body could be causing the SWR readings at channels 1 and 40 to be above 2.0.

Could someone with more experience let us know if a larger ground cable could get us lower SWR readings?

Ultimately, I'd like to know if you can judge performace of the system on SWR measurements alone?

 

[DaveInDenver]

The device we used is Tony's. It's made by Vanco Inc and the only model number I see is a SWR-1.

I have one of those and it generally reads higher than the MFJ-864 I have. I trust the MFJ because it agrees with a Bird Wattmeter that I borrowed to tune a 2m antenna. So, yeah, the SWR-1 is not necessarily accurate, but the reading it gives errs to the safe side. It is nice because you can act like a quick and dirty field strength meter, too, to see how your radiated pattern looks.

 

[DaveInDenver]

I still suspect that the small gauge ground cable from the mount to the body could be causing the SWR readings at channels 1 and 40 to be above 2.0.

Could someone with more experience let us know if a larger ground cable could get us lower SWR readings?

Ultimately, I'd like to know if you can judge performace of the system on SWR measurements alone?

The cable size isn't going to matter much. The more important thing about RF wire is the number of strands (more is better) and it's impedance (not the same as resistance). How do you know this not having a EMI/EMC test facility? You don't, but you can assume that a wire has significant inductance and fairly little capacitance. A far better ground is braided strap, the stuff that looks like a tube. This has much better RF characteristics and will present a better shunt than a wire. For very short connections (say 8" or less) a stranded piece of wire or the shield from a piece of coax can work, but for anything longer a wire will basically look like it has very high impedance and so it's like you have a very poor ground. I would use 1/2" grounding braid for short connections of around 12" or less and 1" to tie the exhaust and sheet metal to the frame or longer than 12" connections.

No, SWR readings are not the only thing you use to judge system performance. In fact the SWR does not really tell you anything except how much of the RF energy is being reflected back down the coax. A low SWR just means that no energy is being lost through connections or a wildly out of tune antenna. But just because all the energy is going out does not guarantee ideal performance. Could easily mean that all the energy is being absorbed. This is why Gary says wisely not to tune your antenna in a garage or near objects. Those objects would be absorbing energy giving you a false SWR. It is a significant part of tuning and optimizing your antenna, but not the only determiner.

 

[hattrik21]

OK I changed out the plug that connects the antenna to the coax and also added the missing plastic space and took more readings. I'll test this later to see if the reported squealing is gone and update later.

Now I'm getting

ch1=1.9, ch20=1.3, ch40=2.1.

Does this seem a little more balanced? I know I'd love to be below 1.5 so maybe I'll look at a better ground strap if this will help. Will it? Thanks everyone

 

[DaveInDenver]

OK I changed out the plug that connects the antenna to the coax and also added the missing plastic space and took more readings. I'll test this later to see if the reported squealing is gone and update later.

Now I'm getting

ch1=1.9, ch20=1.3, ch40=2.1.

Does this seem a little more balanced? I know I'd love to be below 1.5 so maybe I'll look at a better ground strap if this will help. Will it? Thanks everyone

It all helps (grounding is particularly important, true). You're around 2:1 across the band, I wouldn't sweat it too much. IMO 1.5:1 or lower is doing plenty fine. Give it a bit more work and then don't worry.

 

[Scenic WonderRunner]

I hope you get it right!

Sounds like it might be a ground problem to me...but what the heck do I know>?!!!

All I know is.........

I was in the Panamint Mountains this past weekend. I was able to talk with B~Spec........clear up past Trona......which I estimate was over 30 miles as the red tailed hawk flies!...and I was even a callin' from a canyon!

Get your SWR Perfect........and you will be amazed at what your CB Radio can do! B~Spec was!....and he was running 20 watts! I was stock!


http://www.firestik.com/Tech_Docs/Setting_SWR.htm




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